The unit of measuring the brightness-or to use the precise scientific terminology, illuminance-in SI system of measurement is lux or lumen per square meter.Illuminance is measured in foot candles. One foot-candle is equal to 10.76 lux. Please note that lumen is the measure of total light emitted by a source of light.he total amount of visible light is measured in units of lumens. How bright it appears when it falls on a surface area is illuminance. When the same luminous flux falls over a larger area it will appear to be less bright. 1 lx = 1 lm/m^2 which in terms of the basic SI units is 1 cd*sr*m^-2.