A simple method of expressing the intensity (or level of brightness) of GTLS (Gaseous Tritium Light Source) devices is to compare them with known objects and scenes, as in the following table:

Approx. Brightness
(in Foot-Lamberts)
Comparable Visual Object or Scene Approx. Text-Effect
100,000 Upper limit of visual tolerance BLACK - GREY - WHITE
10,000 Fresh snow on a clear day BLACK - GREY - WHITE
1,000 Average earth on a clear day BLACK - GREY - WHITE
100 Average earth on a cloudy day BLACK - GREY - WHITE
10 White paper in good reading light BLACK - GREY - WHITE
1 White paper 1 foot (34cm) from standard candle BLACK - GREY - WHITE
0.01 (= H3 upper) Snow in full moonlight BLACK - GREY - WHITE
0.001 (= H3 lower) Average earth in full moonlight BLACK - GREY - WHITE
0.0001 Snow in starlight BLACK - GREY - WHITE
0.00001 Grass in starlight BLACK - GREY - WHITE
0.000001 Absolute threshold of sight BLACK - GREY - WHITE

GTLS devices are measured by luminous intensity rather than brightness. The term "brightness" is merely the degree to which a unit surface area appears to emit light in the direction of an observer.

For GTLS devices, the corresponding physical measurement of the light actually emitted is called the luminance. This is the measure of the light actually emitted (i.e. the luminous intensity) per unit projected area of surface, the plane of projection being perpendicular to the direction of view.

In engineering, the luminance of an ideally diffusing surface emitting or reflecting one lumen per square foot is called one foot-Lambert (ft-L). And on this scale, H3 light sources [ie. tritium/hydrogen lamps] lie between 0.01 and 0.001 ft-L (equivalent to full moonlight, as in the above table).

For more, read Permaglo Signs: Intensity of GTLS Devices.