The candela, the lumen and the lux

The candela (cd) is the SI unit if light intensity, defined as 1.46x10-3 Watts per steradian at a frequency of 5.40x1014 Hz (in the green). [A sphere encloses 4p steradians.] Candle intensity is low. Light bulbs are much brighter!

In the 19th C, light intensity was measured in candles (eg 2000 candlepower).

The lumen (lm) is the SI unit of luminous flux. Flux is (Intensity x Area). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source with an intensity of 1 cd that radiates equally in all directions.

A Vernier light probe is calibrated in units of illumination called lux.

One lux (lx) is one lumen/m2. A lux meter reading varies inversely with the square of the distance from a point source.