In view of the above, in Step S110 in FIG. 4, on the basis of such 36 pieces of combination data as mentioned above, the first relational equation which represents the relationship between the spectral reflectances of the maximum gradation value and the spectral reflectances of the focused gradation value is obtained. The first relational equation is an approximate equation for obtaining approximate values of the spectral reflectances of the focused gradation value from the spectral reflectances of the maximum gradation value. Note that the first relational equation is obtained by a known method. For example, the first relational equation is obtained by solving simultaneous equations obtained from the 36 pieces of combination data by Gaussian elimination or Gauss-Jordan elimination.
In the present embodiment, a quintic equation is adopted as the first relational equation (the approximate equation). As an example, such a quintic equation as the following Equation (4) is obtained by the process in Step S110 in FIG. 4. Note that, regarding the following Equation (4), y is the reflectance of the focused gradation value (the characteristics-acquired gradation value), and x is the reflectance of the maximum gradation value.
y=?0.321x5+0.7136x4?0.1681x3?0.6897x2+1.0892x+0.375??(4)
