Backlight for switchable directional display

TECHNICAL FIELD

[01] This disclosure generally relates to illumination from light modulation devices, and more specifically relates to control of privacy display.

BACKGROUND

[02] Privacy displays provide image visibility to a primary user that is typically in an on- axis position and reduced visibility of image content to a snooper, that is typically in an off- axis position. A privacy function may be provided by micro-louvre optical films that transmit some light from a display in an on-axis direction with low luminance in off-axis positions. However such films have high losses for head-on illumination and the micro- louvres may cause Moire artefacts due to beating with the pixels of the spatial light modulator. The pitch of the micro-louvre may need selection for panel resolution, increasing inventory and cost.

[03] Switchable privacy displays may be provided by control of the off-axis optical output. [04] Control may be provided by means of luminance reduction, for example by means of switchable backlights for a liquid crystal display (LCD) spatial light modulator. Display backlights in general employ waveguides and edge emitting sources. Certain imaging directional backlights have the additional capability of directing the illumination through a display panel into viewing windows. An imaging system may be formed between multiple sources and the respective window images. One example of an imaging directional backlight is an optical valve that may employ a folded optical system and hence may also be an example of a folded imaging directional backlight. Light may propagate substantially without loss in one direction through the optical valve while counter-propagating light may be extracted by reflection off tilted facets as described in U.S. Patent No. 9,519,153, which is incorporated by reference herein in its entirety.

[05] In a known privacy display the privacy mode is provided by the addition of a removable louver film, such as marketed by 3M Corporation, which may not be reliably or easily fited or removed by users and therefore in practice, is not assiduously atached by the user even' time they are outside the office. In another known privacy display the control of privacy mode is electronically activated but control is vested in the user who must execute a keystroke to enter privacy mode. BRIEF SUMMARY

[06] According to a first aspect of the present disclosure, there is provided an illumination apparatus comprising: a waveguide extending across a plane and comprising: first and second opposed light guiding surfaces arranged to guide light along the waveguide, the second light guiding surface being arranged to guide light by total internal reflection, and an input end arranged between the first and second light guiding surfaces and extending in a lateral direction between the first and second light guiding surfaces; at least one light source arranged to input light into the waveguide through the input end, wherein the waveguide is arranged to cause light from the light sources to exit from the waveguide through the second light guiding surface by breaking total internal reflection; and an optical turning film component comprising: an input surface arranged to receive the light exiting from the waveguide, the input surface extending across the plane; and an output surface facing the input surface, wherein the input surface comprises: a first array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a first array of lines across the plane in which the input surface extends; and a second array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a second array of lines across the plane in which the input surface extends, the first array of lines and the second array of lines extending at different angles projected on to the plane so that the first array of prismatic elements and the second array of prismatic elements deflect the light exiting from the waveguide into different lobes. Advantageously illumination may be provided with high efficiency around two different polar directions.

[07] The first array of lines may have a first arithmetic mean tangential angle projected on to the plane and the second array of lines may have a second arithmetic mean tangential angle projected on to the plane, the second arithmetic mean tangential angle may be greater than the first arithmetic mean tangential angle. Advantageously the turning film may provide different lobes from a common waveguide.

[08] The first array of lines may have an average tangential angle projected on to the plane of 0° from the lateral direction. The first and second arrays of lines may each have tangential angles projected on to the plane of not more than 55° from the lateral direction. The optical aiming film component may have a rectangular shape across the plane and the lateral direction may be along a major or minor axis of the rectangular shape. Advantageously one of the lobes may be provided at or near to the normal to the illumination apparatus and a second lobe may be provided away from the optical axis.

[09] The first away of lines may be straight. The second array of lines may be straight. The output of the illumination apparatus may be provided with a common lobe direction for regions across the illumination apparatus for either of the first or second lobes. The far field illumination pattern may have a reduced angular width, advantageously achieving increased far field lobe fidelity.

[10] lire first array of lines may be curved. The second array of lines may be curved. A common optical window may be provided for regions across the illumination apparatus, advantageously achieving increased uniformity across the illumination apparatus. In a display, the display uniformity may be increased.

[11] Facet angles of respective facets, defined between a normal to the facet and a normal to the plane, are between 40° and 70° and preferably between 47,5° and 62.5°. Advantageously the output from the collimated waveguide may be directed in polar directions around the normal to the illumination apparatus.

[12] In at feast one of the first and second arrays of prismatic elements, the facets on opposite sides of the ridge may have facet angles defined between a normal to the facet and a normal to the plane across which the input surface extends that may be different. Advantageously different lobes may be provided for light input at opposite ends of the waveguide ,

[13] The output surface may be planar. Advantageously cost and complexity' may be reduced.

[14] The at least one light source comprises an array of light sources. Advantageously redundancy may be provided for the light source, increasing lifetime.

[15] The waveguide further comprises a second input end arranged between the first and second light guiding surfaces opposite to the first mentioned input end. The illumination apparatus may further comprise at least one first fight source arranged to input light into the waveguide through the first input end and at least one second light source arranged to input light into the waveguide through the second input end. First and second sets of lobes may be provided with different polar distributions of luminance and/or luminous intensity .

[16] The illumination apparatus may further comprise a control system arranged to control the at least one first light source and the at least one second light source independently. Advantageously different illumination profiles may be achieved. [17] The waveguide may be arranged to cause light from the at least one first light source and the at least one second light source to exit from the waveguide with different angular distributions. The different lobes may have solid angles profiles that are different for light from respective first and second light sources. Advantageously the illumination apparatus may be switched between different polar distributions, each with different lobes.

[18] The waveguide may be arranged to cause light from the at least one first light source and the at least one second light source to exit from the waveguide with a common angular distribution. The different lobes may have solid angles profiles that are the same for light from respective first and second light sources but with different directions. Advantageously the illumination apparatus may be switched between different polar directions, each with different lobes of substantially the same size.

[19] The illumination apparatus may comprise: at least one first light source arranged to provide input light; at least one second light source arranged to provide input light in an opposite direction from the at least one first light source; a waveguide arrangement arranged to receive the input light from the at least one first light source and the at least one second light source and to cause light from the at least one first light source and the at least one second light source to exit from the waveguide arrangement by breaking total internal reflection, wherein the waveguide arrangement comprises at least one waveguide; and an optical turning film component comprising: an input surface arranged to receive the light exiting from a waveguide through a light guiding surface of the waveguide by breaking total internal reflection, the input surface extending across the plane; and an output surface facing the input surface, wherein the input surface comprises: a first array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a first array of lines across the plane in which the input surface extends; and a second array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a second array of lines across the plane m which the input surface extends, the first array of lines and the second array of lines extending at different angles projected on to the plane so that the first array of prismatic elements and the second array of prismatic elements deflect the light exiting from the waveguide into different lobes.

[20] The waveguide arrangement may comprise: a waveguide extending across a plane and comprising: first and second opposed light guiding surfaces arranged to guide light along the optical waveguide, the second light guiding surface being arranged to guide light by total internal reflection, and first and second input ends arranged between the first and second light guiding surfaces and extending in a lateral direction between the first and second light guiding surfaces; wherein the at least one first light source is arranged to input light into the waveguide through the first input end and the at least one second light source is arranged to input light into the waveguide through the second input end, and the waveguide is arranged to cause light from the at least one first light source and the at least one second light source to exit from the waveguide through one of the first and second light guiding surfaces by breaking total internal reflection. Advantageously thickness and cost may be reduced.

[21] The waveguide arrangement may comprise: a first waveguide extending across a plane and comprising first and second opposed light guiding surfaces arranged to guide light along the optical waveguide, the second light guiding surface being arranged to guide light by total internal reflection; and a first input end arranged between the first and second light guiding surfaces and extending in a lateral direction between the first and second light guiding surfaces; wherein the at least one first light source is arranged to input light into the first waveguide through the first input end, and the first waveguide is arranged to cause light from the at least one first light source to exit from the first waveguide through one of the first and second light guiding surface by breaking total internal reflection; a second waveguide extending across the plane in arranged in series with the first w _' aveguide and comprising first and second opposed light guiding surfaces arranged to guide light along the optical waveguide, the second light guiding surface being arranged to guide light by total internal reflection, and a second input end arranged between the first and second light guiding surfaces and extending in a lateral direction between the first and second light guiding surfaces; wherein the at least one second light source is arranged to input light into the second waveguide through the second input end, and the second waveguide is arranged to cause light from the at least one second light source to exit from the second waveguide through one of the first and second light guiding surfaces by breaking total internal reflection, and wherein the first and second waveguides are oriented so that at least one first light source and at least one second light source input light into the first and second waveguides in opposite directions. Advantageously luminance uniformity may be increased.

[22] According to a second aspect of the present disclosure there is provided a backlight apparatus comprising: an illumination apparatus and a rear reflector arranged to receive light exiting from the first surface of waveguide and direct it back through the waveguide. Advantageously output luminance and/or luminous intensity may be increased. [23] According to a third aspect of the present disclosure there is provided a display apparatus comprising: a backlight apparatus and a spatial light modulator arranged to receive light from the backlight apparatus. A display with high luminance directed towards at least two different polar directions may he provided. Advantageously display efficiency and brightness may be increased.

[24] The display apparatus may further comprise at least one display polariser arranged on a side of the spatial light modulator; an additional polariser arranged on the same side of the spatial light modulator as the display polariser; and at least one polar control retarder arranged between the display polariser and the additional polariser, the at least one polar control retarder including a switehab!e liquid crystal retarder comprising a layer of liquid crystal material. Advantageously a privacy display may be provided with high efficiency. The display may be switchable between a privacy mode and a public mode of operation.

[25] According to a fourth aspect of the present disclosure there is provided a vehicle having a display apparatus according to the third aspect mounted therein. The different lobes into which light exiting from the waveguide may be deflected may be lobes directed at locations of two occupants in the vehicle. Advantageously a privacy display may be provided with high security factor for one of the vehicle occupants and high image visibility' for another occupant: and the display may be switched to a public mode of operation with high image visibility for multiple occupants.

[26] The different lobes into which light exiting from the waveguide may be deflected may be lobes respectively directed at a location of an occupant in the vehicle and at a location of an internal surface of the vehicle. Advantageously the security factor to the other occupant may be increased. Light that is reflected from the internal surface may be reflected by the display to the other occupant, further increasing security factor in a privacy mode of operation.

[27] According to a fifth aspect of the present disclosure there is provided an optical turning film component comprising: an input surface and for receiving light exiting from a waveguide through a light guiding surface of the waveguide by breaking total internal reflection, the input surface extending across a plane; and an output surface facing the input surface, wherein tire input surface comprises: a first array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a first array of lines across the plane; and a second array of prismatic elements each comprising a pair of facets defining a ridge therebetween, the ridges extending along a second array of lines across the plane, the first array of lines and the second array of lines extending at different angles projected on to the plane so that the first array of prismatic elements and the second array of prismatic elements deflect the light exiting from the waveguide into different lobes. Advantageously a turning film for use in a privacy display including a switchable privacy display may be conveniently tooled at low cost.

[28] Embodiments of the present disclosure may be used in a variety of optical systems. The embodiments may include or work with or in cooperation with a, variety of illuminators, environmental lighting, interior and exterior automotive illumination, projectors, projection systems, optical components, displays, microdisplays, computer systems, processors, self- contained projector systems, visual and/or audio-visual systems and electrical and/or optical devices. Aspects of the present disclosure may be used with practically any apparatus related to optical and electrical devices, optical systems, presentation systems or any apparatus that may contain any type of optical system. Accordingly, embodiments of the present disclosure may be employed in optical systems, devices used in visual and/or optical presentations, visual peripherals and so on and in computing environments.

[29] Before proceeding to the disclosed embodiments in detail, it should be understood that the disclosure is not limited in its application or creation to the details of the particular arrangements shown, because the disclosure is capable of other embodiments. Moreover, aspects of the disclosure may be set forth in different combinations and arrangements to define embodiments unique in their own right. Also, the terminology used herein is for the purpose of description and not of limitation.

[30] These and other advantages and features of the present disclosure will become apparent to those of ordinary skill in the art upon reading this disclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[31] Embodiments are illustrated by way of example in the accompanying FIGURES, in which like reference numbers indicate similar parts, and in which:

[32] FIGURE 1A is a schematic diagram illustrating a front perspective view of a switchable privacy display;

[33] FIGURE IB is a schematic graph illustrating a front perspective view of a stack of optical components in the apparatus of FIGURE 1 A;

[34] FIGURE 2 is a, schematic diagram illustrating a top view of a privacy display in an automotive vehicle; [35] FIGURE 3A is a schematic diagram iilustrating a side view of a switchabie illumination apparatus with different solid angles in first and second lobe directions;

[36] FIGURE 3B is a schematic diagram illustrating a top perspective view of a collimating waveguide comprising lenticular and stepped prismatic surfaces arranged on the same side of a waveguide arranged to provide different solid angles of output for light input at first and second input sides;

[37] FIGURE 3C is a schematic diagram illustrating a top perspective view of a collimating waveguide comprising linear prismatic and stepped prismatic surfaces arranged on the same side of a waveguide arranged to provide different solid angles of output for light input at first and second input sides;

[38] FIGURE 4A is a schematic diagram illustrating a side view of a switchabie backlight further comprising a rear reflector and with different solid angles in first and second lobe directions;

[39] FIGURE 4B is a schematic diagram iilustrating a top perspective view of a collimating waveguide comprising lenticular and stepped prismatic surfaces arranged on opposite sides of a waveguide arranged to provide different solid angles of output for light input at first and second input sides;

[40] FIGURE 5A is a schematic diagram illustrating a side perspective view' of a turning film comprising first and second arrays of elongate straight prismatic elements;

[41] FIGURE SB is a schematic diagram illustrating a side perspective view of the arrangement of cutting tools to achieve a turning film comprising first and second arrays of elongate prismatic elements;

[42] FIGURE SC is a schematic diagram iilustrating a top perspecti ve view of the optical stack of a switchabie backlight;

[43] FIGURE 5D is a schematic diagram illustrating a cross section of a light turning film component;

[44] FIGURE 6 is a schematic graph illustrating the polar and azimuthal variation of luminance for an illustrative backlight with light input at the first end of the waveguide;

[45] FIGURE 7 is a schematic graph illustrating the polar and azimuthal variation of luminance for an illustrative backlight with light input at the second end of the waveguide;

[46] FIGURE 8A is a schematic graph illustrating the polar and azimuthal variation of transmission of a switchabie retarder arranged between parallel polarisers for switchabie liquid crystal retarders driven for privacy mode; [47] FIGURE 8B is a schematic graph illustrating the polar and azimuthal variation of relative reflection of a switchable retarder arranged between a reflective polarises' and absorbing polariser tor switchable liquid crystal retarders driven for privacy mode;

[48] FIGURE 9A is a schematic graph illustrating the polar and azimuthal variation of visual security factor, S in a privacy mode of operation for a display head-on luminance, of value Y _max measured in nits that is half of the illuminance of value I measured in lux;

[49] FIGURE 9B is a schematic graph illustrating the polar variation of visual security factor, S for zero elevation in a pri vacy mode of operation for a display head-on luminance, of value Y _max measured in nits that is half of the illuminance of value I measured in lux;

[50] FIGURE 10 is a schematic graph illustrating the polar and azimuthal variation of visual security factor, S for the arrangement of FIGURE 1 A in a public mode of operation for a display head-on luminance, of value Y _max measured in nits that is half of the illuminance of value I measured in lux;

[51] FIGURE 11 is a schematic diagram illustrating a rear perspective view' of the first array of elongate prismatic elements wherein the ridges and facets comprise wobble;

[52] FIGURE 12A is a schematic diagram illustrating a side perspective view of a turning film comprising first and second arrays of prismatic elements wherein the first array of prismatic elements comprises curved prismatic elements;

[53] FIGURE 12B is a schematic diagram illustrating a top view of an illumination apparatus comprising a turning film component with first and second arrays of prismatic elements wherein the first array of prismatic elements comprises curved prismatic elements;

[54] FIGURE 13A is an array of schematic graphs illustrating the variation of Security- Factor, S for various different primary user viewing angles in a display of FIGURE IB comprising the turning film of FIGURE 5 A;

[55] FIGURE 13B is an array of schematic graphs illustrating the variation of Security Factor, S for various different primary user viewing angles in a display of FIGURE IB comprising the turning film of FIGURE 12 A;

[56] FIGURE I4A is a schematic diagram illustrating a top perspective view of a collimating waveguide comprising linear prismatic and orthogonal lenticular surfaces arranged on opposite sides of a waveguide arranged to provide similar solid angles of output for light input at first and second input sides;

[57] FIGURE 14B is a schematic diagram illustrating a top perspective view- of a collimating waveguide comprising linear prismatic and orthogonal prismatic surfaces arranged on the same side of a waveguide arranged to pro vide similar solid angles of output for light input at first and second input sides;

[58] FIGURE 15 is a schematic diagram illustrating a side view of a switchable backlight with the same solid angles in first and second lobe directions;

[59] FIGURE 16A is a schematic diagram illustrating a side view of a switchable backlight with the same solid angles in first and second lobe directions comprising first and second stacked waveguides;

[60] FIGURE 16B is a schematic diagram illustrating a front perspective view of the pupillated backlight of FIGURES 16A-B comprising an optical turning film with a first array of prismatic elements that is linear and a second array of prismatic elements that is curved;

[61] FIGURE 16C is a schematic graph illustrating the polar variation of luminance for the backlight of FIGURES 16A-B for light from the first light source;

[62] FIGURE 16D is a schematic graph illustrating the polar variation of luminance for the backlight of FIGURES 16A-B for light from the second light source;

[63] FIGURE 16E is a schematic graph illustrating the polar variation of luminance for the backlight of FIGU RES 16A-R for light from the first and second light sources;

[64] FIGURE 17A is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 14A and turning film of FIGURE 5A for light input from light sources at a first input side for privacy mode of operation;

[65] FIGURE 17B is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 14A and turning film of FIGURE 5A for light input from light sources at a second input side for public mode of operation;

[66] FIGURE 18 is a schematic diagram illustrating a side view of a privacy display m an automotive vehicle;

[67] FIGURE 19A is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film similar to that of FIGURE 5A for light input from light sources at the first input side for privacy mode of operation;

[68] FIGURE 19 El is a schematic graph illustrating the polar and azimuthal variation of output luminance of tire collimated waveguide of FIGURE 3B and off-axis turning film similar to that of FIGURE. 5 A for light input from light sources at the second input side for public mode of operation; [69] FIGURE 19C is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film similar to that of FIGURE 5A tor light input from light sources at both the first and second input sides for public mode of operation;

[70] FIGURE 20 is a schematic diagram illustrating a top view of a segmented backlight;

[71] FIGURE 2IA is a schematic diagram illustrating a top view of a centre stack display in an automotive vehicle;

[72] FIGURE 21B is a schematic diagram illustrating a front perspective view of a dual lobe display;

[73] FIGURE 21 C is a schematic diagram illustrating a side perspective view of a turning film comprising first and second arrays of linear straight prismatic elements wherein the axes of the prismatic elements are each inclined to an axis along the edge of the display;

[74] FIGURE 21 D is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film of FIGURE 21C for light input from light sources at the first input side for efficient mode of operation;

[75] FIGURE 22A is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through the optical stack of FIGURE IB in a privacy mode of operation;

[76] FIGURE 22B is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of FIGURE IB in a privacy mode of operation;

[77] FIGURE 23A is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through the optical stack of FIGURE IB in a public mode of operation;

[78] FIGU RE 23B is a schematic graph illustrating the variation of output luminance with polar direction for the transmitted tight rays in FIGURE 23A;

[79] FIGURE 23C is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of FIGURE IB in a public mode of operation; and

[80] FIGURE 23 D is a schematic graph illustrating the variation of reflectivity with polar direction for the reflected light rays in FIGURE 23C.

DETAILED DESCRIPTION [81] Terms related to optical retarders for the purposes of tire present disclosure will now he described.

[82] In a layer comprising a uniaxial birefringent material there is a direction governing the optical anisotropy whereas all directions perpendicular to it (or at a given angle to it) have equivalent birefringence .

[83] The optical axis of an optical retarder refers to the direction of propagation of a light ray in the uniaxial birefringent material in which no birefringence is experienced. This is different from the optical axis of an optical system which may for example be parallel to a line of symmetry or normal to a display surface along which a principal ray propagates.

[84] For light propagating in a direction orthogonal to the optical axis, the optical axis is the slow' axis when linearly polarized light with an electric vector direction parallel to the slow axis travels at the slowest speed, lire slow axis direction is the direction with the highest refractive index at the design wavelength. Similarly the fast axis direction is the direction with the lowest refractive index at the design wavelength.

[85] For positive dielectric anisotropy uniaxial birefringent materials the slow axis direction is the extraordinary axis of the birefringent material. For negative dielectric anisotropy uniaxial birefringent materials the fast axis direction is die extraordinary axis of the birefringent material.

[86] The temis half a wavelength and quarter a wavelength refer to the operation of a retarder tor a design wavelength Xothat may typically be between 500nm and 570nm. In the present illustrative embodiments exemplary' retardance values are provided for a wavelength of 550nm unless otherwise specified.

[87] The retarder provides a phase shift between two perpendicular polarization components of the light wave incident thereon and is characterized by the amount of relative phase, G, that it imparts on the two polarization components; which is related to the birefringence Dh and the thickness d of the retarder by

[88] In eqn. 1, An is defined as the difference between the extraordinary and the ordinary- index of refraction, i.e.

Δn == n _e - n _o eqn. 2 [89] For a half-wave retarder, the relationship between d, Dh, and l _{0 i} s chosen so that the phase shift between polarization components is For a quarter-wave retarder, the relationship between d, Δn, and λ ₀ is chosen so that the phase shift between polarization components is

[90] The term half-wave retarder herein typically refers to light propagating normal to the retarder and normal to the spatial light modulator.

[91] Some aspects of the propagation of light rays through a transparent retarder between a pair of polarisers will now be described.

[92] The state of polarisation (SOP) of a light ray is described by the relative amplitude and phase shift between any two orthogonal polarization components. Transparent retarders do not alter the relative amplitudes of these orthogonal polarisation components but act only on their relative phase. Providing a net phase shift between the orthogonal polarisation components alters the SOP whereas maintaining net relative phase preserves the SOP. In the current description, the SOP may be termed the polarisation state.

[93] A linear SOP has a polarisation component with a non-zero amplitude and an orthogonal polarisation component which has zero amplitude.

[94] A linear polariser transmits a unique linear SOP that has a linear polarisation component parallel to the electric vector transmission direction of the linear polariser and attenuates light with a different SOP.

[95] Absorbing polarisers are polarisers that absorb one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of absorbing linear polarisers are dichroic polarisers.

[96] Reflective polarisers arc polarisers that reflect one polarisation component of incident light and transmit a second orthogonal polarisation component. Examples of reflective polarisers that are linear polarisers are multilayer polymeric film stacks such as DBEF ^lM or APF™ from 3M Corporation, or wire grid polarisers such as ProFlux ^lM from Moxtek. Reflective linear polarisers may further comprise cholesteric reflective materials and a quarter waveplate arranged in series.

[97] A retarder arranged between a linear polariser and a parallel linear analysing polariser that introduces no relative net phase shift provides full transmission of the light other than residual absorption within the linear polariser.

[98] A retarder that provides a relative net phase shift between orthogonal polarisation components changes the SOP and provides attenuation at the analysing polariser. [99] In the present disclosure an ’A-plate' refers to an optical retarder utilizing a layer of birefringent material with its optical axis parallel to the plane of the layer,

[100] A 'positive A-p!ate' refers to positively birefringent A-plates, i.e. A-plates with a positive An.

[101] In the present disclosure a 'C-plate' refers to an optical retarder utilizing a layer of birefringent material with its optical axis perpendicular to the plane of the layer. A 'positive Opiate' refers to positively birefringent C-plate, i.e. a C-plate with a positive An. A 'negative C-plate' refers to a negatively birefringent C-plate, i.e. a C-plate with a negative An.

[102] O-piate' refers to an optical retarder utilizing a layer of birefringent material with its optical axis having a component parallel to the plane of the layer and a component perpendicular to the plane of the layer. A 'positive O-plate' refers to positively birefringent O- plates, i.e. O-plates with a positive An.

[103] Achromatic retarders may be provided wherein the material of the retarder is provided with a retardance An . d that varies with wavelength l as

Δn . d / λ = k eqn. 3

[104] where k is substantially a constant,

[105] Examples of suitable materials include modified polycarbonates from Teijin Films, Achromatic retarders may be provided in the present embodiments to advantageously minimise colour changes between polar angular viewing directions which have low luminance reduction and polar angular viewing directions which have increased luminance reductions as will be described below.

[106] Various other terms used in the present disclosure related to retarders and to liquid crystals will now' be described.

[107] A liquid crystal cell has a retardance given by An . d where Ail is the birefringence of the liquid crystal material in the liquid crystal ceil and d is the thickness of the liquid crystal cell, independent of the alignment of the liquid crystal material in the liquid crystal cell.

[108] Homogeneous alignment refers to the alignment of liquid crystals in sw itchabie liquid crystal displays where molecules align substantially parallel to a substrate. Homogeneous alignment is sometimes referred to as planar alignment. Homogeneous alignment may typically be provided with a small pre-tilt such as 2 degrees, so that the molecules at the surfaces of the alignment layers of the liquid crystal cell are slightly inclined as will be described below. Pretilt is arranged to minim ise degeneracies in switching of cells.

[109] In the present disclosure, homeotropic alignment is the state in which rod-like liquid crystalline molecules align substantially perpendicularly to the substrate. In discotic liquid crystals homeotropic alignment is defined as the state in which an axis of the column structure, which is formed by disc-like liquid crystalline molecules, aligns perpendicularly to a surface. In homeotropic alignment, pretilt is the tilt angle of the molecules that are close to tlie alignment layer and is typically close to 90 degrees and for example may be 88 degrees.

[110] In a twisted liquid crystal layer a twisted configuration (also known as a helical structure or helix) of nematic liquid crystal molecules is provided. The twist may be achieved by means of a non-parallel alignment of alignment layers. Further, cholesteric dopants may be added to the liquid crystal material to break degeneracy of the twist direction (clockwise or anti-clockwise) and to further control the pitch of the twist in the relaxed (typically undriven) state. A supertwisted liquid crystal layer has a twist of greater than 180 degrees. A twisted nematic layer used in spatial light modulators typically has a twist of 90 degrees.

[111] Liquid crystal molecules with positive dielectric anisotropy are switched from a homogeneous alignment (such as an A-piate retarder orientation) to a homeotropic alignment (such as a C-piate or O-plate retarder orientation) by means of an applied electric field.

[112] Liquid crystal molecules with negative dielectric anisotropy are switched from a homeotropic alignment (such as a Opiate or O-plate retarder orientation) to a homogeneous alignment (such as an A-plate retarder orientation) by means of an applied electric field.

[113] Rod-like molecules have a positive birefringence so that n _e > n ₀ as described in eqn. 2. Discotic molecules have negative birefringence so that n _e < n ₀ .

[114] Positive retarders such as A-pIates, positive O-plates and positive Opiates may typically be provided by stretched films or rod-like liquid crystal molecules. Negative retarders such as negative C-plates may be provided by stretched films or discotic like liquid crystal molecules.

[115] Parallel liquid crystal cell alignment refers to the alignment direction of homogeneous alignment layers being parallel or more typically antiparallel. In the case of pre-tilted homeotropic alignment, the alignment layers may have components that are substantially parallel or antiparallel. Hybrid aligned liquid crystal cells may have one homogeneous alignment layer and one homeotropic alignment layer. Twisted liquid crystal cells may be provided by alignment layers that do not have parallel alignment, for example oriented at 90 degrees to each other.

[116] Transmissive spatial light modulators may further comprise retarders between the input display polariser and the output display polariser for example as disclosed in U.8. Patent No. 8,237,876, which is herein incorporated by reference in its entirety. Such retarders (not shown) are in a different place to the passive retarders of the present embodiments. Such retarders compensate for contrast degradations for off-axis viewing locations, which is a different effect to the luminance reduction for off-axis viewing positions of the present embodiments,

[117] Terms related to privacy display appearance will now be described.

[118] A private mode of operation of a display is one in which an observer sees a low contrast sensitivity such that an image is not clearly visible. Contrast sensitivity is a measure of the ability to discern between luminances of different levels in a static image. Inverse contrast sensitivity may be used as a measure of visual security, in that a high visual security level (VSL) corresponds to low image visibility.

[119] For a privacy display providing an image to an observer, visual security may be given as:

VSL = (Y + R) / (Y - K) eqn. 4

[120] where YSL is the visual security level, Y is the luminance of the white state of the display at a snooper viewing angle, K is the luminance of the black state of the display at the snooper viewing angle and R is the luminance of reflected light from the display.

[121] Panel contrast ratio is given as:

C Y / K eqn. 5

[122] For high contrast optical LCD modes, the white state transmission remains substantially constant with viewing angle. In the contrast reducing liquid crystal modes of the present embodiments, white state transmission typically reduces as black state transmission increases such that

Y + K ~ P.L eqn. 6

[123] The visual security level may then be further given as:

[124] where off-axis relative luminance, P is typically defined as the percentage of head-on luminance, I. at the snooper angle and the display may have image contrast ratio C and the surface reflectivity is p.

[125] The off-axis relative luminance, P is sometimes referred to as the privacy level. However, such privacy level P describes relative luminance of a display at a given polar angle compared to head-on luminance, and is not a measure of privacy appearance.

[12.6] The display may be illuminated by Lambertian ambient illuminance I. Thus in a perfectly dark environment, a high contrast display has VSL of approximately 1.0. As ambient illuminance increases, tire perceived image contrast degrades, VSL increases and a private image is perceived.

[127] For typical liquid crystal displays the panel contrast C is above 100:1 for almost all viewing angles, allowing the visual security level to be approximated to:

[128] The perceptual image security may be determined from the logarithmic response of the eye, such that the security factor, S is given by:

[129] Desirable limits for S were determined in the following manner. In a first step a privacy display device was provided. Measurements of the variation of privacy level, P(0) of the display device with polar viewing angle and variation of reflectivity r(q) of the display device with polar viewing angle were made using photopic measurement equipment. A light source such as a substantially uniform luminance light box was arranged to provide illumination from an illuminated region that was arranged to illuminate the privacy display device along an incident direction for reflection to a viewer positions at a polar angle of greater than 0° to the normal to the display device. The variation 1(θ) of illuminance of a substantially Lambertian emiting lightbox with polar viewing angle was determined by measuring the variation of recorded reflective luminance with polar viewing angle taking into account the variation of reflectivity r(q). lire measurements of R(q), r(9) and 1(0) were used to determine the variation of Security Factor S(0) with polar viewing angle along the zero elevation axis.

[130] In a second step a series of high contrast images were provided on the privacy display including (i) small text images with maximum font height 3mm, (ii) large text images with maximum font height 30mm and (iii) moving images.

[131] In a third step each observer (with eyesight correction for viewing at 1000mm where appropriate) v iewed each of the images from a di stance of 1000mm, and adjusted their polar angle of viewing at zero elevation until image invisibility was achieved for one eye from a position near on the display at or close to the centre-line of the display. The polar location of the observer’s eye was recorded. From the relationship S(0), the security' factor at said polar location was determined. The measurement was repeated for the different images, for various display luminance Y _max , different lightbox illuminance l(q=0), for different background lighting conditions and for different observers.

[132] From the above measurements S < 1.0 provides low or no visual security, 1.0 < S <1.5 provides visual security that is dependent on the contrast, spatial frequency and temporal frequency of image content, 1.5 < S < 1.8 provides acceptable image invisibility ⁷ (that is no image contrast is observable) tor most images and most observers and S ³ 1.8 provides full image invisibility, independent of image content for all observers.

[133] In comparison to privacy displays, desirably wide-angle displays are easily observed in standard ambient illuminance conditions. One measure of image visibility is given by the contrast sensitivity such as the Miehsison contrast which is given by:

[134] and so:

M = ((Y+R) - (K+R)) / ((Y+R) + (K+R)) = (Y-K) / (Y+K+2.R) eqn. 11

[135] Thus the visual security level (VSL), is equivalent (but not identical to) i/M. In the present discussion, for a given off-axis relative luminance, P the wide-angle image visibility, W is approximated as [136] In the present discussion the colour variation De of an output colour +Av’) from a desirable white point (u _w ’, v _w ’) may be determined by the CIELUV colour difference metric, assuming atypical display spectral illuminant and is given by:

[137] Catadioptric elements employ both refraction and reflection, winch may be total internal reflection or reflection from metallised surfaces.

[138] The structure and operation of various directional display devices will now be described, in this description, common elements have common reference numerals, it is noted that the disclosure relating to any element applies to each device in which tire same or corresponding element is provided. Accordingly, for brevity such disclosure is not repeated.

[139] A switcliabie privacy display apparatus will now be described.

[140] FIGURE 1A is a schematic diagram illustrating a front view of a privacy display apparatus 200 comprising a privacy display device 100 that is controlled by a privacy control system 350, 352, 354. The display device 100 displays an image.

[141] Display apparatus 100 comprises a backlight apparatus 20; and a spatial light modulator 48 arranged to receive light from the backlight apparatus 20.

[142] In the present disclosure, the spatial light modulator 48 may comprise a liquid crystal display comprising substrates 212, 216, and liquid crystal layer 214 having red, green and blue pixels 220, 222, 224. The spatial light modulator 48 has an input display polariser 210 and an output display polariser 218 on opposite sides thereof. The output display polariser 218 is arranged to provide high extinction ratio for light from the pixels 220, 222, 22.4 of the spatial light modulator 48. Typical polarisers 210, 218 may be absorbing polarisers such as dichroic polarisers.

[143] Optionally a reflective polariser 208 may he provided between the input display polariser 210 and backlight 20 to provide recirculated light and increase display efficiency. Advantageously efficiency may be increased.

[144] The backlight apparatus 20 a waveguide 1, first and second light sources 15, 17, a rear reflector, 3 and optical turning film component 50. Rear reflector 3 is arranged to receive light exiting from the first surface of waveguide I and direct it back through the waveguide I .

[ 145] Optical stack 5 may comprise diffusers, light turning films and other known optical backlight structures. Asymmetric diffusers, that may comprise asymmetric surface relief features for example, may be provided in the optical stack 5 with increased diffusion in the elevation direction in comparison to the lateral direction may be provided. Advantageously image uniformity may be increased.

[146] Light is output into lobes 440.4, 440B as will be described further herein.

[147] FIGURE IB is a schematic graph illustrating a front perspective view of a stack of some of the optical components in the apparatus of FIGURE 1A. Features of the embodiment of FIGURE IB not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[148] The display apparatus 100 comprises: input display polarise r 210 and output display po!ariser 218 arranged on the output side of the spatial light modulator 48; an additional polariser 318 arranged on the same side of the spatial light modulator as the display polariser 218: and at least one polar control retarder 300 arranged between the display polariser 218 and the additional polariser 318, Polarisers 210, 218, 318 may be absorbing diehroic polarisers.

[149] The display polariser 218 and the additional polariser 318 have electric vector transmission directions 219, 319 that are parallel, and orthogonal to the input polariser 210 transmission direction 211. Reflective polariser 302 further has a polarisation transmission direction 303 that is aligned parallel to the polarisation transmission directions 219, 319.

[150] In FIGURE 1A, additional polariser 318 is arranged on the same side of the spatial light modulator 48 as the display output polariser 218. Alternatively (not shown) the additional polariser 318 may be arranged on the same side as the input polariser 210 and polar control retarder 300 may be arranged between additional polariser 318 and input polariser 210. Alternatively (not shown) plural polar control retarders and plural additional polarisers may be provided on the input side of the spatial light modulator 48. Alternatively (not shown) plural polar control retarders may be provided on the input and output sides of the spatial light modulator 48.

[151] Polar control retarders 300 comprise: (i) a switchable liquid crystal retarder 301 comprising a layer 314 of liquid crystal material arranged between transparent support substrates 312, 316 and arranged between the display polariser 218 and the additional polariser 318: and (ii) at least one passive compensation retarder 330.

[152] FIGURE 1A further illustrates a reflective polariser 302. that is arranged between the output polariser 218 and the polar control retarder 300. The operation of polar control retarders 300 arranged between polariser 218, 302, 318 will be described hereinbelow with respect to FIGURE 22A to FIGURE 23D.

[153] Control of the polar control retarders is achieved by means of driver 350 to change the operating voltage across the liquid crystal layer 314. Controller 352 is provided to control the driver 350 and controller 354 that further controls the driving of light sources 15, 17.

[154] The display device 100 is arranged to display an image and capable of operating in at least a public mode and a privacy mode, wherein in the privacy mode the privacy function is provided and the visibility of tire image to an off-axis viewer is reduced compared to the public mode and the visibility ⁷ of the image to the primary ⁷ user in an on-axis position remains visible in both the privacy and public modes. The control system 350, 352, 354 selectively operates the display device 100 in the public mode or the privacy mode for at least one region of the displayed image, typically the entire displayed image.

[155] FIGURE 2 is a schematic diagram illustrating a top view of a privacy display in an automotive vehicle 650 that has a mounted display apparatus 100 of the present embodiments. Features of the embodiment of FIGURE 2 not discussed in further detail may he assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[156] in some embodiments of the present disclosure different lobes have directions 410, 412 of peak luminance into which light exiting from the waveguide 1 is deflected, the formation of which will be described hereinbelow. The lobes 410, 412 are directed at locations of two occupants 45, 47 in the vehicle 650.

[157] in some embodiments of the present disclosure different lobes have directions 410, 414 of peak luminance into which light exiting from the waveguide 1 is deflected, the formation of which will be described hereinbelow. The lobes 410, 414 into which light exiting from the waveguide 1 is deflected are lobes respectively directed at a location of an occupant 45 in the vehicle and at a location of an internal panel 652 of the vehicle 650.

[158] The structure and operation of illumination apparatus will now be described,

[159] FIGURE 3A is a schematic diagram illustrating a side view of a switchabie illumination apparatus; and FIGURE 3B is a schematic diagram illustrating atop perspective view of a collimating waveguide 1 comprising lenticular microstructure 30 and stepped prismatic surfaces 32, 36 arranged on the same side of a waveguide 1 arranged to provide different solid angles of output for light input at first and second input sides 2, 4. [160] Waveguide 1 is an optical waveguide extending across a plane (x-y plane in FIGURE 3A) that comprises first and second opposed light guiding surfaces 6, 8 arranged to guide light rays 402, 406 along the waveguide 1. The first and second light guiding surfaces 6, 8 are arranged to guide light by total internal reflection, in the embodiment of FIGURE 3 A, the surface 6 is planar and the surface 8 comprises optical microstructures as will be described further with regards to the illustrative embodiment of FIGURE 3B.

[161] Waveguide 1 comprises an input end 2 arranged between the first and second light guiding surfaces 6, 8 and extending in a lateral direction (along the x-axis) between the first and second light guiding surfaces 6, 8.

[162] At least one light source 15 is arranged to input light into the waveguide through the input end 2. Light source may comprise an array of light sources, such as an LED array.

[163] The waveguide 1 is arranged to cause light from the light sources 15 to exit from the waveguide 1 through the second light guiding surface 8 by breaking total internal reflection. That is incident rays 402 on the surface 8 have angles of incidence at the surface 8 that are less than the critical angle at the said surface 8.

[164] The illumination apparatus further comprises an optical turning film component 50 that comprises an input surface 56 arranged to receive the light rays 402 exiting from the waveguide 1, the input surface 56 extending across a plane. The turning film component 50 further comprises an output surface 58 facing the input surface 56. In the embodiment of FIGURE 3A the output surface 58 is planar. In alternative embodiments the output surface may comprise optical structures such as diffusers. Advantageously the size of the light lobes 404, 408 may be adjusted and uniformity of output across the illumination apparatus may be increased.

[165] The input surface 56 comprises: a first array of prismatic elements 52A each comprising a pair of facets 53AA, 53AB defining a ridge 54A therebetween. The ridges 54A extend along a first array of lines across the plane in which the input surface 56 extends.

[166] The illumination apparatus further comprises at least one light source 15 arranged to input light 402 into the waveguide 1 through the input end 2. In operation light rays 402 from light source 15 are input into the waveguide 1 through the input end 2. For a planar input side 2, light ray 402 within the waveguide 1 at the input end 2 is in a light cone 401 that is determined by the critical angle at the input side. In comparison to the present embodiments if surfaces 6,8 were to comprise planar surfaces no light leakage from the waveguide 1 would take place. [167] In the present embodiment surface 8 comprises microstructures that spread the input light cone so that some light leaks through the surface 8 at locations along the waveguide 1. Such microstructures may comprise sloped microprism surfaces 32 and elongate lenticular or prismatic surfaces 30.

[168] Light rays exiting the waveguide 1 close to the critical angle inside the waveguide 1 and are near grazing incidence with the surface 8 and have a narrow output cone angle. Light ray 402 is incident on facet 53AA at which it is refracted and then reflected by TIR at surface 53AB to be deflected in the normal direction. Output light lobe 404 has a nominal direction of peak luminance and solid angle of output with full width half maximum (FWHM) in a given plane that is determined by die extraction microstructures 30, 32 of the waveguide 1 and the surface normal directions of the facets 43AA, 53AB.

[169] In the embodiment of FIGURE 3 A, the waveguide further comprises a second input end 4 arranged between the first and second light guiding surfaces 6, 8 opposite to the first mentioned input end. At least one second light source 17 is arranged to input light ray 406 into the waveguide 1 through the second input end 4.

[170] The waveguide 1 is arranged to cause light from the at least one first light source 15 and the at least one second light source 17 to exit from the waveguide 1 with different angular distributions of lobes 404, 408. In comparison to the light ray 402, the light ray 406 is output through facets 36 of the microstructures. Facets 36 are substantially not seen by the light rays 402 whereas facets 36 have for example a surface normal parallel to the plane of surface 8 provide an output cone 408 that is substantially greater solid angle than the cone 404. Light rays in cone 408 are incident on the turning film component 50 and are extracted by refraction at facets 53AB and reflected by total internal reflection at facets 53AA of the prisms 52. A. Thus the lobe that is determined by the ray 406 and cone 408 may be different to die lobe determined by the ray 402 and cone 404. The direction of the respective lobes may be arranged by adjustment of the microstructures 30, 32, 36 and the prisms 52A of the turning film component 50.

[171] Referring again to FIGURE 1A, the control system 352, 354 is arranged to control the at least one first light source 15 and the at least one second light source 17 independently. Advantageously controllable illumination into lobes 404, 408 may be provided to achieve a switchable directionality- ⁷ illumination apparatus. [172] In an alternative embodiment the first surface 6 of the waveguide 1 may be arranged between the second surface 8 and the turning film component 50. Light output may be achieved through both of the surfaces 6,8 as will be further described below.

[173 ] One example of an alternative waveguide 1 will now' be described.

[174] FIGURE 3C is a schematic diagram illustrating a top perspective view of a waveguide 1 comprising linear prismatic features 29 and stepped prismatic features 32, 36 arranged on the same side of a waveguide I and arranged to provide different solid angles of output for light input at first and second input ends 2, 4.

[175] In operation, the rays 404, 408 may propagate in a similar manner to that illustrated in FIGURE 3.4. In the arrangement of FIGURE 3C, the prismatic features 29 have a higher average facet tilt angle compared to the lenticular features 30 of FIGURE 3B. More waveguidmg light may be leaked into angles dose to the critical angle at the surface 8, and advantageously increased output coupling efficiency along the waveguide 1 may be achieved.

[176] Features of the embodiment of FIGURES 3A-C not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features

[177] The operation of the illumination apparatus similar to that of FIGURE 3A in a backlight 20 w ill now' he described.

[178] FIGURE 4A is a schematic diagram illustrating a side view of a switchable backlight 20 further comprising rear reflector 3 and diffuser 5; and FIGURE 4B is a schematic diagram illustrating a top perspective view' of a collimating waveguide 1 comprising lenticular microstructure 30 and stepped prismatic surfaces 32, 36 arranged on opposite sides of a waveguide 1 arranged to provide different solid angles of output for light input at first and second input sides 2, 4.

[179] FIGURES 4A-B differ from FIGURES 3A-B in several aspects. First, a reflector 3 is arranged to receive light that is output through the second surface 8 of the waveguide 1. in operation light rays 402, 406 may leak through both surfaces 6, 8. Light rays that leak through the first surface 6 are output directly to the turning film component 50 whereas light rays that leak through the second surface 8 are reflected by the rear reflector 3 and pass back through the waveguide 1. Advantageously output efficiency is increased. Such rear reflector may be used with the illumination apparatus of FIGURE 4A as well as the backlight apparatus 20 of FIGURE 4B to increase output efficiency. [180] Further, the lenticular microstructure 30 is arranged on the first surface 6 of the waveguide 1 and the prism microstructures 32, 36 is arranged on the second surface 8 of the waveguide 1. Advantageously a less complex tool may be provided to achieve the microstructures 30, 32, 36.

[181] Further the prismatic structures 32, 36 are arranged between tire waveguide 1 and the rear reflector 3. Advantageously damage due to vibration and applied shear forces may achieve reduced damage to the features such as ridges 54A of the turning film component 50.

[182] Features of the embodiment of FIGURES 4A-B not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features

[183] The structure and operation of the turning film component 50 will now be described.

[184] FIGURE 5A is a schematic diagram illustrating a side perspective view of a turning film component 50 comprising fi rst and second arrays of elongate straight prismatic elements 52A, 52B; and FIGURE SB is a schematic diagram illustrating a side perspective view of the arrangement of cutting tools to achieve a turning film comprising first and second arrays of elongate prismatic elements.

[185] The optical turning film component 50 has a rectangular shape across the plane and the lateral direction is along a major or minor axis of the rectangular shape.

[186] The first array of lines of ridges 54A have an angle gA projected on to the plane in which the input surface 56 extends. The second array of lines of ridges 54B have a second angle gB projected on to the plane, the second angle yB being greater than the first angle yA. The first array of lines of ridges 54A have an angle yA projected on to the plane of 0° from the lateral direction that is along the x-axis and parallel to the major axis of the rectangular shape. In the embodiment of FIGURE 5 A, the first and second arrays of lines are straight.

[187] Considering FIGURE 5 A, the optical turning film component 50 comprises an input surface 56 arranged to receive the light exiting from the waveguide 1 wherein the input surface 56 extends across a plane and output surface 58 faces the input surface 56.

[188] The input surface 56 comprises: a first array of prismatic elements 52A each comprising a pair of facets 53AA, 53AB defining a ridge 54A therebetween, the ridges extending along a first array of lines across the plane in which the input surface 56 extends, which in the embodiment of FIGURE 5 A is the x-y plane.

[189] The input surface 56 further comprises a second array of prismatic elements 52B each comprising a pair of facets 53BA, 53BB defining a ridge 54B therebetween, the ridges 54B extending along a second array of lines across tire plane in which die input surface 56 extends.

[190] The first array of lines and the second array of lines extend at different angles yA, yB projected on to the plane. In the embodiment of FIGURE 5A, yA is parallel to the x-axis direction and thus is zero degrees while yB is non-zero angle from the x-axis direction.

[191] FIGURE 5B illustrates an exemplary method to provide a tool 60 that can be used in die replication of the turning film component 50. Tool 60 may comprise a metal material such as steel, nickel or copper for example. Cutting element 62A, such as a shaped diamond, may cut ridges 54A by translating tool 62A parallel to the x-axis. Cutting element 62B, such as a shaped diamond, may cut ridges 54B by translating tool 62A at angle in the x-y plane and inclined to the x-axis. After cutting, tool 60 has a shape that is inverted compared to the shape of the input surface 56 of the optical turning film component 50. During replication, such as molding, casting or other known replication methods the input surface 56 is formed in die turning film component 50.

[192] FIGURE 5C is a schematic diagram illustrating a rear perspective view of the optical stack of the switchable backlight 2.0 of FIGURE 4A. Ridges 54A of the component 50 are parallel to the x-axis which may be referred to as the lateral direction. Ridges 54B are inclined to the lateral direction in the plane. Features 30 of the waveguide 1 are extended in a direction orthogonal to the lateral direction (y-axis). Light sources 15, 17 are extended in the lateral direction along the first and second input sides 2,4 of the waveguide 1. Lobes 404, 408 are output from the backlight 20.

[193] FIGURE 5D is a schematic diagram illustrating a cross section of a light turning film component. Surface angles a and b are illustrated with respect to the direction 199 of the normal to the plane for facets 53A, 53B in the plane comprising the ridges 54 and the optical axis 199.

[ 194] Features of the embodiments of FIGURES 5A-D not discussed in further detail may ^¬ be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[195] Exemplary lobes 404, 408 from a backlight 20 will now- be described.

[196] FIGURE 6 is a schematic graph illustrating the polar and azimuthal variation of luminance for an illustrative backlight with light input at the first end of the waveguide; and FIGURE 7 is a schematic graph illustrating the polar and azimuthal variation of luminance tor an illustrative backlight with light input at the second end of the waveguide for a collimated waveguide 1 of the type illustrated in FIGURE 3B and the turning film component 50 with the illustrative embodiment of TABLE 1.

[197] FIGURE 6 illustrates polar luminance profile for light from light source 15 at the first input side 2 that is collimated by the waveguide 1. Lobe 404 is provided that has two regions of peak luminance, one near a polar angle of 0° and an azimuthal angle of 0° and the other near a polar angle of 40° and an azimuthal angle of 337.5° (for an azimuthal angle of 0° that is in the easterly direction). By comparison, light input into the waveguide 1 at the second input side 4 from light source 17 has a wider solid angle of output because of output through facet 36 of the waveguide 1. Lobe 408 is provided that has two regions of peak luminance, one near a polar angle of 0° and an azimuthal angle of 0° and the oilier near a polar angle of 45° and an azimuthal angle of 180°.

[198] Referring again to FIGURE 2, the directions, (or lobes) 44 and 46 represent nominal locations for vehicle occupants 45, 47. The head-on occupant 45 near direction 44 sees a high luminance image in the output for light input at first and second light sides, and the off- axis occupant 47 sees a high luminance image for light input at the second side 4 but a low luminance image for light input at the first side 2. Advantageously the backlight may be switched between a privacy mode operation as shown in FIGURE 6 and a share mode operation as shown in FIGURE 7.

[199] FIGURE 6 further illustrates the field of view' 600 of a passenger 45 viewing a 12” display of aspect ratio 8:3 at 700mm, and the field of view' 610 of driver 47 viewing the same display at 45 degrees.

[200] TABLE 1 further illustrates that for the first and second arrays of prismatic elements 52A, 52B, the facets on opposite sides of the ridges 54A, 54B have facet angles aA, bB between a normal to the facet and a normal 199 to the plane across which the input surface extends that are different. Said difference in angles achieves control of location of output lobe directions from the respective prism arrays that has some independence. Control of lobe directions to first and second peak locations at directions 44, 46 for first and second light sources may be achieved. Advantageously observers or occupants may receive desirable luminance profiles in privacy and share modes of operation.

[201] In other embodiments, the first and second arrays of lines may each have tangential angles gA, gB projected on to the plane of not more than 55° from the lateral direction. Facet angles oA, bA, aB, bB of respective facets 53 A A, 53AB, 53BA, 53BB defined between a normal to the facet and a normal 199 to the plane across which the input surface extends may be between 40° and 70° and preferably between 47.5° and 62.5°.

[202] It may be desirable to increase the security factor of a display in privacy mode comprising the backlight 20 of FIGURE 4 A and the light turning film component 50 with the polar output of FIGURE 6. Operation of the display of FIGURE 1A with polar control retarder arrangement of FIGURE IB will now be described.

[203] FIGURE 8A is a schematic graph illustrating the polar and azimuthal variation of transmission of a switchabie polar control retarder 300 arranged between parallel polarisers 218, 302, 318 for switchabie liquid crystal retarders 314 driven for privacy mode; and FIGURE 8B is a schematic graph illustrating die polar and azimuthal variation of relative reflection of a switchabie retarder arranged between a reflective polariser and absorbing polariser for switchabie liquid crystal retarders driven for privacy mode with the illustrative embodiment of TABLE 2.

[204] FIGURE 9A is a schematic graph illustrating the polar and azimuthal variation of visual security factor, S for the arrangement of FIGURE IB, and the backlight with polar output profile of FIGURE 6 in a privacy mode of operation for a display head-on luminance, of value Y _II13X measured in nits that is half of the illuminance of value I measured in lux; and FIGURE 9B is a schematic graph illustrating the polar variation of visual security factor, S for zero ele vation. [205] Field of view 600 illustrates that the advantageously passenger 45 sees the display with high isnage visibility (S < 0,1) across its width, while field of view 610 illustrates that advantageously the driver 47 sees a private image (S > 1.0) across the entire width and an invisible image (S > 1.5) for most of the display area.

[206] FIGURE 10 is a schematic graph illustrating the polar and azimuthal variation of visual security factor, S for the arrangement of FIGURE IB, and the backlight 20 with polar output profile of FIGURE 7 in a public mode of operation for a display head-on luminance, of value Y _max measured in nits that is half of the illuminance of value I measured in lux. Advantageously both occupants 45, 47 see a high luminance image with high image visibility (low security factor, S).

[207] The use of stray light in a vehicle of a display operating in privacy mode will now' be described.

[208] Returning to the polar profile in a privacy mode of operation of FIGURE 6, a peak location near direction 49 is provided in privacy mode. Returning to the lobe directions in the vehicle 650 of FIGURE 2, such light rays 414 near direction 49 are directed towards the internal surface 652 of the vehicle. The different lobes into which light exiting from the waveguide is deflected are lobes respectively directed at a location of an occupant 45 in the vehicle 650 and at a location 652 of an internal surface of the vehicle 650.

[209] On reflection at said internal surface 652, light rays may be directed as light rays 416 by reflection from the display 100 towards the occupant 47. Reflections may by means of front surface Fresnel reflections and by means of reflection from reflective polariser 302 in FIGURE 1A as illustrated in FIGURE 8B.

[210] The internal surface 652 may comprise a reflective material and for example may comprise retroreflective material to increase the luminance of reflected light towards the occupant 47. The luminance of reflected light is conveniently increased, advantageously increasing the security _' factor, S enhanced, particularly in conditions with low ambient light levels such as at night.

[211] It may be desirable to further increase the uniformity of the display.

[212] FIGURE 11 is a schematic diagram illustrating a rear perspective view of the first array of elongate prismatic elements wherein the ridges and facets comprise wobble. For ease of illustration, the wobble is illustrated for first prism array 52.A. The second prism array 52B may further be provided with wobble. Advantageously display uniformity may be increased. Visibility of artefacts arising from manufacturing defects of waveguide 1 may be reduced, advantageously increasing yield and reducing cost. Visibility of defects from damage in use of waveguide 1 may he reduced, advantageously increasing lifetime.

[213] FIGURE I2A is a schematic diagram illustrating a side perspective view of a turning film comprising first and second arrays of prismatic elements wherein the first array of prismatic elements comprises curved prismatic elements. In the embodiment of FIGURE 12A, the first array of lines for ridges 54A are curved and the second array of lines for ridges 54B are straight.

[214] In other embodiments the first and second array of lines for ridges 54A, 54B may be curved.

[215] In the case of curved facets, the first array of lines of ridges 54A have a first arithmetic mean tangential angle gA projected on to the plane and the second array of lines of ridges 54B have a second arithmetic mean tangential angle yB projected on to tire plane, the second arithmetic mean tangential angle yB being greater than the first arithmetic mean tangential angle yA. The first array of lines has an average tangential angle yA projected on to the plane in wliich the input surface extends of 0° from the lateral direction. Features of the embodiment of FIGURE 12A not discussed m further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[216] The operation of the curved ridges 54 of the turning film component 50 will now be described.

[217] FIGURE I2B is a schematic diagram illustrating a top view of an illumination apparatus comprising a turning film component 50 with first and second arrays of prismatic elements 52A, 52B wherein the first array of prismatic elements comprises curved prismatic elements 52A. Output rays 402 from the waveguide 1 are output onto the input surface 56 of the turning film component 50 with ray directions that are near to grazing incidence. The turning film directs the light from across the width of the display to common optical window 26 at a window plane 700 at a distance 726 from the illumination apparatus. When the illumination apparatus is used in a backlight 20 for a display apparatus 100. the distance 726 is arranged to be greater than a typical viewing distance 745 for the observer 45. The use of the term optical window 26 in the present embodiments is distinct and different from the use of the term window when used to refer to sheets or panes of glass or other transparent material such as plastics for use in house window's, car windows and windscreens, and other types of protective windows. Such sheets or panes do not contribute to the creation of desirable viewing regions with improved uniformity as described herein.

[218] Features of the embodiment of FIGURE 12B not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[219] Image uniformity will now be described.

[220] FIGURE 13 A is an array of schematic graphs illustrating the variation of Security Factor, S for various different primary user viewing angles in a display of FIGURE IB comprising the straight facet turning film of FIGURE 5A; FIGURE 13B is an array of schematic graphs illustrating the variation of Security Factor, S for various different primary' user viewing angles in a display of FIGURE IB comprising the curved facet turning film of FIGURE 12A and TABLE 3 and the polar control retarder of TABLE 4 for a display head-on luminance, of value Y _max measured in nits that is half of the illuminance of value I measured in lux.

[221] Shaded regions in FIGURE 13A and FIGURE 13B represent regions of 0 < S < 1.8 where either an observer sees an image with undesirable image contrast (S > 0.1} or an off- axis occupant or snooper sees an image which is not invisible (S < 1.8). Advantageously the curved facets of the present disclosure achieve increased area of display over winch desirable image contrast is seen close to on-axis locations. Advantageously the curved facets achieve increased area of display over which desirable image invisibility is seen at off-axis locations. [222] In other embodiments, the distance 726 may be arranged close to the nominal viewing distance 745. In such embodiments, the uniformity will be further increased in comparison to straight ridges 54.4.

[223 ] It may be desirable to increase the efficiency of illumination to the off-axis occupant of a vehicle in comparison to the arrangement that achieves the lobe profile of FIGURE 7.

[224] FIGURE 14.4 is a schematic diagram illustrating a top perspective view of a collimating waveguide 1 comprising linear prismatic microstructure comprising facets 32A, 32B and orthogonal lenticular surfaces 30 arranged on opposite sides of the waveguide 1 arranged to provide similar solid angles of output for light input at first and second input sides 2, 4 (input side 4 not shown), in comparison to the w aveguide 1 of FIGURE 4B, facets 32, 36 are replaced by facets 32A, 32B which have substantially equal and opposite facet tilt angles with respect to the normal to the plane in which the waveguide 1 extends.

[225] FIGURE 14R is a schematic diagram illustrating a top perspective view ⁷ of a collimating waveguide 1 comprising linear prismatic microstructures 32A, 32B and prisms 29 with ridges that are orthogonal to the microstructures 32A, 32B arranged on the same side of a waveguide I and arranged to provide similar solid angles of output for light input at first and second input sides 2, 4 (input side 4 not shown).

[226] FIGURE 15 is a schematic diagram illustrating a side view of a switchable backlight with the same solid angles in first and second lobe directions. The operation of the waveguide 1 is similar to that shown in FIGURE 3A for example, however in the embodiments of FIGURE 14.A and FIGURE 14B the waveguide 1 is arranged to cause light 402 from the at least one first light source 15 and the at least one second light source 17 to exit from the waveguide 1 with a common angular distribution, that is the lobes 404, 408 have a similar solid angle but are directed in opposite directions from the waveguide 1.

[227] Features of the embodiments of FIGURES 14-16A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[228] Another alternative backlight 20 comprising two waveguides will now be described.

[229] FIGURE 16A is a schematic diagram illustrating a side view of a switchable backlight 20 with the same solid angles in first and second lobe directions 402, 406 comprising first and second stacked waveguides 1A, IB. Waveguides I A, IB may be an alternative to the single waveguide 1 of FIGURE 15 or other dual end injection embodiments described elsewhere herein. Features of the embodiment of FIGURE 16A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[230] In comparison to the arrangement of FIGURE 15, each waveguide IA, IB is provided with light sources 15A, 15B at the respective input ends 2A, 2B. Such waveguides 1A, IB may be arranged to advantageously achieve increased luminance uniformity along the waveguides 1 in comparison to the dual end injection of FIGURE 15.

[231] FIGURE 16B is a schematic diagram illustrating a front perspective view of a backlight 20 comprising an optical turning film 50 with a first array of prismatic elements 54A that is linear and a second array of prismatic elements 54B that is curved. Features of the embodiment of FIGURE I6B not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[232] In the alternative embodiment of FIGURE I6B, the light sources 15B are arranged on the left-hand side of the waveguide IB and the light sources 15A are arranged on the lower side of the waveguide 1A. The peaks 54B are orthogonal to the lateral direction and the peaks 54A are curved along the lateral direction.

[233] The output illumination from an illustrative backlight 20 of FIGURE 16B will now be described.

[234] FIGURE 16C is a schematic graph illustrating the polar variation of luminance for the backlight 20 of FIGURE I6B comprising the waveguides IA, IB similar to that of FIGURE 4B for light from the first light source 15 A; FIGURE I6D is a schematic graph illustrating the polar variation of luminance for the backlight 20 of FIGURE 16B comprising the waveguides 1A.1B of FIGURE 4B for light from the second light source 15B; and FIGURE 16E is a schematic graph illustrating the polar variation of luminance for the backlight of FIGURE 16B comprising the waveguides IA, IB of FIGURE 4B for light from the first and second light sources 15A, 15B.

[235] Advantageously a backlight 20 suitable for use in a switchabfe privacy display 100 of the type illustrated in FIGURE 2 may be provided. A privacy mode to the passenger 45 may ^¬ be provided so the driver 47 cannot see the displayed image by illumination of light source 15B. A low power mode to the passenger 45 may be provided by illumination of light source 15B. A low power mode to the driver 47 may be provided by illumination of light source ISA. A sharing mode to the passenger 45 and driver 47 may be provided by illumination of light sources 15A, 15B. [236] The operation of the waveguides of the types illustrated in FIGURE 14A will now be described together with the turning film component 50 of FIGURE 5 A.

[237] FIGURE 17A is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE I4A and turning film of FIGURE 5A for light input from light sources at a first input side for privacy mode of operation; and FIGURE I7B is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 14A and turning film of FIGURE 5A for light input from light sources at a second input side for public mode of operation, with the parameters of light turning film of TABLE 5.

[238] Advantageously in comparison to the public mode of FIGURE 7, a high brightness public mode may be achieved with higher efficiency.

[239] FIGURE 18 is a schematic diagram illustrating a side view of a display 100 in an automotive vehicle 650. In comparison to the arrangement described hereinbefore, the preferred orientation for the display 100 has a normal 199 to the plane in winch the waveguide 1 and turning film component 50 extend that is nearer to horizontal. In such an arrangement the nominal viewing direction for the occupants 45, 47 is above the axis, and it would be preferable to provide increased efficiency in these directions. Further it would be desirable to reduce the luminance of rays 442 in polar directions around direction 442 that contribute to reflections from windscreen 654.

[240] Features of the embodiment of FIGURES 18 not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[241] Optical profiles suitable for use in the geometry of FIGURE 18 will now be further described.

[242] FIGURE 19A is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film similar to that of FIGURE 5A for light input from light sources at the first input side for privacy mode of operation; FIGURE 19B is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated wav eguide of FIGURE 3B and off- axis turning film similar to that of FIGURE 5A for light input from light sources at the second input side for public mode of operation; and FIGURE 19C is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis taming film similar to that of FIGURE 5 A for light input from light sources at both the first and second input sides for public mode of operation. The profiles of FIGURES 19A-C are provided by backlight 20 of the type illustrated in FIGURE 4A, the optical light turning film 50 of FIGURE 5 A and the facet angles of TABLE 6

[243] FIGURES 19A-B illustrate in privacy and public modes respectively that a look-down location can be provided to the occupants by adjustment of facet angles and orientations. FIGURE 19B further illustrates that the share mode brightness may not be matched between the two directions 44, 46. FIGURE 19C illustrates an embodiment wherein both light sources 15,17 are operated with equal luminous flux, advantageously achieving similar luminance for both occupants.

[244] It may be desirable to provide different outputs for different regions of a display.

[245] FIGURE 20 is a schematic diagram illustrating a top view of some elements of a backlight 20 that is segmented. Features of the embodiment of FIGURE 20 not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[246] First light source 13 comprises first and second parts 15 A, 15B and second light source 17 comprises first and second parts 17A, 17B. Waveguide 1 may be of the type illustrated elsewhere herein, or may be first and second waveguides 1A, IB for example as illustrated in FIGURE 16A, in which case light sources I5AA, 15AB and light sources 15BA, 15BB are provided at respective ends of the waveguide.

[247] Light rays 415A propagating within the waveguide 1 are input with an expanding cone in the lateral direction. The microstructures on the surfaces 6, 8 of the waveguide 1 adjust the ray 415A propagation directions to achieve some coliimation in the lateral direction and thus illumination regions 475A are provided with limited extent in the lateral direction. Such coliimation can achieve partial illumination of the backlight in regions 475A that are determined by the location of the light source along the first input end 2.

[2.48] Similarly light rays 417B propagating within the waveguide 1 are input with an expanding cone in the lateral direction. Some coliimation is provided in the lateral direction and thus illumination regions 477B are provided with limited extent in the lateral direction. Such coliimation can achieve partial illumination of the backlight in regions 477B that are determined by the location of the light source along the input end 4.

[249] By control of light sources 15A, 15B and 17A, 17B, the directionality of output may ^¬ be different for different regions of the backlight 20. In an illustrative example, in one mode of operations, the left side of the display may be arranged for high image visibility to the driver 47 and passenger 45 and the right side of the display may be provided for high image security factor to the driver 47 with high image visibility to the passenger 45. In other illustrative inodes of operation, the whole display may be arranged to be seen by both occupants or the whole display may be arranged to be private to the driver.

[250] The number of light sources 15A-N may be adjusted to increase the number of addressable regions of display control.

[251] It may be desirable to provide a centre stack display (CSD) for a vehicle or similar application.

[252] FIGURE 21A is a schematic diagram illustrating a top view of a centre stack display 100 in an automotive vehicle 650. In comparison to the arrangement of FIGURE 2, occupants 45, 47 are seated either side of the optical axis 199 of the display 100, Features of the embodiment of FIGURE 21A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[2.53] FIGURE 21B is a schematic diagram illustrating a front perspective view of a dual lobe display 100. In comparison FIGURE 1 A, the polar control retarder 300 and polarisers 302, 318 are omitted. Further, as alternatives to the present embodiments described hereinbefore, said retarder 300 and polarisers 302, 318 may more generally be omitted in order to advantageously achieve increased efficiency, although not to achieve image invisibility in typical ambient lighting and display luminance conditions.

[254] FIGU RE 21C is a schematic diagram illustrating a side perspective view of a turning film comprising first and second arrays of linear straight prismatic elements wherein the axes of the prismatic elements are each inclined to an axis along the edge of the display; and FIGURE 21 D is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film of FIGURE 21 C for fight input from light sources at the first input side for efficient mode of operation, with the taming film component 50 of TABLE 7. Features of the embodiment of FIGURES 21B-C not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[255] FIGURE 21D is a schematic graph illustrating the polar and azimuthal variation of output luminance of the collimated waveguide of FIGURE 3B and off-axis turning film of FIGURE 21C for light input from light sources at the first input side for wide angle mode of operation. Directions 44, 46 for the occupants may be offset both vertically and laterally. Advantageously efficiency of the display may be substantially increased in comparison to arrangements with wide optical outputs.

[256] Operation of the switchable retarders of FIGURE 1A and FIGURE IB will now be described.

[257] FIGURE 22A is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through the optical stack of FIGURE 1A in a privacy mode of operation.

[258] When the layer 314 of liquid crystal material 414 is driven to operate in the privacy mode, the retarders 300 provide no overall transformation of polarisation component 360 to output light rays 400 passing therethrough along an axis perpendicular to the plane of the switchable retarder, but provides an overall transformation of polarisation component 361 to light rays 415 passing therethrough for some polar angles which are at an acute angle to the perpendicular to the plane of the retarders.

[259] Polarisation component 360 from the output polanser 218 is transmitted by reflective polarises' 302 and incident on retarders 300. On-axis light has a polarisation component 362 that is unmodified from component 360 while off-axis light has a polarisation component 364 that is transformed by the retarders 300. At a minimum, the polarisation component 361 is transformed to a linear polarisation component 364 and absorbed by additional polariser 318. More generally, the polarisation component 361 is transfonned to an elliptical polarisation component, that is partially absorbed by additional polariser 318.

[260] The polar distribution of light transmission illustrated in FIGURE 8A modifies the polar distribution of luminance output of the underlying spatial light modulator 48. In the case that the spatial light modulator 48 comprises a directional backlight 20 then off-axis luminance may be further be reduced as described above.

[261] Features of the embodiment of FIGURE 22 A not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[262] Advantageously, a privacy display is provided that has low luminance to an off-axis snooper while maintaining high luminance for an on-axis observer.

[263] The operation of the reflective polariser 302 for light from ambient light source 604 will now be described for the display operating in privacy mode.

[264] FIGURE 22B is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of FIGURE 1A in a privacy mode of operation.

[265] Ambient light source 604 illuminates the display device 100 with unpolarised light. Additional polariser 318 transmits light ray 410 normal to the display device 100 with a first polarisation component 372 that is a linear polarisation component parallel to the electric vector transmission direction 319 of the additional polariser 318.

[266] In both states of operation, the polarisation component 372 remains unmodified by the retarders 300 and so transmitted polarisation component 382 is parallel to the transmission axis of the reflective polariser 302 and the output polariser 218, so ambient light is directed through the spatial light modulator 48 and lost. [267] By comparison, for ray 412, off-axis light is directed through the retarders 300 such that polarisation component 374 incident on the reflective polariser 302 may be reflected. Such polarisation component is re-con vested into component 376 after passing through retarders 300 and is transmitted through the additional polariser 318.

[268] Thus when the layer 314 of liquid crystal material is m the second state of said two states, the inflective polariser 302 provides no reflected light for ambient light says 410 passing through the additional polariser 318 and then the retarders 300 along an axis perpendicular to tire plane of the retarders 300, but provides reflected light rays 412 for ambient light passing through the additional polariser 318 and then the retarders 300 at some polar angles which are at an acute angle to the perpendicular to the plane of the retarders 300; wherein the reflected light 412 passes hack through the retarders 300 and is then transmitted by the additional polariser 318.

[269] The retarders 300 thus provide no overall transformation of polarisation component 380 to ambient light rays 410 passing through the additional polariser 318 and then the retarder 300 along an axis perpendicular to the plane of the switchabie retarder, but provides an overall transformation of polarisation component 372 to ambient light rays 412 passing through the absorptive polariser 318 and then the retarders 300 at some polar angles which are at an acute angle to the perpendicular to the plane of the retarders 300.

[270] The polar distribution of light reflection illustrated m FIGURE 8B thus illustrates that high reflectivity can be provided at typical snooper locations by means of the privacy state of the retarders 300. Thus, in the privacy mode of operation, the reflectivity for off-axis viewing positions is increased as illustrated in FIGURE 8B, and the luminance tor off-axis light from the spatial light modulator is reduced as illustrated in FIGURE. 8A.

[271] In the public mode of operation, the control system 710, 752, 350 is arranged to switch the switchabie liquid crystal retarder 301 into a second retarder state in which a phase shift is introduced to polarisation components of light passing therethrough along an axis inclined to a normal to the plane of the switchabie liquid crystal retarder 301.

[272] By way of comparison, solid angular extent 402D may be substantially the same as solid angular extent 402B in a public mode of operation. Such control of output solid angular extents 402C, 402D may be achieved by synchronous control of the sets 15, 17 of light sources and the at least one switchabie liquid crystal retarder 300.

[273] Advantageously a privacy mode may be achieved with low image visibility for off- axis viewing and a large solid angular extent may be provided with high efficiency tor a public mode of operation, for sharing display imagery between multiple users and increasing image spatial uniformity',

[274] Additional poiariser 318 is arranged on the same output side of the spatial light modulator 48 as the display output poiariser 218 which may be an absorbing dichroic polaiiser. The display poiariser 218 and the additional poiariser 318 have electric vector transmission directions 219, 319 that are parallel. As will be described below, such parallel alignment provides high transmission for central viewing locations.

[275] A transmissive spatial light modulator 48 arranged to receive the output light from the backlight; an input poiariser 210 arranged on the input side of the spatial light modulator between the backlight 20 and the spatial light modulator 48; an output poiariser 218 arranged on the output side of the spatial light modulator 48: an additional poiariser 318 arranged on the output side of the output poiariser 218: and a switchable liquid crystal retarder 300 comprising a layer 314 of liquid crystal material arranged between the at least one additional poiariser 318 and the output poiariser 318 in this case in which the additional poiariser 318 is arranged on the output side of the output poiariser 218; and a control system 710 arranged to synchronously control the light sources 15, 17 and the at least one switchable liquid crystal retarder 300.

[276] Control system 710 further comprises control of voltage controller 752 that is arranged to provide control of voltage driver 350, in order to achieve control of switchable liquid crystal retarder 301.

[277] Features of the embodiment of FIGURE 22B not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[278] Advantageously, a privacy display is provided that has high reflectivity ^' to an off-axis snooper while maintaining low reflectivity for an on-axis observer. As described above, such increased reflectivity' provides enhanced privacy performance for the display in an ambiently illuminated environment.

[279] Operation in the public mode will now be described.

[280] FIGURE 23A is a schematic diagram illustrating in side view propagation of output light from a spatial light modulator through tire optical stack of FIGURE 1A in a public mode of operation; and FIGURE 23B is a schematic graph illustrating the variation of output luminance with polar direction for the transmitted light rays in FIGURE 23 A. [281] Features of the embodiment of FIGURE 23A and FIGURE 23B not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[282] When the liquid crystal retarder 301 is in a first state of said two states, the retarders 300 provide no overall transformation of polarisation component 360, 361 to output light passing therethrough perpendicular to the plane of the switchable retarder 301 or at an acute angle to the perpendicular to the plane of the switchable retarder 301. That is polarisation component 362 is substantially the same as polarisation component 360 and polarisation component 364 is substantially the same as polarisation component 361. Thus the angular transmission profile of FIGURE 23B is substantially uniformly transmitting across a wide polar region. Advantageously a display may be switched to a wide field of view'.

[283] FIGURE 23C is a schematic diagram illustrating in top view propagation of ambient illumination light through the optical stack of FIGURE 1A in a public mode of operation; and FIGURE 23D is a schematic graph illustrating the variation of reflectivity with polar direction for the reflected light rays in FIGURE 23C.

[284] Thus when the liquid crystal retarder 301 is in the first state of said two states, the retarders 300 provide no overall transformation of polarisation component 372 to ambient light rays 412 passing through the additional polariser 318 and then the retarders 300, that is perpendicular to the plane of the retarders 300 or at an acute angle to the perpendicular to the plane of the retarders 300.

[285] In operation in the public mode, input light ray 412 has polarisation state 372 after transmission through the additional polariser 318. For both head-on and off-axis directions no polarisation transformation occurs and thus the reflectivity for light rays 402 from the reflective polariser 302 is low. Light ray 412. is transmitted by reflective polariser 302. and lost in the display polarisers 218, 210 or the backlight of FIGURE 1A.

[286] Features of the embodiment of FIGURE 23C and FIGURE 23D not discussed in further detail may be assumed to correspond to the features with equivalent reference numerals as discussed above, including any potential variations in the features.

[287] Advantageously in a public mode of operation, high luminance and low reflectivity is provided across a wide field of view . Such a display can be conveniently viewed with high contrast by multiple observers.

[288] Other types of switchable privacy display will now be described. [289] A display device 100 that may be switched between privacy and public modes of operation comprises an imaging waveguide and an array of light sources as described in U.S. Patent No. 9,519,153, which is incorporated by reference herein in its entirety. The imaging waveguide images an array of light sources to optical windows that may be controlled to provide high luminance on-axis and low luminance off-axis in a privacy mode, and high luminance with a large solid angle cone for public operation.

[290] As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero percent to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.

[291] While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or ail of the above advantages.

[292] Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from tliis disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby, in all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.