1. A method of processing a plurality of source images, comprising the steps of: encoding the plurality of source images in a composite image; rendering the composite image on a substrate by use of a plurality of colorants; and recovering a desired one of the encoded source images from the rendered composite image, such that the recovered source image is made distinguishable, by subjecting the rendered composite image to an illuminant provided from an illuminant source and to a sensor in a detector, so as to detect the resulting recovered source image; wherein the encoding of each source image is performed according to a determination for optimizing the effect of the following on at least one of the encoding, rendering, and recovery steps: (a) the spectral absorption characteristics of the colorants selected for rendering the composite image; (b) the spectral radiance characteristic of the illuminant that is used to illuminate the composite image for recovering the source image; and (c) the spectral response characteristic of the sensor used to sense the rendered composite image during such illumination, for recovery of the desired source image.