A guide to the intensity [brightness] of GTLS (Gaseous Tritium Light Source) devices obtained by reference to the trade literature is that they have a value typically of 1000 micro-Lamberts (ignoring any slight loss through absorption by the Perspex/polycarbonate sign window or the general mode of construction of Alsigns PERMAGLO products). Comparison estimates may be based upon approximations of the performance of Swan/Edison lamps and their electrical ratings [normally expressed in watts]. For example 1000 micro-Lamberts is equivalent to 0.01 watts [i.e. one (1) watt equals 100,000 micro-Lamberts]. Other values may be calculated pro-rata although there are no electric lamps with such low values as fractions of a watt.

An alternate method of expressing the brightness of a tritium (H3) light source is to compare it with known objects and scenes, as in the following table:

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

The position of H3 light sources [ie. tritium/hydrogen lamps] on this scale is between 0.01 and 0.001.

The brightness of a surface is that property by which the surface appears to emit more or less light in the direction of view. This is a subjective quantity. The corresponding physical measurement of the light actually emitted is called the luminance.

Luminance of a surface 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.

Unit: candela/square foot or candela/square metre.

In engineering, the luminance of an ideally diffusing surface emitting or reflecting one lumen/square foot is called one foot-Lambert (ft-L).

Issued with the knowledge of Douglas Williams
(Group President, Alsigns-Surelite Ltd)