Threshold of Illuminance
Alternative term: Visual Threshold
The weakest illuminance produced at the eye which makes it possible to see a given source against a given background luminance.
Note: The internationally accepted value of the threshold of illuminance for observation of a light at night under typical maritime conditions is 2.10-7 lux (0.686 sea-mile candela, 2-1-060). This figure was agreed at the International Technical Conference of Lighthouse Authorities, Paris 1933. In Britain and U.S.A. this value is sometimes approximated by 0.67 sea-mile candela.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM