(Luminance) Contrast (of an object against its background)
1. Subjective assessment of the difference in luminance of an object and its background.
2. The quotient of the difference between the luminance of an object and the luminance of its background by the luminance of the background.
where L1 is the luminance of the background
L2 is the luminance of the object seen against the background.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Celsius (previously Centigrade)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM