Locus filters.

In this paper, directly following from Gage [J. Opt. Soc. Am.23, 46(1993)10.1364/JOSA.23.000046], we study the design of a particular theoretical filter for photography, that we call the locus filter. It is built in such a way that a Wien-Planckian light (of any temperature) is spectrally mapped to another Wien-Planckian light. We provide a physical basis for designing such a filter based on the Wien approximation of Planck's law, and we prove that there exists a unique set of filters that have the desired property. While locus filtered Wien-Planckian lights are on the locus, the amount they shift depends both on the locus filter used and on the color temperature of the light. In experiments, we analyze the nature of temperature change when applying different locus filters and we show that real lights shift more or less as if they were Planckians in terms of the changes in their correlated color temperatures. We also study the quality of the filtered light in terms of distance from the Planckian locus and color rendering index.