(Luminance) Contrast Threshold
The minimum contrast at the eye of a given observer at which an object can be detected.
Note: The contrast threshold is a property of the eye of the individual observer. For most normal observers under ordinary daylight conditions there is good agreement between values for different individuals.
The average value has been taken to be 0.02 in most studies of visibility, and this value is correct for laboratory observation under daylight conditions.
For practical observation in the field, a value of 0.05 is more appropriate and has been adopted internationally.
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