What is lumen？

2023-09-25

lumen (symbol lm) is the international unit of luminous flux. It is defined in terms of Candela spherical Angle (cd times sr). A lumen is a unit of a considerable amount of light emitted at 1 spherical Angle, emitting the same amount of energy in all directions from the light source, and the intensity is 1 CD.

Luminous intensity is referred to as light intensity, the international unit is candela(Candela) short for cd. A point light source with a luminous intensity of 1cd emits a luminous flux of 1lm within a unit solid Angle (1 sphericity). When the light source radiation is uniform, the light intensity is I=F/Ω, Ω is the solid Angle, the unit is sphericity (sr), F is the luminous flux, the unit is lumen, for the point light source by I=F/4π.
Brightness is the brightness of the luminous surface, refers to the luminous intensity of the luminous surface in the specified direction and the ratio of the vertical and specified direction of the luminous surface area, the unit is candela/square meter. For a diffuse scattering surface, although the light intensity and luminous flux in all directions are different, the brightness in all directions is equal. The fluorescent screen of a television is approximately such a diffuse scattering surface, so the image is viewed from all directions and has the same sense of brightness.

The light energy emitted by the source is radiated in all directions, and for the light energy passing through a certain area in a unit time, it is called the radiant energy flux through this area. The frequency of each color of light is different, and the sensitivity of the eye to each color of light is different, even if the radiant energy flux of each color of light is equal, the same degree of brightness can not be produced visually, in each color of light, yellow and green light can arouse the largest bright feeling.
If the green light is used as the level, so that its luminous flux is equal to the radiant energy flux, then for other colors of light, the ability to arouse bright sensation is smaller than that of green light, and the luminous flux is smaller than that of radiant energy flux.
The unit of luminous flux is lumen, which is the transliteration of English lumen, abbreviated as lm.

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