1. A computer-implemented multi-spectral imaging process for computing a reflectance spectral distribution of a captured scene using wide band illumination with known spectral distributions in the presence of ambient illumination having an unknown spectral distribution, the process comprising using a computer to perform the following process actions: selecting a prescribed number of different illumination spectra; capturing multiple images of the scene illuminated by said ambient light using a camera, wherein each image is captured when the scene is further illuminated by a different one of the selected illumination spectra; for each pixel location in the captured images, computing the reflectance spectral distribution of the scene associated with that pixel location based on the relative response between pairs of the radiometric responses of the corresponding pixels in the captured images associated with the pixel location under consideration, given a set of parameters including the added illumination spectra used to capture each of the images and the response function and spectral sensitivity of the camera used to capture the images.