The intensity of visible light is measured in units of candela which replaced the older unit, the candle. One lux is a unit of illumination equal to one lumen per square meter. Irradiance is a measure of radiometric flux per unit area, or flux density.
Illuminance is a measure of photometric flux per unit area.
Radiant Flux is a measure of radiometric power. Luminous Flux is a measure of photometric power. Photometric flux, expressed in lumens, includes only visible light, and is weighted to match the responsivity of the human eye, which is most sensitive to yellow-green. Luminous Efficacy is the total luminous flux divided by the total radiant flux or in the case of a lamp bulb, the total light flux in lumens divided by the electrical power input in watts. It is measured in lumens per watt.