Contrast improvement in liquid crystal light valve projectors

Abstract: The present invention relates to a light valve image projector with a disposition for improving the contrast of an optical image projected on a screen by that projector. The projector comprises at least one illumination system, at least one field lens, at least one set consisting of a polarizer, an LCD and an analyzer and at least a projection lens, and presents at least one optical axis which includes an angle α with the optical axis of the projection lenses. The invention consists in positioning the analyzer (25) so that it includes an angle larger than α with the LCD, at least for the light rays that pass through the contrast limiting area of the LCD. The invention consists furthermore in placing the polarizer so that it includes an angle larger than α with the LCD. Both the analyzer and/or the polarizer may have a flat or a curved surface.

Description: The present invention relates to contrast improvement in liquid crystal light valve projection systems, more particular to a light valve image projector with a disposition for improving the contrast of the by that projector projected optical image. For an enlarged projection of images on a screen by means of a light valve projector, an optical image is put on a light valve and bundled light is sent through said light valve. Said light then passes through an appropriate projection lens system which focuses an image on the screen. An appropriate light valve for such a projection system is a liquid crystal display, commonly referred to as an "LCD". Two kinds of liquid crystal light valve projectors exist: OFF-axis projectors and ON-axis projectors. Fig. 1 shows the optical path in an OFF-axis projection system. Said projection system comprises an OFF-axis projector 1 and a screen 2. The OFF-axis projector 1 comprises an illumination system 3, a field lens 4, a polarizer 5, an LCD 6, an analyzer 7 and a projection lens 8. Said illumination system 3 comprises a lamp, a reflector, and an integrating system which converts the circular and non-uniform light distribution coming from the reflector into a rectangular and uniform illumination of the LCD 6. Said field lens 4 images the illumination system into the entrance pupil of the projection lens 8 and ensures in this way that the light with which said LCD 6 is illuminated is directed into this entrance pupil of the projection lens 8. Said projection lens 8 images the illuminated LCD 6 on the screen 2. The line through the optical centers of said illumination system 3, said field lens 4 and said projection lens 8 is further called the optical axis 9 of the projector 1. In case the OFF-axis projector 1 is a multi-light valve projector (for example a projector with three light valves), the optical path is more complex, because through a set of dichroic mirrors the light is splitted into 3 colours red, green and blue, and after modulation of the light, the colours are recombined again. The OFF-axis projector comprises then an illumination system, three field lenses, three sets consisting of a polarizer, an LCD and an analyzer, and a projection lens. A line through the optical centers of the illumination system, a field lens and the projection lens is an optical axis of the projector. The schematical description of the illumination and imaging system shown in figure 1 is not altered by these colour splitting mirrors. In what follows the state of the art is described as if there were only one illumination system, one field lens, one set consisting of a polarizer, an LCD and an analyzer, and one projection lens. The direction of the optical axis 9 of the projector 1 is different from the direction of the optical axis 10 of the projection lens 8. Both optical axes 9 and 10 include an OFF-axis angle α as shown in Fig. 1. This angle can have any positive value, but in the most common configuration the projector 1 is set up in such a way that the optical axis 10 of the projection lens 8 intersects with the bottom of the image of the screen 2. If the OFF-axis angle α is zero, or in other words, if the direction of the optical axis 9 of the projector 1 is equal to the direction of optical axis 10 of the projection lens 8, the projector is called an ON-axis projector. Fig. 10a shows the minimum configuration of an ON-axis liquid crystal light valve projection system. It comprises an ON-axis projector 11 and a screen 2. The ON-axis projector 11 comprises the same parts as the OFF-axis projector 1 described hereinabove. Said same parts have been denoted by means of the same reference numerals. Fig. 2 shows an image source of as well OFF-axis and ON-axis liquid crystal light valve projectors. Said image source comprises a polarizer 5, an LCD 6 and an analyzer 7. Although represented in Fig. 2 as three separate parts, two of the parts or the three parts can form one unit. The polarizer 5 and the analyzer 7 are both light polarizing elements and have in their planes each an absorbing direction and a non-absorbing direction, the absorbing direction being perpendicular to the non-absorbing direction. The LCD 6 comprises, with reference to the light direction, a front glass plate 15, a back glass plate 16 and sealed in between both glass plates 15, 16 a twisted nematic liquid crystal layer. The side of the glass plates 15, 16 not touching the twisted nematic liquid crystal layer, each have a layer with transparent image forming pixel electrodes. The twisted nematic LCD may be addressed with an active matrix. In such an active matrix TN-LCD, a switching device, such as a thin film transistor or a number of thin film diodes, is integrated on each pixel. In Fig. 2 the non-absorbing direction 17 of the polarizer 5 is perpendicular to the rubbing direction 18 of the front glass plate 15 of the LCD 6. Said rubbing direction 18 defines the preferred direction of the liquid crystal molecules at the interface with the front glass plate 15. The non-absorbing direction 19 of the analyzer 7 is perpendicular to the rubbing direction 20 of the back glass plate 16. In a twisted nematic liquid crystal display, the rubbing directions at the opposite glass plates are perpendicular and therefore, the molecules form a 90 degrees twisted helix in the bulk as shown in Fig. 3a and Fig. 3b. Fig. 3a shows the distribution of the molecules 21 in a twisted nematic LCD between the front glass plate 15 and the rear glass plate 16 in the bright state, and Fig. 3b shows the corresponding distribution in the dark state. To obtain an optimum black level in an image projected by a projection system containing a polarizer, an LCD and an analyzer, the non-absorbing directions of these three elements must be correctly matched. Therefor, the analyzer or the polarizer may be installed rotatable around their normal. The contrast of the image obtained with an LCD as light valve depends strongly on the angle of incidence of the light enterring the LCD, and is by consequence LCD area and by consequence screen area dependent, what is explaned by the following. Fig. 4 shows the typical viewing angle characteristics of a twisted nematic LCD. The graphs are lines of equal contrast between the bright and dark state at the two corresponding driving voltages, with relative values between 3 en 300. The vertical axis gives the vertical angle of incidence between -40° and 40° and is the component of the light ray within the plane perpendicular to both the plane of the LCD and the horizontal image scanning direction of the LCD. The horizontal axis gives angles of incidence between -40° and 40° and is the component of the light ray within the plane perpendicular to the plane of the LCD and the vertical image scanning direction of the LCD. Fig. 4 demonstrates thus the dependence of the contrast in function of the incident angle of the light which enters the LCD. The contrast changes much more in the vertical direction than in the horizontal direction of the screen. Moreover, the maximum contrast is not obtained for light rays with normal incidence but for light rays having a small incident angle in the vertical direction. This angle corresponds with the angle α V shown on Fig. 3b. The angle α V agrees very well with the remaining tilt at dark state of the molecules in the twisted nematic liquid crystal layer. As a consequence of the phenomena as shown in Fig. 4, and refering to Fig. 1 and Fig. 10a which show that light rays enter the LCD with different angles, without further measures the image projected by a projector using as light valve a twisted nematic LCD will show a nonuniform image which is differently contrasted in vertical direction. Fig. 5 shows in another way how the LCD light transmission (vertical axis) depends on the angle of incidence (different graphs) and on the LCD drive voltage (horizontal axis). For angles of light incidence larger than 0°, the light transmission graphs show a minimum. This minimum determines the maximum obtainable contrast at that angle of incidence, as for higher drive voltages the contrast is negative. For larger positive angles of incidence, the minimum transmission value is higher and so the maximum obtainable contrast is lower. For smaller positive angles of incidence, the location of the minimum on the graph is shifted to a higher drive voltage and to a lower transmission value or higher obtainable contrast. For zero and negative angles of incidence of light, there is no minimum of transmission. Another representation of the contrast dependence of the angle of incidence and also including the behaviour of the LCD in function of its drive voltage is given by the graphs "0" in figures Fig. 6a, 6b and 6c. The normalized intensity on the screen is displayed on these figures in function of the LCD drive voltage for a projection system as illustrated by Fig. 1. Fig. 6a shows the intensity measured at the top of the display screen; Fig. 6b shows the intensity in the middle of the screen; Fig. 6c shows the intensity measured at the bottom of the screen. Fig. 6a shows that in areas of the image where the angle of incidence of light is larger, as it is the case at the top of the image in a projection system as illustrated in Fig. 1, for a specific drive voltage of the LCD the intensity of the projected image has a minimum value. For drive voltages above the voltage of minimum intensity, the contrast is even negative. If otherwise the drive voltage should be equal all over the LCD, such an intensity minimum will appear on the screen at a specific angle of incidence and be visible as horizontal darker bar. Fig. 6b and Fig. 6c show that in other areas of the image where the angle of incidence is smaller, there is not such a minimum of intensity or in other words maximum of contrast, and the limitation is determined by the maximum allowed LCD drive voltage. For above reasons, in practice the LCD drive voltages are chosen to get a uniform black level or minimum intensity all over the screen. The limit is thereby determined by what can be obtained in the area of larger angles of incidence, and so does not result in the best contrast possible in other areas of the screen. In order to obtain a better contrast and better uniformity of contrast without loss of other picture quality, various methods of contrast enhancement and apparatus with improved contrast are known. Various apparatus have additional fysical items with reference to the minimum projector set-up as described in Fig. 1. In US-5,576,854 a quarter-wave compensator plate is disposed between the polarizer and the liquid crystal light. This solution is only suitable for the reflective setup described in the patent and is not applicable for projection with TN-LCDs. In US-5,371,559 a stop is disposed inside the projection lens, forming an aperture of the lens assembly decentered from the optical axis of the lenses of the projection lens assembly in the vertical scanning direction of the liquid crystal panel. The blocking of the light rays at the entrance pupil of the lens reduces the efficiency of the projection system, resulting in a lower light output of the projector. In US 5,375,006 contrast improvement in a projector using TN-LCDs is obtained by using (uniaxial) birefringent films. In some of the claims the films are even inclined with respect to the optical axis. This solution however complicates the complete stack of elements of the LCD light modulating device, because two new optical surfaces are introduced. This leads to light loss due to extra reflections. If the films are coupled optically to minimize this light loss, then the LCD itself becomes thicker, and more difficult to cool. Moreover, these thin birefringent films are also sensitive to temperature changes and mechanical stress caused by the temperature differences in a high light-output projector. In JP 07234401A (FUJITSU Ltd), a projection type liquid crytal display is described, of which the surfaces of the analyser and polarizer are parallel to each other and inclined to the optical axis in an OFF-axis projector. It is an object of the present invention to overcome the problems mentioned hereinabove. According to the present invention, a light valve image projector provided with a disposition for improving the contrast of an optical image projected by that projector is provided. The projector comprises at least one illumination system, at least one field lens, at least one set consisting of a polarizer, an LCD and an analyzer, and at least one projection lens, and presents at least one optical axis, each optical axis of the projector including an angle α with the optical axis of the projection lenses. At least one of the analyzers includes with the LCD of the same set consisting of polarizer, LCD and analyzer an angle β a larger than the angle α, at least for the light rays that pass through the contrast limiting area of said LCD. Said angle β a is so that said at least one of the analyzers is not parallel to the polarizer belonging to the same set consisting of analyzer, LCD and polarizer, at least for the light rays that pass through the contrast limiting area of the LCD. The contrast limiting area of an LCD included in a light valve image projector is defined as the part of the surface of the LCD through which light rays pass at that angle with the LCD for which the highest obtainable contrast is minimum. The projector may be a monochrome projector or a multi-color projector. In case there are more sets consisting of a polarizer, an LCD and an analyzer, the angles β a included by each analyzer with the corresponding LCD of the same set, may be different. According to a preferred embodiment also at least one of the polarizers includes an angle β p with the LCD of the same set consisting of polarizer, LCD and analyzer, which angle is larger than the angle α, at least for the light rays that pass through the contrast limiting area of said LCD. In case there are multiple sets consisting of a polarizer, an LCD and an analyzer, the angles β p included by each polaryzer with the corresponding LCD of the same set, may be different. If the angle α mentioned hereinabove is different from zero, the projector is called an OFF-axis projector. If the angle α is equal to zero, the projector is called an ON-axis projector. According to one embodiment of the present invention, at least one of the analyzers of the projector has a flat surface. According to another embodiment, at least one of the analyzers is curved. In that case it has a curvature such that the tangent planes of said analyzer at the points of incidence of the light rays which pass through the contrast limiting area of said LCD of the same set include an angle β a > α with the plane of the LCD. According to a further embodiment at least one of the polarizers has a flat surface. According to yet another embodiment at least one of the polarizers is curved. In that case it has a curvature such that the tangent planes of said polarizer at the points of incidence of the light rays passing through the contrast limiting area of the LCD of the same set include an angle β p > α with the plane of the LCD. A specific embodiment of the present invention is a projector comprising one illumination system, three field lenses, three sets consisting of a polarizer, an LCD and an analyzer, and one projection lens. The invention will be better understood by means of the following drawings, in which : Fig. 1 shows the optical path in an OFF-axis projection system (prior art); Fig. 2 shows the non-absorbing directions of the polarizer and the analyzer as they are mounted in a conventional way in a liquid crystal light valve projector (prior art); Fig. 3a and Fig. 3b show the molecule distribution in a twisted nematic liquid crystal display (prior art); Fig. 4 shows simulated viewing angle characteristics with isocontrast curves for incident angles of light on an LCD (prior art); Fig. 5 shows the light transmission in function of the drive voltage and for different angles of light incidence (prior art); Fig. 6a, Fig. 6b and Fig. 6c show the normalised intensity of an image projected by an OFF-axis projector in function of the drive voltage applied to the LCD respectively at the top, in the middle and at the bottom of the screen; Fig. 7a, Fig. 7b and Fig. 7c show the normalised intensity of an image projected by an ON-axis projector in function of the drive voltage applied to the LCD respectively at the top, in the middle and at the bottom of the screen; Fig. 8a shows an OFF-axis projection system wherein the angle between the analyzer and the LCD is larger than the OFF-axis angle; Fig. 8b shows an OFF-axis projector wherein the angle between the analyzer and the LCD and the angle between the polarizer and the LCD are larger than the OFF-axis angle; Fig. 9 shows an OFF-axis projector wherein the analyzer and the polarizer have a curved surface; Fig. 10a shows the optical path in an ON-axis projection system (prior art); Fig. 10b shows an ON-axis projector wherein the angle between the analyzer and the LCD and the angle between the polarizer and the LCD are larger than 0°; Fig. 11 shows an ON-axis projector wherein the analyzer and the polarizer have a curved surface. A first embodiment of the present invention is shown in Fig. 8a and is an OFF-axis projector 22 wherein the angle β a between the analyzer 23 and the LCD 6 is made larger than the OFF-axis angle α and wherein the polarizer 5 is perpendicular to the optical axis of the projector 9 and includes an angle to the LCD 6 which is equal to the OFF-axis angle α. Fig. 6a, Fig. 6b and Fig. 6c show the normalised intensity of an image projected by the OFF-axis projector in function of the drive voltage applied to the LCD, at the top, in the middle and at the bottom of the screen respectively. The graphs marked "0" correspond to the prior art OFF-axis projection system whereby the polarizer and the analyzer are mounted perpendicular to the optical axis of the projector or in other words have an angle to the LCD surface equal to the OFF-axis angle α; the graph marked "A" corresponds to the set-up covered by this invention and described in this first embodiment whereby the angle β a between the analyzer and the LCD is larger than the OFF-axis angle α. In this first embodiment of the present invention, the contrast is improved over the whole screen as illustrated in all the three figures FIG 6a, Fig. 6b and Fig. 6c, wherein the graphs "A" show lower values of intensity compared to the graphs "0". The result is that in the area which determines the minimum of contrast being the top of the screen, the contrast is increased compared to the prior art version. The contrast in the other areas being the middle and the top of the screen is also improved in a way that the overall contrast compromise is improved compared to the prior art version. A second embodiment of the present invention is shown in Fig. 8b and is an OFF-axis projector 24 wherein the angle β a between the analyzer 25 and the LCD 6 is made larger than the OFF-axis angle α and wherein the angle β p between the polarizer 26 and the LCD 6 is larger than the OFF-axis angle α. FIG 6a, Fig. 6b and Fig. 6c show with graphs "B" on each of the figures the improvement of the contrast of the image obtained through the decrease of the lower intensity levels in function of the LCD drive voltage compared to the lower intensity levels for the prior art OFF-axis projection system (graphs "0") and to the intensity for the first embodiment OFF-axis projector (graphs "A"). The second embodiment has compared to the first embodiment a decreased or in other words improved black level at the top, in the middle and at the bottom of the screen. Having also the polarizer mounted including an angle to the LCD which is larger than the OFF-axis angle α improves so even more the over all black level of the projector and permits to obtain the highest possible contrast all over the screen. Another embodiment of the present invention is shown in Fig. 10b and is an ON-axis projector 27 wherein according to the present invention, the angle β a between the analyzer 28 and the LCD 6 is made larger than 0 degrees and the angle β p between the polarizer 29 and the LCD 6 is made larger than 0 degrees. Fig. 7a, Fig. 7b and Fig. 7c show the normalised intensity of an image projected by the ON-axis liquid crystal light valve projector in function of the drive voltage applied to the LCD at the top, in the middle and at the bottom of the screen respectively. The graph marked "0" corresponds to the prior art projection system whereby the polarizer and the analyzer are mounted perpendicular to the optical axis of the projector or in other words are parallel to the LCD. The graph marked "A" corresponds to the ON-AXIS projection system whereby the angle β a between the analyzer and the LCD is different from 0, and whereby the angle β p between the polarizer and the LCD is equal to 0. The graph marked "B" corresponds to the ON-AXIS projection system whereby both the angle β a and β p are different from 0. Fig. 7a, 7b and 7c show that the ON-axis projector has a lower contrast at the bottom of the screen, because the lower intensity values are higher there than at the top of the screen. The bottom of the screen corresponds to the top of the LCD where incident light rays have a negative incident angle. The viewing angle characteristics of Fig. 4 which show lower contrast for negative incident angles of light explain that the contrast is lower and so most critical at the bottom of the image in the ON-axis projection system. With comparison to graph "0", graph "A" corresponding to an inclination of the analyzer only, and graph "B" corresponding to an inclination of both the analyzer and the polarizer show an improvement of the contrast at the top, in the middle and at the bottom of the screen by the application of the present invention. Other embodiments of the present invention are shown in Fig. 9 and Fig. 11. In these embodiments the surfaces of the analyzer and polarizer are not flat but curved. Compared to the flat versions, the curved versions of analyzer and polarizer introduce a gradually enlarging angle to the LCD plane between a smaller angle and the larger angle as needed by the present invention, the angle being largest at the contrast limiting area side of the LCD. The advantage of these embodiments with curved surfaces for analyzer and polarizer is that space taken inside the projector by the composition of the analyzer and the LCD and by the composition of the polarizer and the field lens is smaller when the surfaces of polarizer and analyzer are curved than when they are flat. The curvatures of the surfaces of analyzer and polarizer do not decrease the contrast uniformity of the projection system as long as the angle between the analyzer or polarizer on the one hand and the plane of the LCD on the other hand has its largest value facing the contrast limiting area side of the LCD and is in the other areas large enough to permit the compensation of the contrast by adaptation of the drive voltage there. This is explained by the following. For an LCD projection system aligned for uniform contrast, the overall contrast is limited to the contrast which can be obtained in the area where the contrast has a minimum in function of the drive voltage as made clear in Fig. 5. The contrast improvement obtained by an embodiment of the present invention must be maximum at that contrast limiting area and can be smaller in other areas as there it can be compensated by adaptation of the drive voltage. Therefor, a smaller improvement of contrast and so a smaller angle than the angle as stated by the present invention is permitted in other areas than the contrast limiting areas. Fig. 9 shows a schematic view of an OFF-axis projector 30, whereby the analyzer 31 and polarizer 32 have a curved surface. The largest curvature corresponds to the bottom side of the LCD 6 which is the LCD side with the contrast limiting area. The angle β a is the angle between the LCD plane and the tangent plane 38 to the curved analyzer 31 at a point of incidence of a light ray, and is larger than α for light rays passing through the contrast limiting area of the LCD 6. The angle β p is the angle between the LCD plane and the tangent plane 39 to the curved analyzer 32 at a point of incidence of a light ray, and is larger than α for light rays passing through the contrast limiting area of the LCD 6. Fig. 11 shows a schematic view of an ON-axis projector, whereby the analyzer 34 and polarizer 35 have a curved surface. The largest curvature corresponds to the top side of the LCD 6 which is the LCD side with the contrast limiting area. The angle β a is the angle between the LCD plane and the tangent plane 36 to the curved analyzer 34 at a point of incidence of a light ray, and is larger than zero for light rays passing through the contrast limiting area of the LCD. The angle β p is the angle between the LCD plane and the tangent plane 37 to the curved analyzer 35 at a point of incidence of a light ray, and is larger than zero for light rays passing through the contrast limiting area of the LCD 6. The embodiments of the present invention as shown by Fig. 8a, 8b and 9 describe OFF-axis projectors whereby the projection lens is positioned at a height lower than the middle of the screen. This applies to what is usually called a table mounted projector. The present invention also applies to OFF-axis projectors whereby the position of the projection lens is higher than the height of screen. This applies to what is usually called a ceiling mounted projector. For a ceiling mounted projector, the figures Fig. 8a, 8b and 9 do apply but should be mirrored vertically, and what is stated about the top of the screen counts then for the bottom of the screen, and vice versa.

Claims: Light valve image projector with disposition for improving the contrast of an optical image projected by said projector (22) said projector (22) comprising at least one illumination system (3), at least one field lens (4), at least one set consisting of a polarizer (5), an LCD (6) and an analyzer (23), and at least one projection lens (8), said projector (22) presenting at least one optical axis (9), each optical axis (9) of the projector including an angle α with the optical axis (10) of the projection lenses, characterized in that at least one of said analyzers (23) includes an angle β a larger than the angle α with the LCD (6) belonging to the same set consisting of analyzer (23), LCD (6) and polarizer (5), at least for the light rays that pass through the contrast limiting area of said LCD (6), said angle β a being so that said at least one of said analyzers (23) is not parallel to the polarizer (5) belonging to the same set consisting of analyzer (23), LCD (6) and polarizer (5), at least for the light rays that pass through the contrast limiting area of said LCD (6). Projector according to claim 1 in which at least one of said polarizers (26) includes an angle β p larger than the angle α with the LCD (6) belonging to the same set consisting of analyzer (25), LCD (6) and polarizer (26), at least for the light rays that pass through the contrast limiting area of said LCD (6). Projector according to any of claims 1 or 2 in which the angle α is different from zero. Projector according to any of claims 1 or 2 in which the angle α is equal to zero. Projector according to any of claims 1 to 4 in which at least one analyzer has a flat surface. Projector according to any of claims 1 to 4, characterized in that at least one analyzer (31) is curved and has a curvature so that the tangent planes of said analyzer (31) at the points of incidence of the light rays passing through the contrast limiting area of the LCD (6) belonging to the same set consisting of analyzer (31), LCD (6) and polarizer (32) include an angle β a > α with the plane of that LCD (6). Projector according to any of claims 1 to 6 in which at least one polarizer has a flat surface. Projector according to any of claims 1 to 6, characterized in that at least one polarizer (32) is curved and has a curvature so that the tangent planes of said polarizer (32) at the points of incidence of the light rays passing through the contrast limiting area of of the LCD (6) belonging to the same set consisting of analyzer (31), LCD (6) and polarizer (32) include an angle β p > α with the plane of that LCD (6). Projector according to any of the preceding claims, characterized in that it comprises one illumination system (3), three field lenses (4), three sets consisting of a polarizer (5), an LCD (6) and an analyzer (7), and one projection lens (8).

Inventors: CANDRY PATRICK, MAXIMUS BART, VAN DEN BOSSCHE BART, CANDRY, PATRICK, MAXIMUS, BART, VAN DEN BOSSCHE, BART
Applicants: BARCO NV, BARCO N.V.
Application year: 1997
Main classification: H04N5
IPC codes: G03B21, G02B5, H04N5, G02F1