The four basic lighting quantities
1. Luminous flux (Φ) is the rate at which light is emitted by a lamp. Measured in lumen (lm), it defines the visible light radiating from a light source in all directions.
2. Luminous intensity (I) is the amount of luminous flux radiating in a particular direction. It is measured in candela (cd).
The spatial distribution of luminous intensity – normally depicted by an intensity distribution curve (IDC) - defines the shape of the light beam emitted by a luminaire, reflector lamp or LED.
3. Illuminance (E) – measured in lux (lx) – is the luminous flux from a light source falling on a given surface.
Where an area of 1 square metre is uniformly illuminated by 1 lumen of luminous flux, illuminance is 1 lux.
The flame of an ordinary candle, for example, produces around 1 lx at a distance of 1 m.
4. Luminance (L) is the brightness of a luminous or illuminated surface as perceived by the human eye.
Measured in cd/m2 or cd/cm2, it expresses the intensity of the light emitted or reflected by a surface per unit area.