WO2005094070A1

WO2005094070A1 - Dual liquid crystal on silicon (lcos) to digital light pulse (dlp) relay

Info

Publication number: WO2005094070A1
Application number: PCT/US2004/005857
Authority: WO
Inventors: Estill Thone Hall, Jr.
Original assignee: Thomson Licensing S. A.
Priority date: 2004-02-26
Filing date: 2004-02-26
Publication date: 2005-10-06

Abstract

A two-stage projection system comprises first and second imagers having a liquid Crystal on Silicon architecture and disposed to receive oppositely polarized light inputs and configured to modulate the oppositely polarized light inputs on a pixel-by-pixel basis in response to one or more video signals to provide oppositely polarized matrices of modulated light pixels. A third imager is disposed to receive the oppositely polarized matrices of modulated light pixels and configured to modulate the oppositely polarized matrices of modulated light pixels using pulse width modulation to form twice-modulated matrices of oppositely polarized light pixels.

Description

Dual Liquid Crystal on Silicon (LCOS) to Digital Light Pulse (DLP) Relay

Field of the Invention The present invention is related generally to projection systems and more particularly to a two-stage projection system using a combination of Liquid Crystal On Silicon (LCOS) and Pulse Width Modulation (PWM) imagers.

Background of the Invention Liquid crystal displays (LCDs), and particularly liquid crystal on silicon (LCOS) systems using a reflective light engine or imager, are becoming increasingly prevalent in imaging devices such as rear projection television (RPTV). In an LCOS system, projected light is polarized by a polarizing beam splitter (PBS) and directed onto a LCOS imager or light engine comprising a matrix of pixels. Throughout this specification, and consistent with the practice of the relevant art, the term pixel is used to designate a small area or dot of an image, the corresponding portion of a light transmission, and the portion of an imager producing that light transmission. Each pixel of the imager modulates the light incident on it according to a gray-scale factor input to the imager or light engine to form a matrix of discrete modulated light signals or pixels. The matrix of modulated light signals is reflected or output from the imager and directed to a system of projection lenses which project the modulated light onto a display screen, combining the pixels of light to form a viewable image. In this system, the gray-scale variation from pixel to pixel is limited by the number of bits used to process the image signal. The contrast ratio from bright state (i.e., maximum light) to dark state (minimum light) is limited by the leakage of light in the imager. One of the major disadvantages of existing LCOS systems is the difficulty in reducing the amount of light in the dark state, and the resulting difficulty in providing outstanding contrast ratios. This is, in part, due to the leakage of light, inherent in LCOS systems. In addition, since the input is a fixed number of bits (e.g., 8, 10, etc.), which must describe the full scale of light, there tend to be very few bits available to describe subtle differences in darker areas of the picture. This can lead to contouring artifacts. One approach to enhance contrast of LCOS in the dark state is to use a COLORSWrrCH™ or similar device to scale the entire picture based upon the maximum value in that particular frame. This improves some pictures, but does little for pictures that contain high and low light levels. Other attempts to solve the problem have been directed to making better imagers, etc. but these are at best incremental improvements. Digital Light Pulse (DLP™) imagers comprise a matrix of micro-mirrors corresponding to pixels of a projected image. The micro-mirrors are pivoted between a position in which they reflect light along a projection path and a position in which they reflect light away from the projection path. The micro-mirrors are individually pulse width modulated (PWM) to provide a desired intensity for each pixel of the image. The amount of time that the micro-mirror is reflecting light along the projection path (i.e., the pulse width) is modulated responsive to a video signal provided to the imager. DLP™ imagers do not filter light by polarization. Instead, they use all of the light from a lamp or light source, regardless of its polarization. Thus, DLP™ imagers typically provide greater lamp efficiency (i.e., a less powerful lamp is required to produce an image of a particular brightness). Stereoscopic projection systems are used, for example, in 3D theaters to create a three-dimensional image by providing different, oppositely polarized images to the eyes of a viewer. In LCOS imager systems, polarized light must be provided to the LCOS imager for modulation. The oppositely polarized light is typically not used. Instead it is directed away from the projection path.

Summary of the Invention According to an exemplary embodiment, the present invention provides a two-stage projection system, comprising first and second imagers having a liquid Crystal on Silicon architecture and disposed to receive oppositely polarized light inputs. The first and second imagers are configured to modulate the oppositely polarized light inputs on a pixel-by-pixel basis in response to one or more video signals, providing oppositely polarized matrices of modulated light pixels. A third imager is disposed to receive the oppositely polarized matrices of modulated light pixels and configured to modulate the oppositely polarized matrices of modulated light pixels using pulse width modulation to form twice-modulated matrices of oppositely polarized light pixels.

Brief Description of the Drawings The invention will now be described with reference to the accompanying figures, in which: Figure 1 shows a block diagram of a stereoscopic projection system according to an exemplary embodiment of the present invention; and Figure 2 shows the two-stage modulation of two oppositely polarized channels by the stereoscopic projection system of Figure 1 according to an exemplary embodiment of the present invention. Detailed Description of the Invention The present invention provides a two-stage projection system using a combination of LCOS and Pulse Width Modulation (PWM) imagers. In an exemplary embodiment of the present invention, illustrated in Figure 1, a two-stage projection system comprises a first imaging stage 1 having two LCOS imagers 30, 40 for modulating oppositely polarized light inputs, and a second imaging stage 2 having a PWM imager 70 disposed to receive the polarized modulated output of both LCOS imagers 30, 40 for temporally modulating both of the previously modulated outputs of the LCOS imager 30, 40. A lamp 10 generates random polarization light (3 in Figure 2) and directs it toward a first imaging stage 1. Lamp 10 may be any lamp suitable for use in an LCOS system. For example a short-arc mercury lamp may be used. The random polarization light may be white light or may be separated into RGB bands of light in the time domain, for example, by a color wheel (not shown). The first imaging stage 1, which serves as an active illumination light modulator for the second imaging stage 2, includes two LCOS imagers, first imager 30 and second imager 40. These imagers 30, 40 actively modulate the light used to illuminate the third imager 70 of the second imaging stage 2. A polarizing beam splitter (PBS) 20 is disposed between the lamp 10 and the two LCOS imagers 30, 40. In the exemplary embodiment shown in Figure 1, the PBS 20 is configured to deflect s-polarized light entering a first face 21 of the PBS 20 toward a second face 22 of the PBS 20, and to allow p-polarized light entering a first face 21 of the PBS 20 to pass through the PBS and out of a third face 23 of the PBS 20. The first LCOS imager 30 is disposed proximate the second face 22 to receive the s-polarized light. The second LCOS imager 40 is disposed proximate the third face 23 to receive the p- polarized light. Thus, substantially all of the light that is directed into the first face 21 of the PBS 20 is received and modulated by one of the two LCOS imagers 30, 40 at the first imaging stage 1 of the projection system. Accordingly, the exemplary projection system is more efficient than a typical imaging system using one LCOS imager and wasting oppositely polarized light, or about half of the total light provided. This allows a smaller lamp to be used, thereby increasing lamp life and conserving energy. The increased efficiency may also be utilized to provide a brighter image. The second imaging stage 2 includes a third imager 70 disposed to receive the output of both the first imager 30 and the second imager 40. The third imager 70 is a pulse width modulation (PWM) imager, such as a DLP™ imager. A Total Internal Reflection (TIR) prism 60 is positioned proximate the third imager 70 to receive the outputs of the first and second imagers, 30 and 40 respectively, and deflect them onto the third imager 70. In applying an active illumination light modulator to a PWM imaging system, one must recognize that the PWM imaging system has two important limitations: (1) a PWM (Pulse width modulated) imager is unsuitable for being the illumination light modulator, because it would provide shortened pulse widths which can not be further modulated by shortening the pulse width and (2) the PWM system does not use polarized light, and therefore, it functions most efficiently if all the light from the lamp (random polarization) is presented to it. This invention solves these problems by using 2 LCOS imagers 30, 40 as the active illumination light modulator. The LCOS imagers 30, 40 modulate the light incident upon them by rotating the orientation of crystals at each pixel, where the portion of light transmitted varies with the crystal orientation. Since the first and second imagers do not use pulse width modulation, the third imager can modulate the already modulated light using pulse width modulation. Also, since the first and second imagers 30, 40 are disposed to modulate each of two oppositely polarized light inputs, virtually all of the light provided to the first imaging stage can be made available to the third imager. A relay lens set 50 is disposed between the first imaging stage 1 and the second imaging stage 2. The relay lens system projects individual pixels of light from the first and second imagers 30 and 40 respectively, onto corresponding micro-mirrors of the third imager 70. A suitable relay lens set is described in co-pending Patent Cooperation Treaty application US03/37978 (filed November 26, 2003 entitled "Two-Stage Projector Architecture) for a system in which the third imager 70 is the same size as the first and second imagers 30, 40, thereby requiring a unity magnification. Projection systems with different size imagers are also contemplated within the scope of this invention, whereby lens set 50 would have a non- unity magnification. A projection lens set 80 is disposed to receive the twice-modulated output of the third imager 70 and project it onto a screen (not shown) to form a viewable image. The projection path of the exemplary projection system will now be described with reference to Figure 2. Lamp 10 generates random polarization light 3, which is directed into a first face 21 of the PBS 20. The random polarization light 3 may be white light as generated by the lamp 10, or may be sequential red, green, and blue (RGB) bands of light separated by, a color wheel (not shown) or other means. The PBS 20 has an internal polarizing surface 24, which deflects light having a first polarization, s-polarized light 4 in Figure 2, out of second face 22 and allows light having an opposite polarization, p-polarized light 5 in Figure 2, to pass through the PBS 20 and out of third face 23. The first LCOS imager 30 is disposed proximate the second face 22 of the PBS 20, such that the s-polarized light 4 is incident upon it. The first LCOS imager 30, modulates the s-polarized light 4 on a pixel-by-pixel basis responsive to a video signal provided to the first LCOS imager 30. The first LCOS imager 30 then rotates the polarization of the modulated light and reflects a matrix of modulated pixels of p-polarized light 6, which passes through the PBS 20 and out of fourth face 25. Similarly, the second LCOS imager 40 is disposed proximate the third face 23 of the PBS 20, such that the p-polarized light 5 is incident upon it. The second LCOS imager 40, modulates the p-polarized light 5 on a pixel- by-pixel basis responsive to a video signal provided to the second LCOS imager 40. The second LCOS imager 40 then rotates the polarization of the modulated light and reflects a matrix of modulated pixels of s-polarized light 7, which is deflected by the polarizing surface 24 out of fourth face 25 of the PBS 20. The matrix of modulated pixels of p-polarized light 6 and the matrix of modulated pixels of s-polarized light 7 are then focused by the relay lens set 50, such that a particular pixel of modulated light is projected onto a corresponding pixel or micro-mirror of the third imager 70. Each micro-mirror of the third imager 70 modulates a pixel of each matrix of modulated pixels of polarized light 6, 7 by pulse width modulation (i.e., modulating the portion of time that the light is reflected along the projection path), thereby reflecting a matrix of twice-modulated p-polarized light 8 and a matrix of twice-modulated s-polarized light 9. The matrix of twice-modulated p-polarized light 8 is modulated once by the first LCOS imager 30 which transmits light according to the orientation of crystals that form the matrix of pixels, which in turn varies with the strength of an electric field created by a signal provided to the first imager 30 and again by the third imager 70 using pulse width modulation. Because the modulation by the first imager 30 is not temporal, it does not preclude the second modulation. Similarly, the matrix of twice-modulated s-polarized light 9 is modulated first by the second LCOS imager 40 and again by the third imager 70. The TIR prism 60 is disposed to deflect the matrices of modulated pixels of polarized light 6, 7 onto the third imager 70 and to allow the light reflected by the third imager 70 to pass through toward the projection lens set 80, such that the projection path bends upon itself. If the first imager 30 and the second imager 40 are fed the same video signal, the matrices of twice-modulated polarized light 8, 9 form the same image superimposed on one another, which may be viewed as a brighter image than would be provided by either matrix of twice-modulated polarized light, alone. Alternatively, the first imager 30 and the second imager 40 may be fed different video signals, such that, viewed through polarizing lenses the matrices of twice-modulated polarized light 8, 9 forms a 3-D image. This system, allows very high contrast, very large bit depth (no contouring with good signal processing), in a compact system. A typical color wheel (not shown) or equivalent, with an integrator (not shown) may be focused with a normal relay optic onto the two LCOS imagers 30, 40. The imager-to-imager relay lens set 50 focuses (on a pixel-by-pixel basis) the LCOS pixels (crystals) onto the PWM pixels (micro-mirrors), and the PWM pixels are projected onto the projection screen (not shown) by the projection lens set 80. The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.

Claims

What is Claimed is;

1. A projection system, comprising: first and second imagers having a liquid crystal on silicon architecture and disposed to receive oppositely polarized light inputs, wherein the first and second imagers are configured to modulate the oppositely polarized light inputs on a pixel-by-pixel basis in response to one or more video signals to provide oppositely polarized matrices of modulated light pixels; and a third imager disposed to receive the oppositely polarized matrices of modulated light pixels, wherein the third imager is configured to modulate the oppositely polarized matrices of modulated light pixels using pulse width modulation to form twice-modulated matrices of oppositely polarized light pixels.

2. A projection system according to claim 1, further comprising a lamp generating randomly polarized light directed toward the first and second imagers.

3. A projection system according to claim 2, further comprising a polarizing beam splitter disposed and configured to provide oppositely polarized light to the first and second imagers.

4. A projection system according to claim 3, wherein the polarizing beam splitter is disposed between the lamp and the first and second imagers.

5. A projection system according to claim 1 further comprising a projection lens set disposed to project the twice-modulated matrices of oppositely polarized light pixels.

6. A projection system according to claim 5, wherein a Total Internal Reflection prism is disposed to deflect the oppositely polarized matrices of modulated light pixels onto the third imager and to pass the twice-modulated matrices of oppositely polarized light pixels to the projection lens set.

7. A projection system according to claim 1 further comprising a relay lens set disposed between the first and second imagers and the third imager to focus the oppositely polarized matrices of modulated light pixels onto corresponding pixels of the third imager.

8. A projection system according to claim 1 wherein the first imager and the second imager are fed a common video signal to form an image with enhanced brightness.

9. A projection system according to claim 1 wherein the first imager and the second imager are fed different video signals to form a 3-D image.

10. A projection system, comprising: a first Liquid Crystal on Silicon (LCOS) imager disposed to receive light input having a first polarization and configured to modulate the first polarization light input on a pixel-by-pixel basis responsive to a video signal to form a matrix of modulated pixels of light having the second polarization; a second Liquid Crystal on Silicon (LCOS) imager disposed to receive light input having a second polarization opposite the first polarization and configured to modulate the second polarization light input on a pixel-by-pixel basis responsive to a video signal to form a matrix of modulated pixels of light having the first polarization; and a Pulse Width Modulated (PWM) imager disposed to receive the oppositely polarized matrices of modulated pixels of light and modulate them on a pixel-by-pixel basis using pulse width modulation to form oppositely polarized matrices of twice-modulated pixels of light.

11. A projection system according to claim 10, further comprising a lamp generating randomly polarized light directed toward the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager.

12. A projection system according to claim 11 , further comprising a polarizing beam splitter disposed and configured to provide oppositely polarized light to the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager.

13. A projection system according to claim 12, wherein the polarizing beam splitter is disposed between the lamp and the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager.

14. A projection system according to claim 10 further comprising a projection lens set disposed to project the twice-modulated matrices of oppositely polarized light pixels.

15. A projection system according to claim 14, wherein a Total Internal Reflection prism is disposed to deflect the oppositely polarized matrices of modulated light pixels onto the Pulse Width Modulated (PWM) imager and to pass the twice-modulated matrices of oppositely polarized light pixels to the projection lens set.

16. A projection system according to claim 10 further comprising a relay lens set disposed between the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager and the Pulse Width Modulated (PWD) imager to focus the oppositely polarized matrices of modulated light pixels onto corresponding pixels of the Pulse Width Modulated (PWM) imager.

17. A projection system according to claim 10 wherein the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager are fed a common video signal to form an image with enhanced light efficiency.

18. A projection system according to claim 10 wherein the first Liquid Crystal on Silicon (LCOS) imager and the second Liquid Crystal on Silicon (LCOS) imager are fed different video signals to form a 3-D image.