Lighting units

The luminous flux F is the basic quantity in lighting technology(Phi). Luminous flux is a measure of the power of the light radiation. It is the amount of light energy radiated by a light source per unit time. The unit of the luminous flux is lumen (lm). One lumen is the luminous flux radiated in a solid angle equal to one radian from a light source equal to one candela. The candela, on the other hand, is the basic SI unit of luminous intensity. Candela is popularly called "candle". The luminous intensity is the ratio of the luminous flux to the spatial angle, in which this stream radiates.

Space angle (lumpy) it is a measure of the opening of the cone, which is a slice of a sphere (drawing).

Full spatial angle, hence the angle defined by the sphere, equals 4π steradians (4π sr). In lighting technology, there is also a quantity called illuminance E, being the ratio of the luminous flux to the area, on which this stream falls

The unit of illuminance is lux (lx), formerly called brightness. One lux is lighting, which gives the luminous flux of one lumen on an equal surface 1 m².