TITLE OF INVENTION BACKLIGHT

TECHNICAL FIELD

The present invention relates to a backlight, for example for use with an at least partially transmissive spatial light modulator. The present invention also relates to a display including such a backlight. Moreover, the present invention relates to a distributed illumination panel that may be used for general illumination.

In particular, an aspect of the present invention relates to maintaining high spatial uniformity of a highly collimated direct view backlight with reduced thickness. The present application claims priority from United States

Application Serial Number 13/204, 128 which was filed on August 5, 201 1.

BACKGROUND ART Figure 1(A) shows a conventional liquid crystal display (LCD) configuration in which collimated white light from the backlight 1 is focused, by means of a lens array 2, through apertures 3 within a thin film transistor (TFT) layer associated with the display electronics. The focusing prevents light from being lost by absorption or scatter in the TFT electronics. A diffusing layer 5 is added above a liquid crystal (LC) cell 6, a plurality of polarizers 7 and 7' and the TFT layer. The diffusing layer 5 increases the angular spread of light rays emitted from the device, thereby increasing the angular range over which the display may be viewed. The properties of diffusing layer 5 are carefully chosen to minimize ambient light reflectance and image blurring while making sure that sufficient angular spread is achieved. Each TFT aperture is associated with a color sub-pixel. Red (8R), green (8G) and blue (8B) color filters, each indexed to a TFT aperture, are used to produce the sub-pixel color from the white backlight. Other sub-pixel color schemes, such as RGBY, are also possible.

Figure 1 (B) shows a conventional alternative configuration of LCD that uses a collimated blue backlight Γ. In this scheme, the backlight light 1 ' is focused by lens sheet 2 through the TFT apertures and into individual chambers 1 1R, 1 1G and 11B associated with red, green and blue sub-pixels, respectively. A red-emitting phosphor is housed in chambers 11R, a green phosphor is housed in chambers 1 1G, and diffusive material is housed in chambers 11B. The phosphors are chosen to give adequate absorption at the blue wavelength of the backlight. The chamber layer 1 1 is separated from the collimated backlight 1' by the LC cell 6, the polarizers 7 and 7' and the TFT layer with apertures 3. A color filter layer 8 reduces ambient light reflectance from the display which would otherwise degrade image contrast.

Focused light from a highly collimated backlight is particularly beneficial for this sort of display since the light must enter the chamber correctly indexed to the TFT aperture it passed through. If it enters an incorrectly addressed chamber, cross talk and consequent image degradation occur. The blue backlight can be replaced with a UV backlight, in which case a blue emitting phosphor is housed in the chambers 1 IB instead of wavelength preserving scattering material. Other sub-pixel color schemes are also possible.

The display types described above require highly collimated and spatially uniform backlights to operate properly. Lightguide-based backlights have the advantage of being thin and requiring relatively a relatively small number of primary light sources such as LEDs. However, attaining highly collimated output with good light extraction efficiency and uniformity has been difficult. Direct-view backlights, which do not involve side-injection of light into a lightguide, are commonly used in large area displays. This type of backlight allows the output to be uniform when the backlight is used in conjunction with a strong diffuser layer. Any collimation is, however, necessarily lost after passing through such a diffuser. Direct view backlights are generally thicker than lightguide based backlights, but are more suitable for applying local dimming techniques to improve efficiency. Even without local dimming, direct view backlights can give higher efficiency than lightguide ones, since the injection of light into a lightguide and the extraction of light from a lightguide are inherently lossy, particularly when uniformity is demanded. Collimated output can be attained using a direct view backlight.

Figure 2 schematically shows a conventional backlight. Figure 2(A) shows a 3-dimensional representation of the geometry. Figure 2(B) shows a cross sectional view with example ray trajectories. The backlight includes a tiled array of single reflection light emitting diodes (SRLEDs). Each SRLED includes an LED 21 that emits downwards towards a parabolic mirror 22. The emitting surface of the LED is placed close to the focus of the parabolic mirror so that the reflected light is well collimated. The spatial uniformity in the light field reflected from the mirror is, however, poor, which will be illustrated by an example below. The output irradiance distribution in a plane normal to the axis of a single SRLED is shown in Figure 3(A) in the case of a downwards emitting point Lambertian light source located at the focus of a parabolic mirror. According to this example, perfectly collimated light results from the mirror reflection. Here, diffraction effects are disregarded because they are small for a mirror many times largre than the light wavelength. The reflectance at the mirror surface is assumed to be independent of the angle of incidence. The irradiance distribution, L(p), is purely a function the radial coordinate, p = (x ² + y ^{2 12} , measured from the central axis of the system. In this example, the irradiance distribution may be represented as

( 1) where R is the radius of curvature of the parabolic mirror at its central point (p=0). It is seen from Figure 3 and Equation ( 1) that the radiance drops off rapidly with radial distance from the system axis, reaching zero at the radial coordinate p=R. Sample ray trajectories, obtained by using a ray tracing algorithm, are shown in Figure 3(B).

In this example, the output irradiance distribution remains unchanged as a function of distance along the system axis due to perfect collimation. A real SRLED will not emit perfectly collimated light due to the finite nature of the emitting surface, imperfections in the mirror geometry, etc. The light irradiance distribution from a tiled array of such SRLEDs will therefore eventually become largely homogenized over an extended region after a sufficient propagation distance from the mirrors. It is essential that this homogenization occurs at the position of the spatial light modulator in an LCD arrangement. The backlight collimation requires large distances to achieve homogenization. An SRLED array backlight has been constructed using SRLEDs each with cross-section dimensions of about 2cm ^χ 2cm measured in a plane normal to its axis. The light from each SRLED is collimated such that 80% of the output power is contained within a cone of half-angle 6° with respect to the axial direction. It was found that significant spatial inhomogeneities remain in the light from the backlight even after 10 cm of propagation. This precludes use of the simple tiled SRLED array as a backlight in commercial LCDs due to stipulations on the maximum allowed thickness.

EP 0802443A1 (M. Ogino et al; published 22/ 10/ 1997) describes a mirror and lens combination that can give collimated and spatially uniform light output. Some of the output light rays are transmitted through a lens without having impinged upon a mirror section. The remaining light rays reflect at a mirror section and may then pass through one or more lenses. The invention is most appropriate for light sources that emit approximately isotropic light. GB 2385191A (J. Slack; published 13/8/2003) describes a backlight comprising an array of single reflection light emitting diodes (SRLEDs) and a lens-based diffuser sheet. The SRLED array gives collimated but spatially non-uniform output. The diffuser sheet spatially homogenizes the output. However, the collimation is lost. EP 02071640A1 (G. L. Abore; published 17/6/2009) describes an array of side-emitting LEDs situated within mirror arrangements. The mirrors re-steer the light towards the output normal. However, only modest collimation is attained. EP 02071640A1 does not adreess the spatial uniformity of the light output. EP 02015126A1 (S. Bernard; published 14/ 1 /2009) describes a collimated backlight in which the angular distribution of light from a primary light source such as an LED is modified by a lens. A second lens is then used to collimate the light field emanating from this inner lens. The second lens is a Fresnel lens. US07808581 (G. Panagotacos; published 5/ 10/2010) describes a backlight that involves a deviator lens arrangement placed in front of a primary light source of small spatial extent. A total internal reflection (TIR) lens is used to collimate the output. One or more diffuser sheets are used to spatially homogenise the output. However, the coUimation is then lost.

EP01762778A1 (M. Shinohara et. al.; published 14/3/2007) describes, amongst other things, injection of collimated light from an array of SRLEDs into a lightguide. Light is then outcoupled from the lightguide by features placed on the lightguide. The lightguide increases the outcoupling rate and helps achieve spatial uniformity. The coUimation and uniformity properties of this backlight are largely set by the lightguide and associated features rather than the output from the SRLED array.

In view of the aforementioned shortcomings associated with conventional backlights, there is a strong need in the art for a direct view backlight which maintains high spatial uniformity of a highly collimated light with reduced thickness.

SUMMARY OF INVENTION

According to an aspect of the present invention, a backlight is provided which includes an array of curved mirror sections; an array of primary light sources, the primary light sources arranged to illuminate a corresponding curved mirror section among the array of curved mirror sections; and a lens array positioned adjacent the array of primary light sources on a side opposite the array of curved mirror sections, wherein the curved mirror sections are shaped to reflect light from the corresponding primary light source so as to illuminate a corresponding lens within the lens array, and the lenses in the lens array are shaped to collimate the light reflected by the corresponding curved mirror sections.

According to another aspect of the present invention, a radiant exitance at a plane immediately above the lens array varies by less than 50% over an area of the backlight. According to yet another aspect of the present invention, the light collimated by the lens array is such that more than 90% of the light power is contained within an angular cone with a half-width of 10 degrees. In accordance with another aspect of the present invention, a central axis of each curved mirror section coincides with a central axis of the corresponding lens and passes through the corresponding primary light source.

According to still another aspect of the present invention, a light emission from each primary light source extends over a polar angular range, Θ, relative to an outward normal from an emitting surface of the primary light source, and the outward normal is parallel to the central axes of the corresponding curved mirror section and primary light source. In accordance with another aspect of the present invention, a total angular spread of the light emission from each primary light source is restricted to the range 0°< Θ <90° as measured in air.

According to yet another aspect of the present invention, a lens cap is placed adjacent each of the primary light sources, the lens cap being configured to alter an emission angular profile of the primary light source to increase light radiance at higher values of Θ.

In yet another aspect of the present invention, the lens cap causes total internal reflection of light rays from the primary light source emitted close to a direction of the central axes of the corresponding curved mirror section and lens.

According to another aspect of the present invention, a surface of each curved mirror section is deformed from being cylindrically symmetric about an axial direction. In accordance with another aspect of the present invention, where a central axis of each curved mirror section coincides with the z-axis of a Cartesian coordinate set and a sag of the surface of the curved mirror section is written z _M (x,y) , a deviation of the surface of the curved mirror section from a parabolic form is represented by:

where the integrals are taken over an extent of the curved mirror section, parameters zp and Rp represent the z-coordinate of the apex of the curved mirror section and the radius of curvature of the curved mirror section at its center, respectively, the integral in the numerator is minimized with respect to the parameters zp and Rp, and the value z^ is the value of zp when the numerator has been minimized.

According to another aspect of the present invention, a value of σ is at least 0.05. In accordance with still another aspect of the present invention, a spatial extent of each primary light source, including packaging and necessary wiring, is less than a tenth of an aperture size of the corresponding lens within the lens array.

In yet still another aspect of the present invention, the array of curved mirror surfaces, the array of primary light sources and the lens array are configured in tiled arrangement.

According to still another aspect of the present invention, the array of curved mirror surfaces, the array of primary light sources and the lens array are configured in lenticular arrangement. In accordance with another aspect of the present invention, the lens array comprises an array of Fresnel lenses. In still another aspect of the present invention, a beam waist of light reflected by each curved mirror section is located between the curved mirror section and the corresponding lens within the lens array.

According to another aspect of the present invention, each curved mirror section and corresponding primary light source and lens form an integrated unit.

According to still another aspect of the present invention, each lens is directly connected to the corresponding curved mirror section, and the corresponding primary light source is embedded within a lens material making up the lens.

In accordance with still another aspect of the present invention, a display is provided including a backlight as described herein.

According to still another aspect of the present invention, an illumination panel is provided including a backlight as described herein. To achieve the foregoing and related ends, embodiments of the present invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS Figures 1 (A) and 1 (B) illustrate two types of conventional display devices that benefit from a highly collimated and spatially uniform backlight. Figure 1(A) shows a type that utilizes a white backlight and Figure 1(B) shows a type that utilizes a blue backlight. Figures 2(A) and 2(b) illustrate a backlight constructed from an array of single reflection light emitting diodes (SRLEDs). Figure 2(A) shows a 3D representation and Figure 2(B) shows a cross sectional view. Figure 2(B) includes example light ray paths.

Figures 3(A) and 3(B) illustrate the light output from an SRLED. Figure 3(A) shows the irradiance distribution and Figure 3(B) illustrates a sample of ray paths found using a ray tracing computer program.

Figures 4(A) and 4(B) illustrate the first embodiment of the present invention. Figure 4(A) shows a schematic representation in 3D and Figure 4(B) shows a cross sectional view. Figure 4(B) includes example light ray paths.

Figures 5(A) and 5(B) illustrate the light output from an example of the first embodiment of the present invention. Figure 5(A) shows the output irradiance distribution from a single unit cell. Figure 5(B) illustrates a sample of ray paths found using a ray tracing computer program. Figure 5(C) shows the far field intensity distribution as a function of angle to the axial direction.

Figure 6(A) and Figure 6(B) illustrate two variations of a second embodiment of the present invention.

Figure 7 illustrates a third embodiment of the present invention. Figure 8 illustrates a fourth embodiment of the present invention. Figure 9 illustrates a fifth embodiment of the present invention. Figure 10 illustrates a lenticular variant of the present invention. Figures 1 1 (A) and 1 1 (B) illustrate the action of active local dimming as provided by the embodiments of the present invention. Figure 11(A) shows the backlight with a subset of the backlight units illuminated. Figure 1 1 (B) shows an example image that could be illuminated using the illumination pattern shown in Figure 1 1(A).

DESCRIPTION OF REFERENCE NUMERALS

1 spatially uniform collimated white-light backlight 1 ' spatially uniform collimated blue-light backlight

2 collimating lens sheet

3 apertures in a TFT layer

4 black mask array

5 diffuser sheet

6 liquid crystal cell

7 polarizer 7' polarizer

8 color filter layer

8R red filter in the color filter layer

8G green filter in the color filter layer

8B blue filter in the color filter layer

1 1 layer containing phosphors

1 1R red emitting phosphor

1 1G green emitting phosphor 1 IB scattering medium

21 primary light source

21 ' elongated primary light source

22 parabolic mirror

22' non-parabolic mirror

22" lenticular mirror

25 collimating lens sheet

25' collimating Fresnel lens sheet

25" lenticular lens sheet

31 refracting lens cap

3 lens cap that TIR reflects rays propagating close to the axial direction

41 beam waist

61 lens made of resin in which a light source 21 is embedded

62 adhesive material used to connect elements of a backlight

71 backlight cell that has been illuminated

72 backlight cell that has not been illuminated

MODES FOR CARRYING OUT THE INVENTION

Various embodiments of the present invention will now be described in detail with reference to the drawings.

According to an aspect of the present invention a backlight is provided which includes an array of curved mirror sections, each section of which is illuminated from above by an LED or other primary light source within an array of primary light sources. Also provided is a lens sheet that collimates the light reflected from the mirror sections. The lens sheet is placed adjacent to the array of primary light sources on a side opposite the array of curved mirror sections, or above the light sources and the mirror sections as shown in Figure 4. The light reflected from each mirror section primarily enters the correctly addressed lens in the lens array. The central axis of each mirror section coincides with that of the correctly addressed lens and passes through the primary light source.

The mirror section shape, lens shape and light source positions are chosen to ensure spatial uniformity in the light output as well as collimation. It is preferred that the collimation is such that more than 90% of the light power is contained within an angular cone with a half- width of 10 degrees. It is preferred that the radiant exitance at a plane immediately above the lens array varies by less than 50% over the backlight area.

The spatial extent of the emitting surface of each primary light source is small in comparison to the aperture size of each lens in the array. This aspect is desirable in order to achieve collimation. It is preferred that the area of the emitting surface of each light source is less than one hundredth of that of each lens. It is also preferred that the spatial extent of the light source, including the packaging and any necessary wiring, is substantially smaller than that of each lens in the array. It is preferred that the maximum cross sectional area of each source including any packaging and wiring is less than a tenth of the aperture size of each lens in the array. Here, the cross-section is understood as being taken in the plane normal to the central axis of the mirror sections and lenses.

The light emission from each primary light source extends over a polar angular range Θ, relative to the outward normal from the emitting surface. It is preferred that this outward normal is parallel to the central axis of the mirror sections and lenses. It is preferred that the total angular spread of the emission from the light source is restricted to the range 0°< Θ <90° as measured in air. This includes the emission profile of most forms of LED, which is approximately Lambertian in radiance within the range 0°< Θ <90°. Isotropic light sources are not appropriate.

In a preferred embodiment, the curved surface of each mirror section and lens is deformed from being cylindrically symmetric about an axial direction. The surface shapes are chosen so that the emission escaping from each lens in the array is uniform over the entire lens aperture. In this way, an extended uniform and collimated backlight can be realized from the array ensemble.

A highly collimated backlight is beneficial in liquid crystal displays (LCDs) since: 1) light traversing the liquid crystal cell is close to being on-axis, thus improving contrast and color balance and 2) it enables light to be focused through thin film transistor (TFT) apertures so that the efficiency of the device is improved. To ensure that the viewing angle range is sufficient, it may be desirable to apply a diffuser sheer above the TFT, as shown in Figure 1(A). An aspect of the present invention attains spatial uniformity with highly collimated output with a much reduced device thickness. Figure 4 illustrates the first embodiment of the present invention. Figure 4(A) shows a 3-dimensional rendering of the geometry and Figure 4(B) shows a cross-section including sample ray paths. In contrast to the backlight of Figure 2, mirrors 22' within the mirror array are not parabolic in shape and do not give collimated output after light from primary light sources 21 is reflected in them. A collimating lens sheet 25 made up of a lens array is used to collimate the light reflected by the mirror array. Each lens in the lens array is registered with a corresponding mirror 22' in the mirror array. The mirror shape, lens shape, light source position and the separation between the mirror and lens sheet are carefully chosen to attain spatial uniformity as well as collimation in the light leaving the lens sheet 25.

The backlight according to the present embodiment includes a tiled array of single reflection light emitting diodes (SRLEDs) each serving as a respective light source. Each SRLED includes an LED 21 as a primary light source that emits downwards towards a corresponding mirror 22' (also referred to herein as a "curved mirror section"). To attain a spatially extended uniform radiance from the SRLED plus lens sheet backlight, the output from each SRLED and lens (representing a "unit cell" in the array) should be uniform over the entire area of the unit cell. To achieve such uniform output, it is preferred that the mirror 22' and lens surface within the unit cell not have cylindrical symmetry. In other words, it is preferred that the sag of these surfaces is not purely a function of the radial distance p from the central axis.

The shape of each mirror 22' differs significantly from the parabolic form used in conventional collimating SRLEDs. If the central axis of a mirror coincides with the z-axis of a Cartesian coordinate set, the sag of the mirror 22' surface can be represented as z _u (x,y) . The deviation of the mirror surface from a parabolic form can be represented as

(2) where the integrals are taken over the extent of the curved mirror. The parameters zp and Rp respectively represent the z-coordinate of the apex of the curved mirror, and the radius of curvature of the mirror at its center. The integral in the numerator is minimized with respect to the parameters zp and Rp. The value appearing in the denominator integral is the value of zp when the numerator has been minimized. It is preferred that the value of the dimensionless parameter σ is at least 0.05.

A simplified model can readily be constructed that gives improved uniformity compared to a standard SRLED. In this model, a uniform output is achieved over a disk region rather than over a region such as a square or hexagon which can be tiled to fill an entire plane. This disk fills as much of the lens aperture area as possible without overlapping the boundaries of the unit cell. The mirror curved surface and the lens curved surface are both modeled as symmetric bi-conic shapes so that their sag is purely a function of the radial coordinate p, being given by

(3)

Here, RM {RL) represent the on-axis radius of curvature of the mirror (lens). The conic constant of the mirror (lens) surface is set by KM (KL) . The distance - z sets the separation between the lens and mirror apexes. z can, without loss of generality, be set to zero. A trivial scale invariance allows all lengths to be expressed in units of one of the length parameters. The mirror on-axis curvature, RM, will be chosen as the unit of length. The emitter will again be taken as a point-like Lambertian emitter directed towards the mirror sections and positioned along the central axis. The axial position of the emitter will be denoted k- With the refractive index of the lens set, the model thus has five parameters ( K _M , R _L , K _L , z _e , z ) - These can be varied to find configurations that give uniform collimated output over a disk region. This can be achieved using an optimization procedure based on minimization of an appropriate cost function that is a function of: 1) a measure of spatial uniformity of light output over the emitting disk region, as; 2) the angular spread in the light output, o¾; 3) the device efficiency η; 4) the emitting disk area, A _e . For each trial system, the radiance distribution exiting the lens layer can found using ray tracing. For the described model, it can also be found analytically if any light reflected from the lens interfaces is considered lost from the system.

Figure 5(A) shows the spatial irradiance distribution for a configuration of the simple SRLED plus lens model. The irradiance is sampled in a plane just above the curved surface of the lens. The parameters are: K _M =-0.9, R _L =2.3RM, KL=-0.9, -¾=0.35i? _M and z£ ⁾ =2.4i? _M . The refractive index of the lens is 1.5 and air exists between the mirror and lens. Figure 5(B) shows a rendering of the system and shows sampled ray paths obtained using a ray tracing computer program. The angular dependence of the far-field intensity emitted from the system is shown in Figure 5(C). The angular spread is less than +/- 1 ⁰ about the axial direction. Figure 5(A) shows that the irradiance uniformity over the disk area is excellent. Many sets of the parameters ( K _M , R _L , K _L , z _e , z ) can give sub +/- \° collimation with near perfect spatial uniformity over a disk region. The model described here as an example employs a point source. The size of the emitting area of a real LED will impact the attainable collimation value.

The simplified example described above is for illustrative purposes and does not define the scope of the invention.

The near Lambertian output from an from a primary light source such as an LED is not ideal for forming a thin backlight with the mirror array and lens sheet. Figure 6 shows a second embodiment where a lens cap 31 is placed over the LED to change the angular properties. Two configurations are shown. In the first configuration, shown in Figure 6(A), the lens cap 31 is purely refractive. More light is sent to higher angles from the axial direction than from the naked emitter. Specifically, the lens cap 31 alters the emission angular profile so that more light radiance is present at higher values of Θ (but within the range 0°<θ<90°) compared to a case in which no les cap is used for the primary light source. In this way, the backlight thickness can be made thinner. It also enables a greater portion of light reflected from the mirror sections to be steered away from primary light sources thus improving the efficiency. The angular redistribution can also reduce the incidence of stray light scattering at the primary light sources. If the backlight is used in a display, such stray scattering could cause degradation in the contrast of images. Figure 6(B) shows a variation in which the lens cap 31' causes total internal reflection of light rays emitted close to the axial direction of the mirror and lens. This can allow more light reflected at the mirrors to be steered away from the primary light sources. Both configurations enable more light to be prevented from impinging on the LED structure after reflecting at the mirror. The form shown in Figure 6(B) is particularly suited for steering light from the LED.

Figure 7 shows a third embodiment in which the top lens sheet is replaced with an array of Fresnel lenses 25'. This allows the backlight to be slightly thinner and reduces the weight of the backlight. These are important design considerations in any mass produced display system.

Figure 8 shows a fourth embodiment in which each lens in the lens sheet 25 is placed above a beam waist 41 formed after reflection in the curved mirror. In other words, the beam waist 41 is located between the curved mirror and the corresponding lens in the lens array. Although this backlight form is thicker than other embodiments described herein, various configurations that give collimated and spatially uniform output may be employed. Figure 9 shows a fifth embodiment in which the mirror 22', lens and LED 21 form an integrated unit. The primary light source 21 is embedded within the lens material 61 that is formed from a resin or a suitable polymer and shaped as a lens. Each lens is directly connected to a curved mirror 22' to make a single composite SRLED and lens unit. It is preferred that no air gaps exist between the lens resin 61 and the curved mirror 22'. The composite units may be connected using a suitable adhesive 62.

Figure 10 shows an embodiment where each mirror and lens section is lenticular in comparison to the tiled arrangement of the embodiment of Figure 4. In this embodiment, each primary light source 21' is elongated and oriented with its long axis parallel to the lenticular axis. Each light source 21' emits substantially towards the downwards half space, i.e. towards a mirror section. This embodiment is a lenticular analogue of embodiment 1 shown in Figure 4. The second through fifth embodiments described above also have lenticular analogues.

The emission power and angular profile of a light source such as an LED shows some variation. To attain the best backlight collimation and uniformity, it may be preferable to partition the light sources in relatively narrow power and angular emission profile bins prior to assembly of the backlight. This is due to the inherent lack of mixing of light from different LEDs in this form of backlight. If emission wavelength variations are sizeable, binning according to emission wavelength may also be required.

It may be possible to compensate for the emitter power variations by appropriately changing the driving power for each LED used. It is understood that narrow binning of the light sources and individual control of the driving power will significantly increase the cost of the backlight. As a cheaper alternative, the spatial profile of the transmittance of the spatial light modulator can be altered to compensate for the residual backlight brightness spatial variations.

The mirror and lens arrangements used in the invention can be fabricated by a variety of methods. A typical scale for a mirror section and lens in a unit cell of the backlight is of order a centimeter so that precision micro-optical fabrication is not required. The mirror arrangement can be made, for example, by metal evaporation coating a suitable substrate prepared with the required surface shapes. The substrate and lens surface relief can be fabricated using, for example, injection molding, blank molding, embossing or grinding. All the above fabrication procedures are known techniques. The mounting of the primary light sources should be accomplished using a connecting element that presents a small cross section to the light flow in the system. Likewise, the wiring that connects the primary light sources to the power source may be chosen to impart a small cross section to the light. Careful relative positioning of the backlight components is desirable to achieve good collimation and uniformity. In particular, alignment between the mirror and lens arrays should be maintained over the entire extent of the backlight.

The embodiments of the present invention allow active local dimming techniques to be applied over highly localized regions. This enables significant improvements in efficiency and also in the depth of the displayed black within an image. Each primary light source may be turned on or off depending on requirements set by the currently displayed image. Alternatively, to simplify the circuit wiring, blocks of the primary light sources may be electrically linked to turn on and off from a single electronic control setting. The power to each primary light source or block of light sources may be continuously variable to further enhance the active local dimming performance. Figure 1 1 (A) shows an example in which only a subset of the primary light sources has been switched on. The backlight is illuminated over a region defined by the corresponding cells 71 in the backlight array, the remaining cells 72 being dark. This pattern would be appropriate for backlighting the example image shown in Figure 1 1(B).

Although described herein primarily in the context of serving as a backlight in an at least partially transmissive display, it will be appreciated that the backlight of the present invention may serve in many different lighting applications. For example, the backlight described herein may be used as a distributed illumination panel in general illumination applications. The embodiments of the present invention encompass any and all such applications.

Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may be made. In particular regard to the various operations performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified operation of the described element, even though not structurally equivalent to the disclosed structure which performs the operation in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention pertain to a backlight that can be used in high efficiency large area displays such as

televisions and computer monitors. The embodiments of the present invention relate to a form of direct view backlight with highly collimated and spatially uniform output. The backlight is thinner than conventional direct view collimated backlights that give spatially uniform radiance. The backlight enables local dimming techniques to be applied over highly localized regions, thus enabling substantial power saving. The backlight can be produced using established fabrication technologies. The invention can also be used in general lighting schemes where a spatially extended uniform and collimated light source is needed. An example application in this field is low-dazzle spotlights.