Measurement of light

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

A formula for the luminous flux
Where Km is the maximum spectral light (apparent) efficiency of radiation, lm/W. In monochromatic radiation, Km value under bright vision condition is 683lm/W (when λ=555nm).
V (λ) – spectral light (apparent) efficiency;
P (λ) — spectral power distribution function of the radiator;
ф — luminous flux, LM (1LM = 1Cd · Sr).

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

light intensity
Units are candela (CD), 1cd=1lm/sr.
Candela is one of the basic units in the international system of units and the legal system of units in our country. Other units of light measurement are derived from Candela. In October 1979, the 16th International Conference on Measurement approved the Kendra redefinition as: a light source emits monochromatic radiation with a frequency of 540×1012Hz (corresponding to monochromatic radiation with a wavelength of 555nm in air). If the radiation intensity in a certain direction is 1/683W· SR-1, the luminescence intensity of the light source in this direction is 1CD.

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

formula 3
The unit of illuminance is lux (1x), and 1lx=1lm/m2. In addition to Lux (LX), the units of illuminance in North America are feet of candlelight (FC), 1FC =10.761x. In engineering also used radiation (PH), radiation (MPH) and so on.

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

For luminescent surface A of any size, if the luminous flux is ф, then the average light emission degree M of surface A 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

Where ρ is the reflection coefficient (reflection ratio) of the illuminated surface;
— the transmission coefficient (transmission ratio) of the exposed surface;
E — illuminance on the secondary luminous surface.

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

formula 7
Where d2 ф (ф, θ) — the optical flux transmitted through the beam element at a given point and including the solid Angle dω in a given direction;
DA — including the beam cross-sectional area at a given point;
θ — Angle between beam cross-sectional area and beam direction.
The symbol for brightness is L and the unit is candela per square meter [CD /m2=lm/ (m-2·sr)].
There are various units of brightness. In addition to the international units of Candela per square meter (CD /m2) or nite, there were once hilti (SB), Apohilti (ASB), and foot-Lambert (ft·L).