The invention is concerned with the use of chiral liquid crystals for the production and/or modulation of polarised light, in particular for use in displays. One of the major drawbacks of colour liquid-crystal display screens is the inefficient use of input energy as a result of the light lost in polarisers and filters. The described invention addresses this shortcoming, though it also has application in monochromatic displays.

The invention makes use of chiral liquid crystals, i.e. liquid crystals having a helical structure. These are exemplified by chiral nematic (cholesteric or N* where the star denotes chirality) and chiral smectic tilted phases such as chiral smectic C phase (SmC*) . Both of these phases can have helical structures. The helix is somewhat like a screw thread and has a pitch, P, associated with it which is the repeat distance along the helix. It corresponds to a rotation around the helix of 27r radians.

Colour displays must be capable of emitting light over a bandwidth of from 380 to 780 nanometres. The wide range of this electromagnetic radiation is the cause of much frustration and technical inconvenience for the designers of thin liquid-crystal displays for use in such products as lap-top computers and televisions. Given the extra problems associated with white-light filtering systems which are employed to produce the red, blue and green pixel elements it is therefore not surprising that only about 4% of the energy input is available to be viewed.

To combat this energy loss attempts have been made to increase the light thrown forward towards the viewer using cholesteric liquid crystals. A promising approach uses the "cholesteric mirror", in which use is made of the reflective and transmissive properties of

cholesteric liquid crystals when used in conjunction with polarised light.

In the basic cholesteric mirror approach, illustrated diagrammatically in Fig. 1, unpolarised light from a source 1 is directed towards a layer 23 of a cholesteric liquid crystal. Such a liquid crystal will pass light of wavelengths which (taking into account the refractive index of the material) are appreciably different from its pitch, and also light of wavelengths close to its pitch and circularly polarised in the opposite direction. Light of the correct wavelength and circularly polarised in the same direction will be reflected. The layer thus acts as a filter. Monochromatic light containing both right- and left-handed polarisations (unpolarised light) will impinge on the chiral filter substrate 23 and light of the same (left-) handedness as the chiral liquid crystal will be almost entirely reflected and that of the opposite handedness will almost entirely traverse the filter, as shown. In order to enhance the light intensity a reflector 11 (either diffuse or specular) may be placed behind the filter either in front of or behind the light emitter. The circularly polarised light will then undergo a iτ phase change at the reflector which effectively reverses its handedness, and it will then be transmitted by the filter. It can then be used to enhance the overall transmitted light. For this arrangement see B. Kerllenevich & A. Coche, conference paper for SID France, August 31 - September 3, 1993.

The right-handedly circularly polarised light traversing the filter 23 can be modulated. In the conference paper a twisted nematic (TN) liquid-crystal layer 12 is used, to which a voltage can be applied by electrodes on each face. This LC layer is of a

material and a thickness making it equivalent to a half-wave plate when no voltage is applied; a supertwisted nematic (STN) LC could be used instead of a TN liquid crystal, the operation being the same for present purposes. The effect of the half-wave plate is to reverse the handedness of the incoming light, as shown. If on the other hand a voltage is applied to the liquid crystal the right-handedly polarised light passes through unchanged. Hence by arranging a right- handed cholesteric mirror on the other side of the TN layer light is either blocked or passed, depending on whether a voltage is applied.

While this approach works well enough for a monochrome display and has the great advantage of not needing polarisers, which are far from 100% efficient, it is not suitable for colour displays since several cholesteric layers would be needed to cover the range of wavelengths. This is one reason why cholesteric liquid crystals have not previously been much used in displays.

SUMMARY OF THE INVENTION In broad terms the invention makes use of cholesteric liquid crystals to block or transmit circularly polarised light and can be used to produce colour displays. This is achieved in one aspect by a system comprising a means for providing circularly polarised light and a modulator made of a cholesteric liquid crystal material towards which the light is to be directed, the modulator being activatable selectively to pass or to block circularly polarised light.

Such an arrangement gives a basically monochromatic output which may itself be adequate for some purposes; however it is particularly effective when the light is substantially monochromatic, in particular ultraviolet, and once modulated is used to

activate substances which can radiate visible light, or change their reflective or absorptive characteristics, or otherwise generate an output. Substances of this type are exemplified by phosphors in the emissive case. Reference may be made here to WO95/27920 for this kind of display. When the invention is incorporated into a display device both monochrome and full-colour display screens can be produced.

The input circularly polarised light can be produced by a conventional "cholesteric mirror" arrangement as described above.

The invention can provide a low-loss liquid- crystal modulator which does not need polarisers and indeed dispenses altogether with anything analogous to an analyzer, needing in the simplest case only a filter and a modulator. This is because the liquid crystal can block the light directly, rather than merely altering its polarisation state for subsequent blocking by another optical element, as is done with conventional TN-type LC displays, for instance. The liquid crystals can be for example chiral liquid crystals of predetermined pitch which are used in conjunction with monochromatic or near-monochromatic light, this being exemplified by sub-visible, short wavelength electromagnetic radiation, commonly referred to as ultra-violet light.

The process advantageously uses ultra-violet light with a wavelength of 365 nm, which with a typical average refractive index of 1.6 needs a chiral liquid crystal of pitch 230 nm. Such pitches are known for chiral nematic liquid-crystal (N*) and chiral Smectic (SmC*) materials.

To optimise the process collimation of the input unpolarised light is preferred. This can be done using lenslet systems. However, since embodiments of the invention typically need no polarizers, in such

embodiments that use phosphor-type output elements these elements can be placed inside the liquid-crystal cell; this greatly reduces the need for collimation and considerably simplifies the arrangement. With the secondary emitters in the form of phosphors ultra-violet light with a wavelength of 365nm is often ideal; single-pitch chiral liquid crystals can be used. For the chosen wavelength, 365nm, a typical pitch value in μm will be:

_P =λ = 0.365μ ^j τ ^? _{= 0j} 228μτn Equa tion (1 ) . n 1.6

However, another possibility would be to use light of a smaller wavelength, e.g. blue light, to activate phosphors emitting at a longer wavelength, such aε red or green; this would avoid the use of UV.

In a different application of the invention a cholesteric mirror system can be used to generate linearly polarised light without using conventional polarizers, for instance by adding a quarter-wave plate. This linearly polarised light can be optically switched by a liquid-crystal electro-optic effect which requires linearly polarised light, such as is used in conventional twisted nematic and supertwisted nematic displays, and is also applicable in conjunction with a number of ferroelectric, electroclinic and other electro-optic effects based on bi-refringence, e.g. simple (non-twisted) nematic planar homeotropic switching. Some dichroic effects can be optimized by lining up the absorbing direction with the light polarisation direction, e.g. in conjunction with nematic liquid crystals without twist and with ferroelectric liquid crystals.

In another aspect therefore the invention provides a light-modulating device including a source of substantially monochromatic ultraviolet light, a first

cholesteric mirror for transmitting light of one circular handedness, a modulatable liquid-crystal layer for selectively altering the polarisation of the transmitted light, and means such as a second cholesteric mirror for passing or blocking the selected polarisation, as the case may be. In one version there are 1/4-wave plates between the cholesteric mirrors and the liquid-crystal modulator, so that the modulator can be a conventional TN, STN or other bi-refringent liquid crystal, the intermediate light being linearly polarised.

Again, the transmitted light can be used to activate secondary emitters such as phosphors, in particular for colour displays. The light source of the present invention can thus be used to produce an ultra-violet light energised, phosphor-radiating, emissive display. This overcomes the difficulty with the known Fig. 1 arrangement in connection with colour displays. DETAILED DESCRIPTION

Embodiments of the invention, including architectures which make use of either circularly or linearly polarised light together with the modulating means, will now be described with reference to the following diagrams, in which:

Fig. 1 shows the known cholesteric mirror principle;

Fig. 2 is a schematic view of a first embodiment of the invention using two cholesteric mirrors in series, one as a polarizing filter and the other as a modulator;

Fig. 3 is a schematic view of a second embodiment using circularly and linearly polarised light with associated polarizers, either conventional or chiral, and

Fig. 4 is another very schematic view showing the

arrangement of Fig. 2 over a wider area as part of a display.

In Fig. 2 a source 1 produces unpolarised, uncollimated electromagnetic radiation, in this example substantially monochromatic or narrow-band ultra-violet light. The light passes through a lens 5, which in a display application can be an element of a lenslet array, from which it emerges as unpolarised, collimated light 9. Left-handed circular polarization is symbolically indicated at 15, and right-handed circular polarization is symbolically indicated at 19. The light impinges on the surface 21 of a cholesteric mirror 23, containing right-handed chiral liquid- crystal material. Light with right-handed polarization 19 is reflected from the surface 21, as shown at 6, while light with left-handed polarization 15 traverses the cholesteric mirror, which therefore acts as a kind of filter. The helical nature of the liquid crystal is shown at 8. For display purposes the arrangement essentially consists of the circular polarizing filter 23 and a chiral modulator 34. The modulator helix should be of the same handedness as the light impinging on it. The circular polarizing filter need not be switched if there is no requirement for modulation at this stage. The wavelength (λ) of the light should satisfy the equation: λ = n x P Equation (2) , where n is the mean refractive index and P is the pitch of the chiral material in both the filter and the modulators.

When no voltage is applied to the modulator the light of the corresponding handedness, will be almost entirely reflected and little light will traverse the modulator. When a field is applied to the modulator the pitch will be unwound above a threshold given by:

where K ₂₂ is the twist elastic constant, Δe is the permittivity anisotropy and E ₀ is the permittivity of free space. The light will then be transmitted. The upper part of Fig. 2 shows a cell where no field is applied and thus no light is transmitted.

Where the pixel cells are in the "off" state the helical structure is maintained as at 34a, while when a field is applied across the cholesteric cell the helix is unwound and the liquid crystal enters the nematic phase as at 34b. For pixel 34a the left-handed polarised light is reflected, indicated by 26, whereas light entering the nematic-state pixel 34b traverses the cholesteric mirror as at 30, with left-handed polarization symbolically indicated at 15. The circular-polarizing filter 23 can be formed from a convenient chiral material, e.g. a polymeric material, with a helix of appropriate handedness to that of the modulator and with a pitch length satisfying equation (2) . This system is basically a circular analogue of the standard display system using linearly polarised light and twisted-nematic liquid crystals with the advantage that no analyzer is necessary. The arrangement shown emits monochromatic circularly polarised light 30, which can be used directly, for instance for a display. However, further advantages can be gained if this essentially monochromatic light is used to activate output elements (not shown) such as phosphors, which in turn emit light to a viewer. In this case the phosphors can be RGB phosphors analogous to those used for instance in a CRT display, which means that an efficient colour display can be produced. This configuration and its advantages are described for

instance in WO 95/27920 (Crossland et al) .

Fig. 3 is an embodiment in which the circularly polarised light undergoes a conversion to linear polarisation and the modulator responds to linear polarisation. Here, as in Fig. 1, the light source 1 produces narrow-band unpolarised, uncollimated light, shown at 3, entering a lens 5, which is again a unit part of a lenslet array. Collimated, unpolarised light is produced, indicated at 9. The left-handed circularly polarised light is symbolically indicated at 15 and right-handed, circularly polarised light is indicated at 10. Collimated light thus impinges on the base 21 of the cholesteric mirror 23. This mirror contains cholesteric liquid crystals with a right- handed chirality (right-handed twist) . The right- handedly polarised component of the collimated, unpolarised light is reflected from the base 21 of the cholesteric mirror 23, and the left-handed polarised component traverses the mirror at 27, with the symbolic indication 15. Thus far the arrangement corresponds to that in Fig. 2.

In this embodiment, the collimated, left-handed, polarised component traverses a quarter-wavelength substrate, shown at 31, and emerges as linearly polarised light, 35, the polarization being shown symbolically at 39. The light 35 enters a half-wave plate (optimum) modulating substrate 43, which may be of conventional (e.g. TN) design, and traverses a plurality of pixels, two of which are indicated at 43a and 43b. In Fig. 3 pixel 43a is switched off and pixel 43b is switched on. The polarised light traversing pixel 43a remains unchanged whereas the polarised light traversing 43b undergoes a rotation of 90 degrees, thus changing its linear polarization. The original polarization is symbolically indicated at 39 and the changed polarization is symbolically indicated at 47.

Light of each linearly polarised form is indicated at 35 and 45 respectively.

The light traversing the device is shown generally at 44 and can be processed in one of two ways, indicated at 44a and 44b.

If process 44a is chosen linearly polarised light of both orientations, 35 and 45, passes through a quarter-wave plate, shown at 53, with the fast axis correctly orientated, whereupon both forms of linearly polarised light are transformed into circularly polarised light. The unmodulated linear form 35 is transformed into left-handed circularly polarised light, 54, with symbolic indication shown at 15, and the linear form 45 is transformed into right-handed circularly polarised light 56 with symbolic indication 19. The two forms of circularly polarised light impinge on the base 65 of a further cholesteric mirror 59. The cholesteric mirror contains liquid crystals of left-handed chirality (twist) . The left-handed circularly polarised light is reflected from the base 65 of the mirror and the right-handed circularly polarised light traverses the cholesteric mirror 59. The emergent light is shown at 61, right-handed circular polarization being symbolically indicated at 19. Of course, the mirror 59 could be right-handed, in which case the image would be "negative". This operation is related to the Kerllenevich & Coche design (Fig. 1) .

If option 44b is chosen a conventional linear polariser, shown at 55, removes the polarised light 35, having an orientation symbolically indicated at 39, whilst the polarised light 45 with orientation symbolically indicated at 47 traverses the polarizer 55 at 61' and is made available to be seen by a viewer 70. In this option one has a simpler structure but retains some of the advantage of having a cholesteric mirror

instead of the first polariser used in a normal TN display.

Fig. 4 shows a part of a display along the lines of Fig. 2; for best results the light 15 should be collimated, but for clarity this is not shown. The transmissive pixels in the modulator 64, i.e. those where a voltage is applied, allow circularly polarised light 63 to pass, for a monochromatic display or for further processing. As mentioned above, in a working display the output light 59, 61, 63 impinges on an array of secondary light-emitting elements which are not shown but which can be, for instance, along the lines of those shown in the international application no. PCT/GB95/770 (Crossland et al.) , namely RGB phosphor dots. Alternatively they can be elements whose absorption/reflection characteristics are altered by the action of the UV light. These, with suitable modulation of the UV light, then produce the desired colour display. This solves the problem of producing a colour display using the highly wavelength-selective cholesteric liquid crystals. However, the invention is not limited to colour displays, nor to the use of ultraviolet light. Clearly many variations of the arrangements shown are possible: the collimator need not be a lens or lenslet array, other applications besides displays are conceivable.