The Impact of UCR on Scanner Calibration

Most present day color scanners, with red, green, and blue filters, are non-colorimetric. This means that their outputs cannot be linearly transformed into CIE tristimulus values for arbitrary input materials. On the other hand, by restricting oneself to a single class of inputs such as photographic, lithographic, or xerographic materials, very accurate scanner calibrations are possible. In this paper, we conjecture that such accurate calibrations can be achieved if the input document is made with only "three colorants", i.e., has only three independent degrees of freedom. The validity of the above conjecture is tested experimentally using a CMYK (four colorant) printer. Usually in CMYK printing, there are only three fundamental degrees of freedom. Even though four colorants are used, the amounts of these colorants are inter-related through the method used for undercolor removal (UCR). The fourth degree of freedom is re-introduced when the method of undercolor removal is varied. To test the above conjecture concerning "degrees of freedom", we evaluated the impact of different UCR methods on scanner calibration accuracy. The paper also proposes an analytic similarity measure for comparing color spectra from different media, which is shown to be in fair agreement with the experimental results.