Common measurement methods and formulas of LED brightness
Like traditional light sources, the optical measurement units of LED light sources are uniform. In order to make readers understand and convenient to use, the relevant knowledge will be briefly introduced below:
1. Luminous flux
Luminous flux refers to the amount of light emitted by the light source in a unit time, that is, the part of the radiant energy that can be felt by the human eye. It is equal to the product of the radiant energy of a band in a unit time and the relative visibility of the band. Since the relative visibility of the human eye to different wavelengths of light is different, when the radiant power of different wavelengths of light is equal, the luminous flux is not equal. The symbol of luminous flux is Φ, and the unit is lumens (Lm).
According to the spectral radiant flux Φ (λ), the luminous flux formula can be derived:
Φ = KmΦ (λ) gV (λ) dλ
In the formula, V (λ) —relative spectral light viewing efficiency; Km—the maximum value of the spectral light viewing efficiency of radiation, in units of Lm / W. In 1977, the Km value determined by the International Metrology Commission was 683Lm / W (λm = 555nm).
2. Light intensity
Light intensity refers to the light energy that passes through a unit area in a unit of time, and the energy is proportional to the frequency, which is the sum of their intensities (that is, the integral). The quotient of the light flux dΦ transmitted in the cube corner element in this direction divided by the cube corner element dΩ
The unit of luminous intensity is candela (cd), 1cd = 1Lm / 1sr. The sum of light intensities in all directions in 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, imaging methods are generally used, and microscopic imaging can be used for chip testing. Luminance is the brightness L of a spot on the light emitting surface of the light source, which is the quotient of the luminous intensity of the face element dS in the given direction divided by the orthographic projection area of ​​the face element in the plane perpendicular to the given direction
The unit of brightness is candela per square meter (cd / m2). When the light emitting surface is perpendicular to the measurement direction, then cosθ = 1.
4. Illumination
Illumination refers to the degree to which objects are illuminated, and is expressed in terms of luminous flux received per unit area. The illuminance is related to the illuminating light source, the illuminated surface and the position of the light source in space. The size is directly proportional to the intensity of the light source and the Yu Xuan 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 panel containing the point divided by the area dS of the panel.
The unit is Lux (LX), 1LX = 1Lm / m2.
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