A photometer in which equality between a radiation under test and a comparison radiation is established visually.
The methods chiefly employed are luminosity match of two adjacent surfaces, adjustment to equality of contrast, or adjustment to minimum flicker.
Reference: C.I.E. (modified)
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