Liquid crystal display device.

0001The invention relates to a display device for use with incident polarized light of a range of wavelengths including a wavelength λ₀, provided with a layer of liquid crystalline material having a positive dielectric anisotropy and maximum birefringence Δn_max between a first transparent supporting plate provided with at least a first transparent drive electrode and a second supporting plate provided with at least a second drive electrode, the layer of liquid crystalline material being switchable via electric voltages on the drive electrodes between two transmission states, the thickness d of said liquid crystalline layer being chosen such that 2d.Δn_max =λ0/2 or d.Δn_max =λ0/2, the device further comprising a compensating layer of birefringent material having a wavelength dependent birefringence and being arranged to compensate birefringence in the layer of liquid crystalline material

0002The invention also relates to a projection display device.

0003A display device of the type mentioned above is described in EP-A-0.320.283. In this device colouring of a super twisted nematic liquid crystal display is prevented by introducing an optically anisotropic medium differing from the liquid crystal layer in at least one of refractive index anisotropy, refractive index dispersion, layer thickness and twist angle. The refractive dispersion is defined as the ratio of the anisotropy at two fixed wavelengths.

0004When using such a display of the type mentioned above between crossed polarizers it transmits no light and is black when a certain minimum voltage is applied. As has been described in EP-A-0.294.899, in a reflective display based on birefringence a very high voltage is needed to obtain full extinction. In said application this is obviated by means of a birefringent compensating layer of a suitable material, producing an opposite phase shift.

0005However, this solution holds for one wavelength or a very narrow wavelength range having a sharp peak. Since the transmission/voltage characteristic curves do not run parallel for all wavelengths, the full extinction is limited to one narrow wavelength range at one selected phase shift. This renders the use of a single display cell in projection display, in which beams of 3 different colours (red, green, blue) are reflected by 3 cells having substantially identical properties (thickness, birefringence), substantially impossible.

0006It is one of the objects of the invention to obviate the above-mentioned problems to a substantial extent. It is another object of the invention to provide a device for projection display in which only one type of display cell for three colour beams is sufficient.

0007It is a further object of the invention to provide a device for projection display using one display cell (provided with, for example, colour filters) in which the complete image is formed by means of one light beam which is not split up.

0008To this end a display device according to the invention is characterized in that the compensating layer comprises a material that has a wavelength dependent phase shift, such that the phase shifts δ_b, δ_g, δ_r of said incident polarised light due to birefringence in the compensating layer for the respective wavelengths λ_b, λ_g, λ_r, corresponding to blue, red and green respectively decrease as a function of increasing wavelength, such that δ_g/δ_r > λ_r/λ_g and δ_b/δ_g > λ_g/λ_b.

0009Notably when the above-mentioned planar ECB effect is used, it appears that the variation in birefringence can be compensated substantially throughout the wavelength region by a similar variation in the compensating layer and more generally parallel transmission-voltage curves can be obtained for devices showing birefringence by applying such a compensating layer. Preferably it holds that the values of the phase shift δ_r, δ_g, δ_b corresponding to wavelengths λ_r(red), λ_g(green) and λ_b(blue) due to the birefringence of the extra layer at their corresponding wavelengths at least satisfy the conditions δ_g/δ_r > λ_r/λ_g and δ_b/δ_g > λ_g/λ_b.

0010In practice this means that it holds for this specific example that 20° < δ_r < 40°, 25° < δ_g < 45° and 30° < δ_b < 50°, respectively.

0011In this manner substantially parallel transmission/voltage characteristic curves are obtained for a part of the voltage range for red light (λ = approximately 658 nm), green light (λ = approximately 546 nm) and blue light (λ = approximately 480 nm).

0012By providing a voltage compensation circuit for at least two colours in a three colour display device, the three transmission/voltage characteristic curves of the total device substantially coincide within the operational range, which renders the device suitable for direct drive by means of the three chrominance signals of, for example, a video signal, because the voltage compensation can be realized by means of a simple voltage difference.

0013A polymer layer having, for example a composition of approximately 60 % of polymethyl methacrylate and 40 % of polyvinylidene fluoride is preferably used for the extra layer of birefringent material.

0014The device thus obtained is very suitable for reflective display, as described in said patent application EP-A 294,899.

0015A further preferred embodiment is therefore characterized in that the second supporting plate is provided with a layer of reflecting material.

0016The device preferably comprises a polariser and an analyser, the angle between the direction of polarisation of the polariser and the direction of orientation of the molecules at the area of the first supporting plate being substantially 45°.

0017These and other aspects of the invention will now be described in greater detail with reference to some embodiments and the drawing.

0018Fig. 1 shows diagrammatically the operation of a device according to the invention, driven in reflection.

0019Fig. 2 shows the variation of the phase shift δ_LC of incident polarised light due to the birefringence in the liquid crystalline materials used in such devices for light beams having three different wavelengths.

0020Fig. 3 shows transmission/voltage characteristic curves for a device without the extra layer of birefringent material.

0021Figs. 4, 5 show transmission/voltage characteristic curves for a device according to the invention.

0022Fig. 6 shows a projection display device.

0023Fig. 1 is a diagrammatic cross-section of a display device 1 provided with a first supporting plate 2 of, for example, glass. The supporting plate 2 is provided with a transparent electrode 3 of indium tin oxide or another suitable material and, if necessary, an orienting layer 4 which may also be used to prevent unwanted reactions between the liquid crystal material 5 and the electrodes. The liquid crystal material 5, for example ZLI 2452 (Merck) is present between the first supporting plate and a second supporting plate 6; the supporting plates are spaced apart by means of spacers which are not shown. The assembly is closed by means of a rim which is not shown either.

0024A matrix of pixels which may consist of reflecting picture electrodes 9 of, for example aluminium is provided on the second supporting plate 6 which may not only be glass but also a semiconductor body. These picture electrodes can be electrically driven by means of switching elements realised in or on the second supporting plate 6.

0025In this example the thickness of the layer 5 may vary between 0.25 »m and, for example, 4 »m, dependent on the type of spacers used.

0026The use of said planar liquid crystal effect (ECB) with positive dielectric anisotropy will now be further described with reference to Figs. 1a-1c. The surface layers 4 and 7 are prepared in such a manner that in the voltageless state the liquid crystalline molecules are directed in a given direction parallel to the surfaces of the supporting plates 2, 6.

0027Linearly polarised light having, for example a direction of polarisation 33 (Fig. 1a) is passed by means of a polariser 8 whose direction of polarisation extends at an angle of 45° to the orientation layer of the liquid crystal molecules (shown diagrammatically by means of the director 32). Since this direction of vibration extends at an angle of 45° to the direction of orientation of the liquid crystal, the incident polarised wave is split into an ordinary wave (with direction of vibration 34, parallel to the orientation direction of the liquid crystal) and an extraordinary wave (with direction of vibration 35 perpendicular to the direction of polarisation 34).

0028The difference in optical path length for the ordinary and extraordinary wave when leaving the liquid crystal is 2 dΔn after reflection (d = thickness of liquid crystal layer; Δn difference in refractive indices of the ordinary and the extraordinary wave). This optical path length difference is maximum in the situation of Fig. 3a because the liquid crystal molecules are oriented substantially parallel to the supporting plates (Δn = Δn_max). The entering and exiting abnormal beams are 180° out of phase (indicated by means of directions of vibration 35 and 35′, respectively) at such a choice of the thickness d that for a chosen wavelength λ0 it holds that 2dΔn_max = 1/2 λ0). The direction of polarisation of the exiting light is then shifted 90° (indicated by 33′ in Fig. 3a) so that this light can pass an analyser 9 whose direction of polarisation is shifted 90° with respect to that of the polariser 31. At V = 0 the maximum quantity of light is reflected.

0029In the situation of Fig. 1c the directors 32 of the liquid crystal are perpendicular to the two supporting plates. An incident polarised bean is not split up in this case and maintains its direction of polarisation (2dΔn = 0). This bean is not passed by the analyser 9.

0030In the intermediate situation of Fig. 1b it holds that 0 < 2dΔ < 1/2 λ0. The exiting beam is now elliptically or circularly polarised and is passed to a greater or lesser extent by the analyser 9, dependent on the applied voltage V and the associated angle between the directors and the supporting plates.

0031For a full extinction in Fig. 1c, i.e. all directors are perpendicular to the supporting plates, a very high (theoretically infinite) voltage is required (curves a, b, c in Fig. 2).

0032In practice such a high voltage can be prevented by choosing an off voltage Vr (Figs. 2, 3) at which a given quantity of light is still reflected corresponding to a phase shift δ<maths id="math0001" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>r</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0001.tif" /></maths> of the polarised light. Full extinction for one wavelength is obtained in this case by extending the device of Fig. 1 with, for example, a phase plate of, for example, Polaroid, cellophane or another suitable birefringent material producing an opposite phase shift -δr.

0033Fig. 2 shows the variation of the phase shift in the liquid crystal material at three different wavelengths, namely in the red δ<maths id="math0002" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>r</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0002.tif" /></maths> at a wavelength of approximately 658 nm, in the green δ<maths id="math0003" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>g</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0003.tif" /></maths> at a wavelength of approximately 546 nm and in the blue δ<maths id="math0004" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>b</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0004.tif" /></maths> at a wavelength of approximately 480 nm. This Figure clearly shows that the three curves do not coincide so that a correction -δ_r at a voltage Vr which causes full extinction in the red still gives rise to a residual transmission in the two other wavelength ranges when using the same voltage. Since the curves (notably for δ^LC &lt; 100°) do not extend parallel either a simple voltage compensation is not readily possible.

0034According to the invention the device is provided with a layer of birefringent material 10 having a wavelength-dependent phase shift, preferably with an optical axis which is perpendicular to the directors 32 in the voltageless state.

0035To determine the wavelength dependence, use is also made of the fact that the characteristic curves for the phase shift δ^LC (Fig. 2) extend substantially parallel for the three different colours at values of δ^LC > 100°. In fact, if this were to hold for all values of δ^LC, substantially identical characteristic curves can be obtained for the three colours by shifting the curves δ<maths id="math0005" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>g</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0005.tif" /></maths> and δ<maths id="math0006" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>b</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0006.tif" /></maths> by means of a voltage compensation circuit over a voltage range of, for example, Δgr (130°) and ΔVgb (130°), respectively, in other words by choosing<maths id="math0007" num=""><math display="block"><mrow><mtext>Vg (0°) = Vr (0°) + ΔVgr (130°)</mtext></mrow></math><img file="EP0459554B1_D0007.tif" /></maths> and<maths id="math0008" num=""><math display="block"><mrow><mtext>Vb (0°) = Vg (0°) + ΔVbg (130°).</mtext></mrow></math><img file="EP0459554B1_D0008.tif" /></maths>

0036It is clearly apparent from Fig. 2 that these shifts for Vg (0°) and Vb (0°) do not lead to δ^LC = 0°, but that a further adaptation is necessary for the birefringence in the blue and the green of the extra layer 10. To this end the starting point is not δ<maths id="math0009" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>g</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0009.tif" /></maths> = δ<maths id="math0010" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>b</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0010.tif" /></maths> = δ<maths id="math0011" num=""><math display="inline"><mrow><mfrac linethickness="0"><mrow><mtext>LC</mtext></mrow><mrow><mtext>r</mtext></mrow></mfrac></mrow></math><img file="EP0459554B1_D0011.tif" /></maths> = 130°, but a slightly different phase shift is taken as a starting point for the green and blue curves, while it is simultaneously assumed that there is a variation in the birefringence of the layer 14.

0037This leads to<maths id="math0012" num=""><math display="block"><mrow><msubsup><mrow><mtext>δ</mtext></mrow><mrow><mtext>r</mtext></mrow><mrow><mtext>LC</mtext></mrow></msubsup><msub><mrow><mtext> = 130°   = 100° + δ</mtext></mrow><mrow><mtext>r</mtext></mrow></msub></mrow></math><img file="EP0459554B1_D0012.tif" /></maths><maths id="math0013" num=""><math display="block"><mrow><msubsup><mrow><mtext>δ</mtext></mrow><mrow><mtext>g</mtext></mrow><mrow><mtext>LC</mtext></mrow></msubsup><msubsup><mrow><mtext> = δ</mtext></mrow><mrow><mtext>r</mtext></mrow><mrow><mtext>LC</mtext></mrow></msubsup><msub><mrow><mtext> - δr + δg = 100° + δ</mtext></mrow><mrow><mtext>g</mtext></mrow></msub></mrow></math><img file="EP0459554B1_D0013.tif" /></maths><maths id="math0014" num=""><math display="block"><mrow><msubsup><mrow><mtext>δ</mtext></mrow><mrow><mtext>b</mtext></mrow><mrow><mtext>LC</mtext></mrow></msubsup><msubsup><mrow><mtext> = δ</mtext></mrow><mrow><mtext>r</mtext></mrow><mrow><mtext>LC</mtext></mrow></msubsup><msub><mrow><mtext> - δr + δb = 100° + δ</mtext></mrow><mrow><mtext>b</mtext></mrow></msub></mrow></math><img file="EP0459554B1_D0014.tif" /></maths>

0038On this basis, voltage adaptations ΔVbg and ΔVgr having a value of<maths id="math0015" num=""><math display="block"><mrow><msub><mrow><mtext>ΔVgr = Vg (100° + δg) - V</mtext></mrow><mrow><mtext>r</mtext></mrow></msub><mtext> (100° + δr)</mtext></mrow></math><img file="EP0459554B1_D0015.tif" /></maths><maths id="math0016" num=""><math display="block"><mrow><msub><mrow><mtext>ΔVbg = Vb (100° + δb) - V</mtext></mrow><mrow><mtext>g</mtext></mrow></msub><mtext> (100° + δr)</mtext></mrow></math><img file="EP0459554B1_D0016.tif" /></maths> can be found. If the drive voltages of a device according to the invention are corrected with these values, the three curves for red, green and blue substantially coincide. In this case it holds that δg/δr > λg/λr and δb/δg > λb/λg in which λr, λb and λg are the dominant wavelengths in the red, blue and green wavelength ranges, respectively.

0039Together with Figs. 4 and 5, Fig. 3a shows the effect of the invention. Fig. 3 shows the transmission/voltage characteristic curves for three wavelengths (λb = 480 nm, blue; λg = 546 nm, green; λr = 658 nm, red) for a device without any extra birefringent layer 10. Notably in the low transmissive part the characteristic curves do not run parallel to each other. In the device of Fig. 4, where the extra layer is present, this is much less the case, and the characteristic curves substantially coincide after voltage compensation per colour (see Fig. 5).

0040The conditions with respect to the chosen phase shifts for the extra birefringent layer 10 associated with this example are found to be eminently satisfied by a layer comprising approximately 60 % of polymethyl methacrylate and approximately 40 % of polyvinylidene fluoride.

0041Dependent on the thickness of the layer the values of δr, δg and and δb varied between values of 20-40°, 25-45° and 30-50°, respectively.

0042Fig. 6 shows diagrammatically a projection device realised with a device 1 as described in Fig. 1.

0043A lamp 40 (shown diagrammatically) emits a light beam which, after having passed collimator lenses 41, 42, is incident on a mirror 43 and is thereby reflected through a small angle with respect to the normal 48 towards the liquid crystal device 1.

0044Via a planoconvex lens 44 the light reaches reflecting or non-reflecting electrodes of the semiconductor device which determine the state of the liquid crystal (light transmissive or non-transmissive) under the influence of drive electronics. The information determining this state may be, for example a TV signal which is shown diagrammatically by means of the antenna symbol 41.

0045After it has been reflected, the beam leaves the device 1 at a small angle with respect to the normal 48 and reaches the image plane 46 via the planoconvex lens 44 and a second lens 45. This plane may coincide, for example, with a projection screen. In this case, the polariser and analyser are arranged, for example, between the mirror 43 and the planoconvex lens 44 and between the lenses 44 and 45, respectively. The light beam may be a single beam of white light and, if necessary, the liquid crystal device 1 may be provided with a colour filter. The light beam may also be one of the 3 composite beams (for example red, green, blue) of a projection device operating with 3 beams. The liquid crystal device 1 is then driven by means of signals associated with the relevant colour.

0046The above described compensation method may of course also be used for two or more colour beams of other composite colours, dependent on its use.