# Measurement of light

Luminous flux ф light source in the unit time to emit light is called the luminous flux of the light source, expressed with ф, the unit is lumen (LM). It is a light-2 solar LED luminescence system metric derived from the effect of radiation on a standard observer.

For bright vision, if the spectral radiation flux of the radiator is ф E (λ), the expression of the luminous flux ф is

Luminous intensity I The luminous flux emitted by a light source in a unit solid Angle in a given direction is defined as the light intensity of the light source in that direction (light intensity for short) and is denoted by I.

If the small light flux in a small solid Angle DW is dф (ф, 0), then the light intensity in this direction is

Illuminance E illuminance (referred to as illuminance) is a measure of the degree of illumination on a surface. It is the flux of light received per unit surface. If the luminous flux on the small area dA is Dф, then the illuminance of the illuminated surface is

The illuminance of 1LX is relatively small, in this illuminance can only roughly identify the surrounding objects, to distinguish small parts of the work is impossible. To give some practical idea of illuminance, here are some examples: on a clear full moon night the ground illuminance is about 0.2lx; Indoor illumination with good daylighting is 100~500lx; In sunny days, the illumination of the ground under the scattered light of the sun (not direct) is about 1000lx. At noon, the surface illuminance of the sun can reach 100000Lx.

The luminous flux emitted by a small surface element dA on the surface of the light source to half a space is dф, then the luminous radiance M of this surface element is

So, light emission is the flux of light emitted per unit area, which is measured in lm/m2. The luminance and illuminance have the same dimension, the difference is that the luminance represents the surface density of the luminous flux emitted by the luminous body, while the illuminance represents the surface density of the luminous flux received by the illuminated object.
For a secondary luminescent surface that emits light by reflection or transmission, the degree of emission is

Luminance L The luminance of the light source in a certain direction L (ф, 0) is the luminous flux emitted by the unit projection plane of the light source in this direction in unit solid Angle. If the light flux in the small area dA and the small solid Angle dω is dф (ф, θ), then the brightness is