Measurement Method Of LED Brightness
Like traditional light sources, the optical measurement units of LED light sources are uniform. In order to make readers understand and use conveniently, the relevant knowledge will be briefly introduced below:
1. Luminous flux
Luminous flux refers to the amount of light emitted by the light source per unit time, that is, the part of the radiant energy that the radiant power can be felt by the human eye. It is equal to the product of the radiant energy of a certain band per unit time and the relative viewing rate of this band. Since human eyes have different relative viewing rates of light of different wavelengths, 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 lumens (Lm).
According to the spectral radiant flux Φ (λ), the luminous flux formula can be derived:
Φ=Km■Φ(λ)gV(λ)dλ
In the formula, V(λ)—relative spectral luminous efficiency; Km—the maximum value of radiated spectral luminous efficiency, in Lm/W. In 1977, the Km value was determined by the International Committee of Weights and Measures to be 683Lm/W (λm=555nm).
2. Light intensity
Light intensity refers to the light energy passing through a unit area in a unit time. The energy is proportional to the frequency and is the sum of their intensities (ie integral). It can also be understood as the luminous intensity I of the light source in a given direction is the light source The quotient of the luminous flux d Φ transmitted in the cube corner element in the direction divided by the cube corner element d Ω
The unit of luminous intensity is candela (cd), 1cd=1Lm/1sr. The sum of the light intensity in all directions in space is the luminous flux.
3. Brightness
In our 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 to measure the chip testing. Luminous brightness is the brightness L of a certain place on the light-emitting surface of the light source, which is the quotient of the luminous intensity of the face element d S in a given direction divided by the area of the face element's orthographic projection on a 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, cosθ=1.
4. Illumination
Illumination refers to the degree to which an object is illuminated, expressed by the 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 proportional to the 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 of a point on the surface is the quotient of the luminous flux d Φ incident on the panel containing the point divided by the area of the panel d S.
The unit is Lux (LX), 1LX=1Lm/m2.