Quantification of dichromatism: a characteristic of color in transparent materials

The color of a material, such as solution of a dye, can change by changing parameters like pH, temperature, illumination direction, and illumination type. Dichromatism -- a color change due to the difference in thickness of the material -- has long been known as a property of only a few materials. Here we show that dichromatism is a common property of many substances and materials, and we introduce a method for its quantification. We defined dichromaticity index (DI) as the difference in hue angle (Delta h(ab)) between the color of the sample at the dilution, where the chroma is maximal, and the color of four times more diluted (or thinner) and four times more concentrated (or thicker) sample. The two hue angle differences are called dichromaticity index toward lighter (DI(L)) and dichromaticity index toward darker (DI(D)), respectively. High dichromaticity was found for materials that were previously known as dichromatic (pumpkin oil, bromophenol).