Abstract:
A diffractive grating modulator system, includes: a diffractive grating modulator active to produce a plurality of output orders of diffraction; an illumination source for directing light onto the diffractive grating modulator; an output system arranged to receive one of the orders of diffraction from the diffractive grating modulator; a detector arranged to receive a different one of the orders of diffraction from the diffractive grating and to produce a signal representing the output of the diffractive grating modulator; an electronic driving system responsive to a data stream for operating the diffractive grating modulator; and a feedback system connected to the detector and the electronic driving system and responsive to the signal for calibrating the diffractive grating modulator.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and system for calibrating a diffractive grating modulator. In particular it relates to systems that have the output system either in one of the first orders of diffraction or in the 0 th  order. The invention also relates to a method for calibrating the system either prior to the application of a data stream or during the application of a data stream. 
     BACKGROUND OF THE INVENTION 
     Diffractive grating modulators are well known in the patent literature. A diffractive spatial light modulator formed in an electro-optic material was disclosed in U.S. Pat. No. 4,281,904, issued Aug. 4, 1981 to Sprague et al., entitled “TIR Electro-Optic Modulator with Individually Addressed Electrodes.” Liquid crystal diffractive modulators have been disclosed in U.S. Pat. No. 4,729,640, issued Mar. 8, 1988 to H. Sakata, entitled “Liquid Crystal Light Modulation Device”; U.S. Pat. No. 4,822,146, issued Apr. 18, 1989 to Yamanobe et al., entitled “Optical Modulation Element”; and U.S. Pat. No. 4,850,681, issued Jul. 25, 1989 to Yamanobe et al., entitled “Optical Modulation Device” These modulators all operated with the light transmitting through the device. 
     Reflective diffraction grating modulators have also been disclosed in U.S. Pat. No. 4,011,009 issued Mar. 8, 1977 to Lama et al., entitled “Reflection Diffraction Grating Having a Controllable Blaze Angle,” U.S. Pat. No. 5,115,344 issued May 19, 1992 to J. Jaskie, entitled “Tunable Diffraction Grating,” and U.S. Pat. No. 5,222,071 issued Jun. 22, 1993 to Pezeshki et al., entitled “Dynamic Optical Grating Device.” More recently, Bloom et al described an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating-light valve (GLV); see U.S. Pat. No. 5,311,360 issued May 10, 1994 to Bloom et al., entitled “Method and Apparatus for Modulating a Light Beam.” This device was later described by Bloom et al with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. See U.S. Pat. No. 5,459,610 issued Oct. 17, 1995 to Bloom et al., entitled, “Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate.” Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783 issued Oct. 14, 1997 to Bloom et al., entitled, “Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate.” 
     Recently, Bloom et al. have disclosed another form of the grating light valve in U.S. Pat. No. 5,841,579 issued Nov. 24, 1998 to Bloom et al. entitled “Flat Diffraction Grating Light Valve.” A method for making this form of the grating light valve is disclosed in U.S. Pat. No. 5,661,592 issued Aug. 26, 1997 to Bornstein et al., entitled, “Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve.” 
     The aforementioned diffractive modulators have been used in various display and printing systems. See U.S. Pat. No. 4,389,659 issued Jun. 21, 1983 to R. Sprague, entitled “Electro-Optic Line Printer”; U.S. Pat. No. 5,237,435 issued Aug. 17, 1993 to Kurematsu et al., entitled “Multicolor Projector Employing Diffraction Grating Type Liquid Crystal Light Modulators”; and U.S. Pat. No. 5,764,280 issued Jun. 9, 1998 to Bloom et al., entitled “Display System Including an Image Generator and Movable Scanner for Same.” 
     Methods for calibration of digital printing and display systems have been disclosed. A system and method for calibrating a Grating Light Valve was published by R. W. Corrigan et al. in “Calibration of a Scanned Linear Grating Light Valve Projection System,” SID &#39;99 Digest, pp. 220-223. E. E. Thompson discloses a calibration method for a printing system based on a digital micromirror device (DMD); see U.S. Pat. No. 5,842,088 issued Nov. 24, 1998 to E. Thompson, entitled “Method of Calibrating a Spatial Light Modulator Printing System.” This method was utilized to detect and compensate for faulty or stuck pixels. A calibration system for a projection display that uses a correction factor approach was disclosed in U.S. Pat. No. 5,032,906 issued Jul. 16, 1991 to G. Um, entitled “Intensity Calibration Method for Scene Projector.” 
     Intensity stabilization methods utilizing diffractive grating modulators (specifically, acousto-optic modulators (AOM)), have been disclosed in U.S. Pat. No. 4,367,926 issued Jan. 11, 1983 to T. Hohki, entitled “Light Beam Intensity Stabilizing Method,” and U.S. Pat. No. 4,928,284 issued May 22, 1990 to D. Bums, entitled “Laser Power Control System.” Both of these methods utilized the same diffracted order for both sampling and output, and used the modulation depth of the AOM for intensity control. Because the same diffracted order is used, these methods reduce the power available to be applied to the print medium, thereby having the potential for degrading the performance of the printers. There is a need therefore for an improved calibration system and method that avoids this problem. 
     SUMMARY OF THE INVENTION 
     The need is met according to the present invention by providing a diffractive grating modulator system that includes: a diffractive grating modulator active to produce a plurality of output orders of diffraction; an illumination source for directing light onto the diffractive grating modulator; an output system arranged to receive one of the orders of diffraction from the diffractive grating modulator; a detector arranged to receive a different one of the orders of diffraction from the diffractive grating and to produce a signal representing the output of the diffractive grating modulator; an electronic driving system responsive to a data stream for operating the diffractive grating modulator; and a feedback system connected to the detector and the electronic driving system and responsive to the signal for calibrating the diffractive grating modulator. 
     It is advantageous that the unique properties of a diffractive grating modulator can be useful for calibration of systems employing these types of modulators. Specifically, the symmetry of diffracted orders generated by a binary diffraction grating makes it possible for one order to be used for output and the second for monitoring. Common systems employ a beam splitter in the path of the output beam to sample the output for calibration. This has disadvantages, including a reduction in output power, and a potential for introduction of optical aberrations, both of which are corrected in the present invention. 
     The invention can be applied to, for example, linear arrays of grating modulator elements used to generate images in printing and projection display applications. The system and method for calibrating can correct for spatial nonuniformities present in the illuminating light beam or in the diffractive grating modulator. Furthermore, temporal variations in the optical power provided by the illumination source can be corrected for using the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is described with reference to the embodiments shown in the drawings. 
     FIG. 1 is an illustration of diffraction from a binary reflective grating; 
     FIG. 2 is a perspective, partially cut-away view of an electro-mechanical grating device; 
     FIG. 3 is a top view of a linear electromechanical grating modulator array; 
     FIG. 4 is a cross-sectional view along plane  4 — 4  indicated in FIG. 3 of two pixels of the linear electromechanical grating modulator array used for pulse width modulation; 
     FIG. 5 is a cross-sectional view along plane  4 — 4  indicated in FIG. 3 of two pixels of the linear electromechanical grating modulator array used for amplitude modulation; 
     FIG. 6 is a diagram of a system that delivers modulated light to a non-zero diffracted order of a diffractive grating modulator, and includes a calibration system according to one embodiment of the present invention; 
     FIG. 7 is a diagram of a system that delivers modulated light to the zeroth order of a diffractive grating modulator, and includes a calibration system according to an alternative embodiment of the present invention; 
     FIG. 8 is a flow chart explaining a method for calibrating a light modulation system according to the present invention that uses a diffractive grating modulator before operation; and 
     FIG. 9 is a flow chart explaining an alternative embodiment of a method of calibrating a light modulation system that uses a diffractive grating modulator during operation; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Periodic corrugations on optical surfaces (i.e. diffraction gratings) are well known to perturb the directionality of incident light. Collimated light incident in air upon a grating is diffracted into a number of different orders, as described by the grating equation:                  sin                   θ   m       =       sin                   θ   0       +       m                 λ     Λ         ,           Eq.  (1)                                
     where λ is the wavelength of the light and m is an integer denoting the diffracted order. FIG. 1 illustrates a reflective grating  10  having an optical beam 11 incident on the grating  10  at an angle θ 0 . The grating surface is defined to have a period Λ, which defines the diffraction angles according to Equation 1. A diffracted beam 13 corresponding to diffraction order m exits the grating  10  at an angle θ m . 
     The diffraction grating  10  pictured in FIG. 1 is a binary grating where the grating profile is a square wave with an optical grating depth d g . The duty cycle is defined as the ratio of the width of the groove L 1  to the grating period Λ. A binary phase grating will have the maximum diffraction efficiency when the duty cycle is equal to 0.5 and R, the reflectivity, is equal to 1.0. 
     For uniform reflectivity and 0.5 duty cycle, Equation 2 gives the theoretical diffraction efficiency, within the accuracy of scalar diffraction theory.                  η   m     =     R                     cos   2          (       π   λ          (         q   m          d   g       -     m                   λ   /   2         )       )                sin   2          (     m                   π   /   2       )           (     m                   π   /   2       )     2           ,           Eq.  (2)                                
     where q m  is a geometrical factor,                q   m     =         cos                   θ   0       +     cos                   θ   m         =     1   +       1   -       (     m                   λ   /   Λ       )     2                     Eq   .                (   3   )                                 for normal incidence (θ 0 =0).  Eq.(3) 
     According to Equations 2 and 3, the optical powers diffracted into two separate orders from a region of the diffraction grating  10  are simple functions of the local intensity of illumination, the reflectivity, and the local diffraction grating characteristics. For example, light from two diffracted orders can be passed through separate optical systems that image the diffraction grating  10  (here assumed to be spatially uniform Λ and d g ). According to equations 2 and 3, the observed spatial distributions of light in the image planes of both imaging systems will have the same profile, differing only by a constant factor of the diffraction efficiencies. 
     When the diffraction grating  10  is illuminated from normal incidence, it diffracts light with equal efficiency into anti-symmetric orders. For example, the maximum efficiency in the first (m=1) order occurs when the optical grating depth d g  is equal to λ/4. Such a grating has equal diffraction efficiencies into the +1 and −1 orders of up to 40% for the gratings of interest (λ/Λ≦0.5), while the remaining light is diffracted into higher odd orders (i.e. ±3, ±5, etc.). If the light beams from two anti-symmetric orders (e.g. the +1 and −1 orders) are passed through the aforementioned imaging systems, the observed spatial profiles will be identical, even in the presence of spatial nonuniformities in the grating characteristics Λ and d g . 
     FIG. 2 is a perspective, partially cut-away view of an electro-mechanical grating device  20  that uses the principles of grating diffraction for the modulation of a light beam. The electromechanical grating device  20  comprises a first and a second set of deformable ribbon elements  22   a  and  22   b  that are formed atop a spacer layer  24  in which a channel  26  is formed. The deformable ribbon elements comprise a ribbon layer  28 , which has an intrinsic tensile stress, and a reflective, conductive layer  30 . The reflective, conductive layer  30  is patterned to form a first and a second conductive region  30   a  and  30   b.  The ribbon layer  28  is patterned according to the first and second conductive regions  30   a  and  30   b  to form the first and second set of deformable ribbon elements  22   a  and  22   b.  According to the patterning, every other ribbon element suspended over the channel  26  belongs to the same set. The first and second conductive region  30   a  and  30   b  perform the dual purposes of enhancing the diffraction efficiency by increasing the reflectivity, and providing a means of applying a voltage to either the first or second set of deformable ribbon elements  22   a  or  22   b.    
     The electromechanical grating device  20  comprises a base  21 . The base  21  comprises a substrate  32 , which is chosen from the materials of silicon, another semiconductor, glass, metal, or plastic. Atop the substrate  32 , a bottom conductive layer  34  is provided. The material for the bottom conductive layer  34  is chosen from the group consisting of aluminum, titanium, gold, silver, tungsten, silicon alloys, and indium tin oxide. The bottom conductive layer  34  provides a means of applying a voltage to the substrate. A protective layer  36  is provided between the bottom conductive layer  34  and the spacer layer  24 . 
     FIG. 3 is a top view of a linear electromechanical grating modulator array  40  that is produced by forming a linear series of the electro-mechanical grating devices. A view plane  4 — 4 , parallel to the length of the linear electromechanical grating modulator array  40 , provides a cross-sectional view of an electromechanical grating modulator as shown in FIGS. 4 and 5. 
     The conductive, reflective layer is patterned to form a first conducting region  30   a,  which provides a common potential to alternate ribbon elements along the entire length of the linear electromechanical grating modulator array  40 . The patterning also forms a series of conductive regions in order to provide voltages to alternate ribbons of each individual element (pixel) of the linear electromechanical grating modulator array. In FIG. 3, which displays six pixels, these are the second, third, fourth, fifth, sixth, and seventh conductive regions  30   b,    30   c,    30   d,    30   e,    30   f,  and  30   g.  According to the patterning, the ribbon layer is also patterned to form a first, second, third, fourth, fifth, sixth, and seventh set of deformable ribbon elements  22   a,    22   b,    22   c,    22   d,    22   e,    22   f,  and  22   g.    
     The first conductive region  30   a  is electrically connected to the bottom conductive layer  34  through a contact  38 . The contact is formed by etching at least one opening through the multilayer that comprises the ribbon layer  28 , the spacer layer  24 , and the protective layer  36 . The opening is filled with a thick conductive layer that is, for example, an aluminum alloy. The thick conductive layer is limited by photolithographic processing and etching methods to a small area enclosing the contact  38 . Since the contact  38  maintains the first conductive region  30   a  on the first set of deformable ribbons  22   a  at the same potential as the bottom conductive layer  34 , these deformable ribbon cannot be electrostatically actuated. 
     FIG. 4 illustrates the operation of the linear electromechanical grating modulator array  40 . FIG. 4 is a cross-section of the device along view plane  4 — 4  indicated in FIG. 3 of two pixels. The two pixels that are displayed are defined by the third and fourth conductive regions  30   c  and  30   d,  which are associated with the third and fourth sets of deformable ribbon elements  22   c  and  22   d.    
     The configuration illustrated in FIG. 4 has no voltage applied to the third conductive region  30   c  with respect to the bottom conductive layer  34 . Hence, there is no electrostatic force to deflect the members of the third set of deformable ribbon elements  22   c.  The surfaces of the first and third sets of deformable ribbon elements  22   a  and  22   c  that oppose the substrate  32  are coplanar, and define an undeflected ribbon surface  44   a.  Furthermore, due to the planar structure of the device, the top surfaces of the first and third sets of deformable ribbon elements  22   a  and  22   c  form a coplanar, mirror-like surface. An incident light beam  48  will be substantially reflected into a 0 th  order reflected light beam  50 . 
     A voltage source  46  applies a voltage to the fourth conductive region  30   d  with respect to the bottom conductive layer  34 . The potential difference produces an attractive electrostatic actuation force that results in the deflection of the centers of the members of the fourth set of deformable ribbon elements  22   d  toward the substrate  32 . The voltage applied by the voltage source  46  is sufficient to pull the members of the fourth set of deformable ribbon elements downward until a mechanical stop is encountered. The linear electro-mechanical grating modulator array  40  illustrated in FIG. 4 is provided with a series of standoffs  42 , which take the form of pedestals or lines atop the protective layer  36 , and below every deformable ribbon element not associated with the first set  22   a.  The standoffs define a stop surface  44   b.  The standoffs  42  serve to decrease the contact surface area when the deformable ribbon elements are completely actuated, as illustrated by the fourth set of deformable ribbon elements in FIG.  4 . The reduced surface area lowers the risk of device failure from stiction forces, which can cause the deformable ribbon elements to remain in contact with the substrate even when the voltage source  46  is removed. 
     Because the first and fourth sets of deformable ribbon elements  22   a  and  22   d  are interdigitated and are located at different heights in the channel  26 , a diffraction grating is formed in this region of the linear electromechanical grating modulator array  40 . An incident light beam  52  illuminates the pixel from normal incidence. The separation of surfaces  44   a  and  44   b  is an odd multiple of λ/4, which, from equations 2 and 3, yields the maximum diffraction efficiency into the first order, and no reflection. Hence, the incident light beam  52  is substantially diffracted with equal efficiency into a +1 order diffracted beam  54   a  and a−1 order diffracted beam  54   b,  with no reflection into the 0 th  order. 
     The linear electromechanical grating modulator array  40 , as illustrated in FIG. 4, can be used to encode information through pulse width modulation (PWM). Using this technique, each pixel in the linear array is dynamically switched between the two states discussed above, and one of the diffracted orders is collected and directed to an output system. One of the states is then a “bright” state, and the other a “dark” state. Data can be encoded with a pixel of the linear modulator array  40  by controlling the length of time the bright state is produced, or by controlling the total amount of time the pixel is in the bright state during a given integration period. 
     FIG. 5 is a cross-section of the linear electromechanical grating modulator array  40  along view plane  4 — 4  indicated in FIG. 3 illustrating a device using an amplitude modulation scheme. The two pixels that are displayed are defined by the third and fourth conductive regions  30   c  and  30   d,  which are associated with the third and fourth sets of deformable ribbon elements  22   c  and  22   d.    
     Similar to the configuration illustrated in FIG. 4, no voltage is applied to the third conductive region  30   c  with respect to the bottom conductive layer  34  in FIG.  5 . The surfaces of the first and third sets of deformable ribbon elements  22   a  and  22   c  that oppose the substrate  32  are coplanar, and define an undeflected ribbon surface  44   a.  An incident light beam  48  will be substantially reflected into a zeroth order reflected light beam  50   a.    
     A voltage source  46  applies a voltage to the fourth conductive region  30   d  with respect to the bottom conductive layer  34 . The potential difference produces an attractive electrostatic actuation force that results in the deflection of the centers of the members of the fourth set of deformable ribbon elements  22   d  toward the substrate  32 . The surfaces of the fourth set of deformable ribbon elements  22   d  that opposes the substrate  32  define a deflected ribbon surface  44   c.  The alternating undeflected and deflected ribbon elements form a diffraction grating with an optical grating depth d g  equal to the separation of surfaces  44   a  and  44   c.    
     An incident light beam  52  that illuminates the pixel defined by the fourth set of deformable ribbon elements  22   d  from normal incidence will be substantially diffracted into a +1 order diffracted beam  54   a,  a −1 order diffracted beam  54   b,  and a 0 th  order reflected beam  54   c.  A first order diffraction efficiency, η 1 , determines the diffraction into the +1 order beam  54   a  and the −1 order beam  54   b,  which will be equal. A 0 th  order diffraction efficiency η 0 , determines the diffraction into the 0 th  order reflected beam  54   c.  Both diffraction efficiencies can be predicted using equations 2 and 3, and are dependent on the optical grating depth d g . Hence, η 1  increases continuously from zero when the fourth set of deformable ribbon elements  22   d  are undeflected to a maximum value when the ribbon elements are deflected to form a diffraction grating with d g  equal to λ/4. Conversely, η 0  decreases continuously from a maximum value when the fourth set of deformable ribbon elements  22   d  are undeflected to zero when the ribbon elements are deflected to form a diffraction grating with d g  equal to θ/4. 
     The linear electro-mechanical grating modulator array  40 , as illustrated in FIG. 5, can be used to encode information through amplitude modulation (AM). Using this technique, each pixel in the linear electro-mechanical grating modulator array  40  forms a diffraction grating with a variable diffraction efficiency, and one of the diffracted orders is collected and directed to an output system. Data can be encoded with a pixel of the linear modulator array  40  by controlling the diffraction efficiency of the grating within that pixel. 
     FIG. 6 is a diagram of a light modulation system  60  that delivers modulated light to a non-zero diffracted order of a diffractive grating modulator  68 , and includes a calibration system according to the present invention. A light source  62 , providing light with the desired wavelength and brightness characteristics, illuminates a set of conditioning optics  64 . The light exiting the conditioning optics  64  produces an illumination beam  66  with the desired properties that illuminates the diffractive grating modulator  68 . The diffractive grating modulator  68  may be an electromechanical grating device or a linear electromechanical grating modulator array  40 , such as those described above, or any other type of diffractive grating modulator, such as devices formed from liquid crystals, acousto-optic materials, electro-optic materials, or semiconductor quantum wells. 
     The light exits the diffractive grating modulator  68  via its various diffracted orders. Those pixels for which the grating has been activated will diffract light predominantly into a +1 order diffracted beam  70   a  and a −1 order diffracted beam  70   b.  The −1 order diffracted beam  70   b  is collected by the output system  72 , which may be, for example, a projection lens system and screen for a display or an imaging lens system and photosensitive media for a printer. The +1 order diffracted beam  70   a  is collected by imaging optics  74 , which images the diffractive grating modulator  68  onto a segmented detector  76 . The segmented detector can take the form of multiple photodiodes, a segmented photodiode, or a CCD array. The magnification of the imaging system produces an image of each pixel of the diffractive grating modulator  68  that covers one or more segments of the segmented detector  76 . Therefore, the segmented detector  76  can be used to monitor the intensity of the light diffracted from each pixel of the diffractive grating modulator  68  into the +1 order diffracted beam  70   a.  Due to the symmetry of the diffraction outlined above, the −1 order diffracted beam  70   b  will receive the same diffracted intensity. 
     The activation of the grating elements in the diffractive grating modulator  68  is accomplished using an activation voltage generator  78 . The activation voltage generator  78  receives inputs from a data stream  80 , which contains the data to be encoded into the modulation of the light, and a control system  82 , which provides a series of correction factors. As a result of a correction factor, the length of a pulse that is sent to a pixel is altered for PWM encoding, or the activation voltage level that is sent to a pixel is altered for AM encoding. The control system  82  is placed in a feedback loop, taking its input from the segmented detector  76 , and providing the correction factors to the activation voltage generator  78 . These correction factors are produced based on the intensity data received from the segmented detector  76 , and are generated in order to provide spatially uniform diffracted intensity profile or constant output power, or a combination of both. 
     Although the system illustrated in FIG. 6 utilizes the +1 and −1 orders, it will be apparent to one skilled in the art that any two non-zero diffracted orders can be used. For normal incidence illumination, the system would be best utilized with two antisymmetric orders, +m and −m. 
     FIG. 7 is a diagram of a light modulation system  60  that delivers modulated light to the 0 th  diffracted order of a diffractive grating modulator, and includes a calibration system according to an alternative embodiment of the present invention. Such a system might be preferable to the system illustrated in FIG. 6 for many applications that require high efficiency due to the higher diffraction efficiency into the 0 th  order than into the +1 or −1 order. A light source  62 , providing light with the desired wavelength and brightness characteristics, illuminates a set of conditioning optics  64 . The light exiting the conditioning optics  64  produces an illumination beam  66  with the desired properties that illuminates the diffractive grating modulator  68 . 
     The light exits the diffractive grating modulator  68  via its various diffracted orders. The 0 th  order reflected beam  70   c  is collected by the output system  72 . The +1 order diffracted beam  70   a  is collected by imaging optics  74 , which images the diffractive grating modulator  66  onto a segmented detector  76 . The magnification of the imaging system produces an image of each pixel of the diffractive grating modulator that covers one or more segments of the segmented detector  76 . Therefore, the segmented detector can be used to monitor the intensity of the light diffracted from each pixel of the diffractive grating modulator  68  into the +1 order diffracted beam  70   a.  Due to the symmetry of the diffraction outlined above, the light delivered to the output system via the 0 th  order diffracted beam will be a scaled, inverse image of the pixels as measured by the segmented detector  76 . 
     The activation of the grating elements in the diffractive grating modulator  68  is accomplished using an activation voltage generator  78 . The activation voltage generator  78  receives inputs from a data stream  80 , which contains the data to be encoded into the modulation of the light, and a control system  82 , which provides a series of correction factors. The control system  82  is placed in a feedback loop, taking its input from the segmented detector  76 , and providing the correction factors to the activation voltage generator  78 . These correction factors are produced based on the intensity data received from the segmented detector  76 , and are generated in order to provide spatially uniform diffracted intensity profile or constant output power, or a combination of both. 
     A flow chart  90  for a calibration method according to the present invention is shown in FIG.  8 . The calibration method is carried out before the operation of a light modulation system that uses a diffractive grating modulator. The calibration method can be applied using the light modulation system  60  illustrated in either FIG. 6 or  7 . A first step  92  is to initialize the correction factors that are provided to the activation voltage generator  78 . As a second step  94 , the diffractive grating modulator  66  is activated in a predetermined pattern. The uniformity and power as measured by the segmented detector  76  are observed in the third step  96 . The fourth step  98  tests to see if the uniformity and power are within the required tolerance. If not, a feedback step  100  generates a new set of correction factors to be provided to the activation voltage generator  78 . The second through fourth steps  94 ,  96 , and  98 , and the feedback step  100  are repeated until the uniformity and intensity are within the specified tolerance. When this occurs, the light modulation system has been calibrated, and the final step  102  initiates the generation of modulated light for the output system  72 . 
     A flow chart  110  for a calibration method according to an alternative embodiment of the present invention for use during the operation of a light modulation system is shown in FIG.  9 . The calibration method can be applied using the light modulation system  60  illustrated in either FIG. 6 or  7 , while the system  60  is providing modulated light to the output system  72 . In the first step  112 , the pixels of the diffractive grating modulator  68  are activated according to the data stream  80  and the most recent set of correction factors. In the second step  114 , the diffracted intensity of each pixel is monitored by the segmented detector  76  and compared to the data applied to that pixel. From this information, an updated correction factor for each pixel is generated by the control system  82  and applied to the activation voltage generator  78 , which comprises the final step  116 . These steps  112 ,  114 , and  116  are repeated throughout the operation of the light modulation system  60  in order to maintain uniformity and power stability. 
     The above system and methods have been described in relation to an array of diffractive grating modulators and a segmented detector. It can be easily seen by one skilled in the art that the above system and methods can also be applied to the calibration of a single diffractive grating modulator using a single (i.e. non-segmented) detector. A system of this sort could be used to provide a stable power output or to calibrate an entire array one pixel at a time. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  10 
                 reflective grating 
               
               
                   
                  11 
                 incident optical beam 
               
               
                   
                  13 
                 diffracted beam 
               
               
                   
                  20 
                 electro-mechanical grating device 
               
               
                   
                  21 
                 base 
               
               
                   
                  22a 
                 first set of deformable ribbon elements 
               
               
                   
                  22b 
                 second set of deformable ribbon elements 
               
               
                   
                  22c 
                 third set of deformable ribbon elements 
               
               
                   
                  22d 
                 fourth set of deformable ribbon elements 
               
               
                   
                  22e 
                 fifth set of deformable ribbon elements 
               
               
                   
                  22f 
                 sixth set of deformable ribbon elements 
               
               
                   
                  22g 
                 seventh set of deformable ribbon elements 
               
               
                   
                  24 
                 spacer layer 
               
               
                   
                  26 
                 channel 
               
               
                   
                  28 
                 ribbon layer 
               
               
                   
                  30 
                 reflective, conductive layer 
               
               
                   
                  30a 
                 first conductive region 
               
               
                   
                  30b 
                 second conductive region 
               
               
                   
                  30c 
                 third conductive region 
               
               
                   
                  30d 
                 fourth conductive region 
               
               
                   
                  30e 
                 fifth conductive region 
               
               
                   
                  30f 
                 sixth conductive region 
               
               
                   
                  30g 
                 seventh conductive region 
               
               
                   
                  32 
                 substrate 
               
               
                   
                  34 
                 bottom conductive layer 
               
               
                   
                  36 
                 protective layer 
               
               
                   
                  38 
                 contact 
               
               
                   
                  40 
                 linear electro-mechanical grating modulator array 
               
               
                   
                  42 
                 standoffs 
               
               
                   
                  44a 
                 undeflected ribbon surface 
               
               
                   
                  44b 
                 stop surface 
               
               
                   
                  44c 
                 deflected ribbon surface 
               
               
                   
                  46 
                 voltage source 
               
               
                   
                  48 
                 incident light beam 
               
               
                   
                  50 
                 0 th  order reflected light beam 
               
               
                   
                  52 
                 incident light beam 
               
               
                   
                  54a 
                 +1 order diffracted beam 
               
               
                   
                  54b 
                 −1 order diffracted beam 
               
               
                   
                  54c 
                 0 th  order reflected beam 
               
               
                   
                  60 
                 light modulation system 
               
               
                   
                  62 
                 light source 
               
               
                   
                  64 
                 conditioning optics 
               
               
                   
                  66 
                 illumination beam 
               
               
                   
                  68 
                 diffractive grating modulator 
               
               
                   
                  70a 
                 +1 order diffracted beam 
               
               
                   
                  70b 
                 −1 order diffracted beam 
               
               
                   
                  70c 
                 0 th  order reflected beam 
               
               
                   
                  72 
                 output system 
               
               
                   
                  74 
                 imaging optics 
               
               
                   
                  76 
                 segmented detector 
               
               
                   
                  78 
                 activation voltage generator 
               
               
                   
                  80 
                 data stream 
               
               
                   
                  82 
                 control system 
               
               
                   
                  90 
                 method for calibrating before operation 
               
               
                   
                  92 
                 first step 
               
               
                   
                  94 
                 second step 
               
               
                   
                  96 
                 third step 
               
               
                   
                  98 
                 fourth step 
               
               
                   
                 100 
                 feedback step 
               
               
                   
                 102 
                 final step 
               
               
                   
                 110 
                 method for calibrating during operation 
               
               
                   
                 112 
                 first step 
               
               
                   
                 114 
                 second step 
               
               
                   
                 116 
                 final step 
               
               
                   
                  d g   
                 optical grating depth 
               
               
                   
                  η m   
                 m th  order diffraction efficiency 
               
               
                   
                  η 0   
                 zeroth order diffraction efficiency 
               
               
                   
                  η 1   
                 first order diffraction efficiency 
               
               
                   
                  λ 
                 wavelength of light 
               
               
                   
                  Λ 
                 grating period 
               
               
                   
                  L 1   
                 width of the groove 
               
               
                   
                  m 
                 diffracted order 
               
               
                   
                  θ 0   
                 angle of incidence 
               
               
                   
                  θ m   
                 angle of diffraction order m 
               
               
                   
                  q m   
                 geometrical factor 
               
               
                   
                  R 
                 reflectivity