Dichromatism (or polychromatism) is a phenomenon where a material or solution's hue is dependent on both the concentration of the absorbing substance and the depth or thickness of the medium traversed.[1] In most substances which are not dichromatic, only the brightness and saturation of the colour depend on their concentration and layer thickness.
Examples of dichromatic substances are pumpkin seed oil, bromophenol blue, and resazurin. When the layer of pumpkin seed oil is less than 0.7 mm thick, the oil appears bright green, and in layer thicker than this, it appears bright red.
The phenomenon is related to both the physical chemistry properties of the substance and the physiological response of the human visual system to colour. This combined physicochemical–physiological basis was first explained in 2007.[2]
In gemstones, dichromatism is sometimes referred to as the 'Usambara effect'.[3]
- ^ Kennard IG, Howell DH (1941) Types of colouring in minerals. Am Mineral 26:405–421
- ^ Kreft S and Kreft M (2007) Physicochemical and physiological basis of dichromatic colour, Naturwissenschaften 94, 935-939. On-line PDF
- ^ https://www.ssef.ch/wp-content/uploads/2019/02/facette-2019.pdf [bare URL PDF]