Commonly used measurement methods and formulas for LED brightness

Commonly used measurement methods and formulas for LED brightness

Like traditional light sources, the optical measurement units of LED light sources are unified. In order to make readers understand and use them conveniently, the following is a brief introduction to the relevant knowledge:

1. Luminous flux

Luminous flux refers to the amount of light emitted by a light source per unit time, that is, the part of the radiation energy that can be felt by the human eye. It is equal to the product of the radiation energy of a certain band per unit time and the relative visibility of the band. Since the relative visibility of the human eye to light of different wavelengths is different, when the radiation power of light of different wavelengths is equal, the luminous flux is not equal. The symbol of luminous flux is Φ, and the unit is lumen (Lm).

According to the spectral radiation flux Φ (λ), the luminous flux formula can be obtained:

Φ=KmΦ(λ)gV(λ)dλ

In the formula, V (λ) is the relative spectral luminous efficiency; Km is the maximum value of the spectral luminous efficiency of radiation, in Lm/W. In 1977, the International Committee for Weights and Measures determined the Km value to be 683 Lm/W (λm = 555 nm).

2. Light intensity

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

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

3. Brightness

In the process of testing the brightness of LED chips and evaluating the safety of LED light radiation, we generally use imaging methods. For chip testing, microscopic imaging can be used for measurement. Brightness is the brightness L of a certain point on the light source's luminous surface, which is the quotient of the luminous intensity of the surface element dS in a given direction divided by the orthographic projection area of ​​the surface element on the plane perpendicular to the given direction.

The unit of luminance is candela per square meter (cd/m2). When the luminous surface is perpendicular to the measurement direction, cosθ=1.

4. Illuminance

Illuminance refers to the degree to which an object is illuminated, and is expressed as the luminous flux received per unit area. Illuminance is related to the illumination source, the illuminated surface, and the position of the light source in space. It is proportional to the light intensity of the light source and the cosine of 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 of a point on the surface is the quotient of the luminous flux dΦ incident on the surface element containing the point divided by the area of ​​the surface element dS.

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