17-139

chroma

colourfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting

NOTE For given viewing conditions and at luminance levels within the range of photopic vision, a colour stimulus perceived as a related colour, of a given chromaticity and from a surface having a given luminance factor, exhibits approximately constant chroma for all levels of illuminance except when the brightness is very high. In the same circumstances, at a given level of illuminance, if the luminance factor is increased, the chroma usually increases.