Estimation of the Device Gamut of a Digital Camera in Raw Performance Using Optimal Color-Stimuli

Using a spectroradiometric model of capture for a digital camera based on the mathematical description of the empirical opto-electronic conversion spectral functions (OECSF), the capture of MacAdam or optimal spectra with fixed illumination level is simulated. This model of capture allows to change freely the f-number of the zoom-lens and/or the photosite integration time of the electronic shutter of the camera, regardless of the spectral composition of the stimulus. If we follow the procedure employed by MacAdam in 1935 working with the CIE1931 XYZ standard observer, these color-stimuli are arranged in decreasing pyramidal form as the luminance factor increases for any chromaticity diagram (CIE-xy, UCS-u'v' or CIE-L*a*b*). These loci are often called MacAdam limits or Rösch color solid. On the other hand, transforming the simulated RGB digital output levels of the optimal colors to XYZ data through the raw colorimetric profile with luminance adaptation of our digital image capture device, the corresponding MacAdam loci for each luminance factor are smaller than those of the colorimetric standard observer. This systematic desaturation of the optimal color-stimuli shows that our color device, in raw performance, desaturates in general the real color-stimuli, so this result justifies the additional use in digital photography of color correction algorithms, more or less complex, in order to reach the colorimetric status of color reproduction. Introduction Optimal or MacAdam color-stimuli are binary (zero or one) spectral profiles of reflectance or transmittance. There are two kinds of these color-stimuli: type 1, when ρ(λ) = 1 for λ ∈ [λ1 , λ2] nm, and, type 2, when ρ(λ) = 1 for λ ∉ ]λ1 , λ2] nm (Fig. 1). Figure 1. Two samples of optimal color-stimuli with luminance factor Y = 50: type 1 (top) and type 2 (bottom). Using a vector space notation the algorithm obtaining the optimal color-stimuli is as follows. Selecting the spectral range equals to [380, 780] nm at 1 nm step, the length of the color-stimuli vectors will be 401. Considering the color-matching functions TXYZ of the CIE-1931 XYZ standard observer and the equal-energy illuminant E, the two types of optimal color-stimuli are redistributed according to its luminance factor Y as follows: IS&T's 2003 PICS Conference