BACKGROUND OF THE INVENTION In recent years, light modulators have been developed using MEMS (micro- electro-mechanical systems) technology in which moveable elements are configurable to direct light. An example of such light modulators is a grating light valve (GLV) taught in U. S. Patent No. 5,311,360 to Bloom et al., in which the GLV is configurable in a reflecting mode and a diffracting mode. The GLV taught by Bloom et al. is isometrically illustrated in FIG. 1. The GLV 10 includes moveable elongated elements 12 suspended over a substrate 14.

A first side view of the GLV 10 of the prior art is illustrated in FIG. 2A, which shows the GLV 10 in the reflecting mode. The moveable elongated elements 12 each include a first reflective coating 16. Interspersed between the moveable elongated elements 12 are second reflective coatings 18. In the reflecting mode, upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a height difference of a half wavelength A/2 of incident light 1. The incident light I reflecting from the second reflecting coatings 18 travels a full wavelength further than the incident light I reflecting form the first reflecting coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, constructively combines to form reflected light R. Thus, in the reflecting mode, the GLV 10 produces the reflected light R.

A second side view of the GLV 10 of the prior art is illustrated in FIG. 2B, which shows the GLV in the diffracting mode. To transition from the reflecting mode to the diffracting mode, an electrostatic potential between the moveable elongated elements 12 and the substrate 14 moves the moveable elongated elements 12 to contact the substrate 14. To maintain the diffracting mode, the electrostatic potential holds the moveable elongated elements 12 against the substrate 14. In the diffracting mode, the upper surfaces of the first and second reflective coatings, 16 and 18, are separated by a quarter wavelength A/4 of the incident light 1. The incident light I reflecting from the second reflecting surfaces 18 travels a half wavelength further than the incident light I reflecting from the first reflective coatings 16. So the incident light I, reflecting from the first and second reflecting coatings, 16 and 18, destructively interferes to produce diffraction. The diffraction includes a plus one diffraction order D+, and a minus one diffraction order D,. Thus, in the diffracting mode, the GLV 10 produces the plus one and minus one diffraction orders, D+, and D,.

A first alternative GLV of the prior art is illustrated in FIGS. 3A and 3B. The first alternative GLV 10A includes first elongated elements 22 interdigitated with second elongated elements 23. The first elongated elements 22 include third reflective coatings 26; the second elongated elements 23 include fourth reflective coating 28. In the reflecting mode, illustrated in FIG. 3A, the third and fourth reflective coatings, 26 and 28, are maintained at the same height to produce the reflected light R. In the diffracting mode, illustrated in FIG. 3B, the first and second reflected coatings, 26 and 28, are separated by the second height difference of the quarter wavelength A/4 of the incident light I to produce the diffraction including the plus one and minus one diffraction orders, D+, and D_,.

A display system utilizing a GLV is taught in U. S. Patent No. 5,982,553 to Bloom et al. The display system includes red, green, and blue lasers, a dichroic filter group, illumination optics, the GLV, Schlieren optics, projection optics, a scanning mirror, and display electronics, which project a color image onto a display screen.

The red, green, and blue lasers, driven by the display electronics and coupled to the GLV (via the dichroic filter group and the illumination optics) sequentially illuminate the GLV with red, green, and blue illuminations. The GLV, driven by the display electronics, produces a linear array of pixels which changes with time in response to a signal from the display electronics, each pixel configured in the reflecting mode or the diffracting mode at a given instant in time. Thus, the GLV produces sequential linear arrays of red, green, and blue pixels with each of the red, green, and blue pixels in the reflecting mode or the diffracting mode.

The red, green, and blue pixels are then coupled to the Schlieren optics which blocks the reflecting mode and allows at least the plus one and minus one diffraction order, D and D-1, to pass the Schlieren optics. Thus, after passing the Schlieren optics, the linear arrays of the red, green, and blue pixels have light pixels corresponding to the pixels at the GLV in the diffracting mode and dark pixels corresponding to pixels at the GLV in the reflecting mode. The projection optics (via the scanning mirror) project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the display system produces a two dimensional color image on the display screen.

An alternative display system utilizing the GLV includes the red, green, and blue lasers; red, green, and blue illumination optics; first, second, and third GLVs ; the dichroic filter group; the projection optics; the scanning mirror; and the display electronics. The red, green, and blue lasers, via the red, green, and blue illumination optics, illuminate the first, second, and third GLVs, respectively. The first, second, and third GLVs produce the linear arrays of the red, green, and blue pixels, respectively, in response to signals from the display electronics. The dichroic filter group directs the linear arrays of the red, green, and blue pixels to the Schlieren optics, which allows at least the plus one and minus one diffraction order, D,, and D_,, to pass the Schlieren optics. The projection optics, via the scanning mirror, project the linear arrays of the red, green, and blue pixels onto the display screen while the scanning mirror, driven by the display electronics, scans the linear arrays of the red, green, and blue pixels across the display screen. Thus, the alternative display system produces the two dimensional color image on the display screen.

Examples of applications for a GLV based display system include a home entertainment system, a boardroom application, and a cinema application among others. In the home entertainment system or the boardroom application, the GLV based display system projects the two dimensional color image onto the display screen located on a wall. In the cinema application, the GLV based display system projects the two dimensional color image from a display booth onto a cinema screen.

A GLV based display system may also be utilized in printing applications. In such a case, the system would not include a scanning mirror, and the printing media, replacing a screen, would move to effectuate printing from a fixed line of light.

The aforementioned GLV based display systems put light in the l diffraction orders. Theoretically, when light is filtered into two diffraction orders, the maximum amount of light that can be transmitted or reflected is equal to only 81 % of the incident light beam. Another problem encountered in this type of system is the need for a more complex separating optics configuration or Schleieren optics. In such a system that filters light into two separate diffraction orders, a separating optical system must have two slits to receive the two orders. This configuration requires a complicated set of separating optics to properly separate the two orders.

Yet another disadvantage to implementing a GLV based system such as this is the requirement of the GLV producing a wide cone of light. In a system that produces light in the i1 diffraction orders, all of the optics between the GLV and the projection screen must have a low F number in order to collect a large amount of light. This means that the optics must have a high optical throughput, thus requiring a larger lens.

This larger lens captures more light, including additional background light, thus producing an image with a lower contrast, thus a less clear picture. Additionally, a larger lens means greater expense.

What is needed is a display system that implements a diffracted light modulator that puts light in a single diffraction order while providing a higher contrast. This system would allow a larger percentage of the incident light to be put in a diffraction order. A light modulator utilizing only one diffraction order would also allow for a less complex and expensive separating optics configuration. Additionally, utilizing such a light modulator would eliminate the need for all of the optics to have a low F number and high optical throughput, thereby reducing the cost of the entire system.

SUMMARY OF THE INVENTION The present invention is a display apparatus and method for providing angled illumination for a single order grating light valve projection system. The display apparatus and method includes a light modulator being optically coupled to illumination optics such that in operation the illumination optics illuminate the light modulator with an off-axis illumination and further such that in operation the light modulator directs light onto an optic axis for a bright pixel, thereby forming on-axis light. Further, the light modulator directs the light away from the optic axis for a dark pixel, thereby forming off-axis light.

The display apparatus and method for providing angled illumination for a single order grating light valve projection system also includes separating optics that are optically coupled to the light modulator such that in operation they separate the off-axis light from the on-axis light, where the on-axis light produces a two dimensional image that is in the preferred embodiment a real image. Alternatively, the two dimensional image is a virtual image.

Lastly, the apparatus and method includes projection and scanning optics that are optically coupled to the separating optics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates an isometric view of a grating light valve (GLV) of the prior art.

FIG. 2 illustrates a side view of the GLV of the prior art.

FIG. 3 illustrates a side view of an alternative GLV of the prior art.

FIG. 4 schematically illustrates a display apparatus of the present invention.

FIG. 5 illustrates a plan view of display optics of the present invention.

FIG. 6 illustrates an elevation view of the display optics of the present invention with the display optics unfolded along an optical axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A display system of the present invention is illustrated schematically in FIG. 4.

The display system 40 includes display optics 42 and display electronics 44. The display optics 42 comprise a laser 46, illumination optics 48, a blazed grating light valve (BGLV) 50, separating optics 52, projection and scanning optics 56, and a display screen 58. The display electronics 44 are coupled to the laser source 46, the BGLV 50, and the projection and scanning optics 56.

The details concerning the BGLV 50 are disclosed in a co-owned, co-pending U. S. patent application, Ser. No. 09/930,838, entitled BLAZED GRATING LIGHT VALVE, and co-owned, co-pending U. S. patent application, Ser. No. 09/930,820, entitled STRESS TUNED BLAZED GRATING LIGHT VALVE. The U. S. patent application Ser. No. 09/930,838, entitled BLAZED GRATING LIGHT VALVE, and U. S. patent application Ser. No. 09/930,820, entitled STRESS TUNED BLAZED GRATING LIGHT VALVE are also incorporated by reference.

The display electronics 44 power the laser 46. The laser 46 emits a laser illumination. The illumination optics 48 focus the laser illumination onto the BGLV 50. The BGLV 50 is located in a first image plane 60. The display electronics 44 control the BGLV 50. The BGLV 50 modulates the laser illumination forming reflected light or diffracted light for a linear array of pixels. The separating optics 52 separates the reflected light from the diffracted light allowing at least an active first diffraction order to pass the separating optics 52.

The display electronics 44 drive a scanning mirror of the projection and scanning optics 56. The projection and scanning optics 56 project the line image onto the display screen 58 and scan the line image across the display screen 58 to form a two dimensional image on the display screen 58. The display screen 58 is located in a third image plane 64.

The display optics 42 of the present invention are further illustrated in FIGS. 5 and 6. FIG. 5 illustrates a plan view of the display optics 42. FIG. 6 illustrates an elevation view of the display optics 42, with the display optics 42 unfolded along an optic axis 70. The laser 46 emits the laser illumination 72 on an off-axis 98. The illumination optics comprise a line generating lens or Powell lens 74, a collimation lens 76, and a cylindrical lens 78. The collimation lens 76 is translated so that upon leaving the illumination optics 48, the light beam is tilted away from the optical axis 98. This variable illumination angle is achieved by translating the collimation lens 76 by a different amount for each color illumination. The desired angle for each color is shown here for a 12.75 micron grating pitch: Wavelength (n-m) Diffraction/Illumination Angle Red 620nm 2.8° Green 532nm 2.4° Blue 457nm 2.05° It will be readily apparent to one skilled in the art that the Diffraction/Illumination Angle may differ according to the grating pitch.

The illumination optics 48 focus the laser illumination 72 onto the BGLV 50 in a focus line having a focus width. Note that FIG. 5 illustrates the laser illumination 72 illuminating the BGLV 50 with an angle of incidence of 45°. Ideally, the angle of incidence is a minimum angle of incidence which allows the laser illumination 72 to illuminate the BGLV 50 while allowing the reflected and diffracted light to reach the separating optics 52. It will be readily apparent to one skilled in the art that other optics arrangements can be used to illuminate the BGLV 50. It will also be readily apparent to one skilled in the art that depiction of lenses in the present invention is not limited to single component lenses and that any given lens can be replaced with a compound lens or a reflective optical element.

The BGLV 50 modulates the laser illumination 72 as the linear array of pixels along the focus line, forming the reflected light Do or the diffracted light, including the active first diffraction order D, for each pixel. Preferably, the BGLV 50 produces a linear array of 1,080 pixels. Alternatively, the BGLV 50 produces more or less than 1,080 pixels. Note that FIG. 6 illustrates the reflected light Do and the active first diffraction order D, for two pixels for illustration purposes. If a given pixel is modulated to reflect light, the reflected light Do will be present and the active first diffraction order D, will not be present. Alternatively, if the given pixel is modulated to diffract light, the active first diffraction order D, will be present and the reflected light Do will not be present. In some instances it is desirable to modulate the given pixel to produce the reflected light Do and the active first diffraction order D, in order to reduce a brightness of the given pixel in a resulting image, which provides a gray scale effect in the resulting image. It will be readily apparent to one skilled in the art that an alternate light modulator which places light off-axis in a first state and on-axis in a second can replace the BGLV 50 of the present invention.

Referring again to FIG. 5, the Schlieren optics 52 include a Schlieren stop 80 located between first and second relay lenses, 82 and 84. The Schlieren stop 80 stops the reflected light R and allows the active first diffraction order D, to pass the Schlieren stop 80. The Schlieren stop 80 is preferably located in a first transform plane 85. Alternatively, the Schlieren stop 80 is located near the first transform plane 85.

The projection and scanning optics 56 comprise a projection lens 86 and the scanning mirror 88. The projection lens 86, via the scanning mirror 88, projects the line image 90 onto the display screen 58. The projection lens 86 also reforms the wavefront having the spatial phase variation across the line image width 92 on the display screen 58. The scanning mirror 88 is preferably located at about a second transform plane 94.

The scanning mirror 88 moves with a first scan motion A and, thus, scans the line image 90 across the display screen 58 with a second scan motion B. Preferably, the first scan motion A is a sawtooth scan motion where a first part of a scan cycle illuminates the display screen 58 and a second part of the scan cycle returns the scanning mirror 88 back to a beginning of the scan cycle. By repeatedly scanning the line image 90 across the display screen 58, a two dimensional image is formed on the display screen 58. It will be readily apparent to one skilled in the art that other scan motions can be used to scan the line image 90 across the display screen 58. It will also be readily apparent to one skilled in the art that a transmissive scanning device such as an objective scanner having zero optical power can replace the scanning mirror 88.

As the line image 90 scans across the display screen 58, the BGLV 50 modulates the linear array of pixels thus producing the two dimensional image made up of a rectangular array of pixels. For a high definition television (HDTV) format, the BGLV 50 modulates 1,920 times as the line image 90 scans across the display screen 58. Thus, the BGLV 50 preferably produces a 1,920 by 1,080 rectangular array forming the two dimensional image for the HDTV format. For other picture formats, the BGLV 50 modulates more or less than the 1,920 times as the line image 90 scans across the display screen 58 depending upon which of the other picture formats is being displayed.

As the line image width 92 scans across the display screen 58, the wavefront having the spatial phase variation produces the multiple speckle patterns with time.

The multiple speckle patterns reduce the speckle that is detected by the eye or the intensity detector of the optical system.

The display optics 42 depicted in FIGS. 4,5, and 6 produce a monochrome image. Color display optics comprise the display optics 42, two additional lasers, two additional illumination optics, two additional BGLV's, and a dichroic filter group. In the color display optics, red, green, and blue lasers illuminate the three BGLV's producing red, green, and blue linear arrays of pixels. The dichroic filter group combines the reflected and diffracted light from the three BGLV's and directs the reflected and diffracted light to the separating optics 52. For the color display optics, the spatial phase variation across the line image width 92 preferably has an optimum amplitude for one of red, green, and blue laser illuminations (e. g., the green laser illumination), or a wavelength that is a specific average of participating wavelengths.

The red, green, and blue wavefronts produce the multiple speckle patterns over time as the line image 90 is scanned across the display screen 58 and, thus, reduce the speckle in the color display optics.

One advantage of the angled illumination is apparent in the projection optics.

A single beam having all three colors on-axis requires a smaller lens, thus allowing less stray into the system. This provides the image with higher contrast yielding an overall clearer picture. Another advantage is that, because all three colors go through the same path in the projection optics, the design is simpler permitting the use of"off- the-shelf'optics as opposed to specially designed optical pieces. Further, this particular technique of varying the angle of the illumination is flexible, variable and consistent with good manufacturing and alignment practices.

One modification to the preferred embodiment may include, but is not limited to, implementing a standard GLV rather than a blazed type. This modification can be implemented if throughput is not an issue, as in some printing applications. In which case, one of the diffraction orders would simply be ignored. Additionally, the technique in the preferred embodiment is also applicable to monochrome systems, since the single color would still be on-axis for the projection system.

It will be readily apparent to one skilled in the art that other various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined by the appended claims.