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Thomas Bangert, Ebroul Izquierdo; Alternatives in chromaticity. Journal of Vision 2017;17(15):38. doi: 10.1167/17.15.38.
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Chromaticity is a very useful way of representing colour by taking the ratio of a set of spectrally restricted visual sensors and mapping that into a two-dimensional space. The chromaticity space for the CIE standard observer is derived from colour matching studies performed on a fairly small number of subjects, and the derived colour matching functions are in many ways inadequate and fail to reflect the abilities not only of the human visual system but those of other animals with excellent colour vision such as birds. While it is useful that chromaticity reflects human colour perception, this is not necessary and in some ways unnecessarily limiting. Chromaticity simply maps pure monochromatic (single wavelength) colours to an approximately circular space which encloses all less pure colors, with achromatic white at the centre. It is highly desirable for the space to be regular so that when a colour is measured its wavelength (the angle) and saturation (the distance) can easily be determined from its chromaticity coordinates. The CIE chromaticity space is irregular both in angle and distance. We have devised a number of alternative colour matching functions which produce chromaticity spaces that exhibit the desirable property of regularity. We show that with three sensors it is possible to have a chromaticity space that is regular by angle or by distance but not both. We also demonstrate a four-sensor chromaticity space that is fully regular, and we suggest that this may provide an explanation why some animals with excellent colour vision are tetrachromatic.
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