1. Field of the Invention
The present invention relates to a two-dimensional colorimeter, and more particularly to a tristimulus two-dimensional colorimeter provided with a spectral sensitivity correction function.
2. Description of the Related Art
Heretofore, there have been used tristimulus two-dimensional calorimeters i.e. colorimeters for directly measuring tristimulus values using an optical filter such as an XYZ filter, or a like optical member, to measure luminance or chromaticity of a two-dimensional light source. Unlike a two-dimensional spectrocolorimeter, the tristimulus two-dimensional calorimeters have a characteristic that spectral sensitivities thereof do not completely match with a color matching function (X(λ), Y(λ), Z(λ)). Accordingly, the tristimulus two-dimensional colorimeters may have an error in measurement values resulting from mismatching between the spectral sensitivities and the color matching function. In other words, the tristimulus two-dimensional calorimeters fundamentally have a measurement error with respect to the color matching function. Also, there are performance differences among individual optical filters or like optical members for use in obtaining tristimulus values. Accordingly, a measurement error concerning the tristimulus two-dimensional colorimeters provided with the individual optical members may occur resulting from the optical member performance differences.
Let us assume that an object image having different colors e.g. colors A, B, and C arranged at respective different positions on a two-dimensional plane, for instance, as shown in FIG. 11A, is measured by a tristimulus two-dimensional colorimeter. In this case, as mentioned above, a measurement error may occur resulting from the mismatching between the spectral sensitivities and the color matching function, or the optical member performance differences (see FIG. 11B, in which “X” indicates a site where a measurement error has occurred concerning the relevant color). In the conventional art, correction i.e. white color calibration (see FIG. 11C) is performed to reduce the measurement error by calculating a calibration coefficient in such a manner that a measurement chromaticity value concerning an object as a calibration reference e.g. a white color (achromatic) light source W is approximated to its true value. In the conventional art, the calculated calibration coefficient is stored in advance, and the measurement value is corrected at the time of measurement, using the stored calibration coefficient.
In the aforementioned white color calibration, a chromaticity value of a color other than the white color may have a measurement error. In the case where a measurement value concerning e.g. the color A obtained by the tristimulus two-dimensional colorimeter is approximated to its true value in order to correct the measurement error, as shown in FIG. 12, the measurement value is corrected by: calculating a calibration coefficient, based on a correlation between the measurement value concerning the color A, and its true value i.e. a value having a high precision as an absolute value obtained by e.g. a spectrocolorimeter, for instance, calculating a calibration coefficient obtained as an inverse number of a ratio of a measurement value versus a true value, which corresponds to an arbitrary calibration coefficient shown in FIG. 12; and by multiplying the measurement value by the calibration coefficient. In the following, calibration of arbitrary colors i.e. colors arbitrarily selected by a user, including the colors B and C, in addition to the color A, is called as “arbitrary correction”. An example of the arbitrary correction with respect to the colors B and C is also shown in FIG. 12. In the conventional arbitrary correction, however, a common arbitrary calibration coefficient is applied to the entirety of measurement results by a two-dimensional measurement. Accordingly, correction with respect to the color A may increase a difference between a measurement value and its true value concerning the colors B and C. As a result, an object image after the correction through visual observation may have different colors from those of an object image before the correction. The same drawback may occur in the case where correction is performed with respect to the color B or the color C, which may also increase a difference between the measurement value and the true value with respect to colors other than the color B or the color C (see FIG. 11D).
As mentioned above, in the case where there exist plural colors within a two-dimensional image to be measured (hereinafter, the image is called as “screen image” according to needs), a measurement error between the measurement value and the true value, resulting from mismatching with the color matching function or optical member performance differences, may occur with respect to the colors within the screen image. Also, since a measurement error characteristic with respect to the true value is different among the colors, even if a measurement error concerning a certain targeted color is corrected, a measurement error may remain concerning the colors other than the targeted color. In a worse case, a measurement error concerning the other colors may be increased. In view of this, it is necessary to correct the measurement errors by providing calibration coefficients individually for the colors to obtain measurement results with less measurement errors with respect to the entirety of the screen image. In the conventional art, however, a common calibration coefficient is applied to the entirety of the screen image, or calibration coefficients are switched over among individual areas within the screen image. The conventional art has no disclosure about an idea of applying optimal calibration coefficients to the colors individually within a common screen image.
In the conventional art, individual screen images are provided with respect to the respective corrected colors. In the above example, three images are formed with respect to the colors A, B, and C. This arrangement fails to apply correction results concerning the respective colors to a common screen image, which makes it impossible to obtain an image i.e. measurement results which are approximated to the respective true values with respect to the entirety of the two-dimensional measurement area. Also, in the case where the arbitrary correction is performed on a real-time basis each time a measurement is conducted, it is necessary to perform the measurement by the number of times equal to the number of colors for arbitrary correction, which may lower processing efficiency, in other words, may increase a measurement period, and may lower usability in operation i.e. operability. Further, in the case where measurement results i.e. image data obtained by an image sensing device are temporarily stored before an arbitrary correction is conducted, the measurement results are obtained by one-time measurement. However, as a post-process, arbitrary correction is required to be performed by the number of times equal to the number of colors for arbitrary correction, which may also lower processing efficiency and operability.