Abstract:
A method for correcting defects ( 300 ) in an imaging system ( 10 ) that comprises several steps. Transmitting a digital image ( 370 ) to at least one spatial light modulator and capturing the resulting image. Comparing variation in intensity between each image pixel and at least one reference image pixel and deriving a correction factor ( 305 ) from the comparison. Determining gain of correction ( 480 ) at each code value ( 460 ) for each image pixel and applying the correction factor ( 580 ) and gain to the digital image ( 510 ).

Description:
FIELD OF THE INVENTION 
   This invention relates in general to a method for correcting defects in an image, an in particular to correcting non-uniformity errors that are the result of projecting onto photosensitive media from a two dimensional projection device such as a spatial light modulator device. 
   BACKGROUND OF THE INVENTION 
   Photographic images are traditionally printed on photographic paper using conventional film based optical printers. The photographic industry, however, is converting to digital imaging. The process uses images obtained from digital cameras, or scanned film exposed in traditional photographic cameras, to create digital image files that are then printed onto photographic paper. 
   The growth of the digital printing industry has led to multiple approaches to digital printing. One of the early methods used for digital printing was cathode ray tube (CRT) based printers such as the Centronics CRT recorder. Another commonly used approach to digital printing is the laser based engine shown in U.S. Pat. No. 4,728,965. Such systems are generally polygon flying spot systems which use red, green, and blue lasers. 
   A more contemporary approach uses a single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049. Spatial light modulators provide significant advantages in cost as well as allowing longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661. 
   Another low cost solution uses LCD modulators. Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661, 5,701,185, and 5,745,156. Most of these designs involve the use of a transmissive LCD modulator such as is depicted in U.S. Pat. Nos. 5,652,661 and 5,701,185. 
   A problem with all of these devices is that they occasionally produced defects in the final image. These defects may be due to, for example, surface finishing of the LCD which results in a high spatial frequency defects. Another defect, again using the LCD as an example, is low spatial frequency defects in the resulting image caused by warping of the plate as it is attached to the LCD during manufacture. 
   It is the purpose of this invention to correct image defects in an area spatial light modulator based printing system. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a method and apparatus for correcting non-uniformity in an AgX printed image projected from an area array device such as a LCD reflective array. The present invention is directed to overcoming one or more of the problems set forth in the background of the invention. 
   Briefly summarized according to one aspect of the present invention, a method for correcting defects in an imaging system comprises the steps of transmitting a digital image to at least one spatial light modulator and capturing a resulting image; comparing variations in intensity between each image pixel and at least one reference image pixel; deriving a correction factor from the comparison; determining gain of correction at each code value for each image pixel; and applying the correction factor and gain to the digital image. 
   The present invention takes the printed results of an area array device, and uses this non-uniform image as the basis for correcting the data path. This may be done by digitally scanning the printed image to receive a pixel-by-pixel digital version of the non-uniform characteristics. The method of acquiring image data may have a different response curve to the data there is, also a singular gain value that is applied to the scanned image making it a correction map for non-uniformity at a singular exposure code value. Consequently, the invention works across all code values from the area array device. The amount of corrections is weighted by utilizing information from the measured response at the image plane. 
   In a second major embodiment the amount of corrections is weighted by utilizing the general response curve of the device. In a third major embodiment a predetermined uniformity map is provided. In a fourth embodiment, multiple defect correction maps are generated. In a fifth embodiment a three dimensional look up table is generated. 
   The above and other objects, advantages, and novel features of the present invention will become more apparent from the accompanying description thereof when considered in conjunction with the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of a reflective LCD modulator system for printing two-dimensional swaths. 
       FIGS. 2   a  and  2   b  are a top plan view and a side view in cross section, respectively, of a reflective LCD modulator. 
       FIG. 3  is a flow diagram of a single gain value for a single code value for a scanner. 
       FIG. 4  is a flow diagram of a single gain value for a single code value for a CCD. 
       FIG. 5  is a flow diagram for creation of a gain correction table. 
       FIG. 6  is a gain curve test image. 
       FIG. 7  is a flow diagram for a gain correction equation. 
       FIG. 8  is a schematic showing application of the correction defect map for a single RGB gain and a gain correction table. 
       FIG. 9  is a graph of a polynomial representing a normalized response for a LCD. 
       FIG. 10  shows gain correction curves derived from the response curve. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present description will be directed in particular to elements forming part of, or in cooperation more directly with, an apparatus in accordance with the present invention. It is understood that the elements not shown specifically or described may take various forms well known to those skilled in the art. 
   Referring to  FIG. 1 , there is illustrated a printer referred to in general by numeral  10 . The photographic printer is comprised of a light emitting diode (LED)  12 , uniformizing optics  45 , polarizing beamsplitter  80  reflective LCD modulator  90 , a data path (not shown) for providing image information to the reflective LCD modulator  90 , and a print lens assembly  110 . The photographic printing system  10  provides a two-dimensional image on light sensitive media  140  located at an image plane  130 . 
   The LEDs are arranged in a two-dimensional array. Each of the LEDs emits one of three distinct wavelengths, red, blue, or green. The LEDs are typically mounted in a circular aperture and are arranged such that the colors of emission are spread evenly about the aperture. The LEDs are spaced such that a significant fraction of the light emitted can be captured in the collection aperture of the illumination optics  75 . Furthermore, redundancy in emitters reduces the system sensitivity to malfunction in any individual LED. 
   The LEDs are operated in a pulsed and color sequential manner. For any given image, groups of LEDs are activated in order of wavelength. For example, the red LEDs are activated and deactivated, the blue LEDs are activated and deactivated, then the green LEDs are activated and deactivated. Following each LED the system may include a collimating lens  14 , or alternatively a Fresnel lens. 
   Each of the LEDs is mapped by the illumination optics  75  to cover the area of the reflective LCD modulator  90  in a uniform and essentially telecentric manner at the modulator plane. The illumination optics  75  is designed to illuminate a nearly square or rectangular aperture. In general, axially symmetric components are employed in the illumination. Following the LEDs are uniformizing optics  45  comprising a field lens  30  that images light to a lenslet array  50  and field lens  60 . The light at the intermediate conjugate plane  65  is broken into a number of portions equivalent to the number of elements in the first portion of lenslet array  50 . The individual portions are then superimposed and magnified by the second portion of lenslet array  50  and the field lens  60 . A condenser lens  70  is positioned immediately before the polarizing beamsplitter  80 . In a single reflective LCD modulator imaging system a linear polarizer  40  may be incorporated in the illumination optics  75  prior to the polarizing beamsplitter  80 . 
     FIGS. 2   a  and  2   b  show a top view and a side view of a reflective LCD modulator. The reflective LCD modulator  90  consists of a plurality of modulator sites  92  that are individually modulatable. Light passes through the top surface of LCD  52 , liquid crystal material  54 , of reflective LCD modulator  90 , is reflected off the back plane of LCD  56  of the modulator, and returns through the modulator. If a modulator site  92  is “on” or bright, during the round-trip through the reflective LDC modulator  90 , the polarization state of the light is rotated. In an ideal case the polarization state of the light is rotated 90 degrees. However, this degree of rotation is rarely easily achieved. If a given modulator site is “off” or dark, the polarization state of the light is not rotated. The light that is not modulated is not passed straight through the polarized beamsplitter  80  but is redirected away from the light sensitive media  140  plane by the polarized beamsplitter  80 . 
   Referring again to  FIG. 1  the reflective LCD modulator  90  and the polarized beamsplitter  80  are followed by print lens assembly  110  and a linear polarizer  120 . This lens assembly provides the correct magnification of the image of the reflective LCD modulator  90  to the image plane  150  where the media light sensitive media  140  is located. The print lens assembly  110  is designed to provide magnification relating to a given image size at the media plane. Once imaged at the media plane, the printer moves the media to a next position and the next image is recorded. In any system on any media on which images are created at multiple wavelengths, the composite tri-color image will be referred to as an image. 
   The most readily available choice of reflective polarization based modulators is the reflective liquid crystal modulator. Such modulators, originally developed for use in projection display, can have resolutions as high as 4000×2000 modulator sites. However, most commercially available spatial light modulators display a number of non-uniformities including roll-off and site defects. 
   The reflective LCD modulator  90  can be designed in a number of different configurations. The most amenable to a low cost printing system is a single chip system used in color sequential mode. Such a LCD may be either specifically designed for color sequential use, often incorporating a faster backplane and slightly different liquid crystal compositions, or can be a single chip with a 60 to 70 frame per second backplane. The latter option is sufficient for printing because the high frame rates are not a necessity and often reduce the bit depth of the resulting image. However, while many liquid crystals are the same basic crystal for all three primary color wavelengths, sometimes, either due to the specific applied voltage or the liquid crystal thickness, operation may differ in the three wavelengths. Specifically, for a given liquid crystal composition, depth, and applied voltage, the resulting polarization rotation on an incident beam may vary with wavelength. The efficiency and contrast of the modulation will vary among the three colors. This optical system is designed to image and pass light with a rotated polarization state. However, the degree of rotation will vary as a function of wavelength. In the bright, or “on” state, this difference in rotation will affect the efficiency of the system. In other words, the percentage of incident light that is actually modulated and imaged on the media plane will vary. This difference in wavelength efficiency can be accounted for by varying the illumination strength, and exposure time. Also, the media requires different power densities in the different wavelengths. More significant problems arise in the dark or “off state”. In this state, the polarization state of the light is not rotated and should not be directed thought the polarizing beamsplitter  80  and imaged. If the polarization state of the light is in fact rotated, light will leak through the imaging system and decrease the contrast. 
   Most spatial light modulators are designed for use in projection systems. The eight bit (or greater), sequence of code values, or integer values corresponding to color levels, may not readily translate into the correct sequence of color intensity for the given application or on print media. The code values themselves are determined by the voltage and addressing of the modulator, and form a monotonically increasing sequence of intensity. Media also has a responsivity that alters the appearance of an image. For instance, the intensity corresponding to a bright red image may translate to a more muted cyan image on photosensitive media. The optical system and modulator are tuned or calibrated such that a known sequence of code values in combination with preset illumination conditions, determine known density values on media. This method of determining operating conditions pertaining to color content is called color calibration. 
   Having calibrated the system such that known intensity values are projected, and known density values are printed, information pertaining to spatial uniformity of resultant images can be determined. Spatial nonuniformities can be divided into two categories, low spatial frequency and high spatial frequency. 
   Low frequency non-uniformities refer to undesirable changes in image intensity at the image or media plane that are coarse or change gradually. A good example of such nonuniformity is roll-off at the edge of an image. Correcting low frequency nonuniformity does not require adjusting data on an image pixel basis. Consequently, the requirements on pixel alignment accuracy are not so stringent. 
   For such low frequency correction a uniform image of a single code value is transmitted to the array device. A single value sent to all the array elements is know as a flat field. In turn, the device exposes photosensitive media  200 . In this situation the media is photographic paper. The processed photographic paper clearly shows the areas of non-uniformity. 
   Referring to  FIG. 3  the next step is to scan the printed flat field with a flat bed reflection scanner  210 . The scanned image has to then be prepared to correct for unique geometric alignment issue  220 . Then the image is low-pass filtered  230  to correct for low spatial frequency non-uniformity. Finally the scanned image is color balanced by a simple offset alignment of the histogram curves  235  for the three individual color channels (RGB)  240 . The image is then resized to match the size of the flat field image  250 . Fiducial marks, or dust spots, are used to align the original flat field image to the scanned image  260 . At this point the system has a defect map  265 . Next an image including offsets determined by the defect map is printed  270 . If the defects are corrected  300  the defect correction map  305  is ready for the next step. If the defects are not corrected  290 , algorithm returns to step  270 , printing the defect map. The reflection scanning method is a lower cost solution, but has advantages for removal of low spatial frequency non-uniformity. 
   High frequency nonuniformities refer to undesirable changes in the image intensity that are noticeable at the pixel level. Such defects require detailed knowledge of each image pixel. For such cases, reflection scanners do not provide adequate resolution or geometric alignment. The “defect map,” or knowledge of the image, is generated with sub pixel accuracy. In other words, each image pixel is oversampled. This data or map can be acquired with a high-resolution CCD camera or some other equivalent image acquisition system. Whichever method is chosen, a pixel level map of image data is created and stored. 
   For high frequency correction, the algorithm of acquisition is shown in  FIG. 4  as a flow chart. The first step is to project the flat field image  360  onto a CCD camera to capture the projected image digitally  370 . The digital image has to then be resized  380  and prepared to correct for unique geometric alignment issue  390 . Then the image is high-pass filtered  400  to correct for high spatial frequency non-uniformity. At this point the system has a defect map  265 . Next an image including offsets determined by the defect map is printed  270 , the defects are corrected  300  the defect correction map  305  is ready for the next step. If the defects are not corrected  290 , a single RGB gain is applied to all pixels, and the algorithm returns to step  270 , printing the defect map. 
   In the first embodiment the color corrected, filtered, and aligned image becomes the defect map. Care should be taken in examining the data from the defect map. When a scanner is used, the data is inverted. When a CCD camera is used, the data is not inverted. In some cases, a map may be provided with the system. 
   The defect map is a pixel by pixel mapping of the spatial variations in image intensity at a given code value, or for a flat field. The code value corresponds to a mid density point in a printed image which demonstrates the image non-uniformity. If the chosen code value is the “worst case” for image artifacts, it will require the most correction. All other code values will requires a fraction or percentage of the maximum correction required at the mid density point. Since it is preferable to working normalized space, it is most convenient if the code value at which the defect map is created is the worst case. Within the flatfield image, with reference to a center image pixel or otherwise declared reference image pixels, relative variations in image pixel intensity can be quantified. Every image pixel within the flat field image is approximately an integer number of code values different from the reference pixel. This difference is the offset. By adjusting the input image data for each pixel at the image plane by the offset from the reference pixel, the projected image data appears uniform at the specified code value. 
   The input image data is most easily adjusted at the spatial light modulator by adjusting either data fed to the spatial light modulator on a site by site basis, or by adjusting the operating conditions such as the amplifier threshold at the spatial light modulator. The image data including the pixel variations serves as the “defect correction map.” 
   Since the array device and the flat bed scanner each have there own gain levels (i.e. gamma curve) there is also an issue of knowing how much correction to apply. Therefore the correction data is also adjusted by a gain value prior to applying it to the image data for projection. This gain value can be quickly determined by doing a series of exposures with different gain values applied. 
   At this point there is sufficient data to correct for defects from an area array device at a single code value. To be able to correct an image that contains a wide tonal range would require considering the effects at multiple code values. If the defect correction map was generated at the worst uniformity case, the gain for any other code value at any pixel can be approximated as a percentage of the gain applied at the code value at which the defect correction map was generated. 
   In a preferred embodiment, a gain response curve is needed for the corrections to work across all the code values. The method used to obtain this gain response curve, depicted in the flow chart of  FIG. 5 , is to print an image  450  in which the code value is varied from the lowest to highest densities in one dimension in stepped increments. The result is an image that looks like stripes of increasing density  490  as in shown in  FIG. 6 . In the other dimension, the gain is incremented in predetermined steps  510 . In printing this test image the observer can subjectively decide the best gain value for a given code value  460 . The determination can also be made by measuring for the lowest standard deviation within a particular code value strip. Alternatively, the intensity distribution at a prescribed code value can be Fourier transformed and the spatial frequency components examined. The gain value that corresponds to minimizing the spatial frequency components corresponding to any pattern noise is the best choice at a given code value. 
   The chosen gain value may be constant or vary from code value to code value linearly or other wise. The algorithm may be implemented by determining a gain for every code value, but this is tedious. It is preferable to use a smaller number of code values and assume reasonable continuity in the gain profile between consequent code values. Once all the gain values are gathered a spline curve-fitting algorithm is used to give us a continuous response curve over the entire density range  470 . 
   The continuous response curve serves as a gain correction table  480  in combination with the defect correction map provides correction data for every image pixel at every code value. This data is applied to the optical system via image data at the point at which pixilated image data is applied. In the case of a spatial light modulator based printing system, the information is carried in the image data sent to the spatial light modulator. 
   In an alternate embodiment, obtaining the response curves for the device accomplishes this. The flow chart for the second embodiment is shown in  FIG. 7 . The response curve  310 , shown in  FIG. 8 , is determined by increasing the signal to the device in known steps (called code values) while monitoring the resulting light output. The corresponding light output of the device is measured producing a response curve though a polynomial or other functional fit  320 . As shown in  FIG. 9 , this curve can be normalized  600 ,  610 ,  620  to represent the curve in relation to the density values on paper with the high end of the curve maximized at the highest possible density  330 . The first derivative of the curves the gain correction curves shown in  FIG. 10  are  630 ,  640 ,  650 . In effect, gain values relate to any code value that the image may contain. This gain correction curve is transformed into a gain correction table  350  for purposes of math efficiency. A curve is produced for each color channel, so effectively there exists separate RGB gain correction tables. Correction is applied as in the previous embodiment. 
   In both embodiments a flow chart representing inclusion of the defect correction map and the gain table into printing system is shown in  FIG. 8 . The image to be rendered  530  includes the defect map, the defect map  560  includes a comparison to a midpoint value  520 , a single gain from the flatfield correction  570 , and a secondary gain table  540  which is based on the pixel value from the image being rendered. The difference between the midpoint value  520  and the pixel value from the defect map  560  becomes the correction amount for that pixel coordinate, however, the composite gains  570 ,  540  are applied to this difference prior to the amount being applied to the image being rendered. The result is a defect corrected image  580  the midpoint is derived from the peak value of the histogram of the defect map, or alternatively from the mean value of all the pixels of the defect map. 
   In another embodiment, the defect map generated at a single code value is not representative of the defects at another value. For such cases, multiple defect correction maps need to be generated. Defect correction maps corresponding to a high intensity/density, a mid intensity/density, and a low intensity/density may be required. The relative gain between mapped code values is generated as in the previous embodiments. 
   In a most complex embodiment, defect maps are created for every code value and stored. Relative gain may be applied between defect correction maps or may included in the acquisition method. The result is a three dimensional table that is dimensioned by the x direction resolution, y direction resolution, and the code value. 
   Because all data collection occurs at the image plane, and all correction is applied to input image data, the stated embodiments are a valid approach for non-uniformity correction with any spatial light modulator (SLM). 
   It should also be noted that the same method of correction can be applied to any printing system that contains an element with individual sites corresponding to image pixels. Such “pixilated devices” allow use of optical systems not only with spatial light modulators, but other arrays such as organic light emitting diode arrays, light emitting diode arrays, laser arrays, and CRTs. The algorithm is effectively hardware independent. 
   The reason that this method of defect corrections for non-uniformity is considered highly effective is because the gain correction table has to be formed only once for a given device. It is assumed that the response curve will not change to any significance for all devices manufactured within the same lot. In changing from one device to another the only algorithm values to be changed are the defect correction map and the singular gain value. Therefore, for any given device we only need to create a defect map and a singular gain value once the gain correction table has been created. 
   The same algorithm values may be allowed to account for media variations. The optical system and algorithm described relate to printers using a single pixilated element such as a spatial light modulator. If the same algorithm is applied to a system with multiple pixilated devices, defect correction maps and relative gain would have to be determined for each device. Image data to each device would be adjusted according to device specific defect correction maps. An example of such a system would be the three-channel spatial light modulator system with separate modulators for each color. The algorithm and method of application would not change. 
   In cases where there is mechanical manipulation of the optical components or the media, such as spatial dithering, the defect map is created at a single position if the motion is within a few pixels. This is because defects are usually spatially specific to devices. The gain however, is measured from the composite or final image to account for any variations due to media response or to the perception of data. However, when the motion is a larger distance, defect maps may be created for each position to account for variations in illumination optics. 
   The optical system described relates primarily to printing. However, the algorithm is independent of media. The defect correction map can be determined at the image plane with a CCD camera, and the gain can be determined from the response (Electro-optic or otherwise) of the pixilated device or modulator. The correction data is applied to the image data. In so doing, the algorithm can be implemented in a projection system. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. For example, an LCD spatial light modulator has been discussed throughout the specifications, however, the invention applies to devices using CRTs, digital micromirror device (DMD), or other imaging devices. While printing systems have been described the invention is equally applicable to projections systems and any application which requires correction of image data. 
   PARTS LIST 
   
       
         10 . Photographic printing system 
         12 . LED 
         14 . Collimating lens 
         30 . Field lens 
         40 . Linear polarizer 
         45 . Uniformizing optics 
         50 . Lenslet array 
         52 . LCD 
         54 . Liquid crystal material 
         56 . LCD 
         60 . Field lens 
         65 . Intermediate conjugate plane 
         70 . Condenser lens 
         75 . Illumination optics 
         80 . Polarizing beamsplitter 
         90 . Reflective LCD modulator 
         92 . Modulator sites 
         110 . Print lens assembly 
         120 . Linear polarizer 
         130 . Image plane 
         140 . Light sensitive media 
         150 . Image plane 
         210 . Flat bed reflection scanner 
         220 . Unique geometric alignment issue 
         230 . Image low-pass filtered 
         235 . Histogram curves 
         240 . Individual color channels (RGB) 
         250 . Flatfield image 
         260 . Scanned image 
         265 . Defect map 
         270 . Defect map printed 
         290 . Defects not corrected 
         300 . Defects corrected 
         305 . Defect correction map 
         310 . Response curve 
         320 . Functional fit 
         340 . Curve 
         350 . Gain correction table 
         360 . Flatfield image 
         370 . Project image digitally 
         380 . Digital image resized 
         390 . Unique geometric alignment issue 
         400 . Image high-pass filtered 
         460 . Code value 
         470 . Density range 
         480 . Gain correction table 
         490 . Increasing density 
         510 . Gain incremented in predetermined steps 
         520 . Midpoint value 
         530 . Image rendered 
         540 . Secondary gain table 
         560 . Defect map 
         570 . Flatfield correction 
         580 . Defect corrected image 
         600 . Curve normalized 
         610 . Curve normalized 
         620 . Curve normalized 
         630 . Gain correction curves 
         640 . Gain correction curves 
         650 . Gain correction curves