Patent Publication Number: US-2007115440-A1

Title: Projection display with screen compensation

Description:
FIELD OF THE INVENTION  
      The present invention relates to projection displays, and especially to projection display control systems that compensate for imperfections in the displayed image.  
     BACKGROUND  
      In the field of projection displays, a designer may select a display screen or surface that has controlled optical properties. In particular, for a high quality displayed image, one may select a display surface free of marks or other optical inconsistencies that would be visible in the displayed image. The projector-to-screen geometry may also be selected to avoid geometric distortion. Moreover, the design and fabrication of display optics and other components may be controlled to avoid distortion introduced by the projection display.  
       FIG. 1  is a diagram illustrating in one dimension the operation of a display system showing the interaction of a video signal with a display surface. An input video signal  102  is provided. As illustrated, the vertical axis of input video signal  102  represents a one-dimensional line through a display image. The horizontal axis represents a pixel level or brightness. Thus, input video signal  102  is shown as consisting of interleaved pixels or lines that vary in brightness value. Vertical line  104  represents an assumed or actual display screen response taken along a corresponding line shown on the vertical axis. As may be seen, the display screen response  104  is assumed to have a uniform response—such that here is substantially no variation in the scattering or transmission of light along the line. A transmitted image  106  is shown along a corresponding line in the vertical axis. As may be seen, the input video image  102 , when convolved with a uniform screen response  104 , creates an output image  106  that is substantially identical with the input video image  102 . Thus the viewer  108  sees the video image substantially as it was intended to be seen.  
       FIG. 2  is another diagram illustrating the operation of a display system made when the display screen includes non-uniformities. A video input  102  is provided as in  FIG. 1 . This time, however, the screen response  202  is non-uniform. As may be seen, some regions scatter or transmit higher amounts of light toward the viewer  108  and other regions scatter or transmit lower amounts of light toward the viewer. When the video input  102  is convolved with the non-uniform screen response  202 , a non-uniform output image  204  results. As may be seen with the exemplary case, the variation in pixel values present in the input video image  102  is superimposed over the screen response  202  to output the non-uniform output image  204 . The non-uniform output image  204  is thus perceived by the viewer  108  as a video image that differs at least somewhat from the image that the video input  102  was intended to depict.  
      Another aspect of variations in image quality delivered to the viewer has to do with a non-ideal geometric relationship between the projector and the screen or between the projector, the screen, and the viewer. An example of such variations corresponds to what is commonly referred to as keystone distortion. In keystone distortion, a screen that is non-normal to the axis of projection will result in image growth in one area relative to another area. Typically, keystone distortion is corrected manually by adjusting a shift lens element to make the edges of the image parallel. In other instances, variations in screen flatness or distance can result in local compression or expansion of pixel placement or variations in image size, respectively.  
      Another aspect of variations in image quality may not-visible to the viewer but may result in higher cost, lower reliability, or reduced availability of a display system. Variations arise from design limitations that place a burden on optimizing projector design to reduce image distortion. In a related aspect, any “damage” or other variations in the relationship between or behavior of projector components can cause a degradation in performance that may not be compensated for.  
     Overview  
      One aspect according to the invention relates to methods and apparatuses for compensating for imperfections in display screen surfaces.  
      According to one embodiment, the scattering or projection properties of a selected display screen are measured. A projection display modifies the value of projected pixels in a manner corresponding to the optical properties of the display screen at respective pixel locations. For example, regions that tend to absorb a given wavelength also tend to scatter less of that wavelength to the eye of the viewer, so pixels that correspond to such regions may be modified to provide a higher output of the wavelength to overcome the reduced scattering. Additionally or alternatively, regions that have a higher than average amount of scattering of a given wavelength may receive projected pixels having reduced power in that wavelength. Thus, variations in the way the pixels are scattered or transmitted from the display screen are compensated for and the perceived image quality may be improved.  
      According to some embodiments, a substantially inverse image of the display screen may be combined with received video data to provide modified video data that is emitted to the display screen. According to other embodiments, received video data may be modified by multiplying input pixel values by the inverse of corresponding screen responses to derive compensated pixel values.  
      According to some embodiments, the light scattering or transmitting properties of a display screen are measured. The measured properties are used to provide a screen compensation bitmap and the screen compensation bitmap is projected onto the screen along with video program material. According to other embodiments, the measured properties are used to provide a screen compensation convolution table that is convolved with input video program material data to derive compensated video program material data.  
      According to one embodiment the properties of the display screen are measured during a dedicated calibration process.  
      According to another embodiment the properties of the display screen are measured substantially continuously.  
      According to one embodiment, the properties of a rear projection screen are compensated for.  
      According to another embodiment, the properties of a front projection screen are compensated for. According to some embodiments, the front projection screen may be a purpose-built projection screen. According to other embodiments, the front projection screen may be a wall, a door, window coverings, a bookshelf, or other arbitrary surface that would otherwise be unsuitable for high quality video projection.  
      According to one embodiment the projection display comprises a scanned beam display or other display that sequentially forms pixels.  
      According to another embodiment the projection display comprises a focal plane display such as a liquid crystal display (LCD), micromirror array display, liquid crystal on silicon (LCOS) display, or other display that substantially simultaneously forms pixels.  
      According to one embodiment, a focal plane detector such as a CCD or CMOS detector is used as a screen property detector to detect screen properties.  
      According to another embodiment, a non-imaging detector such as a photodiode including a positive-intrinsic-negative (PIN) photodiode, phototransistor, photomultiplier tube (PMT) or other non-imaging detector is used as a screen property detector to detect screen properties. According to some embodiments, a field of view of a non-imaging detector may be scanned across the display field of view to determine positional information.  
      According to one embodiment, the projection display comprises a screen property detector. According to another embodiment the screen property detector is provided as a piece of calibration equipment.  
      According to one embodiment screen calibration is performed automatically. According to another embodiment screen calibration is performed semi-automatically or manually.  
      According to some embodiments, compensation data may provide for projecting relatively high quality images onto surfaces of relatively low quality, such as an ordinary wall. This may be especially useful in conjunction with portable computer projection displays, such as “beamers”.  
      According to another aspect, a displayed image monitoring system may sense the relative locations of projected pixels. The relative locations of the projected pixels may then be used to adjust the displayed image to project a more optimum distribution of pixels. According to one embodiment, optimization of the projected location of pixels may be performed during a calibration period. According to another embodiment, optimization of the projected location of pixels may be performed substantially continuously during a display session.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram illustrating the operation of a display system made according to the prior art.  
       FIG. 2  is another diagram illustrating the operation of a display system made according to the prior art when the display screen includes non-uniformities.  
       FIG. 3  is a diagram illustrating a uniform video signal interacting with a non-uniform screen response according to an embodiment.  
       FIG. 4  is a flow chart showing a method for generating a screen compensation pattern according to an embodiment.  
       FIG. 5  is a simplified diagram illustrating a sequential process for projecting pixels and measuring a screen response according to an embodiment.  
       FIG. 6  is a flow chart representing a method for sequentially measuring a screen response according to an embodiment.  
       FIG. 7  is a diagram illustrating a calibrated system illuminating a screen with non-uniform response to produce a flat field response according to an embodiment.  
       FIG. 8  is a block diagram of a scanned-beam type projection display with a capability to compensate for variations in screen properties according to an embodiment.  
       FIG. 9  is a block diagram of an apparatus and method for generating a compensation pattern for a display screen according to an embodiment.  
       FIG. 10  is a diagram illustrating an initial state prior to determining a display surface response.  
       FIG. 11  is a diagram illustrating a state where a display surface response has been fully converged according to an embodiment.  
       FIG. 12  is a diagram illustrating a display surface response that has been converged to a partially compensating state according to an embodiment.  
       FIG. 13  is a flow chart showing a method for converging on a screen compensation pixel value according to an embodiment.  
       FIG. 14  is a diagram illustrating the combination of an input video signal and a screen response to form a compensated output video signal according to an embodiment.  
       FIG. 15  is a diagram illustrating the interaction of a compensated video pattern with a screen response to produce a perceived projected image according to an embodiment.  
       FIG. 16  is a flow chart illustrating a method for determining a compensated video image according to an embodiment.  
       FIG. 17  is a diagram illustrating dynamic updating of a screen compensation map according to an embodiment.  
       FIG. 18  is a block diagram illustrating the relationship of major components of a screen-compensating display system according to an embodiment.  
       FIG. 19  is a block diagram illustrating the relationship of major components of a screen-compensating display controller according to an embodiment.  
       FIG. 20  is a perspective drawing of a detector subsystem according to an embodiment.  
       FIG. 21  is a perspective drawing of a front projection display with screen compensation according to an embodiment.  
       FIG. 22  is a perspective drawing of an exemplary portable projection system with screen compensation according to an embodiment.  
    
    
     DETAILED DESCRIPTION  
       FIG. 3  is a diagram illustrating a uniform video signal  102  interacting with a non-uniform screen response  202  to produce a non-uniform output video s signal  204  having features corresponding to the features of the non-uniform screen response, according to an embodiment. A sensor  302  is aligned to receive at least a portion of a signal corresponding to the output video signal  204 . According to one embodiment, the sensor  302  may be a focal plane detector such as a CCD array, CMOS array, or other technology such as a scanned photodiode, for example. The sensor  302  detects variations in the response signal  204  produced by the interaction of the input video signal  102  and the screen response  202 . While the screen response  202  may not be known directly, it may be inferred by the measured output video signal  204 . It may also be noted that in some applications the output video signal may be affected by other aspects of the projection system including a video signal transmission path, optics, electronics, and other aspects not directly attributable to the screen response  202 . As will be appreciated, embodiments allow the measurements made by the sensor  302  to compensate not only for non-uniform screen response, but also for other system non-uniformities. Furthermore, as will be appreciated; the system may detect and compensate for variations arising from geometric relationships such as a non-ideal geometric relationship between a projection system and screen; variations in screen flatness; a geometric relationship between a projection system, screen and viewer; etc. Thus, strictly speaking, the output video signal  204  includes not only variations arising from the screen response  202 , but also variations arising from other system components.  
      Although there may be differences between the response signal  204  and the actual screen response  202 , hereinafter they may be referred to synonymously for purposes of simplification and ease of understanding.  
       FIG. 4  is a flow chart comprising a method for generating a screen compensation pattern, according to an embodiment. In step  402 , a controller enters a calibration routine. The calibration routine may, for example, be executed at start-up or wake-up of the display, be executed at shut down or between receipt of program signals, be executed upon selection by the viewer, be executed at installation of a projection display system, or alternatively may be executed substantially continuously during operation of the display system. Accordingly, step  402  may be initiated manually or automatically, depending upon the particular application.  
      Proceeding to step  404 , a known pattern is projected onto a display surface. The known pattern may be, for example, uniform or varied, static or dynamic, a special calibration pattern or normal programming. These and other approaches may be used in accordance with embodiments, according to designer or user preferences.  
      Proceeding to step  406 , a sensor assembly such as a focal plane optical sensor is used to measure the image scattered by the display surface or screen. One way for doing this is to simply take one or a series of digital pictures of the displayed pattern. Alternatively, a pattern may be sequentially provided. The use of a sequentially presented calibration pattern will be described more fully below.  
      The measured response of the screen may, for instance, include uniform or local variations in the optical scattering efficiency in one or more projected wavelengths. Alternatively or additionally; the measured response of the screen may include variations in pixel placement; such as when a projected image includes keystone, barrel, pincushion or other “uniform” optical distortions; or when a projected image includes local distortions arising from non-idealities or damage to the optical or other subsystems of the projection system; or when a projected image includes local distortions arising from screen flatness errors; for example.  
      Proceeding to step  408 , the image, an inverted version of the image, a pixel placement distortion model or map, or other data that is characteristic of the measured image from the screen is stored. Some focal plane imagers store a captured image locally so it will be appreciated that step  408  may or may not be a discrete step, according to the particular embodiment.  
      In step  410 , the measured response of the screen is compared to the input data pattern. For example, if one area of a projection surface includes a region that is painted red, then the measured value of pixels in the region may be higher in the red channel and lower in green and blue channels, the latter being absorbed by the paint rather than scattered. One way to compensate for such a painted region may, for example, be to somewhat reduce the level of pixel red values and somewhat increase the level of pixel green and blue values in the region. The amount of reduction or increase in each channel will depend upon the comparison of the measured pattern to the known input pattern.  
      Similarly, geometric variations in pixel placement, or required offsets in pixel placement relative to the input pattern may be stored as a compensation setting.  
      Proceeding to step  412 , the calculated increase and/or decrease of pixel levels in each channel are stored as an updated compensation setting.  
      According to some embodiments, the screen compensation settings are stored as a bitmap corresponding to an inverted image of the projection screen. This allows a fairly simple addition or multiplication of input video pixel values with the corresponding screen compensation pixel values. Thus, areas that are relatively dark may receive higher value (brighter) projected pixels and/or areas that are relatively light may receive lower value (dimmer) projected pixels.  
      According to other embodiments, screen compensation settings may be stored as values in a screen compensation matrix. During projection, the input bitmap may be convolved with the screen compensation matrix to produce an output bitmap. According to the value of the coefficients in the screen compensation matrix, pixel brightness and pixel placement may be modified according to the nature of the measured image distortion. Additionally or alternatively, at least a portion of the screen compensation settings may be stored in other forms. For example, correction of keystone, pincushion, or barrel distortion may be stored as a projection lens shift value, algorithmic coefficients, etc., while pixel brightness compensation and/or local pixel placement compensation is stored as coefficients in the screen compensation matrix.  
      Furthermore, while the flowchart of  FIG. 4  is shown as a discrete calibration routine, calibration may be developed iteratively, continuously, etc. For example, where compensation for pixel placement results in displacement of pixels to locations outside the former, distorted display field of view, a second iteration may be used to determine pixel brightness values within such previously unmeasured regions. Continuous or iterative calibration can be made using rules that vary according to measured displacement from nominal. Such rules can result in fast convergence from large displacements (such as in location or brightness) and then shift to low control gain convergence at small displacements to improve stability of the convergence routine.  
      After storing the updated screen compensation values, the program proceeds to step  414 , wherein the calibration routine is exited. Especially for systems that perform continuous or semi-continuous screen compensation updates, steps  402  and  414  may be omitted and the program simply loop back to step  404  and the process repeated.  
       FIG. 5  is a simplified diagram illustrating a process for sequentially projecting pixels and measuring screen response or simultaneously projecting pixels and sequentially measuring screen response, according to embodiments. Sequential video projection and screen response values  502  and  504 , respectively, are shown as intensities I on a power axis  506  vs. time shown on a time axis  508 . Tic marks on the time axis represent periods during which a given pixel is displayed with an output power level  502 . At the end of a pixel period, a next pixel, which may for example be a neighboring pixel, is illuminated. In this way, the screen is sequentially scanned, either with a pixel light intensity shown by curve  502  or by the detected light intensity value  504 . Thus, it can be seen that in the example of  FIG. 5  the pixels each receive uniform illumination as indicated by the flat illumination power curve  502 . Alternatively, values may be varied and the varied values used for comparison to the measured values.  
      As may be seen from the measured screen response curve  504 , the screen includes non-uniformities that cause a variable light scattering.  
      One advantage of sequential measurement of screen response, as shown in  FIG. 5 , is that a non-imaging detector may be more easily used.  
       FIG. 6  is a flow chart representing a method for sequentially measuring a screen response, according to an embodiment. In step  402 , the program enters a calibration process. Proceeding to step  602 , a pixel count is initialized to a starting pixel. The starting pixel may be selected as a particular pixel, for example such as the topmost, leftmost pixel ( 1 , 1 ), it may be selected as a result of a previously measured anomaly in screen response, it may be randomized to produce a varying calibration pattern, or other conventions may be used. For the present example, it is assumed that the pixel count is initialized to i=1, j=1, where i is the column and j is the row.  
      The program then proceeds to step  604  where the currently selected pixel is illuminated on the projection screen. Such illumination may be at constant level as indicated in  FIG. 5 , or may alternatively be varied from pixel to pixel. Similarly, the pixel may be illuminated with one color, such as red, green, or blue for example; or alternatively may be simultaneously illuminated with plural colors, for example with an RGB signal nominally intended to produce a white-balanced spot. The choice of how to illuminate a pixel may depend upon the particular application and upon the hardware implementation. For example, for applications where a non wavelength-differentiating detector such as an unfiltered PIN photodiode or unfiltered focal plane detector array is used, it may be advantageous to sequentially project individual colors to unambiguously determine the response of the screen to individual colors. For applications where RGB filtered detectors are used, it may be advantageous to project red, green, and blue channels simultaneously to reduce calibration time.  
      Proceeding to step  606  the amount of light scattered off the screen (or in the case of a rear projection screen, transmitted by the screen) at the i,j pixel is detected and measured. As with the flow chart of  FIG. 6 , a number of technologies may be used to detect the screen response. According to one exemplary embodiment, one filtered PIN photodiode is used for each color channel, for example a red filtered PIN photodiode, a green filtered PIN photodiode, and a blue filtered PIN photodiode. The responses of the photodiodes may be normalized for sensitivity in hardware, for example by selecting amplifier gain, or alternatively compensation for sensitivity may be made in software.  
      The particular methods for sequentially detecting pixel values in the combination of steps  604  and  606  may vary according to hardware implementation and/or other design consideration. For example, as indicated above an illuminated pixel may be scanned to select a location for measuring the screen response. A non-imaging detector having a field of view corresponding to possible pixel positions may then be used to measure screen response. To select the next pixel, the illuminated pixel may then be incremented with the non-imaging detector continuing to monitor its field of view. Pixel scanning may comprise modifying a light propagation path, for example as in a scanned beam projection display, or alternatively may comprise selecting a new pixel from a matrix of pixels, for example as in an LCOS, LCD, DMD, or other parallel illumination display technology. Alternatively, a detector field-of-view may be set to a small area, for example corresponding to a single pixel, and the detector scanned across a larger display field of view. In the case of scanning the detector, it may be advantageous to illuminate a number of pixels simultaneously. Alternatively, combinations of pixel scanning and detector scanning may be used.  
      As an alternative to measuring the screen response for single pixels, a plurality of pixels may be measured simultaneously using the method of  FIGS. 5 and 6 . For example, using a non-imaging detector with a field of view substantially equal to the entire display field, pairs, triplets, etc. of pixels may be illuminated. Sequences of pixels illuminated may be selected such that the confounding of individual pixel responses may be canceled over time by statistically evaluating the measured responses. A similar approach can be used to reduce or eliminate confounding arising from measuring plural pixel responses measured by a scanned detector or simultaneously scanned pixels and a scanned detector. According to another embodiment, plural detectors may be used, the individual detectors having fields of view less than the entire display field. In this way, four detectors, each having a detectable field of view approximately equal to one-quarter of the display field can be used while four pixels, one in each field, is projected and its response measured. Pixels near the intersections between detectors may be illuminated singly to remove the confounding of being measured by plural detectors simultaneously.  
      According to another embodiment, detectors may be selected to have small fields of view corresponding to desired angles to the four corners of a display field. Pixels may be illuminated and/or the projection path varied until an appropriate response is received by the four detectors. By offsetting the incidence angle of the pixel source from the detector, a trapezoid may be deduced that is indicative of a correction for keystone compensation. By solving the trigonometry for the baseline between the pixel source and the detector, real keystone correction may be deduced from the apparent angles to the corners of the display.  
      A similar approach to offsetting the incidence angle from the detection angle may be used with an imaging detector such as a focal plane detector to determine geometric variations in screen response, for example such as keystone correction, pincushion/barrel distortion correction, etc.  
      Returning to  FIG. 6 , in step  608  the screen response is stored in memory. As in other embodiments, a number of conventions may be used to indicate screen response. According to one embodiment, an average screen response for all pixels and all color channels is saved. Individual pixel variations are then saved as a code value returned by a sensor analog-to-digital converter above or below the average response. According to another embodiment, the negative value of the individual response is saved, the latter approach allowing simple addition of pixel code values or scaled code values. As used herein, addition or subtraction of code values will be simplified as equivalent as it is understood that addition of a negative value is the same as subtraction of the same positive magnitude. According to another embodiment, the response of an individual pixel is saved as a multiple or divisor compared to the average pixel response. In one approach, the response is stored as a coefficient in a screen compensation matrix or a portion of the response may be stored as a coefficient in a screen compensation matrix, as described above in conjunction with  FIG. 4 .  
      According to another embodiment, screen response is saved as offsets from input pixel values, such as in a LUT. The offsets are allowed to vary as a function of input pixel value. Such an approach allows the processor to accommodate video rate input data by using relatively simple addition/subtraction functions, while the data in the LUT corresponds to a multiplicative relationship between the screen response and the value of the input pixel data. According to still another embodiment, the LUT size may be reduced by saving offsets according to a range of input pixel values, thus providing a trade-off between memory size and the precision of screen compensation, while still allowing for a stepwise multiplicative relationship between input pixel value and screen compensation offset.  
      Proceeding to step  610 , a check is made to see if the last pixel has been measured. This may be the actual last pixel in the entire field of view, or alternatively may be another pixel in a range of pixels chosen for calibration. If the last pixel has been measured, the program proceeds to step  414  where the calibration routine is exited. As an alternative, the pixel value may be incremented again to the first pixel value and the process of steps  604 - 608  repeated. Such an approach allows for continuous calibration. If the last pixel has not been measured, the program proceeds to step  612  where the pixel value is incremented to the next pixel value and the process of steps  604 - 608  are repeated.  
       FIG. 7  is a diagram illustrating a calibrated system illuminating a screen non-uniformly, with the screen having a corresponding non-uniform response to produce a flat field response, according to an embodiment. In  FIG. 7 , a system or alignment of a projection display was determined to produce a screen response  202 . From the previously determined screen response, a screen compensation pattern  702  is determined for an illumination level. When a compensated illumination pattern  702  is shown on or through the projection screen having the screen response  202 , the result is a flat field response  704 . As may be seen from inspection of  FIG. 7 , areas of the screen that scatter or transmit a nominal amount of illumination  202   a ,  202   b , and  202   c  receive a corresponding nominal amount of illumination energy  702   a ,  702   b , and  702   c , respectively. Areas of the screen that scatter or transmit a greater amount of illumination  202   d  and  202   e  receive a corresponding reduced amount of illumination energy  702   d , and  702   e , respectively, the amount of which is scaled according to the screen response. Areas such as  202   f  that scatter or transmit higher than average amounts of illumination energy toward the viewer receive corresponding reduced amounts of illumination energy  702   f . The amount of increase or reduction in illumination energy is made such that the quantity of illumination is balanced by the quantity of scatter or transmission to provide a uniform response  704  that may be visible to the viewer.  
      Of course, the relative amount of illumination increase or decrease called for to fully compensate for the non-uniform screen response may fall outside the dynamic range of the projection display. In such cases, a variety of approaches may be used to best approximate ideal compensation. For example, according to one embodiment when a “dark” feature is found to lie in the left side of the display screen and a “light” feature is found to lie on the right side of the display screen, pixel compensation may be selected to vary the viewed image brightness smoothly across the display screen so as to reduce the visual conspicuousness of the features. According to another embodiment, the system may be used to attenuate the visibility of undesirable features on the display screen, even if the edges of the feature are still faintly visible. According to another embodiment, the overall brightness of the display may be decreased or increased to substantially keep the required pixel brightness within the dynamic range of the display engine. According to another embodiment, the dynamic range of the displayed image may be reduced. User preferences may be accommodated to select between or balance between compensation logic. For example, a user selected “brightness” that is set higher than available dynamic range would indicate may be used to select relatively less screen compensation. As the user gradually reduces the brightness, more and more screen compensation may be invoked as the dynamic range of the projection engine allows.  
       FIG. 8  is a block diagram of an exemplary projection display apparatus  802  with a capability for displaying an image on a surface  811  having imperfections, according to an embodiment. An input video signal, received through interface  820  drives a controller  818 . The controller  818 , in turn, sequentially drives an illuminator  804  to a brightness corresponding to pixel values in the input video signal while the controller  818  simultaneously drives a scanner  808  to sequentially scan the emitted light. The illuminator  804  creates a first beam of light  806 . The illuminator  804  may, for example, comprise red, green, and blue modulated lasers combined using a combiner optic and beam shaped with a beam shaping optical element. A scanner  808  deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light  810 . Taken together, the illuminator  804  and scanner  808  comprise a scanned beam display engine  809 . Instantaneous positions of scanned beam of light  810  may be designated as  810   a ,  810   b , etc. The scanned beam of light  810  sequentially illuminates spots  812  in the FOV, the FOV comprising a display surface or projection screen  811 . Spots  812   a  and  812   b  on the projection screen are illuminated by the scanned beam  810  at positions  810   a  and  810   b , respectively. To display an image, substantially all the spots on the projection screen are sequentially illuminated, nominally with an amount of power proportional to the brightness of an input video image pixel corresponding to each spot.  
      While the beam  810  illuminates the spots, a portion of the illuminating light beam is reflected or scattered as scattered energy  814  according to the properties of the object or material at the locations of the spots. A portion of the scattered light energy  814  travels to one or more detectors  816  that receive the light and produce electrical signals corresponding to the amount of light energy received. The detectors  816  transmit a signal proportional to the amount of received light energy to the controller  818 .  
      According to alternative embodiments, the one or more detectors  816  and/or the controller  818  are selected to produce and/or process signals from a representative sampling of spots. Screen compensation values for intervening spots may be determined by interpolation between sampled spots. Neighboring sampled values having large differences may be indicative of an edge lying therebetween. The location of such edges may be determined by selecting pairs or larger groups of neighboring spots between which there are relatively large differences, and sampling other spots in between to find the location of edges representing features of interest. The locations of edges on the display screen may similarly be tracked using image processing techniques.  
      The light source  804  may include multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In a preferred embodiment, illuminator  804  comprises a red laser diode having a wavelength of approximately 635 to 670 nanometers (nm). In another preferred embodiment, illuminator  804  comprises three lasers; a red diode laser, a green diode-pumped solid state (DPSS) laser, and a blue DPSS laser at approximately 635 nm, 532 nm, and 473 nm, respectively. While some lasers may be directly modulated, other lasers, such as DPSS lasers for example, may require external modulation such as an acousto-optic modulator (AOM) for instance. In the case where an external modulator is used, it is considered part of light source  804 . Light source  804  may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source  804  may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Additionally, while the wavelengths described in the previous embodiments have been in the optically visible range, other wavelengths may be within the scope of the invention.  
      Light beam  806 , while illustrated as a single beam, may comprise a plurality of beams converging on a single scanner  808  or onto separate scanners  808 .  
      Scanner  808  may be formed using many known technologies such as, for instance, a rotating mirrored polygon, a mirror on a voice-coil as is used in miniature bar code scanners such as used in the Symbol Technologies SE 900 scan engine, a mirror affixed to a high speed motor or a mirror on a bimorph beam as described in U.S. Pat. No. 4,387,297 entitled PORTABLE LASER SCANNING SYSTEM AND SCANNING METHODS, an in-line or “axial” gyrating, or “axial” scan element such as is described by U.S. Pat. No. 6,390,370 entitled LIGHT BEAM SCANNING PEN, SCAN MODULE FOR THE DEVICE AND METHOD OF UTILIZATION, a non-powered scanning assembly such as is described in U.S. patent application Ser. No. 10/007,784, SCANNER AND METHOD FOR SWEEPING A BEAM ACROSS A TARGET, commonly assigned herewith, a MEMS scanner, or other type. All of the patents and applications referenced in this paragraph are hereby incorporated by reference  
      A MEMS scanner may be of a type described in U.S. Pat. No. 6,140,979, entitled SCANNED DISPLAY WITH PINCH, TIMING, AND DISTORTION CORRECTION; U.S. Pat. No. 6,245,590, entitled FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING; U.S. Pat. No. 6,285,489, entitled FREQUENCY TUNABLE RESONANT SCANNER WITH AUXILIARY ARMS; U.S. Pat. No. 6,331,909, entitled FREQUENCY TUNABLE RESONANT SCANNER; U.S. Pat. No. 6,362,912, entitled SCANNED IMAGING APPARATUS WITH SWITCHED FEEDS; U.S. Pat. No. 6,384,406, entitled ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE; U.S. Pat. No. 6,433,907, entitled SCANNED DISPLAY WITH PLURALITY OF SCANNING ASSEMBLIES; U.S. Pat. No. 6,512,622, entitled ACTIVE TUNING OF A TORSIONAL RESONANT STRUCTURE; U.S. Pat. No. 6,515,278, entitled FREQUENCY TUNABLE RESONANT SCANNER AND METHOD OF MAKING; U.S. Pat. No. 6,515,781, entitled SCANNED IMAGING APPARATUS WITH SWITCHED FEEDS; U.S. Pat. No. 6,525,310, entitled FREQUENCY TUNABLE RESONANT SCANNER; and/or U.S. patent application Ser. No. 10/984,327, entitled MEMS DEVICE HAVING SIMPLIFIED DRIVE; for example; all hereby incorporated by reference.  
      In the case of a 1D scanner, the scanner is driven to scan output beam  810  along a single axis and a second scanner is driven to scan the output beam  810  in a second axis. In such a system, both scanners are referred to as scanner  808 . In the case of a 2D scanner, scanner  808  is driven to scan output beam  810  along a plurality of axes so as to sequentially illuminate pixels  812  on the projection screen  811 .  
      For compact and/or portable display systems  802 , a MEMS scanner is often preferred, owning to the high frequency, durability, repeatability, and/or energy efficiency of such devices. A bulk micro-machined or surface micro-machined silicone MEMS scanner may be prefered for some applications depending upon the particular performance, environment or configuration. Other embodiments may be preferred for other applications.  
      A 2D MEMS scanner  808  scans one or more light beams at high speed in a pattern that covers an entire projection screen or a selected region of a projection screen within a frame period. A typical frame rate may be 60 Hz, for example. Often, it is advantageous to run one or both scan axes resonantly. In one embodiment, one axis is run resonantly at about 19 KHz while the other axis is run non-resonantly in a sawtooth pattern to create a progressive scan pattern. A progressively scanned bi-directional approach with a single beam, scanning horizontally at scan frequency of approximately 19 KHz and scanning vertically in sawtooth pattern at 60 Hz can approximate an SVGA resolution. In one such system, the horizontal scan motion is driven electrostatically and vertical scan motion is driven magnetically. Alternatively, both the horizontal scan may be driven magnetically or capacatively. Electrostatic driving may include electrostatic plates, comb drives or similar approaches. In various embodiments, both axes may be driven sinusoidally or resonantly.  
      Several types of detectors  816  may be appropriate, depending upon the application or configuration. For example, in one embodiment, the detector may include a PIN Photodiode connected to an amplifier and digitizer. In this configuration, beam position information is retrieved from the scanner or, alternatively, from optical mechanisms. In the case of multi-color imaging, the detector  816  may comprise splitting and filtering to separate the scattered light into its component parts prior to detection. As alternatives to PIN photodiodes, avalanche photodiodes (APDs) or photomultiplier tubes (PMTs) may be preferred for certain applications, particularly low light applications.  
      In various approaches, photodetectors such as PIN photodiodes, APDs, and PMTs may be arranged to stare at the entire projection screen, state at a portion of the projection screen, collect light retro-collectively, or collect light confocally, depending upon the application. In some embodiments, the photodetector  816  collects light through filters to eliminate much of the ambient light.  
      The projection display  802  may be embodied as monochrome, as full-color, or hyper-spectral. In some embodiments, it may also be desirable to add color channels between the conventional RGB channels used for many color displays. Herein, the term grayscale and related discussion shall be understood to refer to each of these embodiments as well as other methods or applications within the scope of the invention. In the control apparatus and methods described below, pixel gray levels may comprise a single value in the case of a monochrome system, or may comprise an RGB triad or greater in the case of color or hyperspectral channels (for instance red, green, and blue channels) or may be applied universally to all channels, for instance as luminance modulation.  
       FIG. 9  is a block diagram of a feedback apparatus for determination of screen response according to an embodiment. The block diagram of  FIG. 9  is able, for example, to generate a compensated illumination pattern  702  shown in  FIG. 7 . Initially, a drive circuit drives the light source based upon a pattern, which may be embodied as digital data values in screen memory  902 . The screen memory  902  drives display engine  809  during calibration. Display engine  809  may for instance comprise an illuminator  804  and scanner  808  as  FIG. 8 . The display engine projects pixels onto a display surface  811 . For each spot or region of display surface, an amount of scattered light is detected and converted into an electrical signal by detector  816 . Detector  816  may include an A/D converter that outputs the electrical signal add a binary value, for instance. The detected signal is inverted by inverter  908 , and is optionally processed by optional intra-frame image processor  910 . The inverted detected signal or processed value is then added to the corresponding value in the screen memory  902  by adder  912 . This proceeds through the entire frame or projection screen until substantially all spots have been scanned and their corresponding screen memory values modified. The process is then repeated for a second frame, a third frame, etc. until substantially all spots have converged to a common amount of scattered light. In some embodiments and particularly those represented by  FIG. 11  below, the converged pattern in the screen memory represents the inverse of the projection screen response, akin to the way a photographic negative represents the inverse of its corresponding real-world image.  
      Inventer  908 , optional intra-frame processor  910 , and adder  912  comprise leveling circuit  913 .  
      The pattern in the screen memory  902  may be read out and  9  may be subjected to optional inter-frame image processing by optional inter-frame image processor  916 . The pattern in the screen memory  902  or the processed value in screen memory may be output to a video source or host system via interface  920 .  
      Optional intra-frame image processor  910  includes line and frame-based processing functions to manipulate and override the control input of the detector  816  and inverter  908  outputs. For instance, the processor  910  can set feedback gain and offset to adapt numerically dissimilar illuminator controls and detector outputs, can set gain to eliminate or limit diverging tendencies of the system, and can also act to accelerate convergence and extend system sensitivity. As was described above, the logic for converging the screen memory may vary according to the degree of divergence a given pixel has with respect to a nominal value. To ease understanding, it will be assumed herein that detector and illuminator control values are numerically similar, that is one level of detector grayscale difference is equal to one level of illuminator output difference.  
      As a result of the convergence of the apparatus of  FIG. 9 , spots that scatter a small amount of signal back to the detector become illuminated by a relatively high beam power while spots that scatter a large amount of signal back to the detector become illuminated with relatively low beam power. Upon convergence, the overall light energy received at from each spot may be substantially equal.  
      One cause of differences in apparent brightness is the light absorbance properties of the material being illuminated. Another cause of such differences is variation in distance from the detector. Optionally, time-of-flight or other distance measurement apparatus and methods may be used to correct for variations in screen compensation that arise due to differences in distance. In many applications it is desirable to project an image onto a relatively flat or smoothly curved surface having no or only moderately varying distance from the detector  816 . In such applications, it may be unnecessary to measure projection surface distance.  
      According to an embodiment, the controller may be programmed to ignore changes in received scattered energy that vary slowly according to position, instead determining compensation values only for regions having relatively sharp transitions in screen response. Such a system may, for example provide screen compensation values sufficient to overcome variations in screen response relative to a local value of a low slope variation in response.  
      Optional intra-frame image processor  910  and/or optional inter-frame image processor  916  may cooperate to ensure compliance with a desired safety classification or other brightness limits. This may be implemented for instance by system logic or hardware that limits the sum total energy value for any localized group of spots corresponding to a range of pixel illumination values in the screen memory. Further logic may enable greater illumination power of previously power-limited pixels during subsequent frames. In fact, the system may selectively enable certain pixels to illuminate with greater power (for a limited period of time) than would otherwise be allowable given the safety classification of a device.  
      While the components of the apparatus of  FIG. 9  are shown as discrete objects, their functions may be split or combined as appropriate for the application. In particular, inverter  908 , intra-frame processor  910 , adder  912 , and inter-frame processor  916  may be integrated in a number of appropriate configurations  
      The effect of embodiments corresponding to the apparatus of  FIGS. 8 and 9  may be more effectively visualized by referring to  FIGS. 10 and 11 .  FIG. 10  illustrates a state corresponding to an exemplary initial state of screen memory  902 . A beam of light  810  produced by a display engine  809  is shown in three positions  810   a ,  810   b , and  810   c , each illuminating three corresponding spots  812   a ,  812   b , and  812   c , respectively. Spot  812   a  is shown having a relatively low scattering or transmission. In this discussion, relative scattering or transmission will be referred to as apparent brightness. Spot  812   b  has a medium apparent brightness and spot  812   c  has a relatively high apparent brightness. These are indicated by the dark gray, medium gray and light gray shading of spots  812   a ,  812   b , and  812   c , respectively.  
      In an initial state corresponding to  FIG. 10 , the illuminating beam  810  may, for example, be powered at a medium energy at all locations, illustrated by the medium dashed lines impinging upon spots  812   a ,  812   b , and  812   c . In this case, dark spot  812   a , medium spot  812   b , and light spot  812   c  return low strength scattered signal  814   a , medium strength scattered signal  814   b , and high strength scattered signal  814   c , respectively to detector  816 . Low strength scattered signal  814   a  is indicated by the small dashed line, medium strength scattered signal  814   b  is indicated by the medium dashed line, and high strength scattered signal  814   c  is indicated by the solid line.  
       FIG. 11  illustrates a case where the screen memory  902  has been converged to a flat-field response, according to an embodiment. After such convergence, light beam  810  produced by display engine  809  is powered at level inverse to the apparent brightness of each spot  812  it impinges upon. In particular, dark spot  812   a  is illuminated with a relatively powerful illuminating beam  810   a , resulting in medium strength scattered signal  814   a  being returned to detector  816 .  
      Medium spot  812   b  is illuminated with medium power illuminating beam  810   b , resulting in medium strength scattered signal  814   b  being returned to detector  816 .  
      Light spot  812   c  is illuminated with relatively low power illuminating beam  810   c , resulting in medium strength scattered signal  814   c  being returned to detector  816 . In the case of  FIG. 11 , the screen memory has been converged such that the scanned beam compensation signals make the screen appear to be a substantially white-balanced region of uniform brightness.  
      It is possible and in some cases preferable not to fully converge the screen memory such that all spots on the projection screen return substantially the same energy to the detector. For example, it may be preferable to compress the returned signals somewhat to preserve the relative strengths of the scattered signals, but move them up or down as needed to fall within a reasonable range of neighboring spots so as to “smear out” abrupt transitions on the projection screen.  FIG. 12  illustrates this variant of operation. In this case, the illumination beam  810  is modulated in intensity by display engine  809 . Beam position  810  a is increased in power somewhat in order to raise the power of scattered signal  814   a  to fall above the detection floor of detector  816  but still result in scattered signal  814   a  remaining below the strength of other signals  814   b  scattered by neighboring spots  812   b  having higher apparent brightness. The detection floor may correspond for example to quantum efficiency limits, photon shot noise limits, electrical noise limits, or other selected limits. Conversely, apparently bright spot  812   c  is illuminated with the beam at position  810   c , decreased in power somewhat in order to lower the power of scattered signal  814   c  to fall below the detection ceiling of detector  816 , but still remain higher in strength than other scattered signals  814   b  returned from neighboring spots  812   b  with lower apparent brightness. The detection ceiling of detector  816  may be related for instance to full well capacity for integrating detectors such as CCD or CMOS arrays, non-linear portions of A/D converters associated with non-pixelated detectors such as PIN diodes, or other selected limits set by the designer. Of course, illuminating beam powers corresponding to other spots having scattered signals that do fall within detector limits may be similarly modified in linear or non-linear manners depending upon the requirements of the application. For instance, in some applications, the apparent brightness range of spots may be compressed to fit the dynamic range of the detector, spots far from a mean level receiving a lot of compensation and spots near the mean receiving only a little compensation. Alternatively, compensation power may be determined as a maximum slope from neighboring pixels, thus producing an image with smoothly varying background features on an otherwise optically noisy projection screen.  
       FIG. 13  is a flow chart showing a method according to an embodiment for converging a pixel value to level appropriate for screen compensation. In step  1302 , the screen memory is initialized. In some embodiments, the buffer values may be set to fixed initial values near the middle, lower end, or upper end of the power range. Alternatively, the buffer may be set to a quasi-random pattern designed to test a range of values. In yet other embodiments, the buffer values may be informed by previous pixels in the current frame. In still other embodiments, the buffer values may be informed by previous frames or previous images.  
      Using the initial screen memory value, a spot is illuminated and its scattered light detected as per steps  1304  and  1306 , respectively. If the detected signal is too strong per decision step  1308 , illumination power is reduced per step  1310  and the process repeated starting with steps  1304  and  1306 . If the detected signal is not too strong, it is tested to see if it is too low per step  1312 . If it is too low, illuminator power is adjusted upward per step  1314  and the process repeated starting with steps  1304  and  1306 .  
      Thresholds for steps  1308  and  1312  may be set in many ways. For example, some or all of the pixels on the projection surface may be illuminated with an output power near the center of the power range of the light source(s), the amount of scattered energy received measured, and the measured values averaged. The average screen response measured by the detector, optionally plus and minus a small amount for steps  1308  and  1312 , respectively, may then be used as thresholds. Alternatively, output power may be varied to fall within the dynamic range of the detector. For detectors that are integrating, such as a CCD detector for instance, illuminator powers with corresponding thresholds that return scattered pixel energies above noise equivalent power (NEP) (corresponding to photon shot noise or electronic shot noise, for example) and below full well capacity may be used. Instantaneous detectors such as photodiodes may be limited by non-linear response at the upper end and limited by NEP at the lower end. Thus these points may be used to select illuminator powers for steps  1308  and  1312 , respectively. Alternatively, upper and lower thresholds may be programmable depending upon video image attributes, application, user preferences, illumination power range, electrical power saving mode, etc. In some embodiments, thresholds are set according to the response of neighboring pixels, with values chosen such that changes in image brightness, white balance, etc. are allowed over moderate distances. Such an approach can result in the ability to use projection screens that would otherwise have scattering or transmission responses that exceed the dynamic range of the illuminators.  
      Thus, upper and lower thresholds used by steps  1308  and  1312  may be variable across the projection screen.  
      After a scattered signal has been received that falls into the allowable detector range, the detector value may be transmitted for further processing, storage, etc. in optional step  1316 .  
      After convergence, screen memory values may be combined with the incoming video image to level the screen response and provide an image superior to what might be otherwise formed on a given projection surface.  
       FIG. 14  is a diagram that shows the combination of an input video pattern  102  with a screen compensation pattern  702  to form a compensated video pattern  1402  according to an embodiment.  FIG. 15  is a diagram illustrating the interaction of a compensated video pattern  1402  with a screen response  202  to produce a projected image  1502  as perceived by a viewer  108 .  
      According to an embodiment, the screen compensation pattern  702  may be combined with the video pattern  102  through addition or subtraction, depending upon the screen compensation format, to form a compensated video pattern  1402 . Such an approach may be especially useful when the affect of screen variable response on the perceivable image is small. That is, variations of a few bits in screen response across the dynamic range of the light sources may be compensated quite efficiently by addition or subtraction of screen compensation offset values to create a compensated video pattern. Such addition or subtraction may be provided in ranges. For example a greater amount may be added or subtracted at high power levels and a corresponding lesser amount added or subtracted at low power levels. Such greater or lesser addition or subtraction values (screen compensation offset values) may be determined algorithmically, for example. Alternatively, screen compensation offset values may be determined by measuring screen response across a range of illumination powers.  
      According to another embodiment, the screen compensation pattern  702  may be combined with the video pattern  102  through multiplication or division operations. For example, for pixel locations corresponding to a region on the projection screen that scatters only half the amount of green light required for proper white balance or alternatively only have the amount of green light as the average screen response, the green code value in the input video signal may be doubled (multiplied by decimal 2).  
      According to another embodiment, compensated video signal pixel values may be determined according to a look-up table (LUT) that is constructed according to screen calibration results. In such a LUT, screen compensation may be gradually decreased at extremes of code values to accommodate dynamic range limitations of the projection display engine. According to another embodiment, the compensated video signal pixel values may be determined by convolving the input video bitmap with a screen compensation matrix.  
      According to another embodiment, compensated video pixel values may be calculated algorithmically.  
      As may be seen by inspection of  FIGS. 14 and 15 , application of screen compensation to an input video signal  102  may result in a perceivable image  1502  that substantially matches the input video image. Alternatively, the compensated video image  1402  may be formed such that the perceivable image  1502  includes fewer screen artifacts than would be present if the input video image  102  was projected.  
       FIG. 16  is a flow chart illustrating a method for generating a compensated video image according to an embodiment. Starting the process at step  1602 , an input video image is received. How this is done, depends upon the embodiment and the application. For example, an image may be received from a computer across a conventional wired or wireless interface as a bitmapped image. Alternatively, the control process of  FIG. 16  may be resident in the image source computer and receiving the input image may comprise reading a display memory. Alternatively, the input image may be received as a video image from a DVD player, VCR, television tuner, or the like as an NTSC, PAL, HDTV, etc. compliant signal. Alternatively, the process of step  16  may be included within a DVD player, VCR, television tuner, or the like. In any case, step  1602  may include converting the image into a format appropriate for modification, for example as a bitmapped image. For illustration purposes, it will be assumed herein that the input image is finally received as a bitmap for display.  
      Proceeding to step  1604 , the process parses through the image to select input pixels and/or channels for possible modification. For example, the process may start with the upper leftmost pixel (e.g. pixel  1 , 1 ) and proceed across columns then down rows until the bottom rightmost pixel (e.g. pixel  800 , 600  for an SVGA image) is processed.  
      Proceeding to step  1606 , the process determines output pixel values for each input pixel value and corresponding screen response for the pixel. According to one embodiment, this is done by accessing a LUT. Other embodiments may use algorithmic determination of the output pixel value in conjunction with a screen map.  
      For example, a screen map value is read for the current pixel. According to one embodiment, the screen map value is stored as an inverted value, such as in the screen map stored in the screen compensation memory  902  of  FIG. 9 . The inverted value is added to the current pixel to derive at least an intermediate value. Thus, spots with high scattering or transmission of a given wavelength channel are stored as relatively small values in the screen map and only a small amount is added to the input pixel value. Conversely, spots with low scattering or transmission of a given wavelength are stored as relatively large values and the input pixel is added to such relatively large values to create extra gain for spots that are not efficient at displaying the wavelength. Following derivation of the at least intermediate value, another uniform value, such as the average response, for example, may optionally be subtracted from the intermediate value to derive the output pixel value.  
      According to another embodiment, the screen map values are stored as a multiplier for each spot. Such a multiplier may be derived, for example, by dividing the converged spot code value by the code value of the illumination power used during calibration. During step  1604 , the multiplier for a spot corresponding to a pixel is read from the screen map and multiplied with the input pixel value to derive an output pixel value. Optionally, an offset may then be added or subtracted from each spot to maximize dynamic range. Alternatively, spots with large multipliers (corresponding to poor scattering or transmission of a given color) may be allowed to reach a maximum value and the image displayed with the best possible compensation, realizing that certain spots may be too inefficient to properly reach the desired apparent brightness, given a maximum power output of the display engine. The addend may additionally be determined through user input whereby a user “dials in” a larger added value for a brighter image or a smaller (perhaps negative) added value for a dimmer image.  
      Proceeding to step  1608 , the derived output pixel value is written to an output buffer for driving the display engine. If the current pixel is not the last pixel in a video frame, step  160  directs the program to step  1612 , which increments the pixel value and then returns to step  1604  where the next pixel is parsed and the output pixel derivation procedure is repeated. If the current pixel is the last pixel in the frame, step  1610  directs the program to step  1602  where a next video frame is read and the whole process repeated.  
      As may be readily appreciated, the process of  FIG. 16  may occur on a number of different hardware embodiments including but not limited to a programmable microprocessor, a gate array, an FPGA, an ASIC, a DSP, discrete hardware, or combinations thereof. The process of  FIG. 16  may further be embedded in a system that executes additional functions or may be spread across a plurality of subsystems.  
      The process of  FIG. 16  may operate with monochrome data or with a plurality of wavelength channels, each channel having, for example, individual coefficients or addends for each spot in the screen map.  
      The process of  FIG. 16  may operate on RGB values. Alternatively, the process may operate using chrominance/luminance or other color descriptor systems.  
      In addition to discrete or separate screen calibration and display functions, systems may dynamically monitor the scattering or transmission of the display screen and update the screen map.  FIG. 17  is a diagram illustrating dynamic updating of a screen compensation map. A compensated video signal  1402  interacts with a screen response  202  to provide a compensated visible image  1502  to a viewer  108  while a sensor  302  simultaneously monitors the image output  1502  of the system. For cases where the screen response remains  202  static, illustrated by the solid lines in the screen response  202 , the displayed image  1502  will remain properly compensated, illustrated by the solid lines in the displayed image  1502 . However, in some cases, the screen response may change. Normal screen aging, soiling, and damage may be the cause of changes. Another cause for change has to do with the display engine moving relative to the projection surface or screen, such as in a hand-held projection display.  
      In cases where there are changes in the screen response  202 , indicated by dashed lines in the screen response  202 , corresponding variations in the displayed image  1502  may result, indicated by dashed lines in the displayed image  1502 .  
      The sensor  302  may continuously monitor the output image  1502 , comparing it to the input video image (not shown) and determine pixels that do not match the desired output indicated by the solid line. In such a case, the sensor measures the variance in apparent brightness. The calibration system, which may for example be embodied as the process of  FIG. 6 , receives the measured value and updates the screen compensation map to accommodate the variations in response. Compensated output signals for subsequent video frames will thus be based on the updated screen map. The process of continuous monitoring and update may operate substantially continuously, upon user triggering, at pre-determined intervals, or according to other schedules as may be appropriate for an application.  
       FIG. 18  is a block diagram illustrating the relationship of major components of an embodiment of a screen-compensating display system  802 . Program source  1802  provides a signal to controller  818  indicative of content to be displayed. Controller  818  drives display engine  809  to display the content onto a display screen (not shown). Sensor  302  receives light scattered or transmitted by the display screen and provides a signal to the controller  818  indicative of the strength of the received signal. The components operate together in a manner described elsewhere herein. The display engine may be of a number of different types. Although a scanned beam display engine is described in detail above, other display engine technologies such as LCD, LCOS, mirror arrays, CRT, etc. may be used in conjunction with the screen compensation system described herein.  
      The major components shown in  FIG. 18  may be distributed among a number of physical devices in various ways or may be integrated into a single device. For example, the controller  818 , display engine  809 , and sensor  302  may be integrated into a housing capable of coupling to a separate program source  1802  through a wired or wireless connector. According to another example, the program source may be a part of a larger system, for example an automobile sensor and gauge system, and the controller, display engine, and sensor integrated as portions of a heads-up-display. In such a system, the controller  818  may perform data manipulation and formatting to create the displayed image.  
       FIG. 19  is a block diagram, according to an embodiment, illustrating the relationship of major components of a screen-compensating display controller  818  and peripheral devices including the program source  1802 , display engine  809 , and sensor subsystem  302  used to form a screen-compensating display system  802 . The embodiment of  FIG. 19  is a fairly conventional programmable microprocessor-based system where successive video frames are received from the video source  1802  and saved in an input buffer  1902  by a microcontroller  1904  operating over a conventional bus  1906 . The microcontroller operates instructions read from read-only memory  1908  to read pixel values from the input buffer  1902  into a random access memory  1910 , read corresponding portions of the screen memory  1912 , and perform operations to derive compensated pixel values, which are written into an output frame buffer  1914 . The contents of the output frame buffer  1914  are transmitted to the display engine  809 , which contains digital-to-analog converters, output amplifiers, light sources, one or more pixel modulators (such as a beam scanner, for example), and appropriate optics to display an image on a screen (not shown). The sensor subsystem  302  measures the amount of light scattered or transmitted by the screen and the values returned from the sensor subsystem  302  are used by the microcontroller  1904  to construct or update a screen map, as described above. A user interface  1916  receives user commands that, among other things, affect the properties of the displayed image. Examples of user control include compensation override, compensation gain, brightness, pixel range truncation, on-off, enter-calibration, continuous calibration on/off, etc.  
       FIG. 20  is a perspective drawing of a detector module  816  made according to an embodiment. Within detector module  816 , the scattered light signal is separated into its wavelength components, for instance RGB.  
      Optical base  2002  is a mechanical component to which optical components are mounted and kept in alignment. Additionally, base  2002  provides mechanical robustness and, optionally, heat sinking. The sampled scattered or transmitted light enters the detector  816  through a window  2004  with further light transmission is made via the free space optics depicted in  FIG. 20 . Focusing lens  2006  shapes the received light  2008  that propagates through the window  2004 . Mirror  2010 , which may be a dielectric mirror, splits off a blue light beam  2012  and directs it to the blue detector assembly. The remaining composite signal  2014 , comprising green and red light, is split by dielectric mirror  2016 . Dielectric mirror  2016  directs green light  2018  toward the green detector assembly, leaving red light  2020  to pass through to the red detector assembly.  
      Blue, green, and red detector assemblies  2022 ,  2024 , and  2026 , respectively, each comprise an appropriate wavelength filter and a detector. The type of detectors used in the embodiment of  FIG. 20  are photomultiplier tubes (PMTs). Specifically, the blue detector assembly comprises a blue filter  2028  and a PMT  2030  for detecting blue light; the green detector assembly comprises a green filter  2032  and a PMT  2034  for detecting green light; and the blue detector assembly comprises a red filter  2036  and a PMT  2038  for detecting red light. The filters serve to further isolate the detector from any crosstalk, which may be present in the form of light of unwanted wavelengths. For one embodiment, HAMMAMATSU model R1527 PMT may give satisfactory results for each of the three channels. This tube has an internal gain of approximately 10,000,000, a response time of 2.2 nanoseconds, a side-viewing active area of 8×24 millimeters, and a quantum efficiency of 0.1. Other commercially available PMTs may be satisfactory as well.  
      For the PMT embodiment of the detector  816 , two stages of amplification, each providing approximately 15 dB of gain for 30 dB total gain, boost the signals to levels appropriate for analog-to-digital conversion. The amount of gain varies slightly by channel (ranging from 30.6 dB of gain for the red channel to 31.2 dB of gain for the blue channel), but this is not felt to be particularly critical because calibration and subsequent processing can maintain white balance.  
      In another embodiment, avalanche photodiodes (APDs) are used in place of PMTs. The APDs used include a thermo-electric (TE) cooler, TE cooler controller, and a transimpedence amplifier. The output signal is fed through another 5× gain using a standard low noise amplifier.  
      As was indicated above, alternative non-imaging light detectors such as PIN photodiodes may be used in place of PMT or APD type detectors. Additionally, detector types may be mixed according to application requirements. Also, it is possible to use a number of channels fewer than the number of output channels. For example a single detector may be used. In such a case, an unfiltered detector may be used in conjunction with sequential illumination of individual color channel components of the pixels on the display surface. For example, red, then green, then blue light may illuminate a pixel with the detector response synchronized to the instantaneous color channel output. Alternatively, a detector or detectors may be used to monitor a luminance signal and screen compensation dealt with through variable luminance gain. In such a case, it may be useful to use a green filter in conjunction with the detector, green being the color channel most associated with the luminance response. Alternatively, no filter may be used and the overall amount of scattering or transmission by the display surface monitored.  
      As may be appreciated, a non-imaging detector system such as that shown in  FIG. 20  may be used in a variety of implementations, including those where pixels are generally displayed simultaneously. In one example, a pixel at a time is progressively displayed during a calibration routine. The response of the screen to each pixel is used to determine the screen map. Various acceleration approaches such as an analysis of variance where multiple pixels are displayed simultaneously may be used. Pixel locations may additional be scanned according to previously measured pixels to measure locations most likely to have display surface non-uniformities.  
      Non-imaging detectors may additionally be used to perform continuous calibration with simultaneous pixel display engines such as LCD, LCOS, etc. According to one embodiment, a sequence of pixels are displayed across the display surface during successive inter-frame periods, i.e. during periods that are normally blanked. One way to do this is to sequentially latch pixels to the value displayed during the previous period or alternatively to offset the period for display into the inter-frame period.  
       FIG. 21  is a perspective drawing of an exemplary front projection display with screen compensation  802 , according to an embodiment. Housing  2102 , which may for example be adapted to be mounted to a ceiling, includes a red pixel display engine  809   a  (of which one can see the output lens), a green pixel display engine  809   b , and a blue pixel display engine  809   c  aligned to project a registered display image onto a projection surface  811 . Display engines  809  may, for example, be LCD, LCOS, binary mirror array (DMD), etc. Corresponding sensors  816   a ,  816   b , and  816   c  are aligned and operable to receive the corresponding red, green, and blue images projected onto the screen  811 . In the example of  FIG. 21 , the sensors  816  are focal plane CCD or CMOS sensors that image the pixels and measure their apparent brightness. Each respective sensor includes a filter to selectively receive the appropriate color channel. While screen  811  is illustrated as a conventional projection screen, it will be appreciated that embodiments may allow projection onto surfaces with optical characteristics that are less than ideal such as a wall, door, etc.  
       FIG. 22  is a perspective drawing of an exemplary portable projection system with screen compensation  802 , according to an embodiment. Housing  2102  of the display  802  houses a display engine  809 , which may for example be a scanned beam display, and a sensor  816  aligned to receive scattered light from a projection surface. Sensor  816  may for example be a non-imaging detector system made as a variant of the sensor system of  FIG. 20 . The display  802  receives video signals over a cable  820 , such as a Firewire, USB, or other conventional display cable. Display  802  transmits detected pixel values up the cable  820  to a host computer. The host computer applies screen compensation to the image prior to sending it to the portable display  802 . The housing  2102  may be adapted to being held in the hand of a user for display to a group of viewers. A user input  1916 , which may for example comprise a button, a scroll wheel, etc., is placed for access to display control functions by the user.  
      Thus the display of  FIG. 22  is an example of a screen compensating display where the display engine  809 , sensor  816 , and user interface  1916  are in one housing  2102 , and the program source  1802  and controller  818  are in a different housing, the two housings being coupled through an interface  820 .  
      According to some embodiments, the detectors  816   a ,  816   b , and  816   c  of  FIG. 21  are offset from their respective corresponding pixel display sources  809   a ,  809   b , and  809   c . Similarly, detector  816  of  FIG. 22  is offset from the projection display engine output  809 . According to an embodiment, the distance between the respective pixel illumination and pixel detection elements represents a baseline from which geometric distortions may be triangulated using simple trigonometry using certain assumptions about the projection screen, such as the screen being parallel to the normal of the mean projection angle in at least one dimension. Alternatively, pairs, triplets, etc. of detectors may be used to provide additional baseline geometries for triangulation of geometric distortion.  
      According to embodiments, the screen compensation system taught herein may be adapted to rear-projection displays or front-projection displays.  
      The preceding overview, brief description of the drawings, and detailed description describe illustrative embodiments according to the present invention in a manner intended to foster ease of understanding by the reader. Other structures, methods, and equivalents may be within the scope of the invention.  
      Compensation for geometric distortions may be driven in a variety of ways, according to the preferences of the embodiment. For example, scanned beam display engines in particular may be driven with offset pixel timing or interpolated/extrapolated pixel locations to compensate for such distortions. Other types of display engines having fixed pixel relationships may be similarly corrected with projection optics to vary pixel projection angle.  
      The scope of the invention described herein shall be limited only by the claims.