Abstract:
An image projection system includes an illumination system and an image display system with at least one reflective image display panel for modulating. An illumination beam is supplied by the illumination system with image information and a projection lens system. The illumination system has an extra optical system that at least partly re-illuminates the image display system with light reflected by the reflective image display panel to the illumination system. The extra optical system redistributes light coming from a pixel of the image display system across a plurality of pixels of the image display system.

Description:
FIELD OF TECHNOLOGY 
     The invention relates to an image projection system comprising, in this order, an illumination system, an image display system with at least one reflective image display panel for modulating, an illumination beam to be supplied by the illumination system with image information, and a projection lens system. 
     BACKGROUND AND SUMMARY 
     An image projection system of the type described in the opening paragraph is known, for example, from U.S. Pat. No. 5,905,545. The image projection system described in this specification comprises an illumination system for supplying an illumination beam, and an image display system with reflective image display panels for modulating this light beam in conformity with image information to be projected. The display panel may be, for example, a digital micro-mirrored display (DMD) with a two-dimensional array of reflective digital light switches which are driven by means of electrodes. Small mirrors for each pixel are used to switch a pixel on or off by changing an angle of the mirrors. The pixels of the DMD can maintain their ‘on’ or ‘off’-state for controlled display times. To achieve intermediate levels of illumination between white and black, pulse-width modulation techniques are used. In the reflective DMD projection systems, the part of the light modulated by the display panel, which part must give rise to dark pixels in the image, is deflected from the light path by the reflective switches and absorbed in the optical system and is thus lost. This is at the expense of the peak brightness in the image. 
     It is an object of the present invention to provide an image projection system in which a relatively high peak brightness is realized. 
     This object is achieved by the image projection system according to the invention, which is characterized in that the illumination system comprises an extra optical system for at least partly re-illuminating the image display system with the light reflected by the image display system to the illumination system, the extra optical system comprising means for redistributing light coming from a pixel of the image display system across a plurality of pixels of the image display system. 
     The present invention relates to a reflective projection system in which light is incident on the display panel and modulated by the display panel before it is projected. The invention is based on the recognition that the light which is modulated by a pixel representing a dark or grey pixel in the image is deflected from the light path but is not absorbed in the display system and is again sent towards the entrance of the optical display and thus recuperated. This light will as yet have an opportunity of being incident on a pixel representing a bright pixel. In the image projection system described above, the light intended for dark or grey pixels is thus not lost but is re-used. Furthermore, to prevent ghost images from being produced during this reuse, the illumination system is provided with means for redistributing of the light. 
     A preferred embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises a lens element and an optical fiber, the lens element being situated between an element of the reflective image display panel and an input of the optical fiber for concentrating reflected light in the optical fiber. In this way, the reflected light which is not used to form a projection image can be efficiently transported back to the illumination system. Total internal reflection in the optical fiber redistributes the recuperated light. 
     A further embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises an optical wedge for combining the light from the radiation source and the reflected light from the reflective image display panel. 
     A further embodiment of the image projection system according to the invention is characterized in that the extra optical system comprises an integrating rod for receiving light from an element of the reflective image display panel, and reflecting means situated at one side of the integrating rod for reflecting the received light back to the image display panel. In this way, the integrating rod acts as a non-imaging mirror device which homogeneously distributes the light on the image display device so that small distortions are reduced in the projected picture. 
     A further embodiment of the image projection system according to the invention is characterized in that the image projection system comprises a further reflective image display panel for modulating a second beam provided by the illumination system, and the extra optical system comprises means for recombining the light reflected from both the reflective image display panel and the further reflective image display panel. For example, a dichroic mirror may be used to recombine the light of different colors from the respective reflective image display panels into a single light beam which can be directed to the illumination system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 shows a first embodiment of an image projection system according to the invention, 
     FIG. 2 shows a second embodiment of an image projection system comprising two integrating rods and 
     FIG. 3 shows a third embodiment of an image projection system comprising two reflective image display panels. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The image projection system  1  shown in FIG. 1 comprises an illumination system  3  for supplying an illumination beam. The illumination system  3  comprises a radiation source  5  and a reflector  7 . The reflector  7  at least partly surrounds the radiation source  5  and ensures that the greater part of the light emitted by the radiation source in a direction away from the system as yet reaches the system. 
     The illumination beam generated by the illumination system  3  is incident on the display system, represented for the sake of simplicity by a single image display panel  23 , and is modulated thereby in conformity with the image information to be displayed. The light modulated and reflected by the display panel is projected on a screen (not shown) by means of a projection lens system represented for the sake of simplicity by a single projection lens  25 . 
     In FIG. 1, light coming from the illumination system  3  is sent via a second reflector  9 , a first optical wedge  11 , an integrator system  13 , a first lens  15 , a folding mirror  17  and first and second relay lenses  19 , 21  towards the reflective image display panel, for example a digital mirrored display (DMD) panel  23 . DMD devices are known per se from U.S. Pat. No. 5,061,049. The mirrors of the DMD image display panel  23  reflect the light either to the pupil of the projection lens  25  or to the entrance of an extra optical system in dependence on the voltage applied across the electrodes. The extra optical system comprises, for example, in said order, an entrance mirror  27 , a third relay mirror  29  and a second optical wedge  33 . The applied voltage controls the angle of the micro-mirrors of the DMD device in conformity with the image information to be displayed. The mirrors of the DMD image display panel  23  can be set to two different states, an on-state and an off-state. The on-state of the mirror is situated, for example, at 10 degrees with respect to a normal on the DMD image display panel  23  and the off-state of the mirror is situated, for example, at −10 degrees with respect to the normal. In the on-state, a light beam impinging on the mirrors of the DMD image display panel  23  at an angle of 20 degrees with respect to the normal is reflected in a direction coincident with the normal. In the off-state, the light beam impinging on the mirrors of the DMD image display panel  23  at an angle of 20 degrees is reflected at an angle of 40 degrees with respect to the normal. This difference is sufficient for a skilled person to dimension the optical elements of the system so as to direct the light in the pupil of the projection lens  25  in the on-state of the mirrors and away from the pupil of the projection lens in the off-state of the mirror. The present invention proposes to provide the illumination system with the extra optical system for at least partly re-illuminating the DMD image display panel  23  with light reflected by the DMD image display panel. Here, light is concerned which is reflected by display elements representing dark or grey pixels in the image. The extra optical system comprises means for redistributing light coming from such a display element across a plurality of display elements. In known systems, the light reflected by such display elements is reflected from the light path by the DMD display panel  23  and is thus lost. In the image projection system according to the invention, this light is recuperated and is given another opportunity of being incident on display elements, giving rise to bright pixels in the image. In operation, in the off-state, the entrance mirror  27  directs the light coming from pixels of the display panel representing dark pixels in the image via the third relay mirror  29  and the second optical wedge  33  towards the entrance of the light-integrating system  13 . 
     The extra optical system may alternatively comprise, in said order, an entrance lens and an optical light guide, for example, an optical fiber made of plastic or glass. 
     In the embodiments of the image projection system according to the invention, shown in FIG. 1, the illumination system may not only comprise a radiation source and a reflector, but also an integrator system. The first lens  15  behind the integrator system ensures that all re-images are superimposed in the plane of the DMD image display panel. 
     The integrator system  13  comprises a first lens plate  31  and a second lens plate  35  and ensures a homogeneous illumination of the display panel  23 . For a detailed description of the principle of an integrator system with two lens plates, reference is made to U.S. Pat. No. 5,098,184. 
     Instead of two integrator plates, the integrator system may alternatively comprise a bar-shaped integrator. The illumination system is then made uniform by multiple total internal reflection on the side walls of the bar. The bar may be in the form of, for example, a quartz bar. 
     The display panel is, for example, a display panel which is sequentially illuminated with a red, a green and a blue beam, while it is simultaneously driven with the image having the color of the corresponding illumination. The extra optical system may also comprise, in said order, an integrating rod and a reflective means. An image projection system comprising such an extra optical system is discussed with reference to FIG.  2 . 
     The image projection system  1  shown in FIG. 2 comprises an illumination system  3  for supplying an illumination beam. The illumination system  3  comprises a radiation source  5  and a reflector  7 . The reflector  7  at least partly surrounds the radiation source  5  and ensures that the greater part of the light emitted by the radiation source in a direction away from the system as yet reaches the system. 
     The illumination beam generated by the illumination system  3  is incident on the display system, represented for the sake of simplicity by a single image display panel  23 , and is modulated thereby in conformity with the image information to be displayed. The light modulated and reflected by the display panel is projected on a screen (not shown) by means of a projection lens system represented for the sake of simplicity by a single projection lens  25 . 
     In FIG. 2, light coming from the illumination system  3  is sent via an integrator system, for example, a quartz rod  13 , a pair of relay lenses  40 , 42 , an aperture stop  60 , a folding mirror  17 , a third relay lens  62  and a total internal reflection (TIR) prism  44  towards the reflective image display panel, for example a DMD image display device  23 . The mirrors of the DMD image display device  23  are designed to be set at two different states. In the on-state, the mirrors of the DMD image display device  23  are designed to reflect the light beam in such a way that the reflected light beam entering the TIR prism  44  is transmitted to the projection lens  25 . In the off-state, the mirrors of the DMD are designed to reflect the incident light beam from a reflective image element back into the direction of incidence. The light is then reflected via an interface  43  of the TIR prism  44  back to the illumination system  3 . By tilting the DMD image display device  23 , the reflected light beam can be directed to the entrance of a second integrating rod  64  near the first integrating rod  13 . The other entrance of the second integrating rod  64  is provided with a reflective means, for example a reflective coating  66  for reflecting the reflected light beam back to the display system. The second integrating rod  64  and the reflective coating  66  are part of the extra optical system for redistribution of the light coming from pixels of the image display system across the pixels of the DMD image display device  23 , and, as a result, the peak brightness of the image projection device is increased. The relay lenses  40 , 42 , 62  are designed in such a way that the reflected light beam creates an image of the aperture stop  70  in the same plane as the aperture stop  68 . Furthermore, the reflective DMD display panel may be, for example, sequentially illuminated with a red, a green and a blue beam, while the display panel  23  is simultaneously driven with the image having the color of the corresponding illumination. Therefore, the illumination system  3  preferably comprises a color wheel  37  driven by a motor  39  for alternately providing the red, green and blue light beams. 
     The image projection system may alternatively comprise two or three display panels. An embodiment using two display panels is shown in FIG.  3 . The image projection system comprises an illumination system comprising, in said order, a radiation source  5 , a reflector  7 , an optical wedge  11 , an integrating system, for example an optical bar  13 , a total internal reflection (TIR) prism  41 , a dichroic mirror  45  for reflecting radiation with a wavelength in the red range and passing radiation in the green and blue ranges, a first reflective image display panel  47  and a second reflective image display panel  49 , for example DMD image display panels and a projection lens  25 . Furthermore, in accordance with the invention, the image projection system comprises an extra optical system for recuperating the light reflected by the first and second DMD image display panels  47 , 49  in the illumination system. This extra optical system comprises, in said order, the coupling lenses  51 , 53 , optical light guides  55 , 61 , for example, an optical fiber made of glass or plastic, and a dichroic mirror  67  for combining the reflected light from the respective first and second DMD image display panels  47 , 49  in a single beam. In operation, light from the illumination system  3  is coupled into the integrating system  13 , for example an optical rod, via the optical wedge  11  and a rotating color wheel  37  driven by a motor  39  for alternating providing yellow and magenta light beams. The optical rod  13  homogeneously distributes the light and directs the light to the total internal reflection (TIR) prism  41 . Since the angle of incidence of the incoming light at the interface  43  of the TIR prism  41  is larger than the critical angle, the prism reflects the light towards the first dichroic mirror  45 . The TIR prism  41  is situated with respect to the incoming light beam in such a way that the light beam can be properly switched by the rotary action of the mirrors of the DMD image display panels  47 , 49 . The first dichroic mirror  45  continuously reflects red light with a wavelength in the red range towards the first DMD image display panel  47  and alternately passes the green or blue light with wavelengths in the green or blue range, respectively, towards the second DMD image display panel  49 . The first DMD image display panel  47  modulates the red light, in conformity with the image information to be displayed, by reflecting the light via the dichroic mirror  45  and the TIR prism  41  in the pupil of the projection lens  25  for projection on a screen (not shown) or via the first lens  51  in the entrance pupil  57  of the first optical fiber  55  for recuperating the light which is not used for projection. The second reflective image display panel  49  alternately modulates the green or blue light simultaneously with the rotating color wheel  37  and in conformity with the image information to be displayed. The rotary action of the mirrors of the second DMD image display panel  49  directs the green or blue light via the first dichroic mirror  45  and the TIR prism  41  in the entrance pupil of the projection lens  25  for projection on the screen (not shown) or via the second lens  53  in the entrance pupil  63  of the second optical fiber  61  for recuperating the light not used for projection. The optical fibers  55 , 61  transport the red light and the respective green or blue light back to the illumination system. The second dichroic mirror  67  combines the red light coming from the output  59  of the first optical fiber  55  with the respective green or blue light coming from the output  65  of the second optical fiber  61 . The combined light is then fed back into the illumination system by the optical wedge  33 . By providing the image projection system with an integrator system ( 13 ) and an optical system ( 51 , 53 , 55 , 61 , 67  ) as described above, the light coming from pixels representing dark pixels in the image will be recuperated, as described above with reference to a single DMD image display panel and, as a result, the peak brightness of the projected image is increased. 
     Instead of DMD image display panels, also other reflective image display panels may be used in the projection system according to the invention, for example, an actuated mirror array (AMA) image display panel known per se from the U.S. Pat. No. 5,729,386. 
     Furthermore, a reflective liquid crystal image display panel may be used. When a liquid crystal display panel is used, the illumination system should provide a polarised light beam for illumination of the display system. Furthermore, an analyzer has to be situated between the liquid crystal display panel and the projection screen. 
     It is to be noticed that the integrating system may be omitted to save costs in exchange for image homogeneity. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative solutions without departing from the scope of the claims.