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Four LED Brightness Calculation Methods

Nov 26, 2019

Four LED Brightness Calculation Methods

First, the luminous flux

Luminous flux refers to the amount of light emitted by a light source per unit time, that is, the portion of radiant energy that the radiant power can be perceived by the human eye. It is equal to the product of the radiant energy of a certain band per unit time and the relative visibility of that band. Since the relative visibility of the different wavelengths of light by the human eye is different, the luminous fluxes are not equal when the radiation powers of the different wavelengths of light are equal. The sign of luminous flux is Φ, the unit is lumens (Lm)

According to the spectral radiant flux Φ(λ), the luminous flux formula can be derived:


In the formula, V(λ)—relative spectral luminous efficiency; Km—the maximum value of spectral optical performance of radiation, in units of Lm/W. The Km value was determined by the International Metrology Commission in 1977 to be 683 Lm/W (λm = 555 nm).

Second, the light intensity

Light intensity refers to the light energy passing through a unit area per unit time. The energy is proportional to the frequency, which is the sum of their intensities (ie, integral). It can also be understood that the luminous intensity I of the light source in a given direction is the light source. The quotient of the luminous flux dΦ transmitted in the solid angle element in this direction divided by the solid angle element dΩ

The unit of luminous intensity is candela (cd), 1 cd = 1 Lm / 1 sr. The sum of the light in all directions of space is the luminous flux.

Third, the brightness

In the process of testing the brightness of LED chips and evaluating the safety of LED light radiation, imaging methods are generally used, and microchip imaging can be used for chip testing. The brightness is the brightness L at a certain point on the light-emitting surface of the light source, which is the quotient of the light-emitting intensity of the face element dS in a given direction divided by the orthographic area of the face element in a plane perpendicular to a given direction.

The unit of brightness is candela per square meter (cd/m2). When the light emitting surface is perpendicular to the measuring direction, then cos θ = 1.

Fourth, the illuminance

Illuminance is the degree to which an object is illuminated, expressed in terms of the luminous flux received per unit area. The illuminance is related to the position of the illumination source, the illuminated surface and the light source in space, and the size is proportional to the light intensity of the light source and the incident angle of the light, and inversely proportional to the square of the distance from the light source to the surface of the illuminated object. The illuminance E at a point on the surface is the quotient of the luminous flux dΦ incident on the panel including the point divided by the panel area dS.

The unit is lux (LX), 1LX = 1Lm/m2.