Patent Publication Number: US-2013249780-A1

Title: Projection display and lack of brightness uniformity compensation method

Description:
FIELD OF THE INVENTION 
     The present invention relates to a projection display apparatus and a brightness irregularity correcting method. 
     BACKGROUND ART 
     Projectors are known as display devices for projecting images onto a screen. 
       FIG. 1  is a diagram showing a configurational example of a display unit for use in a projector.  FIG. 1  shows light source  710 , dichroic mirrors (hereinafter referred to as “DM”)  721  through  724 , total reflection mirrors  731 ,  732 , LCDs (Liquid Crystal Displays)  741  through  743 , cross-dichroic prism (hereinafter referred to as “XDP”)  750 , and projection lens  760 . 
       FIG. 2   a  shows a transmittance characteristic of light transmitted through DM  721 .  FIG. 2   b  shows a transmittance characteristic of light transmitted through DM  722 .  FIG. 2   c  shows a transmittance characteristic of light transmitted through DM  723 .  FIG. 2   d  shows a transmittance characteristic of light transmitted through DM  724 .  FIG. 2   e  shows a transmittance characteristic of red light transmitted through XDP  750 .  FIG. 2   f  shows a transmittance characteristic of blue light transmitted through XDP  750 .  FIG. 2   g  shows a wavelength characteristic of blue light reflected by DM  722 .  FIG. 2   h  shows a wavelength characteristic of green light reflected by DM  723 .  FIG. 2   i  shows a wavelength characteristic of red light reflected by DM  724 . 
     As shown in  FIGS. 2   a  through  2   f , DMs  721  through  724  and XDP  750  have a frequency range (transmissive range) in which light is transmitted therethrough, and a frequency range (reflective range) in which light is reflected thereby. The wavelength at the boundary between the transmissive range and the reflective range is referred to as a cutoff wavelength. 
     In the projector, light emitted from light source  710  does not have a uniform wavelength-dependent intensity distribution, but has peaks at certain wavelengths of red, green, and blue colors. Since the light emitted from light source  710  is not parallel-ray light, it is applied to DMs  721  through  724  and XDP  750  with its transverse cross section becoming progressively wider or narrower. The parallel-ray light refers to light whose rays travel parallel to each other across the transverse cross section thereof. 
       FIG. 3  is a diagram showing the path of light applied to a DM with its transverse cross section becoming progressively narrower. As shown in  FIG. 3 , even though the light is applied to section B of the DM at an incident angle θ of 45 degrees, it is applied to section A thereof at an incident angle θ greater than 45 degrees and to section C thereof at an incident angle θ smaller than 45 degrees. When the incident angle of light changes, the transmittance characteristic of the light transmitted through DMs  721  through  724  and XDP  750  changes. 
       FIG. 4  is a diagram showing the relationship between the incident angle θ and the transmittance characteristic with respect to DM  724 . 
     As shown in  FIG. 4 , the transmittance characteristic of light transmitted through DM  724  is such that as the incident angle θ becomes greater than 45 degrees, the cutoff wavelength shifts into a longer wavelength range, and as the incident angle θ becomes smaller than 45 degrees, the cutoff wavelength shifts into a shorter wavelength range. 
     Therefore, in section A of DM  724 , since the incident angle of light is greater than 45 degrees, the cutoff wavelength is longer than the cutoff wavelength in section B of DM  724 . In section C of DM  745 , since the incident angle of light is smaller than 45 degrees, the cutoff wavelength is shorter than the cutoff wavelength in section B of DM  724 . If the transverse cross section of light becomes progressively wider, then as the incident angle becomes greater or smaller than 45 degrees, the cutoff frequency shifts into a wavelength range that is opposite to the wavelength range described above. 
       FIG. 5  is a diagram showing by way of example shifts of the cutoff wavelength in a projector. In  FIG. 5 , the cutoff wavelength undergoes a shift of 10 nm (nanometers) in DMs  721  through  724 . In DM  724 , for example, section B separates the wavelengths ranging from 590 nm to 750 nm, section A separates the wavelengths ranging from 600 nm to 760 nm, and section C separates the wavelengths ranging from 580 nm to 740 nm. 
     Consequently, in each of DMs  721  through  724  and XDP  750 , the frequency range of separated light varies depending on the location where the light is applied. Inasmuch as the light emitted from light source  710  does not have a uniform wavelength-dependent intensity distribution, the brightness of the projected image tends to vary. 
     The human eye has characteristics such that it senses a wavelength in the vicinity of a green color, i.e., 520 nm, as bright, and senses wavelengths in the vicinity of red and blue colors as dark Therefore, red light reflected by section A of DM  724  has a longer wavelength than green light reflected by section B thereof and hence is perceived as dark by the human eye. Red light reflected by section C of DM  724  has a closer wavelength to green light reflected by section B and hence is perceived as bright by the human eye. Blue light and red light are also similarly perceived by the human eye. 
       FIG. 6  is a diagram showing brightness irregularities of red and blue images which are caused because light emitted from a light source is not parallel-ray light. As shown in  FIG. 6 , the red and blue images have uneven brightness levels in left, central, and right regions thereof, and hence suffer brightness irregularities. 
     Patent document 1 discloses a projector which is capable of reducing color irregularities. The projector disclosed in Patent document 1 includes a plurality of light sources, a modulator for modulating lights emitted from the respective light sources, a prism for combining the lights modulated by the modulator, a projection lens for projecting the combined light from the prism, temperature detectors for detecting the temperatures of the light sources, and a memory for storing the respective temperatures of the light sources and brightness distributions of the color lights from the light sources. 
     The projector disclosed in Patent document 1 controls the modulator to uniformize the brightness distributions of the color lights on a screen based on the temperatures of the light sources which are detected by the temperature detectors and the brightness distributions of the color lights which are stored in memory, to thereby reduce color irregularities that are caused by a change in the temperatures of the light sources. 
     PRIOR TECHNICAL DOCUMENTS 
     Patent Documents 
     Patent document 1: JP2004-226631A 
     SUMMARY OF THE INVENTION: 
     Problems to be Solved by the Invention 
     The projector disclosed in Patent document 1 has been problematic in that although it can reduce color irregularities due to a change in the temperatures of the light sources, it is unable to reduce brightness irregularities of the respective color lights that result from different light paths in prisms to which the color lights are applied. 
     A multi-screen display system combines different images generated by a plurality of projectors into a signal image and projects the single image. The multi-screen display system tends to have the image brightness change significantly due to brightness irregularities of color lights at the junctions between the combined images in the projected image. 
     It is an object of the present invention to provide a projection display apparatus and a brightness irregularity correcting method which correct brightness irregularities of color lights. 
     Means for Solving the Problems 
     According to the present invention, there is provided a projection display apparatus comprising a light source that emits light, separating means that divides the light emitted from said light source into a plurality of color lights, modulators associated respectively with the color lights separated by said separating means, for modulating the color lights depending on an image signal, a prism that combines the color lights modulated respectively by said modulators, a projection lens that projects a which is combined by said prism, holding means that holds correction values for correcting brightness irregularities of the color lights which are caused by said prism, and correcting means that, in response to the image signal received thereby, determines a plurality of brightness correction values for correcting the brightnesses at right and left ends of the combined light depending on the correction values held by said holding means, and that corrects said image signal in order to correct each of the pixels represented by said image signal depending on said brightness correction values. 
     According to the present invention, there is also provided a brightness irregularity correcting method for a projection display apparatus including a light source that emits light, separating means that divides the light emitted from said light source into a plurality of color lights, modulators associated respectively with the color lights separated by said separating means, for modulating the color lights depending on an image signal, a prism that combines the color lights modulated respectively by said modulators, and a projection lens that projects light combined by said prism, said brightness irregularity correcting method comprising holding correction values for correcting brightness irregularities of the color lights which are caused by said prism, in holding means, and in response to the image signal, determining a plurality of brightness correction values for correcting the brightnesses at right and left ends of the combined light depending on the correction values held by said holding means, and correcting said image signal in order to correct each of pixels represented by said image signal depending on said brightness correction values. 
     Advantages of the Invention 
     According to the present invention, it is possible to correct brightness irregularities of color lights. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configurational example of a display unit for use in a projector; 
         FIG. 2   a  is a diagram showing a transmittance characteristic of light transmitted through DM  721 ; 
         FIG. 2   b  is a diagram showing a transmittance characteristic of light transmitted through DM  722 ; 
         FIG. 2   c  is a diagram showing a transmittance characteristic of light transmitted through DM  723 ; 
         FIG. 2   d  is a diagram showing a transmittance characteristic of light transmitted through DM  724 ; 
         FIG. 2   e  is a diagram showing a transmittance characteristic of red light transmitted through XDP  750 ; 
         FIG. 2   f  is a diagram showing a transmittance characteristic of blue light transmitted through XDP  750 ; 
         FIG. 2   g  is a diagram showing a wavelength characteristic of blue light reflected by DM  722 ; 
         FIG. 2   h  is a diagram showing a wavelength characteristic of green light reflected by DM  723 ; 
         FIG. 2   i  is a diagram showing a wavelength characteristic of red light reflected by DM  724 ; 
         FIG. 3  is a diagram showing the path of light, which is not a parallel light ray, applied to a DM; 
         FIG. 4  is a diagram showing the relationship between the incident angle and the transmittance characteristic with respect to a DM; 
         FIG. 5  is a diagram showing by way of example shifts of the cutoff wavelength in a projector; 
         FIG. 6  is a diagram showing brightness irregularities of red and blue images; 
         FIG. 7  is a view showing a multi-screen display system according to a first exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram of projector  1  according to the present exemplary embodiment; 
         FIG. 9  is a diagram showing a detailed configurational example of display unit  50 ; 
         FIG. 10  is a block diagram showing a configurational example of brightness corrector  40 ; 
         FIG. 11  is a diagram illustrative of a calculating process carried out by correction value calculator  421 ; 
         FIG. 12  is a conceptual diagram showing red images to be corrected; 
         FIG. 13  is a view showing a menu screen for setting a correction value for the left end of the red image; 
         FIG. 14  is a diagram showing a brightness correction value set to “−4”; 
         FIG. 15  is a conceptual diagram showing a blue image to be corrected; 
         FIG. 16  is a view showing a menu screen for setting a correction value for the left end of the blue image; 
         FIG. 17  is a diagram showing a brightness correction value set to “+4”; 
         FIG. 18  is a diagram showing a red image produced when a process for correcting brightness irregularity is performed on an image signal having a maximum level; 
         FIG. 19  is a diagram showing a red image produced when the level of an image signal is lowered depending on correction value Ar; 
         FIG. 20  is a diagram showing a blue image produced when a process for correcting brightness irregularity is performed on an image signal having a maximum level; 
         FIG. 21  is a diagram showing a blue image produced when the level of an image signal is lowered depending on correction value Ab; 
         FIG. 22  is a flowchart showing a method for correcting brightness irregularity in the multi-screen display system; 
         FIG. 23  is a view showing an example in which a projector according to a second exemplary embodiment is used; 
         FIG. 24  is a flowchart showing a method for correcting brightness irregularity in the projector; 
         FIG. 25  is a diagram showing brightness correction values for a green image whose both side regions are darker than a central region of the image; 
         FIG. 26  is a diagram showing brightness correction values for a green image whose both side regions are brighter than a central region of the image; and 
         FIG. 27  is a diagram showing brightness correction values for a green image which are to be acquired using a lookup table. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of the present invention will be described below with reference to the drawings. 
       FIG. 7  is a view showing a multi-screen display system according to a first exemplary embodiment of the present invention. 
     The multi-screen display system joins different images produced by projectors  1 ,  2  into a single image and projects the single images onto screen  3 . The multi-screen display system includes projectors  1 ,  2 , image signal distributor  4 , and image signal generator  5 . Projectors  1 ,  2  are identical in structure to each other. 
     Image signal generator  5  generates an image signal for displaying a crescent at the center of the image, and supplies the image signal to image signal distributor  4 . 
     When image signal distributor  4  receives the image signal, it generates two image signals each identical to the supplied image signal. Image signal distributor  4  supplies one of the two image signals to projector  1  and the other to projector  2 . 
       FIG. 8  is a block diagram showing a configurational example of projector  1 . 
     Projector  1  is a projection display apparatus which, in response to an image signal representing an image, projects the image represented by the image signal onto screen  3 . 
     Projector  1  includes video input unit  10 , signal processor  11 , correction value holder  41 , display unit  50 , memory  60 , and CPU (Central Processing Unit)  70 . Signal processor  11  has resolution converter  20 , color corrector  30 , and brightness corrector  40 . 
     Video input unit  10 , resolution converter  20 , color corrector  30 , correction value holder  41 , display unit  50 , and memory  60  are connected to CPU  70  by system bus  80 . System bus  80  comprises a serial bus or a parallel bus. 
     CPU  70  controls video input unit  10 , resolution converter  20 , color corrector  30 , color corrector  30 , brightness corrector  40 , display unit  50 , and memory  60 . Memory  60  comprises a RAM and a ROM, for example. 
     Display unit  50  projects an image represented by an image signal from signal processor  11  onto screen  3 . 
       FIG. 9  is a diagram showing a detailed configurational example of display unit  50 . 
     Display unit  50  includes light source  510 , DMs  521  through  524 , total reflection mirrors  531 ,  532 , LCDs  541  through  543 , XDP  550 , and projection lens  560 . 
     Light source  510  generates white light. Light source  510  applies the white light to DM  521 . 
     DMs  521  through  524  may generally be referred to as separating means. 
     DMs  521  through  524  are used to divide the light emitted from light source  510  into blue, green, and red lights. Specifically, DM  521  divides the light emitted from light source  510  into light having a wavelength of 450 nm or higher. DM  522  separates blue light from the light that has passed through DM  521 . DM  523  separates green light from the light that has passed through DM  522 . DM  524  separates red light from the light that has passed through DM  523 . 
     Total reflection minor  531  applies the blue light separated by DM  522  to LCD  541 . Total reflection mirror  532  applies the red light separated by DM  524  to LCD  543 . 
     Each of LCDs  541  through  543  may generally be referred to as a modulator. 
     LCDs  541  through  543  are provided respectively for the color lights separated by DMs  521  through  524 . LCDs  541  through  543  modulate the color lights with respective image signals from brightness corrector  40 . Specifically, LCD  541  modulates the blue light from total reflection mirror  531  with a blue image signal. LCD  542  modulates the green light from DM  523  with a green image signal. LCD  543  modulates the red light from total reflection mirror  532  with a red image signal. 
     XDP  550  may generally be referred to as a prism. 
     XDP  550  combines the color lights modulated by LCDs  541  through  543 . The light combined by XDP  550  (hereinafter referred to as “combined light”) is applied to projection lens  56 . DMs  521  through  524  and XDP  550  have characteristics such that they change the transmittance characteristic of light depending on the incident angle at which the light is applied thereto. 
     Projection lens  560  projects an image represented by the combined light from XDP  550  onto screen  3 . According to the present exemplary embodiment, the image projected onto screen  3  will be called a projected image. 
     In display unit  50 , since the light emitted from light source  510  is not parallel light ray, the incident angle at which the light is applied to each of DMs  521  though  524  and XDP  550  changes depending on the different paths of light along which the light is applied thereto. Therefore, the color light divided by each of DMs  521  though  524  and XDP  550  contains a mixture of lights in different frequency ranges, and tends to suffer brightness irregularities. 
     Correction value holder  41  may generally be referred to as a holding means. 
     Correction value holder  41  holds correction values A for correcting brightness irregularities of color lights which are caused by DMs  521  though  524  and XDP  550 . 
     Correction values A are set by the user of projector  1  while a white image, i.e., an image which is entirely indicative of white, for example, is projected from projector  1 . 
     According to the present exemplary embodiment, correction value holder  41  holds correction value Ar for red and correction value Ab for blue. Correction value holder  41  may also hold correction value Ag for green. 
     Video input unit  10  receives an analog image signal from image signal distributor  4 . Video input unit  10  converts the received analog image signal into a digital image signal, and supplies the digital image signal to signal processor  11 . 
     Signal processor  11  may generally be referred to as a correcting means. 
     When signal processor  11  receives an image signal from video input unit  10 , it determines a plurality of brightness correction values a(x) for correcting the brightnesses at right and left ends of combined light. For example, signal processor  11  multiplies the distance between each pixel x represented by the image signal and a reference pixel at the left end by correction value A, thereby determining brightness correction value a(x). Signal processor  11  corrects the image signal so that each pixel x represented by the image signal will be corrected depending on brightness correction value a(x). 
     When resolution converter  20  receives an image signal from video input unit  10 , it converts the resolution of an image represented by the image signal into a resolution used by projector  1 . Resolution converter  20  also performs a trapezoidal correction process for correcting a trapezoidal distortion of the projected screen on the image signal. Resolution converter  20  supplies the corrected image signal to y corrector  310 . 
     Color corrector  30  corrects the colors of the image represented by the image signal. Color corrector  30  includes y corrector  310 , partial converter  320 , overall converter  330 , and coefficient holder  340 . 
     γ corrector  310  processes the image signal to match the gradation characteristics of projector  1 . 
     Partial converter  320  processes the image signal to adjust a particular hue such as a skin color, a red color, or the like. 
     Overall converter  330  processes the image signal to correct a hue difference due to the individual variability of projector  1 . Overall converter  330  corrects altogether input image signals in respective colors including red, blue, and green, using matrix coefficients C11 through C33. 
     Overall converters  330  converts input image signals Ri 1 (x, y), Gi 1 (x, y), Bi 1 (x, y) into output image signals Ro 1 (x, y), Go 1 (x, y), Bo 1 (x, y) where x represents a horizontal pixel position in the image and y represents a vertical pixel position in the image, according to the following equation (1): 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               C 
                               11 
                             
                           
                           
                             
                               C 
                               21 
                             
                           
                           
                             
                               C 
                               31 
                             
                           
                         
                         
                           
                             
                               C 
                               12 
                             
                           
                           
                             
                               C 
                               22 
                             
                           
                           
                             
                               C 
                               32 
                             
                           
                         
                         
                           
                             
                               C 
                               13 
                             
                           
                           
                             
                               C 
                               23 
                             
                           
                           
                             
                               C 
                               33 
                             
                           
                         
                       
                       ) 
                     
                      
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Overall converter  330  receives matrix coefficients C11, C22, C33 for adjusting the levels (amplification gains) of the image signals in the respective colors, among the matrix coefficients C11 through C33, from parameter calculator  420 . Overall converter  330  also receives the other matrix coefficients from coefficient holder  340 . 
     Overall converter  330  may convert input image signals Ri 1 (x, y), Gi 1 (x, y), Bi 1 (x, y) into output image signals Ro 1 (x, y), Go 1 (x, y), Bo 1 (x, y) according to the following equation (2): 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               1 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               C 
                               11 
                             
                           
                           
                             0 
                           
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                           
                             
                               C 
                               22 
                             
                           
                           
                             0 
                           
                         
                         
                           
                             0 
                           
                           
                             0 
                           
                           
                             
                               C 
                               33 
                             
                           
                         
                       
                       ) 
                     
                      
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 1 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Brightness corrector  40  performs on the image signal a process for correcting brightness irregularities of the respective color lights which are caused by DMs  521  through  524  and XDP  550 . 
       FIG. 10  is a block diagram showing a detailed configuration of brightness corrector  40 . 
     Brightness corrector  40  includes correction processor  410  and parameter calculator  420 . Parameter calculator  420  includes correction value calculator  421  and coefficient calculator  422 . 
     Correction value calculator  421  calculates a plurality of brightness correction values for each of the color lights based-on the correction values A held by correction value holder  41  and the levels of the image signals from overall converter  330 . Correction value calculator  421  calculates brightness correction values for the pixels at the left end of the projected image using the correction values A while setting the brightness correction value for the central pixel of the projected image to zero, and determines brightness correction values from the left end to the right end of the projected image according to linear interpolation, using the brightness correction values for the pixels at the left end and the central pixel. 
     According to the present exemplary embodiment, correction value calculator  421  calculates an offset value that defines a gradient for calculating a plurality of brightness correction values, depending on the level of the image signal. 
       FIG. 11  is a diagram showing the relationship between the level of the image signal and the offset value. 
     Correction value calculator  421  calculates, as the offset value, a value produced when a value that is twice the absolute value of correction value A is multiplied by the proportion (%) of the level of the image signal. 
     Correction value calculator  421  calculates a brightness correction value a(x) for the xth pixel from the reference pixel at the left end of the projected image, using the offset value, according to the equation (3) shown below. In other words, correction value calculator  421  determines a brightness correction value a(x) by multiplying the distance x between each pixel represented by the image signal and the reference pixel at the left end, by correction value A. 
         a ( x )=correction coefficient×offset value×distance  x− offset value/2  (3)
 
     The correction coefficient is represented by the reciprocal of the number of effective pixels (dots). For XGA, the correction coefficient is 1/1024. 
     For example, correction value calculator  421  calculates brightness correction value ar(x) for red using correction value Ar for red and the level of the image signal, and calculates a brightness correction value ab(x) for blue using correction value Ab for blue and the level of the image signal. 
     Coefficient calculator  422  calculates matrix coefficients C11, C22, C33 using correction value A. According to the present exemplary embodiment, coefficient calculator  422  subtracts a maximum value, either the absolute value of correction value Ar or the absolute value of correction value Ab, from given data, and calculates the difference as matrix coefficients C11, C22, C33. 
     For example, if the image signal is represented by 8 bits, then matrix coefficients C11, C22, C33 are represented by 9 bits. In this case, coefficient calculator  422  calculates matrix coefficients C11, C22, C33 according to the following equation (4): 
       C11, C22, C33=100000000−maximum value among absolute values of Ar, Ab  (4)
 
     If the image signal is represented by 10 bits, then matrix coefficients C11, C22, C33 are represented by 11 bits, and coefficient calculator  422  calculates matrix coefficients C11, C22, C33 according to the following equation (5): 
       C11, C22, C33=10000000000−maximum value among absolute values of Ar, Ab  (5)
 
     Coefficient calculator  422  may subtract a maximum value selected from plural brightness correction values ar(x) or plural brightness correction values ab(x) calculated by correction value calculator  421 , from given data to calculate matrix coefficients C11, C22, C33. 
     Correction processor  410  corrects the image signal in order to correct each of pixels x represented by the image signal depending on brightness correction value a(x). Specifically, correction processor  410  adds brightness correction value a(x) to the image signal if the sign of correction value A is negative (minus), and subtracts correction value a(x) from the image signal if the sign of correction value A is positive (plus). 
     According to the present exemplary embodiment, correction processor  410  receives brightness correction value ar(x) for red and brightness correction value ab(x) for blue from correction value calculator  421 . Correction processor  410  also receives the image signal from overall converter  330 . 
     Correction processor  410  corrects red image signal Ri 2 (x, y) using the sign of correction value Ar and brightness correction value ar(x), and corrects blue image signal Bi 2 (x, y) using the sign of correction value Ab and brightness correction value ab(x). 
     If the sign of correction values Ar, Ab is negative, then correction processor  410  adds brightness correction values ar(x), ab(x) respectively to image signals Ri 2 (x, y), Bi 2 (x, y) according to the following equation (6): 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                     + 
                     
                       ( 
                       
                         
                           
                             
                               α 
                               
                                 r 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             0 
                           
                         
                         
                           
                             
                               α 
                               
                                 b 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     If the sign of correction values Ar, Ab is positive, then correction processor  410  subtracts brightness correction values ar(x), ab(x) respectively to image signals Ri 2 (x, y), Bi 2 (x, y) according to the equation (7) shown below. Therefore, correction processor  410  calculates corrected image signals Ro2(x, y), Go2(x, y), Bo2(x, y). 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         
                           
                             
                               α 
                               
                                 r 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             0 
                           
                         
                         
                           
                             
                               α 
                               
                                 b 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Then, a process of reducing a change in the brightness at the junctions of an image projected by the multi-screen display system will be described below. 
     In order to correct brightness irregularities of color lights of red and blue, projectors  1 ,  2  project white images onto screen  3 . 
       FIG. 12  is a conceptual diagram showing red images to be corrected which are projected from projectors  1 ,  2 . Red image  101  is an image in red which is projected from projector  1 . Red image  102  is an image in red which is projected from projector  2 . 
     As shown in  FIG. 12 , the difference between the brightness at the right end of red image  101  projected by projector  1  and the brightness at the left end of red image  102  projected by projector  2  is so large that the brightness of the overall red images is irregular. The user sets correction value Ar for the brightness irregularity of red image  101  in projector  1 . 
       FIG. 13  shows a menu screen for setting correction value Ar for the left end of the red image. Correction value Ar is set to a value in a range from “−4” to “+4), for example, using a control bar. In order to lower the brightness at the left end of the red image, for example, correction value Ar is set to a minus (−) value. 
       FIG. 14  is a conceptual diagram showing brightness correction value ar(x) at the time correction value Ar is set to “−4”. As shown in  FIG. 14 , correction value calculator  421  determines brightness correction value ar(x) so as to keep the brightness of the central pixel of the red image unchanged and to change the brightness linearly from the left end to the right end of the screen. Correction processor  410  processes the image signal according to the equation (6), using the sign (−) of correction value Ar and brightness correction value ar(x), thus adding brightness correction value ar(x) to the red image signal. 
     Correction of brightness irregularities of a color light of blue with projector  2  will be described below. 
       FIG. 15  is a conceptual diagram showing blue images to be corrected which are projected from projectors  1 ,  2 . Blue image  103  is an image in blue which is projected from projector  1 . Blue image  104  is an image in blue which is projected from projector  2 . 
     As shown in  FIG. 15 , the difference between the brightness at the right end of blue image  103  projected by projector  1  and the brightness at the left end of blue image  104  projected by projector  2  is so large that the brightness of the overall blue images is irregular. The user sets correction value Ab for the brightness irregularity of blue image  104  in projector  2 . 
       FIG. 16  shows a menu screen for setting correction value Ab for the left end of the blue image. Correction value Ab is set to a value in a range from “−4” to “+4), for example, using a control bar. In order to raise the brightness at the left end of the blue image, for example, correction value Ab is set to a plus (+) value. 
       FIG. 1-7  is a conceptual diagram showing brightness correction value ab(x) at the time correction value Ab is set to “+4”. As shown in  FIG. 17 , correction value calculator  421  determines brightness correction value ab(x) so as to keep the brightness of the central pixel of the blue image unchanged and to change the brightness linearly from the left end to the right end of the screen. Correction processor  410  processes the image signal according to the equation (7), using the sign (+) of correction value Ab and brightness correction value ab(x), thus subtracting brightness correction value ab(x) from the blue image signal. 
     Therefore, the multi-screen display system is capable of reducing a change in the brightness which appears significantly at the junctions between the images projected from projectors  1 ,  2 . However, when image signals having a maximum level are input to projectors  1 ,  2 , the image signals are clipped by correction processors  410  of projectors  1 ,  2 . 
       FIG. 18  is a diagram showing a red image produced when a process to correct a brightness irregularity is performed on an image signal having a maximum level. As shown in  FIG. 18 , the brightness irregularities of red image  105  indicated by the dot and dash lines remain uncorrected because the corrected image signal is clipped. 
     In projector  1 , signal processor  11  lowers the level of the image signal depending on the maximum value of the absolute value of correction value Ar, and corrects the image signal per pixel depending on brightness correction value Ar(x). Specifically, coefficient calculator  422  calculates matrix coefficients C11, C22, C33 according to the equation (4), and overall converter  330  multiplies the red, blue, and green image signals by matrix coefficients C11, C22, C33 which are of the same value, thus simultaneously adjusting the levels (gains) of the image signals of the respective colors. Correction processor  410  then performs a process to correct the brightness irregularity on the adjusted image signals. 
       FIG. 19  is a diagram showing a red image produced when the level of an image signal is lowered depending on correction value Ar. As shown in  FIG. 19 , even when an image signal having a maximum level is input, red image  109  indicated by the dot-and-dash lines has its brightness irregularities corrected because the corrected image signal is not clipped. 
       FIG. 20  is a diagram showing a blue image produced when a process to correct brightness irregularity is performed on an image signal having a maximum level. As shown in  FIG. 20 , blue image  108  indicated by the dot-and-dash lines has its brightness irregularities uncorrected because the corrected image signal is clipped. 
     In projector  2 , signal processor  11  lowers the level of the image signal depending on the maximum value of the absolute value of correction value Ab, and corrects the image signal per pixel depending on brightness correction value ab(x). 
       FIG. 21  is a diagram showing a blue image produced when the level of an image signal is lowered depending on correction value Ab. As shown in  FIG. 21 , even when an image signal having a maximum level is input, blue image  112  indicated by the dot-and-dash lines has its brightness irregularities corrected because the corrected image signal is not clipped. 
     Operation of the multi-image display system will be described below. 
       FIG. 22  is a flowchart showing a method for correcting brightness irregularity in the multi-screen display system. 
     The function in projectors  1 , 2  that is used to correct brightness irregularity is turned on (step S 811 ), and projectors  1 ,  2  project white images onto screen  3  (step S 812 ). 
     While the white images are being projected from projectors  1 ,  2  onto screen  3 , correction value Ar for red is set in projector  1  and held by correction value holder  41  thereof (step S 813 ). When correction value Ar is held by correction value holder  41 , coefficient calculator  422  of projector  1  calculates matrix coefficients C11, C22, C33 depending on correction value Ar (step S 814 ). 
     Thereafter, overall converter  330  of projector  1  processes the image signal according to the equation (1) using the matrix coefficients from coefficient calculator  422  (step S 815 ). In other words, overall converter  330  simultaneously lowers the levels of the red, blue, and green image signals depending on the absolute value of correction value Ar. 
     Correction value calculator  421  of projector  1  calculates brightness correction value ar(x) for red per pixel using the levels of the image signals from overall converter  330  and correction value Ar (step S 816 ). 
     Correction processor  410  of projector  1  adds brightness correction value ar(x) to or subtracts brightness correction value ar(x) from the red image signal depending on the sign of correction value Ar (step S 817 ). In other words, correction processor  410  performs a process to correct brightness irregularity for the red light using the sign of correction value Ar and brightness correction value ar(x). 
     Consequently, brightness corrector  40  determines a plurality of brightness correction values ar(x) to correct the brightnesses at the right end and the left end of the combined light depending on correction value Ar, and corrects the red image signal so as to correct each of the pixels represented by the image signal depending on brightness correction values ar(x). 
     When adjustment of correction value Ar for red is finished for projector  1  (step S 818 ), correction value Ab for blue is set in projector  2  and held by correction value holder  41  thereof (step S 819 ) while the white images are being projected from projectors  1 ,  2  onto screen  3  (step S 819 ). 
     When correction value Ab is held by correction value holder  41 , coefficient calculator  422  of projector  2  calculates matrix coefficients C11, C22, C33 depending on correction value Ab (step S 820 ). Thereafter, overall converter  330  of projector  2  processes the image signal according to the equation (1) using the matrix coefficients from coefficient calculator  422  (step S 821 ). In other words, overall converter  330  simultaneously lowers the levels of the image signals of the respective colors depending on the absolute value of correction value Ab. 
     Correction value calculator  421  of projector  2  calculates brightness correction value ab(x) for blue per pixel using the levels of the image signals from overall converter  330  and correction value Ab (step S 822 ). 
     Correction processor  410  of projector  2  adds brightness correction value ab(x) to or subtracts brightness correction value ab(x) from the blue image signal depending on the sign of correction value Ab (step S 823 ). In other words, correction processor  410  performs a process to correct brightness irregularity for the blue light using the sign of correction value Ab and brightness correction value ab(x). 
     Consequently, brightness corrector  40  determines a plurality of brightness correction values ab(x) to correct the brightnesses at the right end and the left end of the combined light depending on correction value Ab, and corrects the blue image signal so as to correct each of the pixels represented by the image signal depending on brightness correction values ab(x). 
     When the adjustment of correction value Ab for blue is finished for projector  2  (step S 824 ), the method to correct brightness irregularity for the multi-screen display system is finished. 
     According to the first exemplary embodiment, in projector  1 , correction value holder  41  holds correction value Ar for red, and when signal processor  11  receives an image signal, it determines a plurality of brightness correction values ar(x) to correct the brightness at the right and left ends of the projected image (combined light) depending on correction value Ar held by correction value holder  41 , and corrects the image signal so as to correct each of the pixels represented by the image signal depending on brightness correction values ar(x). In projector  2 , correction value holder  41  holds correction value Ab for blue, and when signal processor  11  receives an image signal, it determines a plurality of brightness correction values ab(x) to correct the brightness at the right and left ends of the projected image (combined light) depending on correction value Ab held by correction value holder  41 , and corrects the image signal so as to correct each of the pixels represented by the image signal depending on brightness correction values ab(x). 
     The multi-screen display system can therefore reduce a change in brightness which appears significantly at the junctions between the images projected from projectors  1 ,  2  due to brightness irregularities of the respective color lights which are caused by DMs  521  through  524  and XDP  550 . 
     According to the first exemplary embodiment, furthermore, correction value holder  41  holds correction value A for red or blue, and signal processor  11  simultaneously lowers the levels of the image signals of the respective colors depending on correction value A held by correction value holder  41 , thereby correcting the image signals depending on brightness correction values a(x). 
     Therefore, even when signal processor  11  receives an image signal having a maximum level, it can correct the image signal depending on brightness correction values a(x) without clipping the image signal which has been corrected. Accordingly, projectors  1 ,  2  can appropriately correct brightness irregularities caused by the differences between the paths of the color lights. 
     According to the first exemplary embodiment, moreover, signal processor  11  multiplies the distance between each pixel x represented by the image signal and a reference pixel at the left end by correction value A, thereby determining brightness correction value a(x), and corrects the image signal per pixel depending on brightness correction value a(x). 
     Consequently, projectors  1 ,  2  are capable of correcting a red or blue image signal in accordance with the characteristics of brightness irregularities of the color light which are caused by the difference between the paths of the red and blue lights. 
     The multi-image display system according to the present exemplary embodiment is illustrated as having two projectors. However, the multi-image display system may have three or more projectors. 
       FIG. 23  is a view showing an example in which a projector according to a second exemplary embodiment is used. 
     Projector  1  receives an image signal from image signal distributor  4 , and projects an image of a crescent represented by the image signal onto screen  3 . 
     According to the present exemplary embodiment, while a white image is being projected from projector  1  onto screen  3 , correction value Ar for red is set in projector  1  by the control bar shown in  FIG. 13 . For example, if correction value Ar is set from “0” to “−4” in order to lower the brightness at the left end of the red image, then correction value calculator  421  determines brightness correction value ar(x) so as to keep the brightness of the central pixel of the red image unchanged and to change the brightness linearly from the left end to right end of the screen. Correction processor  410  processes the image signal according to the equation (6) or equation (7), depending on the sign of correction value Ar, thus adding brightness correction value ar(x) to the red image signal. 
     Then, while the white image is being projected from projector  1  onto screen  3 , a correction value is set in projector  1  by the control bar shown in  FIG. 16 . For example, if the correction value is set from “0” to “+2” in order to raise the brightness at the left end of the blue image, then correction value calculator  421  determines brightness correction value ab(x) so as to keep the brightness of the central pixel of the blue image unchanged and to change the brightness linearly from the left end to the right end of the screen. Correction processor  410  processes the image signal according to equation (6) or equation (7), depending on the sign of correction value Ab, thus adding brightness correction value ab(x) to the blue image signal. 
     Parameter calculator  420  compares the absolute value of correction value Ar and the absolute value of correction value Ab with each other, and calculates matrix coefficients C11, C22, C33 according to equation (4) using a greater maximum value of the compared values. Parameter calculator  420  may calculate matrix coefficients C11, C22, C33 according to equation (4) using a maximum value among plural brightness correction values ar(x) and plural brightness correction values ab(x). 
     Overall converter  330  simultaneously lowers the levels of the image signals according to equation (1) using matrix coefficients C11, C22, C33 from parameter calculator  420 . Correction processor  410  can thus correct the image signals without clipping the image signals. 
       FIG. 24  is a flowchart showing a method for correcting brightness irregularity in the projector  1 . 
     The function in projector  1  that is used to correct brightness irregularity is turned on (step S 911 ), and projector  1  projects a white image onto screen  3  (step S 912 ). 
     While the white image is being projected from projector  1  onto screen  3 , correction value Ar for red is set in projector  1  and held by correction value holder  41  thereof (step S 913 ). 
     When correction value Ar is held by correction value holder  41 , coefficient calculator  422  calculates matrix coefficients C11, C22, C33 depending on correction value Ar (step S 914 ). 
     Thereafter, overall converter  330  processes the image signal according to equation (1) using the matrix coefficients from coefficient calculator  422  (step S 915 ). 
     Correction value calculator  421  calculates brightness correction value ar(x) for red per pixel depending on the image signals from overall converter  330  and correction value Ar (step S 916 ). Correction processor  410  adds brightness correction value ar(x) to or subtracts brightness correction value ar(x) from the red image signal depending on the sign of correction value Ar (step S 917 ). In other words, correction processor  410  performs a process to correct brightness irregularity for the red light using the sign of correction value Ar and brightness correction value ar(x). 
     When the adjustment of correction value Ar for red is finished (step S 918 ), correction value Ab for blue is set in projector  2  and held by correction value holder  41  thereof (step S 919 ) while the white image is being projected from projector  1  onto screen  3  (step S 919 ). 
     When correction value Ab is held by correction value holder  41 , coefficient calculator  422  confirms whether or not the absolute value of correction value Ab for blue is greater than the absolute value of correction value Ar for red (step S 920 ). If the absolute value of correction value Ab is greater than the absolute value of correction value Ar, then coefficient calculator  422  calculates matrix coefficients C11, C22, C33 depending on correction value Ab (step S 921 ). 
     Thereafter, overall converter  330  processes the image signal according to equation (1) using the matrix coefficients from coefficient calculator  422  (step S 922 ). Correction value calculator  421  calculates brightness correction value ab(x) for blue per pixel depending on the levels of the image signals from overall converter  330  and correction value Ab (step S 923 ). 
     If the absolute value of correction value Ab is equal to or smaller than the absolute value of correction value Ar in step S 920 , correction value calculator  421  also calculates brightness correction value ab(x) for blue per pixel (step S 923 ). 
     Thereafter, correction processor  410  adds brightness correction value ab(x) to or subtracts brightness correction value ab(x) from the blue image signal depending on the sign of correction value Ab (step S 924 ). In other words, correction processor  410  performs a process to correct brightness irregularity for the blue light using the sign of correction value Ar and brightness correction value ab(x). 
     When the adjustment of correction value Ab for blue is finished (step S 925 ), the method for correcting brightness irregularity is finished. 
     According to the second exemplary embodiment, correction value holder  41  holds correction value Ar for red and correction value Ab for blue, and when signal processor  11  receives an image signal, it simultaneously lowers the levels of the image signals in the respective colors depending on a maximum value subtracts a maximum value, either the absolute value of correction value Ar or the absolute value of correction value Ab, from given data. 
     Therefore, projector  1  can prevent image signals in both blue and red colors from being clipped when it corrects these image signals. Consequently, projector  1  is capable of reducing brightness irregularities of both red and blue colors. 
     According to the first and second exemplary embodiments, the method for correcting brightness irregularity is performed on the red and blue lights. However, it may be performed on the green light. In this case, if the sign of correction value Ag is negative, then correction processor  410  adds brightness correction value ag(x) to the green image signal according to the equation (8) shown below. If the sign of correction value Ag is positive, then correction processor  410  subtracts brightness correction value ag(x) from the green image signal according to the equation (9) shown below. 
     
       
         
           
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                     + 
                     
                       ( 
                       
                         
                           
                             
                               α 
                               
                                 r 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               α 
                               
                                 g 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               α 
                               
                                 b 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     ( 
                     
                       
                         
                           
                             R 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             G 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             B 
                             
                               o 
                                
                               
                                   
                               
                                
                               2 
                                
                               
                                 ( 
                                 
                                   x 
                                   , 
                                   y 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               R 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               G 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               B 
                               
                                 i 
                                  
                                 
                                     
                                 
                                  
                                 2 
                                  
                                 
                                   ( 
                                   
                                     x 
                                     , 
                                     y 
                                   
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                     - 
                     
                       ( 
                       
                         
                           
                             
                               α 
                               
                                 r 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               α 
                               
                                 g 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                         
                           
                             
                               α 
                               
                                 b 
                                  
                                 
                                   ( 
                                   x 
                                   ) 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
       FIGS. 25 and 26  are diagrams showing examples in which brightness correction value ag(x) for green is calculated. With respect to green images, since the human eye perceives a wavelength in the vicinity of 520 nm among wavelengths ranging from 495 to 590 nm, as bright, when the cutoff wavelength is shifted due to the different path of the green light, the central area of the screen and the both ends of the screen look different in terms of brightness. However, the difference in brightness (brightness irregularity) between the central area of the screen and the both ends of the screen is smaller than the difference in brightness on red and blue images. 
     For example, green image  113  shown in  FIG. 25  has both screen sides darker than the central area of the screen. Alternatively, green image  114  shown in  FIG. 26  has both screen sides brighter than the central area of the screen. 
     Therefore, in order to perform a process to correct brightness irregularity for green light, correction value Ag1 for the left end of the screen and correction value Ag2 for the right end of the screen should preferably be settable independently, and correction calculator  421  should preferably calculate brightness correction values ag(x) individually for the left end and the right end of the screen, using correction value Ag1 and correction value Ag2 for both ends of the screen. Correction value Ag1 for green and correction value Ag2 for green are called first and second green correction values, respectively. 
     Specifically, signal processor  11  determines left brightness correction value ag1(x) by multiplying the distance between each pixel on the left side of a central pixel and the central pixel by corrective value Ag1. Signal processor  11  determines right brightness correction value ag2(x) by multiplying the distance between each pixel on the right side of a central pixel and the central pixel by corrective value Ag2. Signal processor  11  then corrects the green image signal depending on rightness correction value ag1(x) and brightness correction value ag2(x). 
     Therefore, it is possible to appropriately correct brightness irregularities of green image  113  shown in  FIG. 25  or green image  114  shown in  FIG. 26 . 
       FIG. 27  is a diagram showing another example in which brightness correction values ag(x) are calculated. In  FIG. 27 , a lookup table containing brightness correction values ag(x) associated with respective pixels is stored in memory  60 . Correction value calculator  421  acquires brightness correction values ag(x) from the lookup table. Projector  1  is thus able to make brightness correction values ag(x) curvilinear in the central area of the screen, unlike  FIGS. 25 and 26 , making it possible to correct green image  115  more appropriately. 
     The configurations illustrated in the exemplary embodiments described above are shown by way of example only, and the present invention is not limited to the illustrated configurations. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       1 ,  2  projector 
       3  screen 
       4  image signal distributor 
       5  image signal generator 
       10  video input unit 
       11  signal processor 
       20  resolution converter 
       30  color corrector 
       310  γ corrector 
       320  partial converter 
       330  overall converter 
       340  coefficient holder 
       40  brightness corrector 
       41  correction value holder 
       410  correction processor 
       420  parameter calculator 
       421  correction value calculator 
       422  coefficient calculator 
       50  display unit 
       510  light source 
       521 - 524  DM 
       531 ,  532  total reflection mirror 
       541 - 543  LCD 
       550  XDP 
       560  projection lens 
       60  memory 
       70  CPU