Abstract:
A proposition is to perform a high accurate chromatic coordinate conversion of a chromaticity value generated by a color sensor into a chromaticity value of a predetermined chromaticity coordinate system, and to perform an accurate color-reproduction. A chromatic coordinate converting apparatus includes a recording unit recording information representing a chromaticity coordinate system unique to the color sensor in advance, and a converting unit performing a chromatic coordinate conversion of a first chromaticity value being a chromaticity value generated by the color sensor into a second chromaticity value of the predetermined chromatic coordinate system based on the information recorded by the recording unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-189710, filed on Jul. 20, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present embodiments relate to a chromatic coordinate converting apparatus performing a chromatic coordinate conversion of a chromatic value generated by a tricolor imaging device and so on into a chromatic value represented by a predetermined chromaticity coordinate system (a general chromaticity coordinate system reflecting human visual characteristics such as a chromaticity coordinate system of a CIE calorimetric system), a computer readable recording medium, and a creating method of a chromatic coordinate converting table. Besides, the present embodiments relates to an imaging apparatus including the chromatic coordinate converting apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    An RGB color imaging device of single panel type is used for a general electronic camera. A spectral sensitivity characteristic of this color imaging device is approximated to a color-matching function of a CIE-XYZ calorimetric system, but they are not matched exactly. 
         [0006]    Accordingly, in the electronic camera, for example, an RGB value generated by the color imaging device is performed a chromatic coordinate conversion into an XYZ value by a 3×3 matrix. A method in which the spectral sensitivity characteristic of the color imaging device is taken into consideration to optimize elements of the matrix is disclosed in Patent Document 1. It becomes possible to perform comparatively accurate color reproduction for various display devices by using the XYZ value in which the chromatic coordinate conversion is performed with this matrix (Patent Document 1: Patent Publication No. 3803441). 
         [0007]    However, it cannot be said that this matrix can perform the chromatic coordinate conversion of RGB values of all color generated by the color imaging device accurately. For example, when an optimization is performed so that the chromatic coordinate conversion of the RGB value in standard chroma is performed accurately, a conversion error of the RGB value in high chroma should be tolerated. 
       SUMMARY 
       [0008]    A proposition of the present invention is to provide a chromatic coordinate converting apparatus capable of performing a chromatic coordinate conversion of a chromaticity value generated by a color sensor into a chromaticity value of a predetermined chromatic coordinate system with high accuracy, a computer readable recording medium, and a creating method of chromatic coordinate converting table. Besides, another proposition of the present invention is to provide an imaging apparatus capable of surely performing an accurate color-reproduction. 
         [0009]    A chromatic coordinate converting apparatus of the present invention includes a recording unit recording information representing a chromaticity coordinate system unique to a color sensor in advance, and a converting unit performing a chromatic coordinate conversion of a first chromaticity value being a chromaticity value generated by the color sensor into a second chromaticity value of a predetermined chromaticity coordinate system based on the information recorded by the recording unit. 
         [0010]    Incidentally, the converting unit may obtain the second chromaticity value by estimating a spectroscopic characterization of incident light for the color sensor based on the information and the first chromaticity value, and thereafter, by representing the spectroscopic characterization by the predetermined chromaticity coordinate system. 
         [0011]    Besides, the recording unit may record information of a plurality of reference chromaticity values generated by the color sensor in accordance with each of a plurality of reference lights of which spectroscopic characterizations are already known, and the converting unit may estimate the spectroscopic characterization of the incident light by performing a weighting synthesis of the spectroscopic characterizations of the plurality of reference lights with a weighting ratio in accordance with a relation between the plurality of reference chromaticity values and the first chromaticity value. 
         [0012]    Besides, the plurality of reference lights are desirable to be composed of a plurality of monochromatic lights different from one another within a visual light range. 
         [0013]    Besides, the plurality of reference lights may be composed of a plurality of monochromatic lights different from one another within a visual light range and white light. 
         [0014]    Besides, the converting unit may estimate a part of the spectroscopic characterization of the incident light by assuming a closed curve connecting the plurality of reference chromaticity values in the unique chromaticity coordinate system, and one straight line passing on the first chromaticity value, and by performing the weighting synthesis of the spectroscopic characterizations of the reference lights having colors corresponding to intersection points of the closed curve and the straight line among the plurality of reference lights, and estimate a whole of the spectroscopic characterization of the incident light by repeating the estimation while changing the straight line to obtain a plurality of estimated results, and by superimposing the above-stated estimated results. 
         [0015]    Besides, the converting unit may take off the estimated result from an object of the superimposing when the straight line is a straight line intersecting a red-violet light of the closed curve. 
         [0016]    Besides, the superimposing is a weighting superimposing, and the converting unit may set the weighting ratio of the weighting superimposing in accordance with a spectroscopic characterization of a light source illuminating an object of detection of the color sensor. 
         [0017]    Besides, an imaging apparatus of the present invention includes a color imaging device, and any one of chromatic coordinate converting apparatuses according to the present invention performing a chromatic coordinate conversion of a chromaticity value generated by the color imaging device. 
         [0018]    Besides, a computer readable recording medium of the present invention for causing a computer to execute a chromatic coordinate converting program includes a recording operation recording information representing a chromaticity coordinate system unique to a color sensor in advance, and a converting operation performing a chromatic coordinate conversion of a first chromaticity value being a chromaticity value generated by the color sensor into a second chromaticity value of a predetermined chromaticity coordinate system based on the information recorded in the recording operation. 
         [0019]    Incidentally, the second chromaticity value may be obtained by estimating a spectroscopic characterization of incident light for the color sensor based on the information and the first chromaticity value, and thereafter, by representing the spectroscopic characterization by the predetermined chromaticity coordinate system, in the converting operation. 
         [0020]    Besides, a plurality of reference chromaticity values generated by the color sensor in accordance with each of a plurality of reference lights of which spectroscopic characterizations are already known may be recorded, in the recording operation, and the spectroscopic characterization of the incident light may be estimated by performing a weighting synthesis of the spectroscopic characterizations of the plurality of reference lights with a weighting ratio in accordance with a relation between the plurality of reference chromaticity values and the first chromaticity value, in the converting operation. 
         [0021]    Besides, the plurality of reference lights are desirable to be composed of a plurality of monochromatic lights different from one another within a visual light range. 
         [0022]    Besides, the plurality of reference lights may be composed of a plurality of monochromatic lights different from one another within a visual light range and white light. 
         [0023]    Besides, a part of the spectroscopic characterization of the incident light is estimated by assuming a closed curve connecting the plurality of reference chromaticity values in the unique chromaticity coordinate system and one straight line passing on the first chromaticity value, and by performing a weighting synthesis of spectroscopic characterizations of reference lights having colors corresponding to intersection points of the closed curb and the straight line among the plurality of reference lights, and a whole of the spectroscopic characterization of the incident light may be estimated by repeating the estimation while changing the straight line to obtain a plurality of estimated results and by superimposing the above-stated estimated results, in the converting operation. 
         [0024]    Besides, the estimated result when the straight line is a straight line intersecting a red-violet light of the closed curve may be taken off from an object of the superimposing, in the converting operation. 
         [0025]    Besides, the superimposing is a weighting superimposing, and the weighting ratio of the weighting superimposing may be set in accordance with a spectroscopic characterization of a light source illuminating an object of detection of the color sensor, in the converting operation. 
         [0026]    Besides, a creating method of chromatic coordinate converting table of the present invention includes notifying a correspondence between a chromaticity coordinate system unique to a color sensor and a predetermined chromaticity coordinate system, and creating the chromatic coordinate converting table to perform a chromatic coordinate conversion of a first chromaticity value being a chromaticity value generated by the color sensor into a second chromaticity value of the predetermined chromaticity coordinate system based on the correspondence. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a configuration chart of a camera system. 
           [0028]      FIG. 2  is a view describing spectroscopic characterizations of a plurality of reference lights. 
           [0029]      FIG. 3  is a view showing spectral sensitivity characteristics of a color imaging device. 
           [0030]      FIG. 4  is a view representing RGB values of the reference lights by points on an rg chromaticity coordinate system. 
           [0031]      FIG. 5  is a view showing normalization coefficients K. 
           [0032]      FIG. 6  is a view describing a meaning of a positional relationship on the rg chromaticity coordinate system. 
           [0033]      FIG. 7  is an operation flowchart of a computer  21  relating to a chromatic coordinate conversion. 
           [0034]      FIG. 8  is a view describing step S 5 . 
           [0035]      FIG. 9  is a view describing step S 6 . 
           [0036]      FIG. 10  is a view describing another case of the step S 5 . 
           [0037]      FIG. 11  is a view describing another case of the step S 6 . 
           [0038]      FIG. 12  is an image of a spectroscopic characterization S 0  calculated at the step S 9 . 
           [0039]      FIG. 13  is a color-matching function of a CIE 1931 XYZ colorimetric system. 
           [0040]      FIG. 14  is a view describing a modification example of the step S 9 . 
           [0041]      FIG. 15  is a view describing a modification example of step S 4  to the step S 9 . 
           [0042]      FIG. 16  is a view describing another modification example of the step S 4  to the step S 9 . 
           [0043]      FIG. 17  is a view describing a characteristic data and a modification example of the step S 4  to the step S 9 . 
           [0044]      FIG. 18  is a view describing a characteristic data and another modification example of the step S 4  to the step S 9 . 
           [0045]      FIG. 19  is a view describing a characteristic data and still another modification example of the step S 4  to the step S 9 . 
           [0046]      FIG. 20  is a view describing a lookup table. 
           [0047]      FIG. 21  is a view describing a correspondence table between an RAW data and a device independent color. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiments 
       [0048]    Hereinafter, a first embodiment of the present invention is described. The present embodiment is an embodiment of a camera system. 
         [0049]      FIG. 1  is a configuration chart of a camera system. As shown in  FIG. 1 , the camera system includes an electronic camera  10 , a computer  21 , a monitor  22   a , an inputting device  22   b , and so on. 
         [0050]    A color imaging device of single panel type  11  is included in the electronic camera  10 . The color imaging device  11  is, for example, an RGB color imaging device in a Bayer array. 
         [0051]    A pixel interpolation processing (a deBayer processing) is performed for an image data generated by the color imaging device  11  at the electronic camera  10 . Here, the image data after the interpolation processing is an image data of an RAW image, and hereinafter, referred to as an “RAW data”. This RAW data is written to an image file at the electronic camera  10 , and stored into a memory  12  (a card memory and so on) of the electronic camera  10 . At this time, the RAW data is written to an image storage area A 1  of the image file as shown by (A) in  FIG. 1 , and type information of the electronic camera  10  is written to a tag area At of the image file. After that, the electronic camera  10  performs a communication with the computer  21 , and transfers the image file to the computer  21 . 
         [0052]    A chromatic coordinate converting program to perform a chromatic coordinate conversion of the RAW data is installed on the computer  21 . Hereinafter, contents of this chromatic coordinate converting program are described as operations of the computer  21 . 
         [0053]    The computer  21  stores types of color imaging devices mounted on each of various types of electronic cameras. Besides, the computer  21  stores characteristic data D c1 , D c2 , D c3 , . . . of various types of color imaging devices as shown by (B) in  FIG. 1 . 
         [0054]    The computer  21  refers to the tag area At of the image file of the RAW data at the time of the chromatic coordinate conversion of the RAW data, recognizes the type of the electronic camera which obtained the RAW data, and recognizes the type of the color imaging device mounted on the electronic camera. The computer  21  selects the characteristic data corresponding to the color imaging device from among the characteristic data D c1 , D c2 , D c3 , and sets as the characteristic data to be used for the chromatic coordinate conversion. 
         [0055]    Here, there are chromaticity characteristic data and summational sensitivity characteristic data in the characteristic data of the color imaging device. The chromaticity characteristic data is the one in which responses of a spectral sensitivity characteristic of the color imaging device at each wavelength are shown by sequence of points of an rg chromaticity coordinate system. The summational sensitivity characteristic data is the one derived from a total sum of the responses of the spectral sensitivity characteristic of the color imaging device at each wavelength. 
         [0056]    These characteristic data are, for example, obtained by a manufacturer of the electronic camera mounting the color imaging device as described below. 
         [0057]    At first, the manufacturer makes a plurality of reference lights having equal intensity and different colors incident on the electronic camera. Here, the plurality of reference lights are set to be 41 kinds of monochromatic lights with different wavelengths by 10 nm each within a visual light range (monochromatic light with wavelength of 380 nm, monochromatic light with wavelength of 390 nm, monochromatic light with wavelength of 400 nm, . . . , and monochromatic light with wavelength of 780 nm) as shown in  FIG. 2 . Besides, half breadths of spectroscopic characterizations S 380 , S 390 , S 400 , . . . , and S 780  of the individual reference lights are set to be 10 nm respectively. 
         [0058]    Further, the manufacturer refers to RGB values of the RAW data in which the electronic camera generates individually in accordance with the respective reference lights. The respective RGB values corresponding to the respective reference lights are, for example, as shown by solid lines in  FIG. 3 . These solid lines represent the spectral sensitivity characteristics of the color imaging device. 
         [0059]    Besides, a sum of these RGB values (hereinafter, called as “an RGB brightness value”) are as shown by a dotted line in  FIG. 3 . This dotted line represents the summational sensitivity characteristic of the color imaging device. 
         [0060]    Next, the manufacturer converts the respective RGB values corresponding to the respective reference lights into rg values by an expression (1). 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           r 
                           = 
                           
                             R 
                             / 
                             
                               ( 
                               
                                 R 
                                 + 
                                 G 
                                 + 
                                 B 
                               
                               ) 
                             
                           
                         
                       
                     
                     
                       
                         
                           g 
                           = 
                           
                             G 
                             / 
                             
                               ( 
                               
                                 R 
                                 + 
                                 G 
                                 + 
                                 B 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0061]    Accordingly, chromaticities of the respective reference lights are represented by points P 380 , P 390 , . . . , and P 780  on an rg chromaticity coordinate system unique to the color imaging device, as shown in  FIG. 4 . These points P 380 , P 390 , . . . , and P 780  are the chromaticity characteristic data of the color imaging device. 
         [0062]    Incidentally, a subscript “k” of a point P k  represents a wavelength of the reference light corresponding to the point P k . For example, the point P 380  represents the chromaticity of the reference light with the wavelength of 380 nm by the rg chromaticity coordinate system unique to the color imaging device, and the point P 780  represents the chromaticity of the reference light with the wavelength of 780 nm by the rg chromaticity coordinate system unique to the color imaging device. Hereinafter, each of these points P 380 , P 390 , . . . , and P 780  are called as “reference points”, and the rg chromaticity coordinate system unique to the color imaging device is just called as the “rg chromaticity coordinate system”. 
         [0063]    Besides, the manufacturer generates a coefficient group K 380 , K 390 , . . . , and K 780  corresponding to reciprocals of the RGB brightness values based on the summational sensitivity characteristic of the color imaging device (the dotted line in  FIG. 3 ). These coefficient group K 380 , K 390 , . . . , and K 780  are the summational sensitivity characteristic data of the color imaging device. A subscript “k” of a coefficient K k  represents a wavelength of the reference light corresponding to the coefficient K k . For example, the coefficients K 380 , K 780  are the coefficients corresponding to the reference lights with the wavelengths of 380 nm and 780 nm. 
         [0064]    Here, a case is considered when two kinds of reference lights with the wavelengths of 380 nm and 780 nm are respectively incident on the color imaging device. At this time, the RGB values generated by the color imaging device become different from one another even if the intensity of the reference lights are the same (refer to the dotted line in  FIG. 3 ). However, if spectroscopic characterizations of the reference lights are multiplied by the coefficients K 380 , K 780  in advance, the RGB brightness values of these reference lights become equal with each other. Namely, the coefficient K k  is a normalization coefficient to normalize the RGB brightness value of each reference light. Accordingly, the coefficient K k  is just called as a “normalization coefficient” in the following. 
         [0065]    Next, a meaning of a positional relationship on the rg chromaticity coordinate system is considered. 
         [0066]    As shown in  FIG. 6 , when two color lights represented by two arbitrary points P 1  (r 1 , g 1 ), P 2 (r 2 , g 2 ) on the rg chromaticity coordinate system are represented by the RGB values, each of them become as shown in an expression (2). Incidentally, here, it is assumed that the RGB brightness values of the two color lights are equal, and the RGB brightness value of these color lights is set as “A” (A=R+G+B). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                       1 
                     
                     : 
                     
                       A 
                        
                       
                         ( 
                         
                           
                             
                               
                                 r 
                                 1 
                               
                             
                           
                           
                             
                               
                                 g 
                                 1 
                               
                             
                           
                           
                             
                               
                                 1 
                                 - 
                                 
                                   r 
                                   1 
                                 
                                 - 
                                 
                                   g 
                                   1 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       P 
                       2 
                     
                     : 
                     
                       A 
                        
                       
                         ( 
                         
                           
                             
                               
                                 r 
                                 2 
                               
                             
                           
                           
                             
                               
                                 g 
                                 2 
                               
                             
                           
                           
                             
                               
                                 1 
                                 - 
                                 
                                   r 
                                   2 
                                 
                                 - 
                                 
                                   g 
                                   2 
                                 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0067]    Accordingly, when the two color lights represented by the points P 1 , P 2  on the rg chromaticity coordinate system, and having the equal RGB brightness value are performed a weighting synthesis with a weighting ratio of “a:b”, an RGB value of a synthesized light becomes as shown by an expression (3). 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                     3 
                   
                   : 
                   
                     ( 
                     
                       
                         
                           
                             
                               aAr 
                               1 
                             
                             + 
                             
                               bAr 
                               2 
                             
                           
                         
                       
                       
                         
                           
                             
                               aAg 
                               1 
                             
                             + 
                             
                               bAg 
                               2 
                             
                           
                         
                       
                       
                         
                           
                             aA 
                             - 
                             
                               aAr 
                               1 
                             
                             - 
                             
                               aAg 
                               1 
                             
                             + 
                             bA 
                             - 
                             
                               bAr 
                               2 
                             
                             - 
                             
                               Abg 
                               2 
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0068]    When the RGB value of this synthesized light is represented by a point P 3  on the rg chromaticity coordinate system, it becomes as shown in an expression (4). 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                     3 
                   
                   : 
                   
                     ( 
                     
                       
                         
                           
                             ar 
                             1 
                           
                           + 
                           
                             br 
                             2 
                           
                         
                         
                           a 
                           + 
                           b 
                         
                       
                       , 
                       
                         
                           
                             ag 
                             1 
                           
                           + 
                           
                             bg 
                             2 
                           
                         
                         
                           a 
                           + 
                           b 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0069]    It can be seen from the expression (4) that this point P 3  is the point internally dividing a line segment P 1 P 2  with an internal division ratio of “b:a” on the rg chromaticity coordinate system. 
         [0070]    Namely, when the two color lights represented by the points P 1 , P 2  on the rg chromaticity coordinate system, and having the equal RGB brightness value are performed the weighting synthesis with the weighting ratio of “a:b”, a color of the synthesized light is represented by the point P 3  internally dividing the line segment P 1 P 2  on the rg chromaticity coordinate system with the internal division ratio of “b:a”. Accordingly, the color of the light represented by the arbitrary point P 3  on the rg chromaticity coordinate system becomes the same color with the synthesized light in which the RGB brightness values of the two lights corresponding to both end points P 1 , P 2  of the arbitrary line segment P 1 P 2  passing on the point P 3  are aligned, and thereafter synthesized with the internal division ratio of “a:b”. 
         [0071]    Based on the above, a procedure in which the computer  21  performs a chromatic coordinate conversion of the RAW data is described in detail. 
         [0072]      FIG. 7  is an operation flowchart of the computer  21  relating to the chromatic coordinate conversion. This chromatic coordinate conversion is to convert an RAW data (RGB value) generated by the color imaging device into a data (Yxy value) represented by a CIE 1931 XYZ colorimetric system. 
         [0073]    Step S 1 : The computer  21  displays a list of image files storing the RAW data on the monitor  22   a , to make a use specify one of them. The computer  21  recognizes contents specified by the user via the inputting device  22   b , reads the RAW data of the specified image file, and selects a characteristic data ( FIG. 8 ) to be used for the chromatic coordinate conversion. 
         [0074]    Step S 2 : The computer  21  selects a first pixel of the read RAW data as a pixel to be converted. 
         [0075]    Step S 3 : The computer  21  converts an RGB value of the pixel to be converted into an rg value by the expression (1). A point P in  FIG. 8  represents the rg value after conversion by a point on the rg chromaticity coordinate system. Hereinafter, this point P is called as a “point to be converted”. 
         [0076]    As shown in  FIG. 8 , the point to be converted P positions inside a closed curve S even if any color the light which actually incident on the pixel to be converted of the color imaging device (hereinafter, just called as “incident light”) has. Incidentally, the closed curve S is a closed curve created by connecting the reference points P 380 , . . . , P 780  in wavelength order, and thereafter, connecting the reference point P 780  and the reference point P 380 . 
         [0077]    Incidentally, information of a brightness component is lost when the RGB value is converted into the rg value in this step, but the information of the brightness component is extracted separately in a later step S 12 . 
         [0078]    Step S 4 : The computer  21  selects the reference point corresponding to a minimal wavelength (reference point P 380 ) as a start point P s  from among the reference points P 380 , P 390 , . . . , P 780  shown in  FIG. 8 . 
         [0079]    Step S 5 : The computer  21  calculates a line segment P s P e  starting from the start point P s  shown in  FIG. 8  and intersecting the closed curve S via the point to be converted P, and respectively calculates a length “a” of the line segment P s P, a length “b” of the line segment PP e . 
         [0080]    Step S 6 : The computer  21  detects the reference light corresponding to the start point P s  (here, the reference light with the wavelength of 380 nm) and the reference light corresponding to the end point P e  (here, the reference light with the wavelength of 590 nm), multiplies the spectroscopic characterizations S 380 , S 590  of these reference lights by the normalization coefficients K 380 , K 590  as shown in  FIG. 9 , and thereafter, performs the weighting synthesis with the weighting ratio of “b:a”. The weighting synthesis is represented by an expression (5). 
         [0000]        S   c   =bK   380   S   380   +aK   590   S   590   (5) 
         [0081]    It is conceivable that the spectroscopic characterization Sc obtained by this weighting synthesis represents a part of the spectroscopic characterization of the incident light. 
         [0082]    Step S 7 : The computer  21  judges whether all of the reference points S 380 , S 390 , . . . , S 780  are already selected as the start point P s  or not. When not all of the reference points are selected, the process goes to step S 8 , and when all of them are already selected, the process goes to step S 9 . 
         [0083]    Step S 8 : The computer  21  changes the start point P s  to the next reference point, and goes back to the step S 5 . 
         [0084]    Accordingly, the calculations of the spectroscopic characterization Sc in the steps S 5 , S 6  are repeated until all of the reference points P 380 , P 390 , . . . , P 780  are selected as the start point P s . 
         [0085]    Here, in the above-stated step S 5 , there is a possibility in which the end point P e  positions between the two reference points as shown in  FIG. 10 . In  FIG. 10 , a state is shown in which the reference point P 460  is selected as the start point P s , and the end point P e  positions between the reference points P 640 , P 650 . In the step S 5  in this case, it is necessary to calculate not only the lengths “a, b”, but also a length “a′” of a line segment P 640 P e  and a length “b′” of a line segment P e P 650  respectively. In this case, three spectroscopic characterizations S 460 , S 640 , S 650  become objects to be synthesized in the weighting synthesis of the next step S 6 . 
         [0086]    Incidentally, the weighting synthesis can be shown by an expression (6). 
         [0000]    
       
         
           
             
               
                 
                   S 
                   = 
                   
                     
                       
                         bK 
                         460 
                       
                        
                       
                         S 
                         460 
                       
                     
                     + 
                     
                       a 
                        
                       
                         ( 
                         
                           
                             
                               
                                 b 
                                 ′ 
                               
                               
                                 
                                   a 
                                   ′ 
                                 
                                 + 
                                 
                                   b 
                                   ′ 
                                 
                               
                             
                              
                             
                               K 
                               640 
                             
                              
                             
                               S 
                               640 
                             
                           
                           + 
                           
                             
                               
                                 a 
                                 ′ 
                               
                               
                                 
                                   a 
                                   ′ 
                                 
                                 + 
                                 
                                   b 
                                   ′ 
                                 
                               
                             
                              
                             
                               K 
                               650 
                             
                              
                             
                               S 
                               650 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0087]    This expression (6) represents that the weighting synthesis is performed with the weighting ratio of “b:a” on the one in which the two spectroscopic characterizations S 640 , S 650  are multiplied by the normalization coefficients K 640 , K 650 , and thereafter, the weighting synthesis is performed with the weighting ratio of “b′:a′”, and the one in which the spectroscopic characterization S 460  is multiplied by the normalization coefficient K 460 . 
         [0088]    Step S 9 : The computer  21  calculates a spectroscopic characterization S 0  by superimposing the plurality of spectroscopic characterizations S c  calculated at each of the steps S 5 , S 6 . This superimposing may be either a weighting superimposing or a simple superimposing, but here, the latter one is adopted for simplicity. 
         [0089]    An image of the calculated spectroscopic characterization S 0  is as shown in  FIG. 12 . A form of curve of this spectroscopic characterization S 0  is the one in which total form of the curve of the spectroscopic characterization of the incident light is estimated. Incidentally, the form of the curve drawn in  FIG. 12  is an example. 
         [0090]    Step S 10 : The computer  21  obtains an XYZ value by taking a scalar product between the spectroscopic characterization S 0  (refer to  FIG. 12 ) and a color-matching function of the CIE 1931 XYZ colorimetric system (refer to  FIG. 13 ). This XYZ value corresponds to the one representing the color of the incident light by the CIE 1931 XYZ calorimetric system. However, a brightness component of the XYZ value is not used for the calculation. 
         [0091]    Step S 11 : The computer  21  converts the XYZ value into an xy value by an expression (7). 
         [0000]    
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           x 
                           = 
                           
                             X 
                             / 
                             
                               ( 
                               
                                 X 
                                 + 
                                 Y 
                                 + 
                                 Z 
                               
                               ) 
                             
                           
                         
                       
                     
                     
                       
                         
                           y 
                           = 
                           
                             Y 
                             / 
                             
                               ( 
                               
                                 X 
                                 + 
                                 Y 
                                 + 
                                 Z 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0092]    Step S 12 : The computer  21  generates a Y value from the RGB value of the pixel to be converted by an expression (8). Respective characters “s”, “t”, “u” in the expression (8) are values which can be determined depending on the spectral sensitivity characteristic of the color imaging device, and they are optimized in advance. 
         [0000]        Y=sR+tG+uB   (8) 
         [0093]    The obtained Y value represents the brightness value of the incident light. The computer  21  therefore obtains a Yxy value by combining the Y value and the xy value obtained at the step S 11 . As a result, the RGB value of the pixel to be converted is performed the chromatic coordinate conversion into the Yxy value independent from the characteristics of the color imaging device. 
         [0094]    Step S 13 : The computer  21  judges whether the chromatic coordinate conversion relating to all pixels of the RAW data are completed or not, advances the process to step S 14  when it is not completed yet, and finishes the flowchart when it is completed. 
         [0095]    Step S 14 : The computer  21  changes the pixel to be converted to the next pixel, and returns to the step S 3 . Accordingly, the steps S 3  to S 12  are repeated until the chromatic coordinate conversions of all pixels of the RAW data are completed. 
         [0096]    As stated above, the computer  21  of the present embodiment records the response of the color imaging device for the plurality of reference lights ( FIG. 2 ) of which spectroscopic characterizations are already known as the characteristic data of the color imaging device ( FIG. 8 ) in advance. 
         [0097]    The computer  21  calculates the spectroscopic characterization S 0  of the incident light based on the RGB value and the characteristic data ( FIG. 8 ) (steps S 3  to S 9 ), then represents the spectroscopic characterization S 0  ( FIG. 12 ) by the CIE 1931 XYZ colorimetric system ( FIG. 13 ) again (step S 10 ), and obtains the xy value (step S 11 ) when the RGB value of the RAW data is performed the chromatic coordinate conversion. Accordingly, an accuracy of the chromatic coordinate conversion by the computer  21  is high. 
       Modification Example of Embodiment 
       [0098]    Incidentally, the computer  21  of the above-stated embodiment sets an object of the superimposing in the step S 9  as all of the spectroscopic characterizations S c  calculated at the steps S 5 , S 6 , but there is a case when the spectroscopic characterization S c  is preferable to be taking off from the object of the superimposing when the end point P e  positions on a red-violet light U as shown in  FIG. 14  depending on a type of a light source illuminating a subject. Incidentally, the red-violet light U is a comparatively long straight line connecting the reference point P 380  and the reference point P 780 . 
         [0099]    Besides, the computer  21  of the above-stated embodiment performs the superimposing in the step S 9  as the simple superimposing, but it may be the weighting superimposing. In case of the weighting superimposing, the spectroscopic characterization of the light source illuminating the subject may be reflected to the weighting ratio of the superimposing. For example, when the light source is red-tinged, a weighting value of the spectroscopic characterization S c  to which the red color reference light is relating is set to be large. It is possible to surely enhance an estimation accuracy of the spectroscopic characterization of the incident light independent from the type of the light source. 
         [0100]    Besides, the computer  21  of the above-stated embodiment asks light segments with various angles passing on the point to be converted P in the step S 4  to the step S 9 , but one line segment P s P e  passing on the point to be converted P and in parallel with the red-violet light U may be asked instead of these line segments as shown in  FIG. 15 . In this case, the computer  21  detects two to four reference points positioning in a vicinity of the start point PS and the end point P e , and regards the weighting synthesis of the spectroscopic characterizations of two to four reference lights corresponding to the two to four reference points as the spectroscopic characterization of the incident light. 
         [0101]    Besides, the computer  21  of the above-stated embodiment asks the light segments with various angles passing on the point to be converted P in the step S 4  to the step S 9 , but a first line segment PP n  heading for the reference point nearest from the point to be converted P and a second line segment P V1 P V2  passing on the point to the be converted P and perpendicular to the line segment PP n  may be asked as shown in  FIG. 16  instead of these line segments. In this case, the computer  21  detects three to five reference points positioning in a vicinity of the points P V1 , P V2 , P n , and regards the weighting synthesis of the spectroscopic characterizations of three to five reference lights corresponding to the three to five reference points as the spectroscopic characterization of the incident light. 
         [0102]    Besides, the computer  21  of the above-stated embodiment assumes the plurality of monochromatic lights as the plurality of reference lights (refer to  FIG. 2 ), but a combination of white light and the plurality of monochromatic lights may be assumed. In this case, the manufacturer adds a reference point (a white point) P w  corresponding to the white light to the characteristic data of the color imaging device, as shown in  FIG. 17 . Incidentally, the white point P w  positions in a vicinity of a center of the closed curve S. 
         [0103]    In the step S 4  to the step S 9  in this case, the computer  21  asks a line segment P w P e  starts from the white point P w , passing through the point to be converted P, and intersects the closed curve S, detects two to three reference points (one of them is the white point) positioning in a vicinity of the points P w , P e , and regards the weighting synthesis of the spectroscopic characterizations of two to three reference lights (one of them is the white point) corresponding to the two to three reference points as the spectroscopic characterization of the incident light. 
         [0104]    Besides, the computer  21  of the above-stated embodiment records a relation between the plurality of reference lights of which spectroscopic characterizations are already known (refer to  FIG. 2 ) and the reference points P 380 , P 390 , . . . , P 780  (refer to  FIG. 8 ) in advance, but further, the computer  21  may record a relation between a change of half breadths ω of the plurality of reference lights (refer to  FIG. 18 ) and a movement of the plurality of reference points P 380 , P 390 , . . . , P 780  (refer to  FIG. 19 ). As shown by arrow heads in  FIG. 18  and  FIG. 19 , the larger the half breadth ω becomes, the narrower a distribution range of the reference points P 380 , P 390 , . . . , P 780  becomes, and a size of the closed curve S becomes small. 
         [0105]    In the step S 4  to the step S 9  in the above case, the computer  21  detects a value ω 0  of the half breadth ω when the closed curve S passes on the point to be converted P while changing the half breadth ω and the size of the closed curve S working with the half breadth ω. The computer  21  detects one or two reference point(s) positioning in a vicinity of the point to be converted P under the state as stated above, and regards the weighting synthesis of the spectroscopic characterization of one or two reference light(s) corresponding to the one or two reference point(s) (however, the value of the half breadth ω is ω 0 ) as the spectroscopic characterization of the incident light. 
         [0106]    Besides, the computer  21  of the present embodiment performs the chromatic coordinate conversion by the calculation, but a lookup table in which each rg value on the rg chromaticity coordinate system and each xy value on the xy chromaticity coordinate system are corresponded with each other may be used as shown in  FIG. 20 . A lookup table capable of performing a high accurate coordinate transformation can be obtained if the above-stated any one of the chromatic coordinate conversion methods is adopted to create the lookup table. 
         [0107]    In this case, the computer  21  may generate information of each point on the rg chromaticity coordinate system from the information stored in the lookup table by an interpolation calculation if necessary. 
         [0108]    Besides, when the RAW data after the chromatic coordinate conversion (Yxy data) is outputted to the monitor  22   a , the computer  21  of the above-stated embodiment may convert the Yxy data into the RGB data complying with the characteristic of the monitor  22   a . As a result, an accurate color-reproduction becomes possible regardless of what kind of characteristics the color imaging device and the monitor  22   a  have. 
         [0109]    Besides, when the computer  21  of the above-stated embodiment outputs the Yxy data after the chromatic coordinate conversion to a not-shown printer, the computer  21  may convert the Yxy data into a CMYK data complying with a characteristic of the printer. Accordingly, the accurate color-reproduction becomes possible regardless of what kind of characteristics the color imaging device and the printer have. 
         [0110]    Besides, any one of the above-stated chromatic coordinate conversion methods are also effective when a correspondence table between the RAW data generated by the color imaging device and device independent colors (refer to  FIG. 21 ) is created. This can be realized by a software program, a work sheet, and so on. 
         [0111]    Besides, in the above-stated embodiment, the function of the chromatic coordinate conversion is mounted on the computer  21 , but the similar function of the chromatic coordinate conversion may be mounted on other apparatuses such as an electronic camera, a printer with monitor, and a television monitor. 
         [0112]    Besides, when an automatic white balance control function (namely, a function referring to a rg chromaticity of each small area of a photographed image, and performing a light source judgment based on the rg chromaticity) is mounted on the electronic camera, the light source judgment is good to be performed after the rg chromaticity of the small area is performed the chromatic coordinate conversion. As a result, it becomes possible to perform the light source judgment not on the chromatic coordinate system unique to the color imaging device, but on the predetermined chromaticity coordinate system. 
         [0113]    Besides, a human skin discrimination is preferable to be performed after the chromatic coordinate conversion of the chromaticity of the small area is performed, when a function performing the human skin discrimination is mounted on the electronic camera. As a result, it becomes possible to perform the human skin discrimination not on the chromaticity coordinate system unique to the color imaging device, but on the predetermined chromaticity coordinate system (a general chromaticity coordinate system reflecting human visual characteristics such as the chromaticity coordinate system of the CIE colorimetric system). 
         [0114]    The many features and advantages of the embodiments are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the embodiments that fall within the true spirit and scope thereof. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the inventive embodiments to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope thereof.