9 Brightness vs Illumination
This describes the total Quantity of light output from a source. The unit for luminous flux is the Lumen (Lm), and this represents how much visible light is emitted from the lamp in every direction.
The term Candela (cd) is often associated with Luminous Flux, but specifically, Candela refers to the amount of light emitted in a particular direction. Both Candela and Lumens are used in lighting equations, and they have an equivalency of:
This means that a light source would have to emit 12.57 lumens in every direction in order to guarantee that 1 candela of illumination was measured in any particular direction.
Luminous Flux Density
The density of light flux measured with a photometer on a surface is referred to as illumination or luminous flux density. The unit it is measured in is called the Lux (Lx) or foot-candle (fc) and represents how bright a surface is.