Improving color calibration, and similar operations, by generating an optimal raw RGB color chart, in the raw RGB domain, for an imaging device with a given type of image sensor, such as for a digital still camera. Calibration is performed in response to a constraint which takes into account the spectral sensitivity of the image sensor used on the camera as well as spectral reflectance and spectral radiance of multiple illuminants. Calibration is performed using the raw RGB color chart which is illuminant independent and image sensor specific and which can match the calibration performance of known calibration standards, such as Macbeth 24.