Analysis of characteristic parameters of LED

Light intensity is defined as the luminous flux emitted per unit solid angle, with the unit being candela (cd). Generally speaking, a light source will emit its luminous flux in different directions with different intensities. The intensity of visible light radiation emitted per unit solid angle in a specific direction is called light intensity.

Chromaticity

The human eye's perception of color is an intricate process. In order to quantify the description of color, the International Commission on Illumination (CIE) recorded the visual sensation caused by the human eye to radiant energy of different wavelengths based on the visual experiment of standard observers, and calculated the color matching function of the three primary colors of red, green and blue. After mathematical conversion, the so-called CIE1931ColorMatchingFunction (x((), y((), z(())) was obtained. Based on this color matching function, several color measurement definitions were subsequently developed, allowing people to describe and use color.

According to the CIE1931 color matching function, the stimulation value of the human eye to visible light is expressed as XYZ, and the x, y values ​​are converted into CIE1931 (x, y) chromaticity coordinates through the following formula. Through this unified standard, the description of color can be quantified and controlled.

x,y: CIE1931 chromaticity coordinates (ChromaticityCoordinates)

However, since the color gamut constructed by (x, y) chromaticity coordinates is non-uniform, it is difficult to quantify color differences. Therefore, in 1976, CIE converted the CIE1931 chromaticity coordinates to form a color gamut that is close to a uniform chromaticity space, allowing color differences to be quantified. That is, CIE1976 UCS (Uniform Chromaticity Scale) chromaticity coordinates, represented by (u', v'), and the calculation formula is as follows:

Dominant wavelength (λD)

It is also one of the methods to express color. After obtaining the chromaticity coordinates (x, y) of the device under test, mark it on the CIE chromaticity coordinate diagram (as shown below), connect the E light source chromaticity point (chromaticity coordinates (x, y) = (0.333, 0.333)) with the point and extend the connection line. The wavelength value where this extension line intersects with the spectrum trajectory (horseshoe) is called the dominant wavelength of the device under test. However, it should be noted that under this marking method, the same dominant wavelength will represent multiple different chromaticity points, so it is more meaningful when the chromaticity point of the device under test is adjacent to the spectrum trajectory, and the color characteristics of white light LEDs cannot be described in this way.

Purity

It is an auxiliary representation when describing color with dominant wavelength. It is defined as the percentage of the straight-line distance between the chromaticity coordinates of the DUT and the chromaticity coordinates of the E light source and the chromaticity coordinates of the spectral locus (Spectral Locus) of the dominant wavelength of the DUT. The higher the purity, the closer the chromaticity coordinates of the DUT are to the spectral color of the dominant wavelength. Therefore, the higher the purity of the DUT, the more suitable it is for describing its color characteristics with the dominant wavelength. LED is an example.

Color Temperature

When the radiation energy distribution of a light source is the same as the radiation energy distribution of a standard black body (BlackBodyRadiator) at a certain absolute temperature, the chromaticity of the light source is the same as the chromaticity of the black body radiation. At this time, the chromaticity of the light source is expressed by the corresponding absolute temperature, which is called the color temperature (ColorTemperature). The chromaticity presented by the black body radiation at each temperature can be marked on the chromaticity diagram as a curve, which is called the Planckian Locus (Planckian Locus). The higher the temperature of the standard black body, the more blue stimulation the light radiated by it produces to the human eye, and the red stimulation component is relatively reduced. However, in actual measurement, no light source has the same radiation energy distribution as the black body. In other words, the chromaticity of the light source to be measured usually does not fall on the Planckian Locus. Therefore, the chromaticity coordinates of the light source to be measured are calculated to be closest to a certain coordinate point on the Planckian Locus. The black body temperature of this point is defined as the correlated color temperature (CorrelatedColorTemperature; CCT) of the light source, which is usually obtained using the CIE1960UCS (u,v) chromaticity diagram and described with the color difference △uv. It should be noted that this representation method is only meaningful when the chromaticity of the light source is close to the Planck locus. Therefore, for LED measurement, it is only applicable to the color description of white light LEDs.