Illuminance is a measure of how much light is striking a surface: luminous flux per unit area. It is measured in lux.
Illuminance is useful as a measure of how brightly a surface is lit. For example, on an overcast day, outside surfaces have an illuminance of approximately 1000 lux; inside an office might be more like 400 lux and under a full moon might be only 0.2 lux.
Construct an illuminance value from a number of lux. One lux is equal to one lumen per square meter. See here for a table of illuminance values in lux for common environments.
Convert an illuminance value to a number of lux.
Construct an illuminance value from a number of foot-candles. One foot-candle is equal to one lumen per square foot.
Convert an illuminance value to a number of foot-candles.