Spectral responsivity vs. action spectrum in digital photography

We propose an experimental and theoretical methodology based on spectroradiometric procedures to determine the spectral sensitivity of digital still cameras (DSC). The opto-electronic conversion spectral functions (OECSF's) are obtained from normalized digital level (NDL) vs. spectral exposure data, fitted by a typical sigmoidal transition curve. At this stage, we define spectral responsivity r(λ, H) and action spectrum a(λ, NDL), which are conceptual derivations of the general concept of spectral sensitivity of any image sensor. Then, the real spectral responsivity and action spectrum are represented as 3-dimensional functions, where r(λ, H = constant) and a(λ, NDL = constant) profiles are scaled relatively to determine the valid linear output range of the digital image capture device. This range would serve to design the colorimetric profile of the camera with CIE-1931 XYZ standard observer, which is considered as a linear color image sensor.This research was supported by the Comisión Interministerial de Ciencia y Tecnología (CICYT) (Spain) under grants TAP96-0887 and TAP99-0856