Patent Publication Number: US-2006007406-A1

Title: Equipment, systems and methods for control of color in projection displays

Description:
FIELD OF THE INVENTION  
      This invention relates to the projection of images and more specifically to methods and equipment for matching the color between projection displays.  
     BACKGROUND OF THE INVENTION  
      Spatial Light Modulator (SLM) based projection displays are often used in applications where the reduction of color variation in a display and the matching of colors between displays is important. Displays used in the presentation of high quality images, such as for motion picture applications, require good color matching since the control of color is an important part of the expressive dimension of the film images. Users of these displays desire that in every theatre the colors reproduced by the projection system should match those that were determined during the post production of the motion picture.  
      In other applications multiple displays may be used at the same time, for example when the spatial resolution of a single projection display system of a given type is inadequate. Multiple projection displays may also be required where the projection surface covers a large area or a surface of a shape that cannot be covered by a single projection display system with the desired brightness and image quality.  
      In such situations it is common to employ multiple projection displays in a tiled arrangement. Two or more projection displays are arranged so that their images are adjacent and form a matrix of horizontal and vertical elements so that the resulting composite image has a higher resolution and brightness than would result if a single projection display were used to cover the same projection surface area. Subdivision of the display also allows the projection surface to change in shape or distance from the projection points without requiring excessive depth of focus or special distortion correction from the projection lenses. Multiple displays may also be fully superimposed upon each other to obtain increased brightness or other benefits from the combination of superimposed images such as the suppression of sampling artifacts. In order for these arrangements of multiple displays to have maximum image quality the color characteristics of each of the projectors should be well matched.  
      Projection displays based on spatial light modulators (SLMs) such as deformable mirror devices (DMDs) commonly employ multiple SLMs in order to produce a color display using additive means based on three primary colors. These systems frequently use so-called dichroic filter elements to divide the light from the illuminating light source into three spectral bands that correspond to the desired primary colors, conventionally red, green and blue. Three SLM devices are then used one for each primary color, to modulate the intensity of the divided light, which is then recombined into a single beam and projected onto the display screen by the projection lens. The SLM devices are driven by an input signal that conveys the brightness for each pixel of each of the three SLM devices so that the desired continuous tone color image is formed on the display screen.  
      In order to produce an image with uniform and consistent color, the characteristics of the dichroic color filters should be very carefully matched. In addition, the angle of illumination for each of the filters should be very carefully controlled due to the fact that the wavelengths of the dichroic filter&#39;s passband depend on the angle of incidence. It is difficult to precisely control the color balance of displays equipped with dichroic filters due to the angle dependent nature of the filter characteristics and the inevitable manufacturing tolerances that arise in any mass produced system. Usually these filters are contained within an optical combining assembly that does not permit selection or adjustment of the filters for reasons of practicality and stability. As a result such displays can exhibit color shifts across the display such that, for example, an input signal representing a uniform white field is displayed with a slightly bluish tint at one edge, and a slightly reddish tint at the other edge.  
      Projection displays based on SLM devices commonly employ electronic circuitry to permit control of the appearance of the image. These controls include a means for adjusting the overall contrast or gain, black level, tint and saturation of the display. It is also common for controls to be provided that adjust the gain and black level of each color channel separately. These controls are also commonly used to adjust the color balance of the display, for example to set the displayed white to a particular tint, and to ensure that a displayed grayscale has a neutral appearance. An additional means of adjusting the projector color channels may also be provided that consists of a look-up table that receives the input pixel values for each color channel and for each input pixel value outputs a new pixel value to the SLM devices. This look up table may be used to alter the relative brightness of each channel as well as the input pixel value to image pixel brightness transfer function of each channel.  
      The color balance of an additive display can therefore be adjusted by altering the relative brightness of each channel of the display. However, this color adjustment is achieved by reducing the maximum brightness of one or more of the color channels of the display which in turn reduces the maximum brightness of the display. Furthermore, achieving a desired overall color balance for a group of displays in a multiple projection display configuration may require lowering the brightness of all of one or more of the red, green and blue channels of all of the multiple projection displays, further reducing the brightness of the composite display.  
      Second, adjusting color shift and color balance by manipulating the relative brightness of the three primaries is only effective in the general case for displayed colors that contain some proportion of all three of the primaries. Saturated colors or colors that contain only one or two of the three primaries cannot in general be matched between displays by adjusting the brightness of the red, green and blue components of each display.  
      An improvement in color matching can be obtained by mapping input colors to display colors using for example a three dimensional matrix operation or a three dimensional look-up table. However, this method of matching colors between two or more displays requires that the displayed colors fall within the common gamut of all of the displays. This has the effect of reducing the gamut of colors that can be displayed. This is shown for example in U.S. patent application 2002/0041708 A1 to Pettitt. This patent application shows a method for matching multiple projectors to a “standard color gamut” which is of necessity a subset of the gamuts of the projectors to be matched as shown in  FIG. 5  of the application. While Pettitt makes use of matrix methods to map input signal colors to specific brightness values for each color channel of the projector, a three dimensional look up table that maps input values to new values to be supplied to the SLM devices for the three channels would also suffice. Systems such as Pettitt that perform color correction by modifying the pixel brightness values supplied to the SLM devices in the projector can only match colors which are desaturated by the addition some of each of the other two primaries.  
      Finally, the adjustment of the brightness of the projector display channels cannot compensate for color shifts across the display since the adjustment acts equally on all pixels of the display.  
      U.S. Pat. No. 5,386,253 to Fielding describes a method for improving the uniformity of the projected image in a SLM based projection display. In Fielding, a sensor observing the far field is used to measure the brightness of regions of the projected image and this information is used to correct the brightness distribution on the screen by modifying the pixel brightness values supplied to regions of pixels of the SLM. This modification in pixel brightness may be used to alter the brightness of regions of the projected image to achieve the appearance of any desired brightness distribution. The method in Fielding cannot increase the brightness of a given region or area of the screen above that available for that given area in the uncorrected system. As a result, modifying the pixel brightness of areas of the projected image to achieve, for example, a flat field of uniform brightness will typically limit the brightness of the display to that of the least bright area of the projected image.  
      Fielding provides separate pixel value modifying means for each of three SLM devices used in a color projector. The method in Fielding is intended to ensure that the brightness of the pixels of each color channel of the projector is uniform. This reduces the effect of a color shift across the display, subject to the same limitation previously noted for overall color balance adjustments which is that such an adjustment is only generally effective for displayed colors that contain some proportion of all three of the primaries.  
      Fielding also makes reference to the fact that any overall gain adjustment applied to the pixel values in order to improve the overall uniformity of the display should be the same for all three color channels in order to avoid changing the color balance. It should be apparent to those skilled in the art that a different overall gain adjustment could be applied to the pixel values for each color channel, and that this adjustment essentially duplicates the color channel gain adjustments commonly provided in SLM based projectors.  
      U.S. Pat. No. 6,115,022 to Mayer, III et al. describes a method like that in Fielding where separate adjustment of the red, green and blue pixel values may be used to correct for color shifts in the displayed image. This method has several important limitations. First, as in Fielding the method cannot increase the brightness of the primary colors above that produced by an uncorrected system, only a reduction in brightness can be performed on each color channel. The correction of color shift in general requires reducing one or more of the red, green and blue pixel brightnesses in areas of the display where pixels are brighter to match the brightness in the areas where pixels are not as bright. Likewise the matching of adjacent displays by this method will result in additional reduction of brightness. Furthermore, achieving a desired overall color balance for the composite display may require lowering the brightness of all of the pixels in one or more of the red, green and blue channels of all of the multiple projection displays, further reducing the brightness of the composite display.  
      Second, adjusting color shift and color balance by manipulating the relative pixel brightnesses of the three primaries is only effective in the general case for displayed colors that contain some proportion of all three of the primaries. This means that saturated colors or colors that contain only two of the three primaries cannot in general be matched between displays by adjusting the pixel brightnesses of the red, green and blue components of each display.  
      Where methods such as Mayer, III et al. are applied to displays with high fundamental consistency such as CRT displays where the primary colors are determined by the phosphors used in the CRT and most of the color imbalance is electronic in origin, the likelihood is high that the primary colors will match. For the reasons given above this does not apply to SLM based displays where the three primary colors are produced by dichroic filters.  
      The prior art has not provided a solution that completely solves the problems of color uniformity and color shifts in the individual displays. In addition the methods of the prior art impose limitations on brightness and they are not effective in matching the primary colors of such displays.  
      As a result, the performance of SLM based projection displays is less than satisfactory due to color variations in the displays and the poor matching of the colors between projection displays.  
     SUMMARY OF THE INVENTION  
      The present invention seeks to resolve these issues of uniformity and color matching by introducing equipment, systems and methods that allow for the control of the spectral energy distribution of the input light without reducing the overall brightness of the display. Equipment, systems and methods are disclosed that utilize secondary illumination sources, which add additional light, to reach the desired chromaticity for each primary color. Further equipment, systems and methods are disclosed that utilize adjustable bandpass filters in combination with the illumination source to control the amount of primary color in the input light in order to reach the desired chromaticity for each primary color. Further equipment, systems and methods are disclosed for the correction of field dependant color variation across the field of SLM based projectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a projection system according to the prior art.  
       FIG. 2  is a diagram showing the color characteristics of SLM based projection displays according to the system of  FIG. 1 .  
       FIG. 3  is a more finely scaled diagram showing the color characteristics of SLM based projection displays according to the system of  FIG. 1 .  
       FIG. 4  is a graph of the shifts in the spectral transmission of a color filter used in the system of  FIG. 1  as a result of changing the angle of incidence of the light reaching the filter.  
       FIG. 5  is a graph of the color differences in the display white point produced by changing the angle of incidence of the light reaching the filters used in the system of  FIG. 1 .  
       FIG. 6  shows the color gamuts produced by the color filters used in the system of  FIG. 1 .  
       FIG. 7  illustrates an exemplary embodiment of a system for reducing the color variation of the display and for adjusting the colors of each of the displays in the system of  FIG. 1 .  
       FIG. 8  shows graphs of the spectral energy distributions used for the secondary illumination sources in the systems of  FIGS. 7, 9  and  10 .  
       FIG. 9  illustrates another exemplary embodiment of a system for reducing the color variation of the display and for adjusting the colors of each of the displays in the system of  FIG. 1 .  
       FIG. 10  illustrates another exemplary embodiment of a system for reducing the color variation of the display and for adjusting the colors of each of the displays in the system of  FIG. 1 .  
       FIG. 11  illustrates the spectral energy distribution of a lamp used in the systems of  FIGS. 9 and 10 .  
       FIG. 12  is a diagram showing the color adjustment method of the inventions of  FIGS. 7, 9  and  10 .  
       FIG. 13  is a diagram showing the effect of varying the angle of incidence on the primary color filters in the system of  FIGS. 7, 9  and  10 .  
       FIG. 14  is a second diagram showing the effect of varying the angle of incidence on the primary color filters in the system of  FIGS. 7, 9  and  10 .  
       FIG. 15  is a block diagram of an apparatus for adjusting the brightness of a display based on the inventions of  FIGS. 7, 9  and  10 .  
       FIG. 16  is a block diagram describing an exemplary method for adjusting the colors of a display using the inventions of  FIGS. 7, 9  and  10 .  
       FIG. 17  is a diagram showing the effect of the invention of  FIG. 7  in adjusting the colorimetry of the system in  FIG. 1 .  
       FIG. 18  is a graph showing the relative spectral powers of the illuminating light sources used in the system of  FIG. 7  to make the color adjustments shown in  FIG. 16 .  
       FIG. 19  shows the adjusted color gamut for the system of  FIG. 1  produced by the system of  FIG. 7 .  
       FIG. 20  is a graph showing the passbands of filters used in the green channel of the systems of  FIGS. 7, 9  and  10  with an additional means of adjusting the color.  
       FIG. 21  is a graph showing the chromaticity coordinates of the primary and secondary illuminating light sources for the green channel of the systems of  FIGS. 7, 9  and  10  with the filter passbands of  FIG. 20 .  
       FIG. 22  is a graph of the spectral transmission of three filters of the type used in the system of  FIG. 1 , showing the effects of wavelength shifts on the transmission of these filters.  
       FIG. 23  shows the effect of wavelength shift on the spectral transmission of a bandpass filter.  
       FIG. 24  shows an alternative system for adjusting the colors of a display system.  
       FIG. 25  is a diagram showing the effect of the invention of  FIG. 24  in adjusting the colors of the system in  FIG. 1 .  
       FIG. 26  shows the spectral transmission of filters for use in an alternative filter arrangement for the system of  FIG. 24 .  
       FIG. 27  illustrates a method to adjust the field dependent color variation of a SLM based projector according to the present invention.  
       FIG. 28  is a detail of the method used in the invention of  FIG. 27  in adjusting the field dependent color variation of the system in  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      In order to improve the clarity of the description the present invention will be described using the example of two projectors used together to form a composite display where the two projected images are arranged side by side in the horizontal direction. This is a subset of a more complex system that may involve more than two projectors arranged in configurations where the composite image is produced from a matrix of images superimposed or arranged horizontally, vertically or both. It should be understood that the inventions disclosed herein may be applied to the more complex configurations and to the general application of adjusting the color of a projection display in the case where one display is used alone.  
       FIG. 1  illustrates in schematic form the plan view of a projection system using two SLM based projection displays to form a composite image according to the prior art. Various types of SLM devices may be employed including deformable mirror devices (DMDs), or reflective or transmissive liquid crystal devices, and in this example DMD type SLM devices are shown. The image to be displayed is divided into two halves, a left half and a right half, each being of the same height, but each being one half of the total width of the final image. The composite image is formed on display screen  100  which receives the left and right projected image halves from two projection systems, a left hand projector,  115  and a right hand projector  135 . The left hand projector receives an image input signal corresponding to the left half of the desired image and the right hand projector receives an image input signal corresponding to the right half of the desired image.  
      Each projection system is identical and may be described in detail with reference to the left hand projector,  115 , as follows. The numbers in parenthesis refer to the corresponding elements of the right hand projector,  135 , in  FIG. 1 .  
      An input video or image data signal  114  ( 134 ) representing one half of the image to be displayed is supplied to input circuit  112  ( 132 ) which provides various facilities known to those skilled in the art for separation of composite inputs into discrete red, green and blue or “RGB” signal components as required by the input format, facilities to extract image frame timing information and facilities such as contrast control, color balance adjustment, image scaling and other features known to those skilled in the art. The output of circuit  112  ( 132 ) is three discrete signals  111  ( 131 ) corresponding to the three color components RGB of the image and a frame timing signal  113  ( 133 ). These signals are supplied to display control and formatting circuit  110  ( 130 ) which in turn supplies the control signals  109  ( 129 ) required by the SLM devices  106 ,  107  and  108  ( 126 ,  127  and  128 ). Each SLM device consists of a two dimensional array of modulating elements or pixels, and by means of various control signals each pixel modulates the intensity of a corresponding part of the light to be projected so as to form the desired pattern of pixel brightnesses that correspond to the image to be projected. Each SLM device corresponds to one of the three color components of the image to be displayed, and color separating and re-combining device  105  ( 125 ) provides the optical components necessary to filter input white light into three spectral color bands that correspond to the red, green and blue portions of the visible spectrum, this color separated light then illuminates SLM devices  106 ,  107  and  108  ( 126 ,  127  and  128 ) with red, green and blue light respectively. The control signals  109  ( 129 ) cause the individual picture elements to be controlled so as to modulate the intensity of the red, green or blue light falling on the SLM, which in turn is re-combined by color separating and re-combining device  105  ( 125 ) into a single image of overlaid red, green and blue components which is in turn projected by lens  104  ( 124 ) onto the screen  100 . It will be known to those skilled in the art that  FIG. 1  omits for the sake of clarity a number of details of the construction of a projector, including the illuminating light source and the details of color separating and re-combining device  105  ( 125 ) which varies in its detailed configuration and components according to the type of SLM used.  
      The left hand projector  115  in  FIG. 1  produces a projected image  102  on the screen  100 , which proceeds from the lens  104  as more or less a cone of light as shown in  FIG. 1  by the dashed lines connecting  104  to  102 . Similarly, right hand projector  135  in  FIG. 1  produces a projected image  122  on the screen  100 , which also proceeds from lens  124  as more or less a cone of light as shown by the dashed lines connecting  124  to  122 .  
      SLM based color projectors commonly employ a color separation and re-combining system using dichroic bandpass filters to separate white light into three spectral bands (corresponding to red, green and blue colors) prior to illuminating the SLMs and then recombine the modulated light from each of the three SLMs prior to the projection lens. The arrangement of the dichroic filters commonly uses a combination of each filter&#39;s selective reflection and transmission properties.  
      The exact wavebands associated with these properties are a function of the angle of the incident light. Color uniformity across the projected image therefore requires uniformity in the angles at which light reaches each of the dichroic filters in the color separation and re-combining system. This is achieved by an illumination relay with a telecentric input and output. The telecentric condition insures that all points on the dichroic filter see the same angular distribution of light from the source.  
      The system of  FIG. 1  can provide adjustment of the color balance of the projectors in a multiple projection display by modifying the relative brightnesses of the red, green and blue image channels using the normal projector facilities for gain adjustment of the red, green and blue image channels. But this method has two important limitations. First, achieving a desired overall color balance for the composite display may require lowering the brightness of one or more of the red, green and blue brightnesses of both of the two projection displays, reducing the brightness of the composite display.  
      Second, adjusting color shift and color balance by manipulating the relative brightness of the three primary colors is only effective in the general case for displayed colors that contain some proportion of all three of the primaries. This means that saturated colors or colors that contain only two of the three primaries cannot in general be matched between displays by adjusting the brightness of the red, green and blue components of each display.  
      In order to appreciate the requirements and benefits of an improved means of adjusting the color of a projection display, a method is required that can quantify the visibility and range of color variations or differences found in projection displays and also permit evaluation of the effects of adjustments performed using the improved means of adjusting colors. This can be done using a psychophysically based system of color measurement and color difference evaluation.  
      A variety of techniques for evaluating the visibility of color differences are available in the literature known to those skilled in the art. These techniques are based on some form of three dimensional color space where equal increments of movement along each axis produce perceptually uniform changes in the color sensation experienced by most color normal viewers. A uniform color space allows the measurement of color differences and the comparison of the magnitudes of color differences. Using the techniques of color differences the color variations in a single display and the color variations expected between projectors can be analyzed and the effectiveness of methods for correcting those differences can be evaluated.  
      The International Commission On Illumination, abbreviated CIE after the French “Commission Internationale De L&#39;eclairage”, is recognized by the International Organization for Standardization (ISO) as an international standardization body. Division 1 of the CIE has terms of reference which include the establishment of colorimetric systems. The CIE has standardized color matching functions that allow the numerical representation of color stimuli seen by a human observer in a consistent way that represents the color matching properties of the human visual system. Spectral energy distributions such as those that result when a light source is modulated by SLM devices and filtered by a color separating and re-combining device in a SLM based projector can be converted to numerical values using these color matching functions in appropriate calculations that are known to those skilled in the art.  
      The resulting numerical values or chromaticities can be plotted on various diagrams that are also standardized by the CIE. One of these is the xy diagram, which plots the chromaticity values in terms of a coordinate pair that represents the chromatic component of the stimulus independent of its luminance.  
      On the CIE xy diagram straight lines connect the additive mixtures of stimuli represented by points on the diagram. These mixtures follow a “center of gravity” rule where the location of the resulting color stimulus is located by dividing the line in proportion to the amounts of the stimuli. For example, an equal mixture of two color stimuli results in a new color stimulus located at the midpoint of the line connecting the two initial stimuli.  
      The CIE has also established methods for predicting the magnitude of perceived color differences. The CE L*u*v* uniform color system (abbreviated LUV) is an approximately uniform color space that can be used to graphically depict the relationship of different colors. LUV is a linear transformation of CIE tristimulus values. In the examples provided herein, the color matching functions for the CIE  1931  2° observer have been used to compute tristimulus values. LUV chromaticities can be plotted in two dimensions at a selected value of L* in terms of u* and v* which in this form represents a projective transformation of the CIE xy diagram. CIE tristimulus values are converted to LUV values using equations well known in the art. The LUV values also take into account the chromatic adaptation of the observer by incorporating the chromaticity of a selected white point which always plots at the 0,0 point on a u*v* diagram.  
      The u*v* diagrams also have the property that straight lines connect the additive mixtures of stimuli represented by points on the diagram. Because an additive color mixing system is being analyzed this permits straightforward computer modeling of the color correction methods used by the present invention. Although the most current work shows that LUV contains some important defects when used to predict color appearance attributes, the u*v* diagram predicts color differences related to correlated color temperatures near the Planckian loci of the CIE xy diagram better than the alternative uniform color spaces. Also the u*v* diagram is the most straightforward to use with additive color systems because of its linear treatment of additive color mixing as previously described. Although the use of a more sophisticated color appearance model might change the magnitudes and character of the color differences measured for projection display systems, the principles of this invention for the further adjusting of color would remain unaffected and other color appearance models could be used by those skilled in the art.  
      The color adjustment requirements for matching projection displays can be understood by considering the source of color variations between displays. There are four major sources of color variation in a well designed SLM based projection display. These are the lamp and reflector; the transmission spectra of the various filters and mirrors used in the optical system; field dependent color shifts due to variations in the angle of incidence of the light that reaches the color filters in the color separating and recombining device; and the color of the glass and coatings used in the lens system. The purpose of the present invention is to address the effects of variations in the first three groups, that is, color variations due to the characteristics of the concentrating reflector used with the illuminating lamp, field dependent color shifts due to variations in the angle of incidence of the light that reaches the color filters in the color separating and re-combining device and changes in the transmission spectra of filter components.  
      As previously discussed, the filters commonly used in the color separating and re-combining device used in SLM based color projectors are dichroic filters. These filters have a spectral reflectance and transmittance that is a function of the incident angle of the light passing through the filter. In typical color separation and re-combining systems the angle of incidence used is other than 0 degrees. The wavelength shift for a dichroic filter is approximated by the following equation (1):  
               λ   s     =       λ   n     *         n   2     -       sin   2     ⁢           ⁢   θ                   (   1   )             
          where:     λ s =wavelength resulting from tilt angle θ    λ=the wavelength at zero angle of incidence     n=the effective refractive index of the dichroic coating stack        

      As the equation shows a filter used at non-zero angles of incidence when tilted to greater angles will shift its transmission spectrum towards the shorter wavelengths and when titled to lesser angles will shift its transmission spectrum towards the longer wavelengths.  
      In the following description, a pair of projectors is considered, arranged as shown in  FIG. 1 . One projector is considered to be the reference, and its white point for nominal lamp spectral emission and the design center for the filter components is used as the illuminant white point for the calculation of u*v* coordinates and the subsequent calculation of delta E color difference values according to the formula (2): 
 
Δ E =√{square root over (Δ L*   2   +Δu*   2   +Δv*   2 )}  (2) 
          where:     ΔE=delta E color difference     ΔL*=L* reference −L* shifted |    Δu*=|u* reference −u* shifted |    Δv*=|v* reference −v* shifted |       

       FIG. 2  is a u*v* diagram in the plane where L*=100, showing the u*,v* coordinates of the Planckian spectral loci  201 , the u*,v* coordinates of the pure spectral colors  202 , and the color coordinates of the white point  203  for a typical SLM based projector using DMD devices.  FIG. 3  is a plot of the region shown in dashed outline at  204  in  FIG. 2 . Again,  301  are the Planckian loci and  303  is the white point for typical three SLM based projector using DMD devices. The circle  304  shows the radius of two delta E units of color difference from the white point  303 . The line  305  shows the direction of color shift for the projector white point caused by a variation in the tilt angle of the illuminating light to a greater angle than intended on the red, green and blue dichroic filters used in the color separating and re-combining device of the projector.  
       FIG. 4  shows the effect of increasing the angle of incidence for the illumination of the green filter in the color separating and re-combining device of the projector by 3 degrees and 6 degrees. The curve  401  is the transmission at the correct angle of incidence, and curves  402  and  403  respectively correspond to 3 and 6 degree increases in the angle of incidence. The magnitude of the shift for the dominant wavelength of the filter is approximately 2.5 nanometers of wavelength for the angle of incidence increase of 6 degrees. A shift of this amount for all three colors corresponds on the diagram of  FIG. 3  to the first X  306  outside of the two delta E circle  304  on the line  305 . The delta E values for each of the three primaries and the white point for a shift of 2.5 nanometers are as follows:  
                              Delta E                                     R   G   B   White Point                       7.186151   3.327496   3.76714   3.13456                        
       FIG. 5  shows the delta E value for the white point of the display as a function of increasing angle of incidence for the three dichroic filters. It should be understood that the angle of incidence changes used in these examples serve as a proxy for more complex variations. In general, it is possible that, depending on the design of the color separating and re-combining device in the projector, a more complex variation in angle of incidence for the dichroic filters may arise. In some cases, it may be possible to fully optimize the angle of incidence for at least one of the filters. The variation of the angle of incidence also serves as a proxy for other sources of shift in the filter&#39;s transmission spectra such as those caused by variations in the coating thickness for various layers of the complex multi-layer stacks generally used in such filters. In the manufacturing of dichroic filters used in SLM based projectors for color separation and re-combining a dominant wavelength and overall passband tolerance of ±5 nm is considered a very tight tolerance, near the limits of repeatability.  
      The delta E methods used in these examples are best suited to evaluate color differences for adjacent areas of color, such as those found on either side of the seam region of a tiled display. The magnitude of delta E that corresponds to a visible difference is not an absolute. Color differences are significantly affected by viewing conditions. A delta E of two corresponds reasonably well to the smallest visible color difference between white points on two projection displays in a tiled configuration at screen luminance levels of 12 to 16 foot lamberts. The bit depth of the display limits the ability of a display that might be used to show a color simulation of color differences. On the 8 bit per color displays typical of most computers a simulated color difference of three is just visible under ideal viewing conditions. However, a projection display suited to high quality applications uses either a 10 bit per color logarithmic data format or a 14 to 16 bit linear format in order to provide the dynamic range and fidelity required.  
      The other effect of color shifts in the color separating and re-combining device of the projector is to alter the gamut of displayable colors.  FIG. 6  compares the gamut of a reference projector  601  to the gamut of a projector  602  where the passbands of the three color separation and re-combining filters are shifted by 5 nanometers.  
      The present invention allows the adjustment of color in an SLM based projection system by controlling the spectral energy distribution of the light entering the color separating and re-combining device. This can correct for variations in the color of the input light caused by variations in the lamp and reflector system and also correct for variations in the colors produced by the color filters in the color separating and re-combining device. The present invention exploits the realization that for an additive color mixing system comparatively broad band color filters are used in the color separating and re-combining device, which in turn produce broad band color stimuli that are perceived by the human observer whose visual system also has broad band responses to color. By adding narrowband light energy within the passband of each of the broad band color filters in the color separating and re-combining device into the illuminating light input to the color separating and re-combining device the perceived color can be altered. If both the optical power and the wavelength range of the narrowband source are adjustable then color variations in the system can be controlled and the primary colors of the resulting display matched to the desired standard colors.  
      A first exemplary embodiment of this invention is shown in schematic diagram form in  FIG. 7 , which forms the illumination system of a projector incorporating the methods of the present invention. At  701  the main illumination source consists of a reflector assembly and a high pressure Xenon arc lamp. An elliptical reflector and a spherical retro-reflector combination is shown, but other reflector and lamp combinations may be used and are known to those skilled in the art. The unwanted infrared component of the light from  701  is removed by selectively reflecting filter  702 . Light  703  from filter  702  is then directed to illumination integrating bar  714  by light mixing system  713 . The output of integrating bar  714  is then focused into the desired illumination cone by relay  715  and then directed into color separating and re-combining device  716  where it illuminates the SLM devices. The color separating and re-combining device  716  is analogous to the color separating and re-combining device  105  shown in FIG.  1 , and the balance of the projection optical system including the SLMs, electronics and projection lens may be inferred by reference to  FIG. 1 .  
      In a typical projector the main illumination source may be a Xenon arc lamp with an input power rating of 3 kW or more. This source provides the main source of illumination for the projected image. Secondary illumination sources are provided for color correction. These sources consist of lamp and reflector assemblies  704 ,  707  and  710  in conjunction with wavelength selecting filters  705 ,  708  and  711 . Lamp and reflector assembly  704  and wavelength selecting filter  705  produce an illuminating light  706  with an optical power of approximately 20% of that of the main source with a wavelength distribution confined to the red portion of the spectrum, for example curve  801  on graph  800  in  FIG. 8 . Light  706  is then mixed with the light from the main source  701  and directed to illumination integrating bar  714  by light mixing system  713 .  
      Similarly, lamp and reflector  707  and wavelength selecting filter  708  produce an illuminating light  709  with an optical power of approximately 20% of that of the main source with a wavelength distribution confined to the green portion of the spectrum, for example curve  802  on graph  800  in  FIG. 8 . The illuminating light  709  is then mixed with the light from the main source  701  and the light from secondary source  704  and directed to illumination integrating bar  714  by light mixing system  713 .  
      Similarly lamp and reflector  710  and wavelength selecting filter  711  produce an illuminating light  712  with an optical power of approximately 20% of that of the main source with a wavelength distribution confined to the blue portion of the spectrum, for example curve  803  on graph  800  in  FIG. 8 . The illuminating light  712  is then mixed with the light from the main source  701  and the light from secondary sources  704  and  707  and then directed to illumination integrating bar  714  by light mixing system  713 .  
      As a result, the total illumination received by integrating bar  714  is the sum of the light from each of the four lamp and reflector systems and associated filters. Each of the secondary illumination sources is provided with a controlling device so that the contribution of each source to the total illuminating light entering  714  may be adjusted. This can be accomplished, for example, by controlling the power supplied to each of the secondary illumination lamps in the lamp and reflector combinations  704 ,  707  and  710  or by controlling the amount of light  706 ,  709  and  712  that reaches the integrating bar by means of a variable optical attenuator such as an adjustable aperture or a variable neutral density filter. Secondary power control signals are calculated, for example, as described below with reference to  FIG. 15 .  
      The light mixing system  713  in  FIG. 7  can be constructed in various ways. A simple example is a mirror arrangement, such as a four sided pyramid that shares the input aperture of the integrator rod  714  between the four sources. There are efficiency considerations in such a sharing arrangement. The area of the SLM devices on the color separating and re-combining device  716  and the f-number of the projection lens normally determine the limiting étendue in the illumination system of the projector. The integrator rod input aperture is usually matched to this étendue, taking into consideration any magnification that may be provided by the relay optics  715 . The étendue of the illuminating source is generally much larger and as a consequence only a portion of the total flux from the illumination source is coupled into the projector&#39;s illumination system.  
      A system like that in  FIG. 7  must share the SLM étendue between the main and secondary sources and this sharing will have an impact on the efficiency of the system. The biggest impact will be on the efficiency of the main source since it will have the largest arc and therefore it will have the greatest mismatch to the étendue of the SLM.  
      For wider aspect ratios used in some displays, such as those required for motion picture applications, the input aperture of the integrator rod may be under filled in one direction by the main source  701 . It is then possible to arrange the mixing system  713  to fill in the edges of the input aperture with the light from the secondary sources  704 ,  707  and  710 .  
      As is shown below, it is possible that for some applications it will not be necessary to provide 3 secondary sources, and in those cases the problem of sharing the SLM étendue is reduced. Other alternative configurations for the light mixing system  713  will be known to those skilled in the art, and these may be employed without departing from the spirit of the invention.  
      A second exemplary embodiment is shown in  FIG. 9  that arranges the main and secondary illumination sources so that the SLM étendue does not have to be shared between the sources. Main illumination source  901  again consists of a reflector assembly and a high pressure Xenon arc lamp. An elliptical reflector and a spherical retro-reflector combination is shown, but other reflector and lamp combinations may be used and are known to those skilled in the art. The unwanted infrared component of the light  902  from  901  is removed by selectively reflecting filter  903 . Light  904  from filter  903  then proceeds to illumination integrating bar  917 . The output of integrating bar  917  is then focused into the desired illumination cone by relay  918  and then directed into color separating and re-combining device  919  where it illuminates the SLM devices. The color separating and re-combining device  919  is analogous to the color separating and re-combining device  105  shown in  FIG. 1 , and the balance of the projection optical system including the SLMs, electronics and projection lens may be inferred by reference to  FIG. 1 .  
      Lamp and reflector assembly  905  produces secondary illumination light  906 , which is folded through 90 degrees by wavelength selective reflector  907 . The portion of the light  906  that is reflected by  907  becomes illuminating light  908  which has an optical power of approximately 20% of that of the main illumination source with a wavelength distribution confined to a narrow portion of the red region of the spectrum, for example curve  805  on graph  804  in  FIG. 8 . Wavelength selective reflector  907  reflects the desired portion of the light from secondary illumination source  905  and transmits all of the light  904  from the main illumination source  901  outside of the portion of the spectrum that corresponds to the reflected light  908 . The input aperture of illumination integrating bar  917  is now fully available to sources  901  and  905 , with the loss of light  904  from the main source  901  confined to the narrow portion of the spectrum that corresponds to the reflected light  908 .  
      Lamp and reflector assembly  909  produces secondary illumination light  910 , which is folded through 90 degrees by wavelength selective reflector  911 . The portion of the light  910  that is reflected by  911  becomes illuminating light  912  which has an optical power of approximately 20% of that of the main source with a wavelength distribution confined to a narrow portion of the green region of the spectrum, for example curve  806  on graph  804  in  FIG. 8 . Wavelength selective reflector  911  reflects the desired portion of the light from secondary source  909  and transmits all of the light  904  from the main source  901  outside of the portion of the spectrum that corresponds to the reflected light  912 . Wavelength selective reflector  911  also transmits all of the light  908  from secondary source  905 . The input aperture of illumination integrating bar  917  is now fully available to sources  901 ,  905  and  909 , with the loss of light  904  from the main source  901  confined to the narrow portions of the spectrum that correspond to the reflected light  908  and reflected light  912 .  
      Lamp and reflector assembly  913  produces secondary illumination light  914 , which is folded through 90 degrees by wavelength selective reflector  915 . The portion of the light  914  that is reflected by  915  becomes illuminating light  916  which has an optical power of approximately 20% of that of the main illumination source with a wavelength distribution confined to a narrow portion of the blue region of the spectrum, for example curve  807  on graph  804  in  FIG. 8 . Wavelength selective reflector  915  reflects the desired portion of the light from secondary source  913  and transmits all of the light  904  from the main illumination source  901  outside of the portion of the spectrum that corresponds to the reflected light  916 . Wavelength selective reflector  915  also transmits all of the light  908  from secondary illumination source  905  and all of the light  912  from secondary illumination source  909 . The input aperture of illumination integrating bar  917  is now fully available to sources  901 ,  905 ,  909  and  913  with the loss of light  904  from the main illumination source  901  confined to the narrow portions of the spectrum that correspond to the reflected light  908 , reflected light  912  and reflected light  916 . The resulting spectrum for the transmission of light  901  through filters  907 ,  911  and  915  is approximated by curve  809  on graph  808  in  FIG. 8 . Each of the secondary illumination sources is provided with a controlling device so that the contribution of each source to the total illuminating light entering  917  may be adjusted. This can be accomplished, for example, by controlling the power supplied to each of the secondary illumination lamps in the lamp and reflector combinations  905 ,  909  and  913  or by controlling the amount of light  908 ,  912  and  916  that reaches the integrating bar by means of a variable optical attenuator such as an adjustable aperture or a variable neutral density filter. Secondary power control signals are calculated, for example, as described below with reference to  FIG. 15 .  
      A third exemplary embodiment is shown in  FIG. 10  that also arranges the main and secondary illumination sources so that the SLM étendue does not have to be shared between the sources. Main illumination source  1001  again consists of a reflector assembly and a high pressure Xenon arc lamp. An elliptical reflector and a spherical retro-reflector combination is shown, but other reflector and lamp combinations may be used and are known to those skilled in the art. Selectively reflecting mirror  1003  transmits the unwanted infrared component of the light  1002  while reflecting the desired illuminating light  1004  through 90 degrees. Illuminating light then proceeds to illumination integrating bar  1017 . The output of integrating bar  1017  is then focused into the desired illumination cone by relay  1018  and then directed into color separating and re-combining device  1019  where it illuminates the SLM devices. The color separating and re-combining device  1019  is analogous to the color separating and re-combining device  105  shown in  FIG. 1 , and the balance of the projection optical system including the SLMs, electronics and projection lens may be inferred by reference to  FIG. 1 .  
      Lamp and reflector assembly  1005  produces secondary illumination light  1006 , which passes through wavelength selective reflector  1007 . Wavelength selective reflector  1007  reflects the unwanted portion of the light from secondary illumination source  1005  and transmits the desired portion which becomes illuminating light  1008  which has an optical power of approximately 20% of that of the main illumination source with a wavelength distribution confined to a narrow portion of the red region of the spectrum, for example curve  805  on graph  804  in  FIG. 8 . Selectively reflecting mirror  1003  transmits the light  1008 , and reflects all of the light  1004  from the main source  1001  outside of the portion of the spectrum that corresponds to the transmitted light  1008 . The input aperture of illumination integrating bar  1017  is now fully available to main source  1001 , with the loss of light  1004  from the main illumination source  1001  confined to the narrow portion of the spectrum that corresponds to the transmitted light  1008 .  
      Additional secondary illumination sources may be added as shown in  FIG. 10 , where lamp and reflector assembly  1009  produces secondary illumination light  1010  and in combination with wavelength selective reflector  1011  produces illuminating light  1012  which has an optical power of approximately 20% of that of the main source with a wavelength distribution confined to a narrow portion of the green region of the spectrum, for example curve  806  on graph  804  in  FIG. 8 . Similarly lamp and reflector assembly  1013  produces secondary illumination light  1014  and in combination with wavelength selective reflector  1015  produces illuminating light  1016  which has an optical power of approximately 20% of that of the main illumination source with a wavelength distribution confined to a narrow portion of the blue region of the spectrum, for example curve  807  on graph  804  in  FIG. 8 . Since each of the secondary sources  1005 ,  1009  and  1013  will have smaller lamps, and therefore smaller arcs with a correspondingly smaller étendue, the available étendue of the SLM devices as represented by the input aperture of integrator  1017  may be shared by suitable shaping of the intensity distribution from each of the secondary illumination sources. These sources may be arranged for example at the vertices of a triangle (represented in the plan view of  FIG. 10  by the partial overlapping of the representations of the three secondary sources) and a portion of the total acceptance angle of the integrator rod allocated to each of the sources. Each of the secondary illumination sources is provided with a controlling device so that the contribution of each source to the total illuminating light entering  1017  may be adjusted. This can be accomplished, for example, by controlling the power supplied to each of the secondary illumination lamps in the lamp and reflector combinations  1005 ,  1009  and  1013  or by controlling the amount of light  1008 ,  1012  and  1016  that reaches the integrating bar by means of a variable optical attenuator such as an adjustable aperture or a variable neutral density filter. Secondary power control signals are calculated, for example, as described below with reference to  FIG. 15 .  
      The designs of  FIGS. 9 and 10  may be optimized by considering for example the spectral energy distribution of main illuminating source  901 , which for a Xenon lamp is similar to that shown at  1101  in  FIG. 11 . As the figure shows, some portions of the curve contain less of the total power, and if the secondary illumination source wavelengths are placed in these regions the loss of power from the main source will be reduced.  
      It is also important to note that in the systems of  FIGS. 7, 9  and  10  the spectral energy distribution of each of the secondary illumination sources will be filtered by the color filters in the color separating and re-combining device of the projector. This affects the choice of spectra for the secondary illumination sources, and it is important with respect to efficiency that these sources be located in a spectral region where the passband of the corresponding color separating and re-combining device filter has reasonably high transmission.  
      The use of broad band widths for the filters of the color separating and re-combining device is desirable for efficient use of the light from a white light source such as a Xenon lamp. The broad band widths also reduce the tendency for light to scatter in the color separation and re-combining system, and reduce the effect of shifts of passband wavelength since the eye averages the total light through each filter. Narrow band sources have the disadvantage of being less efficient and more sensitive to wavelength shifts since the color shift due to a change in their wavelengths is more easily seen.  
      However, if the angle of incidence on the wavelength selecting reflectors  907 ,  911  and  915  (or  1007 ,  1011  and  1015 ) is made adjustable, variation in color from the secondary illumination sources can be eliminated. As would be known to one skilled in the art, some means must also be provided for compensating the change in the direction of reflection so that the light remains focused on the input of integrating bar  917  (or  1017 ).  
      Another embodiment of the apparatus of  FIGS. 9 and 10  can be realized by eliminating the main illumination source  901  or  1001  entirely, and increasing the power of illumination sources  905  ( 1005 ),  909  ( 1009 ) and  913  ( 1013 ). In some applications this may prove to be a more efficient arrangement, particularly since the photopic weighting of powers for the three sources results in the green source having a higher total flux requirement than the red and blue sources, which improves the overall efficiency of such an arrangement by requiring less power from the red and blue secondary sources.  
      It is also desirable that the intensity distributions of the main and secondary illumination sources be matched so that unwanted non-uniformities in color do not arise in applications where the subsequent optical system may modify the intensity distribution of the combined sources.  
      The operation of the apparatus of  FIG. 7  may be understood with reference to  FIG. 12 . This discussion also applies to the systems of  FIGS. 9 and 10  and their corresponding components.  FIG. 12  shows a CIE xy diagram using the color matching functions for the  1931  2 degree observer. The solid line forming triangle  1201  connects three points  1202 ,  1203  and  1204  which are the x and y chromaticity values for the red, green and blue color filters of the color separating and re-combining device  716  in  FIG. 7 . This triangle represents the gamut of colors that can be formed by all combinations of brightnesses of the three color channels of a projection system that employs the color separating and re-combining device  716  of  FIG. 7  if it were illuminated by the main illumination source without the contribution of the secondary illumination sources (all secondary illumination optical powers are set to zero).  
      Similarly, the dashed line forming triangle  1211  connects the three points  1212 ,  1213  and  1214  which are the x and y chromaticity values for red, green and blue secondary illumination sources and filters  704  and  705 ,  707  and  708 , and  710  and  711  in  FIG. 7 . Triangle  1211  represents the gamut of colors that would be formed by all combinations of various optical powers of the three secondary sources  704 ,  707  and  710  as filtered by the filters  705 ,  708  and  711  in  FIG. 7  and subsequently by the color filters of the color separating and re-combining device  716  of  FIG. 7  without the contribution of the main illumination source.  
      The three color channels of the projector control the SLM devices to modulate the light that is directed to the screen by the color separating and re-combining device and the projection lens. The light entering the color separating and re-combining device is the sum of the main illumination source and the secondary illumination sources as previously described. The color that is displayed when the brightness of all three channels of the projector are driven to their maximum value, or 100% of full scale, is by convention called the white point of the display. The white point for the gamut of the projector with the optical power of the secondary sources set to zero is shown at  1205 . The chromaticity of the white point of any three primary color gamut is computed as follows in formula (3):  
               WPx   =       Rx   +   Gx   +   Bx     3       ⁢     
     ⁢     WPy   =       Ry   +   Gy   +   By     3               (   3   )             
          where 
            Rx, Ry=chromaticities of the red primary     Gx, Gy=chromaticities of the green primary     Bx, By=chromaticities of the blue primary     WPx, Wpy=chromaticities of the white point where R=G=B=100%    
               

      In other words, the white point of the display is the centroid of the triangle formed by the three primaries. Primary means a set of three spectral energy distributions that are selected such that none of the three spectral energy distributions can be matched by a mixture of the other two. In terms of the chromaticity diagram this results in a triangle, since by definition a triangle is formed by three non-collinear points. The selection of primaries for an image projection system is not arbitrary. In general the primaries are selected so that the gamut formed by the three primaries includes all of the colors that the system is required to reproduce.  
      As previously discussed the color of this white point can be adjusted by changing the gain of one or more of the color channels in the projector so that an input pixel brightness value of 100% for each of red, green and blue is displayed with pixel brightness values of less than 100% for one or more of the three colors according to the desired white point. However, as also discussed this reduces the maximum brightness of the display and also in general can only correct the color balance for neutral tones and other colors that are mixtures of all three of the primaries.  
      The addition of a second set of primaries to the system allows the color balance of the display to be altered without reducing the pixel brightness for any of the three colors, and permits adjustment of the primary chromaticities of the display resulting in an actual shifting of the white point and the associated gamut. This can be understood with reference to  FIG. 12  as follows.  
      The vector  1223  connecting the points  1203  and  1213  is the line along which all combinations of mixture for the green primary and the green secondary source will be found. The ratio of the optical powers of these two sources is equal to a proportion of the distance along the vector  1223 . The mixture that is formed when each source is at the same optical power is located at the midpoint of the vector  1223 . In the example system of  FIG. 7  the maximum optical power of the secondary sources was selected to be approximately 20% of the optical power of the main source. This limits the distance along the vector  1223  that the mixture can travel from the primary  1203  to the secondary  1213  to that shown by the “X” at  1226  in  FIG. 12 .  
      Similarly the vector  1222  connecting the points  1202  and  1212  is the line along which all combinations of mixture for the red primary and the red secondary source will be found. The ratio of the optical powers of these two sources is equal to a proportion of the distance along the vector  1222 . The mixture that is formed when the two sources are at the same optical power is located at the midpoint of the vector  1222 . In the example system of  FIG. 7  the maximum optical power of the secondary illumination sources was selected to be approximately 20% of the optical power of the main illumination source. This limits the distance along the vector  1222  that the mixture can travel from the primary  1202  to the secondary  1212 .  
      The vector  1224  connecting the points  1204  and  1214  is the line along which all combinations of mixture for the blue primary and the blue secondary source will be found. The ratio of the optical powers of these two sources is equal to a proportion of the distance along the vector  1224 . The mixture that is formed when the two sources are at the same optical power is located at the midpoint of the vector  1224 . In the example system of  FIG. 7  the maximum optical power of the secondary sources was selected to be approximately 20% of the optical power of the main illumination source. This limits the distance along the vector  1224  that the mixture can travel from the primary  1204  to the secondary  1214 .  
       FIG. 13  is a CIE xy diagram which shows the effect of varying the angle of incidence on the chromaticities of each of the primary color filters in the color separating and re-combining device  716  in  FIG. 7 . The triangle at  1301  shown with a solid line connects the loci of the three primary chromaticities  1305 , red,  1308 , green, and  1311 , blue for the three primary filters at the nominal angle of incidence and represents the gamut of colors that can be displayed with these primaries. The resulting white point for these three primary chromaticities is shown at  1314 . The triangle  1302 , shown with a dashed outline connects the loci of the three primary chromaticities  1304 , red,  1307 , green, and  1310 , blue for the three primary filters at an angle of incidence greater than the nominal angle of incidence and represents the gamut of colors that can be displayed with these primaries. The resulting white point for these three primary chromaticities is shown at  1313 . The triangle  1303 , shown with a dotted outline connects the loci of the three primary chromaticities  1306 , red,  1309 , green, and  1312 , blue for the three primary filters at an angle of incidence less than the nominal angle of incidence and represents the gamut of colors that can be displayed with these primaries. The resulting white point for these three primary chromaticities is shown at  1315 .  
      The diagram in  FIG. 13  shows the expected effect of varying the angle of incidence or shifting the spectral passband of each filter. The effect is to move the chromaticity of the primaries more or less along the spectral loci, towards the longer wavelengths for a decrease in the angle of incidence, and towards the shorter wavelengths for an increase in the angle of incidence. Formula (1) given earlier also predicts that the wavelength shift with a change of angle of incidence will be greater for longer wavelengths, but  FIG. 13  appears to suggest that the shift is greatest for the green primary.  
       FIG. 14  shows the same information as  FIG. 13 , but plotted on a u*v* diagram, centered on the white point  1404  of the three primaries with the nominal angle of incidence. The triangle  1401  and its vertices represent the chromaticities of the primaries at the nominal angle of incidence, and the triangle  1402  represents the effect of an increased angle of incidence while the triangle  1403  represents the effect of a decreased angle of incidence. Here the distance traversed by each primary is more nearly equal because of the more uniform character of the u*v* diagram.  
      Because longer wavelengths are shifted more for a given change of angle of incidence there is also a change in the width of the passband for a dichroic filter. As the angle of incidence increases the longer wavelength side of the passband moves further towards the shorter wavelengths than does the short wavelength side of the passband. This results in a slight narrowing of the passband as the angle of incidence increases. This accounts for a non-linear movement of the chromaticities which is most easily seen for the blue primary in  FIG. 14 .  
      It should now be clear that a preferred method of determining the optical power settings for the secondary sources is to first determine the chromaticities of the primary sources alone, and then to add the required amount of each secondary source to the primary source so as to bring the resultant mixture as close as possible to the desired chromaticity for each primary. When this is done the resulting white point will also be located at the desired chromaticity.  
      The chromaticity coordinates for a mixture of two colors can be calculated as follows in formula (4):  
                 M   x     =       C   ⁢           ⁢     1   x       -     [       (       C   ⁢           ⁢     1   x       -     C   ⁢           ⁢     2   x         )     *       α   ⁢           ⁢   C   ⁢           ⁢   2         α   ⁢           ⁢   C   ⁢           ⁢   1     +     α   ⁢           ⁢   C   ⁢           ⁢   2           ]         ⁢     
     ⁢       M   y     =       C   ⁢           ⁢     1   y       -     [       (       C   ⁢           ⁢     1   y       -     C   ⁢           ⁢     2   y         )     *       α   ⁢           ⁢   C   ⁢           ⁢   2         α   ⁢           ⁢   C   ⁢           ⁢   1     +     α   ⁢           ⁢   C   ⁢           ⁢   2           ]                 (   4   )             
          Where     Mx,My=x,y chromaticity coordinates of the mixture of color C 1  and C 2      C 1   x ,C 1   y =x,y chromaticity coordinates of the color C 1      C 2   x ,C 2   y =x,y chromaticity coordinates of the color C 2      αC 1 =amount of color  1      αC 2 =amount of color  2         

      The color amounts are in arbitrary units, typically a range of 0 to 1 is used. For example, the chromaticity for the green primary source  903  in  FIG. 9  may be represented by C 1 , and the chromaticity for the green secondary source  913  in  FIG. 9  may be represented by C 2 . The amount of C 1  is then 1, and the amount of C 2  (using the instance of a secondary source with 20% of the power of C 1 ) is 0.2.  
      A preferred embodiment of the inventions in  FIGS. 7, 9  and  10  would use secondary sources with chromaticities that are located along the vector representing the anticipated color shift of the primaries in the color separating and re-combining device, and located in the direction opposite that of the expected shift. Optimization of such a design would require selection of tolerances and specifications for these filters that would produce an appropriate bias in the chromaticity range of the color separation and re-combining primaries, and also allow for tolerances in the color filters of the secondary sources.  
      It is possible that a given system may not require the adjustment of all three primary chromaticities. While three sources provides the most general configuration, if the nature of the color shifts exhibited by a particular display system are carefully evaluated in a particular application only one or two secondary sources may be required. It should also be clear that if the requirement is to match the white point, without fully correcting the colors of the primaries, then a single optimally positioned secondary source would allow adjustment of the white point along the vector connecting that secondary source with the uncorrected white point of the display.  
      Referring again to  FIG. 13 , in general the selection of chromaticities for the secondary sources is made in such a way as to ensure that the gamut of the system can be adjusted in the required directions and over the required range while keeping the power of the secondary sources as low as practical, particularly when two or more secondary sources are required. A system providing the most general operation will preferably have equal lengths for all three of the vectors connecting the main and secondary sources, providing the greatest possibilities for shifting the primary chromaticities in any direction.  
      It is also preferable that the systems of  FIGS. 7, 9  and  10  incorporate a facility for adjusting the overall brightness of the display in an achromatic fashion. That is, the facilities provided for brightness adjustment should act to maintain the ratio of flux levels between the main illumination source and the secondary illumination sources as the display brightness is adjusted. This adjustment system may be described by reference to the block diagram of  FIG. 15 .  
      The overall brightness of the display is controlled by master brightness control  1501 . This may be, for example, a software selected value, adjusted as a percentage of full scale from 0 to 100 percent. Similarly  1502  is the main source optical power control, also software selected value ranging from 0 to 100 percent. The function at  1503  is a multiplier which causes the main source optical power command  1504  to be formed as the product of the master brightness control value  1501  and the main source optical power control value  1502 . The main source optical power command  1504  may be for example a binary number corresponding to the selected optical power that is in turn supplied to a digital to analog converter and the resulting voltage used to control the main source lamp power via an adjustable output lamp power supply.  
      Control  1505  is the optical power control for the red secondary source. This may be a software selected value, adjusted as a percentage of full scale from 0 to 100 percent, that is set to the value determined by the calculations previously described to adjust the chromaticity of the red channel of the display system. This value is processed by multiplier function  1506  which forms the red secondary source optical power command  1507  as the product of the red secondary source optical power control value  1505  and the master brightness control value  1501 . The red secondary source optical power command  1507  may be for example a binary number corresponding to the selected optical power that is in turn supplied to a digital to analog converter and the resulting voltage used to control the red secondary source lamp power via an adjustable output lamp power supply.  
      Similarly control  1508  is the optical power control for the green secondary source. This may be a software selected value, adjusted as a percentage of full scale from 0 to 100 percent, that is set to the value determined by the calculations previously described to adjust the chromaticity of the green channel of the display system. This value is processed by multiplier function  1509  which forms the green secondary source optical power command  1510  as the product of the green secondary source optical power control value  1508  and the master brightness control value  1501 . The green secondary source optical power command  1510  may be for example a binary number corresponding to the selected optical power that is in turn supplied to a digital to analog converter and the resulting voltage used to control the green secondary source lamp power via an adjustable output lamp power supply.  
      Similarly control  1511  is the optical power control for the blue secondary source. This may be a software selected value, adjusted as a percentage of full scale from 0 to 100 percent, that is set to the value determined by the calculations previously described to adjust the chromaticity of the blue channel of the display system. This value is processed by multiplier function  1512  which forms the blue secondary source optical power command  1513  as the product of the blue secondary source optical power control value  1511  and the master brightness control value  1501 . The blue secondary source optical power command  1513  may be for example a binary number corresponding to the selected optical power that is in turn supplied to a digital to analog converter and the resulting voltage used to control the blue secondary source lamp power via an adjustable output lamp power supply.  
      The multiplier functions  1503 ,  1506 ,  1509  and  1512  cause the master brightness control  1501  to adjust the optical power of all four sources in proportion, maintaining the same relative balance between them as the overall brightness of the display is varied by the master brightness control.  
      The adjustment of the systems of  FIGS. 7, 9  and  10  may be accomplished according to the procedure of  FIG. 16 . First in step  1600  the input image channel gains are set to maximum (100 percent) for all three input channels, red, green and blue. The master brightness control ( 1501  in  FIG. 15 ) is also set to full scale (100 percent). The optical power of all three secondary sources is then set to zero at step  1602 . Next at step  1604  the projector is supplied with a full white input signal and the optical power of the main illumination source is adjusted to set the desired display brightness. At step  1606  a full red input signal is supplied to the projector so that all pixels of the red image SLM in the projector are driven to full brightness. The spectral energy distribution of the red image is then measured in step  1608 . Similarly a full green input signal is supplied to the projector in step  1610 , and the spectral energy distribution of the green image is measured in step  1612 . Similarly a full blue input signal is supplied to the projector in step  1614 , and the spectral energy distribution of the blue image is measured in step  1616 . The tristimulus values for the red, green and blue images are then computed using the CE color matching functions in step  1618 . The CIE xy values for the red, green and blue primaries are then computed from the tristimulus values in step  1620 .  
      The CIE xy coordinates of the secondary illumination sources are then obtained in steps  1622  through  1640  by setting the main illumination source optical power to zero and then measuring the spectral energy distribution of a full white image illuminated in turn by each of the secondary illumination sources alone and computing the tristimulus values for these spectral energy distributions and converting them to CIE xy coordinates. The vector representing the adjustment range possible for each of the three colors is then the line connecting the CIE xy coordinates of the primaries computed in step  1620  with the CIE xy coordinates of the secondary illumination sources computed in step  1640 . Assuming the CIE xy coordinates for the desired primaries are known the required secondary illumination source optical powers may be determined by finding where the shortest line from each of the desired CIE coordinates intersects the vector between the main source chromaticities and the secondary illumination source chromaticities for each primary. The location of the nearest point on the main source—secondary source line to the desired CIE coordinate may be calculated as follows:  
             d   =                (       C   ⁢           ⁢     2   y       -     C   ⁢           ⁢     1   y         )     ⁢     (       C   ⁢           ⁢     3   x       -     C   ⁢           ⁢     1   x         )       -       (       C   ⁢           ⁢     2   x       -     C   ⁢           ⁢     1   x         )     ⁢     (       C   ⁢           ⁢     3   y       -     C   ⁢           ⁢     1   y         )                      (       C   ⁢           ⁢     2   x       -     C   ⁢           ⁢     1   x         )     2     +       (       C   ⁢           ⁢     2   y       -     C   ⁢           ⁢     1   y         )     2                   (   5   )                        C   ⁢           ⁢   1   ⁢   CM     ⇀          =                  C   ⁢           ⁢   1   ⁢           ⁢   C   ⁢           ⁢   3     ⇀          2     -     d   2                 (   6   )               k   =              C   ⁢           ⁢   1   ⁢   CM     ⇀                   C   ⁢           ⁢   1   ⁢           ⁢   C   ⁢           ⁢   2     ⇀                    (   7   )                 CM   x     =       [       k   *     ⁢            C   ⁢           ⁢     1   x       -     C   ⁢           ⁢     2   x                ]     +     C   ⁢           ⁢     1   x                 (   8   )                 CM   y     =       [       k   *     ⁢            C   ⁢           ⁢     1   y       -     C   ⁢           ⁢     2   y                ]     +     C   ⁢           ⁢     1   y                 (   9   )             
 
 where: 
      C 1  is the CIE coordinate for the main source primary chromaticity     C 2  is the CIE coordinate for the secondary source chromaticity     C 3  is the CIE coordinate for the desired primary chromaticity  
                 
 
 is the vector between C 1  and C 2   
                 
 
 is the vector between C 1  and C 3  
    d is the shortest distance between C 3  and  
                 
 
                 
 
 is the vector along C 1  to the perpendicular vector between  
                 
 
 and C 3  with length d 
    k is the ratio of the length of  
                 
 
 to the distance between C 1  and C 2  
    CM is the CIE coordinate for the point on  
                 
 
 that is nearest to C 3  
   

      Once the coordinate CM is known, then the amount of C 2  required to form the mixture with C 1  that will result in the chromaticity CM may be calculated. The main source optical power will be left at the value set in step  1604 , so the amount of the color represented by C 1  will be 1, and the amount of C 2  may be computed using the following equation:  
               α   ⁢           ⁢   C   ⁢           ⁢   2     =     k     1   -   k               (   10   )             
          where:     k is from equation (7) above     αC 2  is the amount of the color represented by CIE coordinate C 2         

      These computations are performed in steps  1642  and  1644  in  FIG. 16 . The secondary source optical powers are then set in step  1646  and in step  1648  the main source is returned to the setting established in step  1604 . The brightness is then adjusted in step  1650  using the master brightness control as required to compensate for the additional light from the secondary sources.  
       FIG. 17  shows the action of the system of  FIG. 7  in adjusting the display gamut for the case of the 6 degree shift shown in  FIG. 6 . In this case the white point is shifted as shown in  FIG. 17  from its original location  1702  to a new location  1703  which is within two delta E units of the reference projector white point  1701 . This results in the delta E values for the three primaries and the white point as shown in the following table:  
                              Delta E                                     R   G   B   White Point                       2.095151   1.971248   2.07642   1.445626                        
      The relative spectral distributions for the main illumination source and the settings of the secondary illumination sources are shown for this example in  FIG. 18 . The main illumination spectral energy distribution is normalized to 1 and shown as the curve  1801 . The secondary illumination spectra are shown at their respective proportional powers as  1802  for red,  1803  for green and  1804  for blue.  FIG. 19  shows the resulting restoration of the color gamut location  1902  with respect to the reference color gamut  1901 .  FIG. 19  shows that the color coordinates of the display primaries have been re-aligned by the addition of the secondary sources in the system of  FIG. 7 . Consequently the matching of saturated colors is achieved. As discussed above, all SLM based projection systems have a finite and non-zero black level. Because the color balance is achieved prior to the SLM devices, the color match will obtain all the way to the minimum displayable value and to the display black level. This is not true for systems that manipulate color by modifying the input signal since black is the result when no signal is displayed.  
      Another embodiment of the systems of  FIGS. 7, 9  and  10  incorporates adjustable dichroic filters for each of the secondary illumination sources. In this embodiment the angle of each secondary filter with respect to the incident beam of the secondary illumination source is made adjustable, allowing the resulting spectral energy distribution of each secondary source to be shifted towards longer or shorter wavelengths.  FIG. 20  shows the passbands for the green primary and green secondary color filters. The solid line at  2001  indicates the passband of the green primary color filter in the color separating and re-combining device  716  in  FIG. 7 . The dotted line at  2002  indicates the passband of the green secondary color filter  708  in  FIG. 7 . The heavy solid line  2003  indicates the shift of the green primary passband when the angle of incidence on this filter is increased by 6 degrees. The heavy dotted line at  2004  shows the result of making a complimentary shift in the angle of incidence on the secondary color filter.  
       FIG. 21  shows the chromaticity coordinates that result for the primary and secondary sources based on the passbands shown in  FIG. 20 . The point  2101  corresponds to the passband for the green primary filter shown at  2001  in  FIG. 20 . The point  2102  corresponds to the passband for the green secondary filter shown at  2002  in  FIG. 20 . Similarly, the point  2103  corresponds to the passband for the green primary filter with an increase in the angle of incidence of 6 degrees as shown at  2003  in  FIG. 20 , and point  2104  corresponds to the passband for the green secondary filter when shifted by a complimentary amount as shown by the passband at  2004  in  FIG. 20 .  
      In order to move the shifted chromaticity  2103  back towards the unshifted chromaticity  2101 , it is necessary for the secondary source to be located at coordinates that represent a passband shifted towards the longer wavelengths. This can be accomplished by adjusting the angle of incidence on the green secondary filter to produce such a shift. This is illustrated in  FIG. 21  where the vector  2105  connecting points  2101  and  2103  in  FIG. 21  is the line along which the chromaticity of the green primary moves as the angle of incidence on the filter is changed. The arrow  2106  shows the direction that the chromaticity moves as the angle of incidence is increased. Similarly, the vector  2107  connecting points  2102  and  2104  is the line along which the chromaticity of the green secondary moves as the angle of incidence on the filter is changed. The arrow  2108  shows the direction that the chromaticity moves as the angle of incidence is decreased.  
      The resulting chromaticity of the green channel of the projector will lie somewhere between the vectors  2105  and  2107  of  FIG. 21  when the primary and secondary source spectra are mixed. The range of variation and the target chromaticity for the green primary will determine the preferred location of the two vectors as can be appreciated from  FIG. 21 . It should also be understood that the foregoing are for the purposes of illustration only, the magnitudes of the variations to be corrected in an actual system are not necessarily represented in  FIG. 21 , but the spirit of the invention carries over into any configuration of primary and secondary spectra and resulting chromaticities that can be realized for the purposes of adjustment of the colors in a projection display.  
      The systems of  FIGS. 7, 9  and  10  are based on conventional arc lamp sources. While screen size and illuminance requirements make the use of a high output source such as a Xenon arc lamp for the main illumination source preferable, the secondary illumination sources could be Xenon or other types of arc lamps as well or alternatively the secondary sources could be incandescent sources, lasers or a light emitting diodes (LEDs). If incandescent sources, lasers or light emitting diode (LED) array sources are used in the systems of  FIGS. 7, 9  and  10  suitable changes to the optical configuration of the secondary illumination source lamp and reflector assemblies would be needed as known to those of skill in the art. The use of lasers or LED arrays as secondary sources confers a particular benefit in that these sources provide by direct emission a selected spectral band, corresponding to the red, green or blue portions of the spectrum. Both incandescent and LED sources also offer the advantage of simple direct electronic control of brightness.  
      A third alternative configuration is suggested by further consideration of the effect of wavelength shifts as shown by the line  305  in  FIG. 3 . The effect of these shifts on the passbands of the three filters used in the color separation and re-combining filters used in the system of  FIG. 1  is shown in  FIG. 22 . For example, the reference spectral transmission curve for the red filter is shown at  2201 , and the shifted version at  2202 . These plots are for shifts due to an increase in the tilt angle of the filters, but similar shifts arise from variations in the thickness of layers in the coating stack of the filter. Similarly, the reference spectral transmission curve for the green filter is shown at  2203 , and the shifted version at  2204 , and the reference spectral transmission curve for the blue filter is shown at  2205 , and the shifted version at  2206 . In all cases, the shift is shown as being towards the blue end of the spectrum as would result from increasing the angle of incidence of the light reaching each filter.  
      The curve for a bandpass filter is shown in  FIG. 23 . The reference curve for the spectral transmission of this filter is shown at  2301 , and the shifted curve for the spectral transmission of this filter is shown at  2302 . This curve shows a similar shift towards the blue end of the spectrum as those shown by the curves in  FIG. 22 .  
      An alternative embodiment for color correction is described with reference to an exemplary embodiment shown in  FIG. 24 , which forms the illumination system of a projector incorporating the methods of the present invention. Illumination source  2401  consists of a reflector assembly and a high pressure Xenon arc lamp. An elliptical reflector and a spherical retro-reflector combination is shown, but other reflector and lamp combinations may be used and are known to those skilled in the art. The unwanted infrared component of the light from  2401  is removed by selectively reflecting filter  2402 . Illuminating light  2403  then passes through adjustable bandpass filter  2404  and then enters illumination integrating bar  2405 . The output of integrating bar  2405  is then focused into the desired illumination cone by illumination relay  2406  and then directed into color separating and re-combining device  2407 . The color separating and re-combining device  2407  is analogous to the color separating and re-combining device  105  shown in  FIG. 1 , and the balance of the projection optical system including the SLMs, electronics, and projection lens may be inferred by reference to  FIG. 1 .  
      Adjustable bandpass filter  2404  is capable of adjustment in angle with respect to the optical axis (conventionally the θ angle) of the illumination system so that the angle of incidence of light  2403  can be varied. Filter  2404  has bandpass characteristics similar to that shown in  FIG. 23 . This characteristic varies in the manner shown in  FIG. 23  as the tilt angle of this filter is varied with respect to incident light  2403 . By adjusting the angle of this filter the bandpass characteristic can be made to cut off more or less of the blue portion of the spectrum of the light entering integrating bar  2405 . At the same time, the filter will cut off more or less of the red portion of the spectrum. This adjustment operates for the adjustment of color in a fashion that is similar to that of the system of  FIG. 7  as shown by the u*v* diagram of  FIG. 25 .  FIG. 25  is a plot of a region from a diagram similar to  FIG. 2  and occupies the same area as the dashed outline at  204  in  FIG. 2 . At  2501  are the Planckian spectral loci and  2503  is the white point for typical three SLM based projector using DMD devices. The circle  2504  shows the radius of two delta E units of color difference from the white point  2503 . The line  2505  shows the direction of color shift for the projector white point caused by a variation in the tilt angle of the illuminating light on the red, green and blue dichroic filters used in the color separating and re-combining device of the projector. The point  2506  shows the shifted white point coordinates of the second projector in the system of  FIG. 1 , and the point  2507  shows the shifted white point adjusted by the system of  FIG. 24 .  
      It is also possible to add a second filter capable of adjustment similar to filter  2404 .  FIG. 26  is an example of the spectral transmission curves that can be used for two adjustable filters in the system of  FIG. 24 . Filter curve  2601  is for a first high pass filter that adjusts the boundary of the red spectral cut-off for the illuminating light. Filter curve  2602  is a for a second low pass filter that adjusts the boundary of the blue spectral cut-off for the illuminating light. These two filters provide a more flexible adjustment capable of achieving the same benefits as shown in  FIG. 25 .  
      The system of  FIG. 24  can be adjusted using similar methods to those described in  FIG. 16 . In this case it is necessary only to determine the tristimulus values of the white point of the projector when the projector is driven by a full white input signal and then to adjust the angle of the filter or filters at  2404  until the desired white point chromaticity is obtained.  
      So far the discussion of the effects of tilt have been considered to be uniform over the field of illumination. Projectors that employ dichroic filters for color separation and re-combining use illumination system designs that attempt to ensure that all points on each color filter see the same angular distribution. This is done by employing for example an illumination optical system that produces a telecentric image of the source of illumination for the filters in the color separating and re-combining device. With a telecentric configuration the image chief ray angles for all field points are zero and the marginal rays have essentially the same angle for each field point.  
      A variation in angle of incidence of as little as 6° or ±3° will result in a delta E shift of 3 units, enough to be visible under the conditions where certain embodiments of the present invention will be used. SLM based projectors of the type used in the system of  FIG. 1  have been known to exhibit such color shifts, with the overall image area having, for example, a horizontal shift from right to left of three delta E or more, causing one side of the projected image to have an overall blue color cast when compared to the other side. Configuring a system with two projectors, each with opposite color shifts, increases the visibility of the seam between them, which is undesirable.  
      By placing an adjustable bandpass filter, similar to  2404  in  FIG. 24  and capable of similar θ angle adjustment, within the illumination relay, for example near the pupil of a telecentric illumination relay, the color shift across the screen may be controlled by adjusting the angle of incidence on the bandpass filter. This is shown in  FIG. 27 , which corresponds to a portion of the illumination system of  FIG. 24 .  2701  in  FIG. 27  is an illumination integrating bar, which corresponds to  2405  in  FIG. 24 . The illumination relay  2702  corresponds to  2406  in  FIG. 24 .  2703  is the color separation and re-combing system, which corresponds to  2407  in  FIG. 24 . The adjustable bandpass filter is shown at  2705 , the telecentric stop of the relay is shown at  2704 , and the final lens of the relay at  2706 .  
       FIG. 28  is a detail of relay  2702 , showing the stop at  2801  and the adjustable bandpass filter at  2802 . Lens  2803  corresponds to the final relay lens element  2706  in  FIG. 27 . A ray bundle from an object field point above the optical axis is shown at  2804 , with the principle ray shown in a dashed line, and the marginal rays shown in solid lines. Similarly, a ray bundle along the optical axis is shown at  2805 , and a ray bundle from an object field point below the optical axis is shown at  2806 . As can be seen from  FIG. 28  the rays  2804  from a field point above the optical axis strike the adjustable filter  2802  at a more oblique angle than rays at the optical axis  2805  or below the optical axis  2806 . This results in a greater wavelength shift for the rays above the optical axis. These rays correspond to one edge of the projector display after the light passes through the color separating and re-combining device and is focused on the projection screen by the projection lens. The rays  2805  correspond to rays to the center of the projector display and the rays  2806  correspond to the opposite edge of the projector display from the rays at  2804 . The rays at  2805  and  2806  experience a progressively lesser wavelength shift due to the reduced angle at which they intersect the filter  2802 . By proper adjustment of the angle of the filter  2802  a color shift across the display can be compensated by the filter  2802 . If the relay or the portion of the relay containing the adjustable filter  2802  is made to rotate about the optical axis, permitting adjustment in the conventional φ direction then the orientation of the color shift correction may be adjusted, for example from horizontal to a diagonal across the display.  
      The system of  FIG. 27  can be adjusted using similar methods to those described in  FIG. 16 . In this case it is necessary only to determine the tristimulus values of the white point of the projector at opposite edges when the projector is driven by a full white input signal and then to adjust the angle and rotation of the filter at  2705  until the desired match between white point chromaticities at each edge is obtained.  
      It should be understood that the foregoing is for the purposes of illustration only and the principles of this invention can be applied to a single projector, two or more projectors, and to projectors arranged in configurations where the composite image is produced from a matrix of images arranged horizontally, vertically or both. The invention can also be applied to projectors that do not rely on dichroic filters for the separation and/or recombination of color since the methods of color adjustment are independent the filter types used in the projector for color separation and/or recombination. The present invention is intended to embrace all such alternative configurations, all of which can be implemented without departing from the spirit of the present invention.