ABSTRACT
There are four interrelated units. These describe the flow of light, its intensity in space,
illuminance at a point, and the luminance of a surface.
FLOW AND INTENSITY
The first unit of light is the lumen (lm). It describes luminous flux, the total flow of light
from a source, just as the flow of heat from a radiator can be described in watts (W). The
output from a lamp is given in lumens. For example:
100 W incandescent lamp 1360 lm
58 W fluorescent tube 5200 lm
400 W high-pressure sodium lamp 48 000 lm
These are typical values, but light output depends on the details of a lamp’s construction,
and it decreases as the lamp ages.
