Abstract:
An electronic projector has a projection system that includes a spatial light modulator (SLM) for importing image information to the projected light beam. The light beam leaving the SLM is prepolarized in a defined orientation and the projected light is polarized in the same orientation so as to effectively block any light that has been scattered within the projector and become depolarized. The SLM may be a digital mirror device (DMD). Reflective surfaces within the projector may be covered or coated with material that alters the polarization of reflected stray light from the defined orientation or depolarizes the reflected stray light.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/031,839 entitled “Image Projection System” which was filed under 35 U.S.C. §371 on Jan. 22, 2002 now abandoned as the U.S. national phase entry of International Application No. PCT/CA00/00800 filed Jul. 6, 2000 and claims priority to Canadian Application No. 2,277,656, which was filed on Jul. 19, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to image projection systems, and is concerned more particularly with systems that include a spatial light modulator (SLM) for imparting image information to a projected light beam. Systems of this type typically are used for large screen televisions, which are often referred to as “electronic projectors”. 
     BACKGROUND OF THE INVENTION 
     In a typical electronic projector, the SLM may be a liquid crystal device (LCD) comprising a matrix of individually addressable liquid crystal pixels. Each pixel can be switched between a transmissive mode in which incident light from the light source passes through the pixel and is projected, and a non-transmissive mode. In the non-transmissive mode, the light may be absorbed or reflected away from the projection lens. In any event, each pixel has an “on” state and an “off” state. By appropriately controlling the pixels in accordance with stored data, image information is imparted to the projected light beam. 
     U.S. Pat. No. 5,584,991 (Levis et al.) discloses an example of a LCD projection system. 
     Another example of an SLM that includes an active matrix of pixels is known as a deformable (or digital) mirror device (DMD). In this case, the matrix comprises an array of tiltable mirrors, each of which positioned on a hinge element above electrodes that allow the mirror to be electrostatically deflected between two positions. The device is operated in a binary manner so that each mirror switches between an “on” state and a “off” state. The mirror angularly deflects the incident light beam so that the beam is either reflected through the projector optics, or not. 
     U.S. Pat. No. 5,061,049 (Hornbeck) discloses an example of a DMD device, which is hereby incorporated in its entirety by reference. U.S. Pat. No. 5,535,047 (Hornbeck) discloses further improvements to the DMD device of U.S. Pat. No. 5,061,049 and is hereby incorporated in its entirety by reference. 
     Known projection systems for producing 3D images in which light from a light source is modulated by an SLM and then polarized suffer the disadvantage that there is often a limit on the amount of light flux that can be directed onto the SLM. This limit is caused by, for example, limitations associated with the heating effect of the radiant flux, or saturation due to high luminous flux. This limit prevents increasing the light flux directed onto the SLM to overcome the losses introduced by the polarization of the light leaving the SLM. 
     Another problem with SLMs is that there is a tendency for some of the incident light to be scattered or reflected, which reduces the overall contrast of images projected onto the screen. 
     An object of the present invention is to address these disadvantages with the aim of improving the contrast of the projected images. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the improvement of contrast in electronic projectors utilizing DMDs by reducing the amount of scattered light that reaches the projection screen. The contrast improvement results from polarizing the input light to the DMD and selecting appropriate materials and surface treatments for the projector&#39;s components so that the unwanted reflection and diffraction effects that produce scattered light also depolarize or change the orientation of the polarization of the light. This allows a second polarizer at the output of the projection optics to discriminate between the desired light and the unwanted scattered light. For 3D applications, polarizing the light before the DMD reduces the heat load on the DMD, this allows higher illumination levels to be used to compensate for the loss of brightness due to polarization. The DMD can be operated within its stress ratings and illuminated to the maximum level by polarized light. 
     Generally speaking, the first polarizer means pre-polarizes or “characterizes” the light. Light that is subsequently scattered within the projector and depolarized will be partially blocked (up to a maximum of 50%) by the second polarizer means. Accordingly, the contrast ratio of the projected image will be increased by a factor of up to 2. 
     This is distinct from systems such as LCD projectors where the use of polarized light is essential in order to obtain pixel intensity control from the electrically alterable polarization property of the liquid crystal medium. In LCD projectors, the input light to the LCD is polarized, either prior to the LCD or by a polarizer that is integral to the LCD assembly. A second polarizer then analyzes the output light of the LCD according to the amount of alteration performed by the LCD on the input polarization. 
     Projectors based on DMD devices do not require polarized light. The use of polarization with DMD devices has been thought of as undesirable as it reduces by half the amount of light that the projection system can deliver to the screen. However, in a system for projection of 3D images, two sets of images are produced, one for each eye, and are characterized or coded by orthogonally polarized light. In a traditional system, the light is usually polarized after the projector lens, resulting in an efficiency loss of roughly 50%. This loss of efficiency requires high input light levels to be used, which can lead to excessive heating of the DMDs. This invention avoids this excessive heating by polarizing the light before the DMDs in the projector, therefore reducing the radiant flux and associated heating of the DMD. 
     An advantage of the invention is that it is somewhat easier to characterize the unwanted “noise” (scattered light) by polarization than by trying to characterize the signal in some other way. Inefficiencies in the polarizing material are below significance since the amount of noise is relatively small compared to the signal. Inefficiencies such as inequities in performance depending on wavelength or angle of incidence can be tolerated much more readily when applied to the noise component of the overall signal. 
     Additional significant improvements in the contrast ratio of the projected image are obtained by controlling the surface properties of materials used within the projector where light may be scattered so that those surface properties will further rotate the polarization or depolarize the stray or unwanted light when it is reflected from those surfaces. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention by way of example, and in which: 
     FIG. 1 is a schematic illustration of an electronic projector in accordance with a preferred embodiment of the invention; 
     FIG. 2 is a schematic perspective view of a pixel of a DMD that may be used in the projector of FIG. 1; 
     FIG. 3 shows the reflected intensity of two orientations of polarized light from a dielectric surface; 
     FIG. 4 illustrates the polarization states of light reflected from a dielectric surface; and 
     FIG. 5 illustrates the effect of using suitably coated surfaces with polarized light to reduce reflections. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 illustrates schematically the principal components of a projection system in accordance with the invention. Reference numeral  20  denotes a light source that projects a beam of light  22  onto a projection screen  24  via a projection lens  26 . The light source  20 , projection lens  26  and screen  24  are essentially conventional. Also conventional is a beamsplitter arrangement comprising an assembly of prisms  30  that optically splits the light beam  22  into red, green and blue components (R, G, B). The respective components are directed by the beamsplitter to three corresponding DMDs  32 . 
     The DMDs are essentially identical but deal with different portions of the spectrum. In other words, the light that enters the beamsplitter is split into red, green and blue components which are delivered to the respective R, G and B DMDs. The beamsplitter then in effect “re-assembles” the R, G and B components of the light beam and directs them together into the projection lens  26  for projection onto the screen  24 . 
     Each of the DMDs  32  comprises an array of reflective digital light switches (mirrors) that are integrated onto a silicon chip capable of addressing the switches individually. Each switch represents a single pixel in the array and can be individually switched on or off in accordance with digital information that is provided to the chip by an appropriate hardware and software controller. Each individual pixel in each DMD is controlled to impart appropriate image information to the light beam that is projected onto the screen  24 . 
     FIG. 2 shows a single one of the mirrors of a DMD and part of the silicon chip used to control the mirrors. Since DMDs are known, detailed information with respect to the construction and operation of the DMD is not provided. Reference may be made to the U.S. Pat. No. 5,061,049. For present purposes, it is sufficient to note that FIG. 2 shows the mirror at  34  and that the mirror is mounted at the outer end of a post  36  mounted on a hinge and yoke structure (not shown) above electrodes  35  that allow the element to be electrostatically deflected between two tilted positions, in which the mirror either reflects light into the projection lens  26  (FIG. 1) or away from the projection lens. In FIG. 2, the mirror is shown in full lines in one of its tilted positions and in ghost outline in the other of its tilted positions. 
     A portion of the silicon chip on which the mirror is mounted is denoted by reference numeral  38 . The chip includes individual memory cells, one for controlling each mirror. By virtue of the construction of the DMD, the top surface  39  of the chip  38  below each mirror has surface portions that are at different elevations and have a variety of different irregular shapes, as indicated generally by reference  40  in FIG.  2 . Gaps between each mirror that are inherent in the construction of the DMD mirror array allow light to reach this top surface. Some of this light is reflected from the top surface  39 , which causes scattered light. Pursuant to an aspect of the invention that is to be described later, the under-mirror substrate layer  39  and some of the elevated surface portions  40  are treated or coated with a material that has the property of further rotating the polarization or depolarizing the light that is reflected from that surface. 
     In accordance with a primary aspect of the invention, the projection system includes first polarizer means for polarizing, in a defined orientation, light input to each DMD and second polarizer means for polarizing, in the same defined orientation, light after it has passed through the projection lens. The first polarizing means pre-polarizes or “characterizes” the light in a defined orientation. Light that is subsequently scattered within the projector is altered in polarization or depolarized by the surface properties of the projector components. The scattered light is then blocked by the second polarizer means and will not impair the contrast of the images that are projected onto the screen. 
     In the embodiment shown in FIG. 1, the first polarizer means is indicated by a polarizing filter P 1  in the beam of light that enters the beamsplitter from the light source  20 . For example, the polarizer may be positioned between lens elements  44  that configure the light beam appropriately before the light enters the beamsplitter. In this way, the light is pre-polarized or “characterized” by polarizer P 1 . 
     Polarizer P 2  is also a polarizing filter and in this embodiment is positioned at the outer end of projection lens  26 . Polarizer P 2  has a defined orientation that is the same as the defined orientation of polarizer P 1 . Accordingly, polarizer P 2  will block and prevent projection onto the screen of any light that has become de-polarized or altered in polarization as the light beam passed through the optical system of the projector. It will of course be understood that polarizer P 2  could be located, for example, prior to the projection lens or within the projection lens  26  (e.g. between the lens element of the projection lens). 
     Similarly, the location of polarizer P 1  can change. Preferably, the light is pre-polarized before it reaches the SLM(s) of the projection system. However, it is important merely that the light be polarized as it leaves the SLM(s). 
     In summary, the arrangement of first and second polarizers provided by the invention has been found to lead to significant improvements in the contrast ratio of the images that are projected onto the screen. It has also been found that additional significant improvements in contrast ratio can be achieved by controlling the surface properties of materials used within the projector where light may be scattered so that those surface properties will further rotate the polarization or depolarize the stray or unwanted light when it is reflected from those surfaces. 
     It is well known that specular reflections from metallic surfaces preserve the polarization of the incident light while reflections from dielectric surfaces obey the relationship shown in FIG.  3 . This figure shows that incident rays with a polarization parallel to the plane of incidence are reflected with greater efficiency than incident rays with a polarization perpendicular to the surface. This is illustrated in FIG. 4 where the orientation of polarization of the light is described according to convention by the direction of the electric field vector. 
     In some circumstances it is possible to select dielectric materials to coat surfaces positioned in the projector so that when polarized light is incident on these surfaces it is not reflected due to the orientation of the polarization vector in the incident light. This is shown in FIG.  5 . Surface coatings may also be found that rotate the polarization of the light upon reflection by for example 90 degrees. 
     Additionally, diffuse reflectors of both metallic and dielectric materials depolarize the incident light. Various methods for treating surfaces, such as chemical etching or micro bead blasting can be used to achieve surfaces with diffuse reflecting characteristics. 
     In FIG. 1, the undulating lines denoted by reference numeral  46  indicate typical areas in which such coatings or surface treatments may be applied. 
     One significant area is the top surface  39  of the memory chip  38  of each DMD. Thus, reverting to FIG. 2, the top surface  39  and the elevated surfaces  40  need to be coated or otherwise have properties that will further rotate the polarization or depolarize the stray or unwanted light. These surfaces then effectively screen reflection of light that may “miss” or partially miss the mirror  34  and that would otherwise give rise to significant optical noise within the projector. 
     An additional benefit of the invention is that it reduces the heat load on DMDs in those situations where the light output by the projector is required to be polarized in a particular orientation. This is the case for example in a 3-D projection system where two sets of images are produced, one for each eye, and are characterized or coded by orthogonally polarized light. In a traditional system, the light is usually polarized after the projector lens, resulting in an efficiency loss of roughly 50%. This loss of efficiency requires high input light levels to be used, which can lead to excessive heating of the DMDs. The invention avoids this excessive heating by polarizing the light before the DMDs in the projector, therefore reducing the radiant flux and associated heating of the SLM. 
     In conclusion, it should be noted that, while the preceding description relates to a particular preferred embodiment of the invention, the invention is not limited to this embodiment. A number of modifications have been indicated specifically and others would be apparent to a person skilled in the art. In addition, it should be noted that while the described embodiment relates to a projection system that includes three DMDs, projection systems can be configured using different numbers of DMDs, for example, one or two. Different configurations are possible depending on the intended application of the projection system and the characteristics that are required of the system. Generally speaking, one and two DMD systems require time multiplexing of colour.