WO2005060268A1

WO2005060268A1 - Brightness regulation in lcd projection systems

Info

Publication number: WO2005060268A1
Application number: PCT/IB2004/052788
Authority: WO
Inventors: Adrianus J. S. M. De Vaan
Original assignee: Koninklijke Philips Electronics, N.V.; U.S. Philips Corporation
Priority date: 2003-12-16
Filing date: 2004-12-13
Publication date: 2005-06-30
Also published as: TW200527114A

Abstract

An LCD display includes a light source and an LCD dimming device disposed between the light source and a polarization beam splitter (PBS). A method of forming a high-contrast, dark image on a screen of an LCD display device includes providing an LCD dimming device between a light source and a PBS, and selectively altering the molecular orientation of liquid crystal molecules in the LCD dimming device. This selective altering of the orientation of the liquid crystal molecules effects a polarization vector at the output of the LCD dimming that is reflected away from LCD panels in the remaining optical path, and thus reduces the intensity level of the light that is illuminating the matrix display panel of the system.

Description

BRIGHTNESS REGULATION IN LCD PROJECTION SYSTEMS

Liquid crystal (LC) technology has been applied in projection displays for use in projection televisions, computer monitors, point of sale displays, and electronic cinema to mention a few applications. A more recent application of LC devices is the reflective LC display on a silicon substrate, forming a liquid crystal on silicon (LCOS) structure. Silicon-based reflective LC displays often include an active matrix array of complementary metal-oxide-semiconductor (CMOS) transistors/switches that are used to selectively rotate the axes of the liquid crystal molecules. As is well known, by application of a voltage across the LC cell, the direction of the LC molecules can be controlled and the state of polarization of the reflected light is selectively changed. As such, by selective switching of the transistors in the array, the LC medium can be used to modulate the light with image information. This modulated light can then be imaged on a screen by polarization and projection optics thereby forming the image or 'picture.' In many LCD systems, the light from a source is selectively polarized in a particular orientation prior to being incident on the LC layer. The LC layer may have a voltage selectively applied to orient the molecules of the material in a certain manner. The polarization of the light that is incident on the LC layer is then selectively altered upon the light is traversing through the LC layer. The LCD system often includes a polarization beam splitter (PBS). The light that is reflected by the LCD and that has a certain polarization state will be reflected from or transmitted through the PBS, depending on the structure of the system, and is then incident on the projection optics. This light forms the bright-state pixels of the image. Contrastingly, light of a polarization that is orthogonal to the bright state pixels is prevented to reach the projection optics, and become the dark state light. In this manner, an image may be formed from a plurality of bright and dark state pixels, where a scrolling image may be formed by scrolling the input signals to the matrix of transistors in the LCD. Known LCD display systems have less than ideal contrast levels. This is because the darkest level is determined by light leakage of the polarizers. This may also be due to residual birefringence in the liquid crystal layer due to imperfect molecular orientation profiles, and due to misalignment of the optical axes of the individual polarization dependent parts. Moreover, when the image is a dark image, because the darkest level is set, the contrast in the achieved image can be unacceptable since hardly any details are visible. Finally, known LCD display systems may employ polarizers or other retarders to improve the picture performance by enhancing the contrast or viewing angle dependency. However, these elements eventually may become ineffective due degradation of the materials by heating over time. In accordance with an example embodiment, an LCD display includes a light source and an LCD dimming device disposed between the light source and a polarization beam splitter (PBS). In accordance with another example embodiment, a method of forming a high-contrast, but darker image on a screen of an LCD display device includes providing an LCD dimming device between a light source and a PBS, and selectively altering the molecular orientation of liquid crystal molecules in the LCD dimming device. This selective altering of the orientation of the liquid crystal molecules effects a polarization vector at the output of the LCD dimming that is reflected away from LCD panels in the remaining optical path, and thus reduces the intensity level of the light that is illuminating the matrix display panel of the system. As a consequence the amount of light that can pass the total light path is reduced, which reduces the amount of light that reaches the human eye of the viewers. The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig. 1 is a schematic view of an LCD projection display device in accordance with an example embodiment. Fig. 2 is a perspective view of the LCD display device of Fig. 1 showing the polarization state of light emerging from the LCD dimming device in a high intensity output mode. Fig. 3 is a is a perspective view of the LCD display device of Fig. 1 showing the polarization state of light emerging from the LCD dimming device in a low intensity output mode. Fig. 4 is a perspective view of the LCD display device of Fig. 1 showing the polarization state of light emerging from the LCD dimming device in a medium intensity output mode. Fig. 5 is a schematic view of an LCD projection display device in accordance with an example embodiment. In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the example embodiments. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention. Finally, wherever practical, like reference numerals refer to like features. Briefly, the example embodiments relate to a method and apparatus for selectively providing a dark (sometimes referred to as 'low light'), high contrast image on a display screen in an LCD display device. Illustratively, an LCD dimming device selectively alters the magnitude of orthogonal polarization vectors emerging therefrom so the intensity of the image is selectively altered to a desired intensity level. As will become clearer as the present description continues, the contrast is maintained at a relative high level because in displaying a dark image the LCD dimming device reduces the illumination level of the light on the LCD panels. As such, the intensity level of the darkest pixels within the displayed image is reduced when the maximum obtainable system brightness is not required. Fig. 1 shows an LCD projection display device (LCD display) 100 in accordance with an example embodiment. It is noted that the LCD projection display device 100 may be incorporated into a front projection device. The LCD display 100 includes an illumination system (or illumination unit) 101, which may be a gas discharge light source (e.g., a high pressure mercury lamp, a noble gas arc lamp or metal halide lamp), an LED array or other known light source for emitting light comprising red, green and blue (R, G, B) light that is used in display devices. Moreover, the illumination system 101 may include a parabolic mirror or similar device. The LCD display 100 may include various well-known elements used in LCD imaging. Light from the illumination system 101 traversing a set of beam shaping optics 102-103 which shapes the lightbeam such that the displays are illuminated with a rectangular shaped and homogeneous distributed light beam. Next the light from 103 is incident on a Polarization Conversion System (PCS) 104. The PCS converts the unpolarized light that is emitted by the illumination unit 101 into a polarized lightbeam without a major light-loss. Illustratively, the PCS 104 comprises reflective based polarization splitting optics (e.g. a set of polarizing beam splitters) that splits one polarization mode of the lightbeam by reflecting this mode to another direction than the other polarization mode. At certain locations in the PCS 104, the two orthogonal polarization modes are geometrically separated, such that one of the two modes can be converted into the opposite mode using polarizing rotating elements. It is noted that elements 101-104 are well within the purview of one of ordinary skill in the art, and as such details of the elements and their function are omitted so as to not obscure the description of the example embodiments. For example, details of many of these elements may be found in U.S. Patent 6,327,093 to Nakanishi, et al, the disclosure of which is specifically incorporated herein by reference. Upon emerging from the PCS 104, the light is incident on a relay lens 108 and is reflected by a reflective surface 109, which is illustratively a mirror. The light is incident on a second relay lens 110, and then traverses an LCD dimmer 111, which is illustratively a transmissive LCD panel. After traversing the LCD dimmer 111, the light is incident on a polarization beamsplitter 112. As described more clearly as the present description continues, the LCD dimmer 111 and the PBS 112 serve to control the intensity of light that is incident on the display panels 115-117, and thus set the maximum intensity of the light that is able to reach the projection lens 113 and the displayed image. Light of the appropriate polarization directly traverses the PBS 112, and is incident on the prism system 114, comprised of prism elements that direct the red light to a red liquid crystal panel 115, a green liquid crystal panel 116 and a blue liquid crystal panel 117. After modulation by the respective panels 115-117, the light is transmitted to the PBS 112, where it is separated by polarization state, and bright state light (bright pixels) of each color is reflected by the PBS 112 towards the projection lens 113. Dark state light (dark pixels) of a polarization state that is orthogonal to the state of polarization of the reflected light to the projection lens 113 is transmitted away from the projection lens 113. In this manner, the dark and bright state light form an image on a display screen (not shown). As referenced previously, the LCD dimmer 111 is useful in reducing the overall illumination on the display screen for 'dark' images, such as night video recordings or photographs. In known systems, there are significant problems when dark photos or video scenes are shown. For example, in known systems, this reduction is tantamount to reducing only the light level of the bright state light. To this end, in known systems, the darkest pixels maintain their intensity level independently of the image content. Stated differently, the intensity of the darkest pixels is for dark and bright images the same. As such, in dark images, the contrast between the brightest and darkest pixels is poor, and in some cases the image appears indiscernible. The example embodiments solve these and other shortcomings of the known devices. The LCD dimmer 111 is an LCD device (cell) that functions to selectively alter the polarization state of light that traverses the dimmer. In an example embodiment, the dimmer is a 90° twisted nematic cell, although other LCD devices may be used depending on the display architecture. It is noted that the LCD dimmer 111 may be integral with the PCS 104 in an example embodiment. While the 90° twisted nematic cell is exceedingly beneficial, being easy to manufacturer and the most common effect used in LCD's, other devices may be used as the dimmer. For example, the display system might consist of a single panel which is time sequentially illuminated with Red; Green and Blue light-flashes. In an example embodiment a Ferro-Electric cell is beneficially implemented since the Ferro-Electric display effect is comparatively fast; and the contrast optimization could be done for the Red; Green and Blue lightflashes individually. In another example embodiment, the dimmer might be a device that provides an Electrical Controlled Birefringence (ECB) effect. The ECB effect has the advantage that the spectral output of the ECB cell can be controlled by the voltage over the LCD layer; and such the dimming effect can be adjusted dependent on the maximum required light intensities of the Red, Green and Blue contribution in the image. In any event, the alteration of the polarization by the dimmer 111 is used to selectively prevent light of a particular polarization state from being sent to the prism system 114. This results in a dark image on the display screen. Alternatively, the LCD dimmer may not induce any alteration in the light incident thereon, and all of the light is transmitted to the system 114. This results in a high intensity or normal image on the display screen. Finally, the light may be elliptically polarized so that it has vector components that are transmitted by the PBS 112, and components that are reflected away in a direction opposite of the projection lens 113 by the PBS 112. In this case the image formed is a medium brightness image on the screen. It is noted that in these and other embodiments, a polarizer (e.g., polarizer 518 shown in Fig. 5) may be disposed between the LCD dimmer 111 and the PBS 112. This polarizer may be used in case the polarization states of the light incident on the LC dimmer is of insufficient quality (e.g. in the dark state of the dimmer, the spectral distribution of the light- output of the dimmer becomes colored due to birefringence issues in the optical parts in the illumination system.) The additional polarizer will usefully absorb these impurities in the lightbeam. This polarizer is illustratively a reflective-type polarizer, such as: an interference coating polarizer; a Brewster plate; a birefringent layer; a holographic element; or a wire grid polarizer. In the present and additional embodiments, LCD dimming device may contain a birefringent compensation layer, which substantially optimizes a contrast of the LCD dimmer 111 in a main direction of the light beam traversing through the dimmer. Fig. 2 shows the LCD display system 100 of Fig. 1 with the LCD dimmer in a high intensity output mode. In this example embodiment, the orientation of the molecules of the liquid crystal of the LCD dimmer 111 is such that the input polarization vector 202 of light comprised of RGB light 105-107 (shown schematically as 201) traverses the dimmer 111 substantially unaffected. As such, the output polarization vector 203 is substantially unchanged compared to the input polarization vector. In this example embodiment, the LCD dimmer 111 has substantially no effect on the polarization state of the light that traverses it. To wit, in the direction 205 the PBS 112 reflects approximately only 1% of the light. The light that emerges having a polarization state 203 (shown conceptually as 204) is then incident on the PBS 112, and is virtually completely transmitted therethrough, with virtually none of the light being reflected at this stage in a direction 205. As mentioned before, this light 204 traverses the system 114 and is modulated into dark (reflected in the direction 205 at this stage) and bright pixels (reflected to the projection optics 113) for image formation. As can be appreciated, in this state where the LCD illustratively has an on-state voltage applied thereto, the output to the projection lens forms a bright or normal brightness image on the display screen. Fig. 3 shows the LCD projection display of Fig.1 in a low- intensity output state for forming a dark image at the display screen. In the example embodiment shown in Fig. 3, the orientation of the molecules of the liquid crystal material of the LCD dimmer is such that light having an incident polarization in one direction will emerge from the LCD dimmer having a polarization vector that is orthogonal to the input polarization vector. For example, the incident light 105-107 (shown conceptually as 301) in the example embodiment has an input polarization state (Pj„) shown as vector 302. Upon traversal of the LCD dimmer 111, the polarization state of substantially all of the light is rotated by 90° and emerges from the LCD dimmer having an output polarization state (P_out) having a vector 303 as shown. With a substantial portion of the light emerging from the LCD dimmer 111 having a polarization state that is orthogonal to the transmission axis of the PBS 112, virtually all of the light 301 is reflected in the direction 304, which is opposite to the direction of the projection optics 113. Moreover, only a small portion, on the order of approximately 1%, of the light 301 emerges from the LCD dimmer 111 having its polarization state aligned with the transmission axis of the PBS 112. This light (shown conceptually as 305) is incident on the prism system for modulation by the LC panels 115-117. As described before, depending on the action of the LC panels 115-117, the light that returns to the PBS 112 is either reflected towards the projection optics 113, or in the direction 304, with the former being the brightest state, and the latter being the darkest state. However, the light 305 that is modulated by the LC panels 115-117 represents a small portion of the light that would be transmitted to the projection optics 113 or in the direction 304 but for the action of the LCD dimmer. In an example embodiment, a light trap, which absorbs the light that is reflected by the PBS between the LC dimmer and the LC panels, is included. In the example embodiments, the overall intensity of the light to the LC panels is reduced by the LCD dimmer 111, the darkest pixels have become darker, and the contrast is improved compared to known devices in ways described presently. As mentioned earlier, in known LCD-based display devices, the control of black state light is limited by the ability of the polarizers and the liquid crystal electro -optic effect. The polarizers, LC material and alignments are not ideal, and some of the light intended to be prevented to hit the display screen leaks through. As such, in known systems, regardless of the brightness level, the intensity of the darkest pixels is basically at a constant level. This translates into reduced contrast in the displayed images when the image has no pixels at its maximum brightness. For example, if an LCD panel can provide a contrast of 100:1, and it is displaying an image having its brightest pixels at 50% of the brightest level of light output (or 50% of the maximum, referred to as 50% max), the LCD is driven accordingly. However, while the brightest pixel is at 50% max, the darkest pixel is still at 1% max and the contrast in the displayed image reduced to only 50:1. In case the brightest pixel is at 5% max, the darkest pixels remain at 1% max and the contrast achieved in the displayed image has even dropped to only 5:1. As can be appreciated in very low-light images the contrast in known LCD systems is unacceptable low and all details in the image are lost. However, in accordance with the example embodiments described herein, the intensity of the brightest pixels is controlled by the LCD dimmer and not by the LC panels (e.g., 115- 117). This means that the light is reduced in intensity before reaching the LC panels. In case that the brightest pixels in an image is 50%; the dimmer dims the illumination level of the light onto the LCD to 50%; which also reduces the intensity of the darkest pixels in the displayed image with 50%. The result is that the brightest pixels are at 50% max, while the darkest pixels are now reduced to only 0.5% max resulting in a contrast in the displayed image of 100:1 In this particular example the contrast in the achieved image is doubled compared to the known methods and apparati. In the second case where the brightest pixel is at 5% max; the dimmers dims the illumination level of the light onto the LCD to 5% max; and the dimmer also dims the darkest pixel with 95% max. The result is that the brightest pixels are at 5%, while the darkest pixels are now reduced to only 0.05% max resulting to a contrast in the displayed image of again 100:1 In this particular example the contrast in the achieved image is improved 20 times compared to the known apparati and methods. Another useful aspect of the present example embodiments relates to the dependence of the contrast ratio on the viewing angle. As is well known, the contrast can be compromised when viewing of an image from an LCD is at other than 0° degrees to the normal to the plane of the LCD. However, because the LCD dimmers of the illustrative embodiments described herein are not viewed directly, it does not adversely impact the contrast in this manner. Fig. 4 shows the LCD display dimmer 100 of Fig. 1 with the LCD dimmer in an intermediate output state in accordance with an example embodiment. As is well-known, the electro-optic effect of liquid crystal materials is dependent on the voltage applied to the liquid crystal. For example, the light-transmittance versus voltage for a 90-degree twisted nematic LCD panel decreases precipitously with applied voltage. As such, the transmittance and thus the brightness may be reduced to a chosen level by application of the voltage corresponding to this desired level. The LCD dimmer 111 may also be used to continuously vary the brightness between a high brightness level (e.g., as in the example embodiment of Fig. 2) and a low brightness level (e.g., as in the example embodiment of Fig. 3). In the example embodiment of Fig. 4, the orientation of the molecules of the liquid crystal material of the LCD dimmer is such that light having an incident linear polarization will emerge from the LCD dimmer having elliptically polarized light; that is light having polarization vector components that are parallel and perpendicular to the input polarization state. For example, the incident light 105-107 (shown conceptually as 401) in the example embodiment has an input polarization state (Pj_n) shown as vector 402. Upon traversal of the LCD dimmer 111, the light emerges having a polarization state that is an elliptical state (P_out) 403 as shown. It is noted that in the present exemplary embodiment, the P_out is substantially circularly polarized, which means the magnitude of the polarization vector component of P_outthat is parallel to the input polarization vector 402 is substantially equal to the magnitude of the polarization vector component of P_out that is perpendicular to the input polarization vector 402. However, as can be readily appreciated, elliptical states of other eccentricities may be chosen by selecting another application voltage to the LCD dimmer. In the example embodiment described in connection with Fig. 4, with approximately one-half of the light energy emerging from the LCD dimmer 111 having a polarization state that is orthogonal to the transmission axis of the PBS 112, approximately 50% of the light 401 is reflected in the direction 404, which is opposite to the direction of the projection optics 113. Moreover, approximately 50% of the light 401 emerges from the LCD dimmer 111 having its polarization state aligned with the transmission axis of the PBS 112. This light (shown conceptually as 405) is incident on the prism system 114 for modulation by the LC panels 115-117. As described before, depending on the action of the individual pixels in the LC panels 115-117, the light that returns from the pixels to the PBS 112 is either reflected towards the projection optics 113, or in the direction 404, with the former being the bright state pixels, and the latter being the dark state pixels. Fig. 5 shows an LCD projection display 500 in accordance with an example embodiment. The LCD display 500 shares many of the features of the example embodiments described above in connection with the example embodiments of Figs. 1-4 above. As such, in the interest of clarity of description these commonalities will not be repeated. The display 500 is a scrolling color display device, which includes an illumination source 501, that includes a parabolic reflector and a light element such as those described above. Light from the illumination unit 501 traversing a set of beam shaping optics 502-503 which shapes the lightbeam such that the displays are illuminated with a rectangular shaped and homogeneous distributed light beam. Next the light from 503 is incident on a polarization Conversion System (PCS) 504. The PCS converts the unpolarized light that is emitted by the illumination unit 101 into a polarized lightbeam without a significant loss of light energy. The display also includes wavelength selective mirrors 505, which may be dielectric stack reflectors or dichroic mirrors, well known in physical optics. These wavelength selective mirrors will reflect light of one wavelength (e.g., red light 506) and transmit all other wavelengths (e.g., blue and green light 507). Continuing along these lines, the green light 508 is reflected and the blue light 509 is transmitted as shown. Rotating prisms 510 are used to scan the image in a manner well known in the art, and mirrors 511 and lenses 512 are useful in collimation and focusing of the light as needed. Finally, the wavelength selective mirrors 513 function in the same manner as and are of the same type as the mirrors 505; however, the mirrors 513 are used for the recombination of wavelengths to the PBS 514. The PBS 514 functions in a known manner to direct light to the LC panel 515, which is illustratively an LCOS panel and to direct modulated light to the projection optics 516. As the functions of these various elements are well-known, further description thereof is omitted in the interest of clarity of description. Finally, it is noted that the features described in connection with the present example embodiment as well as other features and descriptions may be found in U.S. Patent 6,563,551 to Janssen, et al. The disclosure of this patent is specifically incorporated herein by reference. An LCD dimmer 517 is disposed between the illumination source 501 and the PBS 517. The LCD dimmer 517 usefully functions to provide control of the brightness of the image formed on a display screen (not shown), while maintaining sufficient contrast, even in low-light images. The function of the LCD dimmer 517 is substantially identical to that of the LCD dimmer described in connection with example embodiments above. The example embodiments having been described in detail, it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure. Such modifications and variations are included in the scope of the appended claims.

Claims

CLAIMS:

1. An apparatus (100, 500) including an LCD display (115,116,117), comprising: an illumination unit (101), which includes a lightsource and a polarization conversion system (104); and an LCD dimming device (111) disposed between the illumination unit and a polarization beam splitter (PBS) (112).

2. An apparatus as recited in claim 1, wherein the LCD display is a direct view display.

3. An apparatus as recited in claim 1, wherein the LCD display is a projection display.

4. An apparatus according to claim 3 wherein the LCD display is a LCOS display.

5. An apparatus as recited in claim 3, wherein the LCD display includes a single matrix.

6. An apparatus as recited in claim 3, wherein the LCD display includes a plurality of matrices.

7. An apparatus as recited in claim 3, wherein the LCD display is a transmissive display.

8. An apparatus as recited in claim 1, wherein the LCD dimming device is integrated with the polarization conversion system.

9. An apparatus as recited in claim 1, including a light trap, which absorbs the light that is reflected by the PBS between the LC dimmer and a matrix display panel (115, 116, 117).

10. An apparatus as recited in claim 1, wherein the LCD dimming device is adapted to provide a 90-degree twisted nematic electro-optical LC effect.

11. An apparatus as recited in claim 1, wherein the LCD dimming device is adapted to provide a Ferro-Electric electro-optical LC effect.

12. An apparatus as recited in claim 1, wherein the LCD dimming device is adapted to provide an Electrical Controlled Birefringence electro-optical LC effect.

13. An apparatus as recited in claim 1, wherein the LC dimmer contains an LC layer, sandwiched between transparent, conducting electrodes.

14. An apparatus as recited in claim 1, wherein the LC dimmer contains a birefringent compensation layer, which optimizes a contrast of the LCD dimmer in a main direction of the light beam traversing through the dimmer.

15. An apparatus as recited in claim 1, including a polarizer (518) between the LCD dimmer and the PBS.

16. An apparatus as recited in claim 15, wherein the polarizer is a reflective-type polarizer.

17. An apparatus as recited in claim 16, wherein the polarizer is one of: an interference coating polarizer; a Brewster plate; a birefringent layer; a holographic element; or a wire grid polarizer.

18. An LCD display apparatus (100, 500), comprising: a light source (101) and an LCD dimming device (111) disposed between the light source and a polarization beam splitter (PBS) (112).

19. An LCD display apparatus as recited in claim 18, including a polarizer (518) between the LCD dimming device and the PBS.

20. A method of forming a dark, high-contrast image on a screen of an LCD display device (115, 116, 117), the method comprising: providing an LCD dimming device (111) between a light source (101) and a polarization beam splitter (PBS) (112); and selectively altering the orientation of liquid crystal molecules in the LCD dimming device.