The Goniophotometry of Printing Ink

In the initial prototyping of a video-based densitometer, it was found that the calculated density of an ink patch varied slightly with the position of the patch within the field of view. Numerous potential explanations were investigated and dismissed including: the effects of scattered light, incomplete correction for uneven illumination and electronic distortions in the camera or frame grabber. The theory was proposed that the effect may be due to changes in the sample's reflectance with the angles of illumination and of detection. This paper a) describes the eucentric goniophotometer which was built to test this theory, b) gives results from an experiment where the goniophotometer was used to simulate video measurement of density, and c) provides a theory to explain the goniophotometric aspects of ink on paper.