Abstract:
A depixelizer for enhancing an image generated by a spatial light modulator having an array of pixels. The depixelizer comprising a translatable stage having the spatial light modulator attached thereto. The stage being moveable in a first axis of motion and a second axis of motion. The movement of the stage in at least one axis oscillates the spatial light modulator and enhances the image generated thereby.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to a system and method of eliminating the appearance of pixels on an image, and more particularly to a mechanical method of removing such pixel images. 
     Currently, spatial light modulators (SLM&#39;s) are used to create digital image displays. The most common type of a SLM is the liquid crystal display (LCD) used in laptop computers and projectors. The LCD consists of an array of pixels (i.e., picture elements) that individually modulate a quantity of light impinging thereon. The individual pixels modulate the light from off to on or some level therebetween. The LCD may utilize polarization rotation to modulate the light level at each pixel. Alternatively, another class of spatial light modulators use a Schlieren system whereby incident light is deflected on or off an aperture to thereby modulate the light level. The SLM may be used in a projector to generate large images on a screen. In this respect, light is directed through the SLM to form the image on the screen. 
     The sharpness of an image is determined by the contrast ratio produced between two adjoining pixels. The contrast ratio is not dependent upon the resolution of the image, or whether discrete pixels or the grain in film creates the image. Generally, the contrast ratio for video is 200:1, while the contrast ratio for cinema is greater than 600:1. In order to maintain the contrast ratio for a SLM, any optics downstream of the SLM must have a transfer function two times greater than the limiting resolution of the modulator (i.e., Nyquist limit). 
     In a spatial light modulator, the array of pixels are generally arranged in rows and columns. Accordingly, each pixel is spatially isolated from an adjoining pixel by a black border. The active area of the pixel, as compared to the area of the pixel pitch, is known as the aperture ratio. The aperture ratio typically ranges from 92% to 45%. As will be recognized, the area around the pixel has no picture information in it and is optically opaque. Accordingly, the borders around the pixels create an array of black lines and columns superimposed around the pixels. This effect is commonly referred to as the screen door effect, as the image is similar to looking through a screen door. 
     Another deficiency with the current SLM&#39;s is the visual effect known as aliasing. Because the pixels are generally square and arranged in columns and rows, a line drawn on the display will appear as a stair-step if drawn at any angle other than horizontal or vertical. The worst case occurs when the pixels are square and the line is drawn at 45°. The line will appear as isolated squares joined at their corners. Accordingly, aliasing is the effect of a non-horizontal or non-vertical line appearing as a stair-step when produced by the spatial light modulator. 
     In rear projection video displays it is common practice for the screen to comprise an array of lenticular lenses separated by black strips. The black strips absorb light transmitted from the observers side of the screen and make the screen look blacker than it would otherwise be. By making the screen look blacker, the effective contrast ratio of the produced image is increased. However, if the spatial light modulator and the screen array are close in spatial frequency, a Moiré pattern will occur at a very low frequency. The Moiré pattern is a series of interference fringes which degrade the image on the screen. 
     The present invention addresses the above-mentioned deficiencies in prior art display systems by providing a system which removes undesired artifacts. Accordingly, the present invention reduces or illuminates the occurrence of the screen door effect through mechanical means. Additionally, the present invention eliminates Moiré patterns by shifting the temporal frequency of the projected image above the threshold that the human eye detects. Further, the present invention reduces the effects of aliasing by mechanically overlapping the corners of the adjacent pixels. As such, the present invention provides a projection system whereby the created image appears smoother without reducing the contrast ratio thereof. 
     BRIEF SUMMARY OF THE INVENTION 
     A depixelizer for enhancing an image generated by a spatial light modulator having an array of pixels. The depixelizer comprises a translatable stage having the spatial light modulator attached thereto. The stage is moveable in a first axis of motion and a second axis of motion. The movement of the stage in at least one axis oscillates the spatial light modulator and enhances the image generated thereby. 
     In the preferred embodiment, the first axis is generally perpendicular to the second axis and the stage is configured to be moveable in both the first and second axes of motion simultaneously. In order to translate the stage in both the first and second axes, the stage of the depixelizer comprises an outer stage moveable in the first axis of motion and an inner stage moveable in the second axis of motion. 
     In accordance with the present invention, the depixelizer further includes a first actuator attached to the outer stage and operative to translate the outer stage in the first axis of motion. Similarly, the depixelizer includes a second actuator attached to the inner stage and operative to translate the inner stage in the second axis of motion. As will be recognized, the inner stage is disposed within the outer stage by a set of inner stage mounting fingers attached to the inner stage and the outer stage. Accordingly, the second actuator is attached to the inner stage and the outer stage. Similarly, the depixelizer includes a frame that the outer stage is disposed within. The first actuator is attached to the outer stage and the frame by a set of outer stage mounting fingers. 
     Further, the depixelizer of the present invention includes an inner stage actuator mount spring attached to the outer stage and the second actuator and an outer stage actuator mount spring attached to the frame and the first actuator. The inner and outer stage actuator mount springs dampen vibration between the frame and the inner and outer stages. Typically, the inner stage mounting fingers, the outer stage mounting fingers, the inner stage, the outer stage, and the frame are integrally formed from a single sheet of metallic material. 
     In accordance with the present invention, there is provided a method of depixelizing an image generated by a spatial light modulator having a plurality of pixels with a prescribed pitch. The method comprises the step of oscillating the spatial light modulator in the direction of a first axis by a distance less than ½ the pitch of the pixels. Further, the method comprises oscillating the spatial light modulator in the direction of a second axis by a distance less than ½ the pitch of the pixels. Typically, the second axis will be generally perpendicular to the first axis. In the preferred embodiment, the spatial light modulator will be oscillated by the depixelizer in the direction of the first axis and the second axis to produce an elliptical motion of the spatial light modulator. The spatial light modulator will be oscillated in the frequency range of from about 30 Hz to 180 Hz in order to reduce the contrast ratio of the spatial light modular from 300:1 to about 2:1. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     These as well as other features of the present invention will become more apparent upon reference to the drawings wherein: 
     FIG. 1 is a perspective view of a depixelizer constructed in accordance with a preferred embodiment of the present invention and used in conjunction with a display system; 
     FIG. 2 is an exploded perspective view of the depixelizer shown in FIG. 1; and 
     FIG. 3 is a front elevational view of the depixelizer shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only, and not for purposes of limiting the same, FIG. 1 perspectively illustrates a depixelizer  10  used in conjunction with a light source  12 , focusing lens  14  and projection screen  16 . Mounted to the depixelizer  10  is a spatial light modulator (SLM)  18  such as a liquid crystal display (LCD). The SLM  18  is operative to modulate the light emitted from light source  12 . In this respect, light from light source  12  travels along axis “A” through the SLM  18  and focusing lens  14 . The SLM  18  modulates the light from the light source  12  into the desired effect. The focusing lens  14  directs the light from SLM  18  onto the projection screen  16  for viewing. The SLM  18  is in electrical communication with an electronic device which modulates the SLM  18  to create the desired image. The depixelizer  10  is being described as being operative with the SLM  18  for a projection system, as shown in FIG.  1 . However, it will be recognized that the depixelizer  10  may be used with direct view SLM&#39;s  18  in laptop computers or other types of direct view devices (i.e., computer displays). 
     As previously mentioned, the SLM  18  suffers from various drawbacks because artifacts such as the screen door effect, aliasing, and Moiré patterns are created thereby. Of these artifacts, the screen door effect is amplified by the projection of the image from the SLM  18  onto the projection screen  16 . As seen in FIG. 1, the focusing lens  14  enlarges the image created by the SLM  18  such that the image will be larger than the SLM  18 . Accordingly, the array of black lines and columns superimposed around the pixels of the SLM  18  will be magnified and visible to a viewer. 
     In order to reduce the screen door effect, the depixelizer  10  mechanically oscillates the SLM  18  in a prescribed direction and prescribed rate. Referring to FIGS. 2 and 3, the depixelizer  10  comprises a platform  20  for supporting the SLM  18 . The platform  20  is attached to and extends perpendicularly from a base  22 . The attachment of the platform  20  to the base  22  is accomplished through the use of fasteners  24 . The platform  20  is milled from a single sheet of metallic material such as aluminum. The platform  20  is milled to define an outer frame  26 , an outer stage  28 , and an inner stage  30 . 
     Referring to FIG. 3, the outer frame  26  is generally rectangular such that a rectangular opening is formed therein. Disposed within the opening of the outer frame  26  is the outer stage  28 . As seen in FIG. 3, the outer stage  28  is generally rectangular and supported by outer stage mounting fingers  32   a ,  32   b ,  32   c , and  32   d . The outer stage mounting fingers  32   a - 32   d  are integrally connected to both the outer frame  26  and outer stage  28 . The outer stage mounting fingers  32   a - 32   d  are milled from the platform  20  such that they are thin, flexible, and resilient. Accordingly, the outer stage mounting fingers  32   a - 32   d  provide a prescribed amount of flexure. 
     As seen in FIGS. 2 and 3, the outer stage mounting fingers  32   a - 32   d  are thin beams oriented along a similar direction. Specifically, each of the outer stage mounting fingers  32   a - 32   d  are generally parallel to the x-axis of the platform  20  as shown in FIG.  3 . Each of the outer stage mounting fingers  32   a - 32   d  flexes in a direction perpendicularto the x-axis because each of the outer stage mounting fingers  32   a - 32   d  are oriented parallel to the x-axis. Accordingly, as will be recognized, the outer stage  28  will be translatable in the direction of the y-axis shown in FIG.  3 . The outer stage  28  will therefore oscillate in the direction of the y-axis by the flexure of the outer stage mounting fingers  32   a - 32   d , as will be explained below. 
     Disposed within the interior of the outer stage  28  is the inner stage  30 . The inner stage  30  is supported to the outer stage  28  through inner stage mounting fingers  34   a ,  34   b ,  34   c , and  34   d . The inner stage mounting fingers  34   a - 34   d  are integrally connected to both the inner stage  30  and the outer stage  28 . The inner stage mounting fingers  34   a - 34   d  are oriented generally parallel with the y-axis of the platform  20  and perpendicular to outer stage mounting fingers  32   a - 32   d , as shown in FIG.  3 . In this respect, each of the inner stage mounting fingers  34   a - 34   d  are generally thin, resilient and oriented parallel to they-axis. The inner stage mounting fingers  34   a - 34   d  permit the inner stage  30  to translate in the direction of the x-axis. In this respect, the inner stage  30  may oscillate in the direction of the x-axis by the flexure of the inner stage mounting fingers  34   a - 34   d . As seen in FIG. 2, the inner stage  30  is generally rectangular with an opening formed therein. The SLM  28  is mounted within the opening of the inner stage  30 . Accordingly, the oscillation of the inner stage  30  in the x-direction and the oscillation of the outer stage  28  in the y-direction will oscillate the SLM  18  attached to the inner stage  30 . 
     In order to oscillate the outer stage  28  and inner stage  30 , the depixelizer  10  includes an outer stage actuator  36  and an inner stage actuator  38 . The outer stage actuator  36  is a voice coil having one end attached to the outer stage  28  and the other end attached to an outer stage actuator mount spring  40 . The outer stage actuator mount spring  40  is attached to the outer frame  26 . The outer stage actuator mount spring  40  is a thin, resilient/flexible beam. The outer stage actuator  36  is operative to change axial length in response to an electrical signal applied to signal line  42 . An electrical signal applied to signal line  42  selectively increases or decreases the axial length of the outer stage actuator  36  such that the outer stage actuator  36  translates the outer stage  28  in the direction of the y-axis of the platform  20 . The outer stage actuator mount spring  40  dampens vibration of the outer stage actuator  36  such that the outer frame  26  and platform  20  do not vibrate excessively. 
     The inner stage actuator  38  is attached to the inner stage  30  and an inner stage actuator mount spring  44 . The inner stage actuator mount spring  44  is attached to the outer stage  28  and is resilient/flexible. In this respect, the inner stage actuator  38  is operative to translate the inner stage  30  in the direction of the x-axis. The inner stage actuator is a variable length voice coil. The axial length of the voice coil may be selectively increased or decreased by the application of an electrical signal to the signal line  46 . The inner stage actuator mount spring  44  prevents vibration from the inner stage actuator  38  from reaching the outer stage  28 . 
     The SLM  18  is attached to one side of the inner stage  30  through the use of spacers  48 . As seen in FIG. 2, the spacers  48  are disposed between the SLM  18  and the inner stage  30 . In this respect, a space or gap is formed between the inner stage  30  and the SLM  18 . In order to balance the weight of the SLM  18 , there is provided a counterweight  50  attached to a side of the inner stage  30  opposite the SLM  18 . The counterweight  50  is a rectangular frame sized to balance the weight of the SLM  18 . 
     The depixelizer  10  further includes an air nozzle  52  attached to the outer frame  26  of the platform  20 . The air nozzle  52  directs a flow of air over the SLM  18 . As mentioned above, the SLM  18  is spaced relative to the inner stage  30  such that a gap is formed therebetween. The air nozzle  52  is positioned to direct a flow of air over both sides of the SLM  18 . The flow of air from the air nozzle  52  cools the SLM  18  during operation thereof. 
     As previously mentioned, the inner stage actuator  38  and the outer stage actuator  36  are operative to oscillate respective ones of the inner and outer stages  28 ,  30 . The oscillation of the inner and outer stages  28 ,  30  thereby results in the oscillating movement of the SLM  18 . The oscillation of the SLM  18  reduces the screen door effect. Specifically, as mentioned above, the SLM  18  comprises a plurality of pixels arranged in columns and rows. Each of the pixels is surrounded by a black border that absorbs or reflects light back to its source. Accordingly, a matrix of light spots separated by the unlit borders is created. In the preferred embodiment, the inner and outer stage actuators  36 ,  38  oscillate the SLM by less than ½ a pixel pitch in a basic sinusoidal pattern. A majority of the light from the light source  12  strikes the central portion of each pixel and is unaffected by the oscillating movement of the SLM  18 . However, as the SLM  18  oscillates, light striking the border portion of each pixel is uncovered and is now free to be transmitted through the focusing lens  18  onto the screen  16 . The oscillation of the SLM  18  causes the black borders to pass light 50% of the time. The contrast ratio between the black border and each pixel is typically greater than 300:1 without oscillation thereby creating a hard edge detectible to the eye. However, with the oscillation of the SLM  18 , the contrast ratio between the black border and each pixel is reduced to less than 2:1 thereby eliminating the screen door effect. 
     In addition to reducing the screen door effect, the speed of oscillation of the SLM  18 , by the inner and outer stage actuators  36 ,  38 , is above a threshold detectable by the eye (i.e., between 30 Hz and 180 Hz). By setting the speed of oscillation of the SLM  18  between 30 Hz and 180 Hz, the SLM  18  will not create any Moirè patterns because the movement of the SLM  18  is not detectible by the eye. Furthermore, because the SLM  18  is oscillated in two directions (i.e., the x and y axis), a multitude of motions can be created. In the preferred embodiment, the SLM  18  is translated in an alternating elliptical motion set at plus and minus 45% in order to reduce aliasing (stair-step effect). 
     In order to reduce vibration of the depixelizer  10 , the spring rate of the outer stage mounting fingers  32   a - 32   d  and inner stage mounting fingers  34   a - 34   d  is determined such that the product of the driven mass and spring rate will resonate at a prescribed frequency. In the preferred embodiment, the SLM  18  weighs approximately 0.5 kilograms and will be modulated at a frequency of 30 Hz. Both the inner stage actuator and outer stage actuator  36 ,  38  have the same mass and natural frequency as the SLM  18  and act as counter balances. The inner and outer stage actuators  36 ,  38  typically include a air gap disposed therein and can travel up to 50 microns in length. In the preferred embodiment, the product of the spring rate of the inner and outer stage mounting fingers  32   a-d ,  34   a-d  and the driven mass is set to resonate as the same frequency as the inner and outer stages  28 ,  30 . 
     In order to detect the motion of the inner and outer stages  28 ,  30 , the depixelizer  10  further includes an outer stage sensor  54  and an inner stage sensor  56 . Both the inner and outer stage sensors  54 ,  56  are position sensors such as Hall or Reed effect sensors operative to detect the motion of respective inner and outer stages  28 ,  30 . In this respect, the outer stage sensor  54  is attached to the outer frame  26  such that it can measure the relative movement of the outer stage  28 . Similarly, the inner stage sensor  56  is attached to the outer stage  28  and measures the relative movement of the inner stage  30 . By determining the motion of the outer stage  28  and the inner stage  30 , it is possible to control the motion of the inner and outer stage actuators  36 ,  38  such that the SLM  18  is oscillated at a prescribed rate. 
     Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. For example, for projectors which require three SLM&#39;s  18 , it will be necessary to use three depixelizers  10  to produce the final image. In such systems, an offset voltage may be applied to each actuator of each depixelizer  10  in order to align the three different SLM&#39;s  18  without additional mechanical means of alignment. Thus, the particular combination of parts described and illustrated herein is intended to represent only a certain embodiment of the present invention, and is not intended to serve as a limitation of alternative devices within the spirit and scope of the invention.