Reproduction Model with Luminance Adaptation for Digital Cameras

From real spectroradiometric data, we propose an algorithm to obtain camera opto-electronic conversion functions (camera OECFs) for different values of the lens aperture N from the opto-electronic conversion spectral functions (OECSFs). Working in the linear range of the inverse OECFs, we have found that the slope vs. lens aperture function is fitted by a second order polynomial. This happens also to the offset vs. lens aperture function. Therefore, we have included the inverse OECFs with luminance adaptation into a basic reproduction model, consisting in a 3x3 transform between the raw RGB space and CIE-XYZ space, opening the possibility of transforming any digital camera into an absolute tele-colorimeter. Introduction A digital image capture device (scanner or camera) is not a color measuring tool as a spectrophotometer or a telespectroradiometer. Although it encodes the spectral information enclosed in a scene into three RGB color channels, the RGB digital data associated to a visual stimulus are not the same than would encode a human observer, who is characterized by the CIE-1931 XYZ standard observer. The color characterization of digital image capture devices consists of calculating the colorimetric profile between RGB device space and the CIE-1931 XYZ space associated to the same color stimulus under uncontrolled illumination conditions, for variations of chromaticity and intensity of the illumination. In this way, the digital image capture device should perform as an absolute tele-colorimeter because the color output data would be in cd/m, i.e., the color device would be simultaneously a colorimeter and a luminance meter. From the digital output levels DOLk (k = R, G, B color channels) we could obtain the absolute tristimulus values CIE-XYZ (in cd/m), as are obtained by a telespectradiometer (Eq.1). c T t XYZ XYZ t m K Z Y X =     