Light source common light quality parameters (1): chromaticity coordinates, color temperature and color deviation values

At this year's Guangya Exhibition, brand companies pay more attention to the quality and design of light, and pay more attention to the improvement of color rendering index CRI, R9, color fidelity Rf, color saturation Rg and so on. Then, today we will talk about common light quality parameters.

Light quality parameters are extremely important indicators to measure the quality of light sources. Common light quality parameters include color temperature, color rendering index, illuminance, glare index, light control, illumination uniformity, brightness, etc., and several articles will be introduced in the following articles. Each light quality parameter based on the spectral power distribution (SPD) gives a further understanding of the light source. Introduced today are chromaticity coordinates, color temperature and color deviation values.

Black body radiation

When it comes to light sources, it is necessary to introduce blackbody radiation, which was mentioned in the previous issue. Blackbody, also known as Planck radiator, is a thermal radiation that can completely absorb incident radiation of any wavelength, and the object with the largest emissivity is called absolute blackbody. The spectral distribution of blackbody radiation depends entirely on its temperature and its spectral radiation. The emission degree is shown in the following formula and figure. As long as the temperature is constant, the spectral distribution of the black body can be calculated. As can be seen from the figure below, 1 at temperature, there is a maximum value of the radiation emission of the black body, and the maximum value moves with the temperature to the short wave direction, and the color of the short wave is blue, so the light color is blue and cold; At any wavelength, the high temperature black body radiance is absolutely greater than the low temperature black body spectrum radiance, whether or not this wavelength is the spectral maximum radiance. Blackbody is of great importance in radioscopy, photometry and colorimetry. In optical radiation measurement, the black body is often used as the original standard to calibrate other radiators for use as a measurement standard, such as a tungsten incandescent lamp with a color temperature of 2856K as specified by the International Commission on Illumination (CIE), that is, a standard light source A.

Where c1=2×π×h×c2=3.74183×10-16W/m2, c2=h×c/k=1.4388×10-2m·K, λ is the wavelength, and T is the absolute temperature of the black body (unit K, T =t+273), c is the speed of light, h is the Planck attempt, and K is the Boltzmann constant.

0.1-100μm blackbody spectral radiation emittance

Blackbody spectral radiation emittance in the visible range of 380-780 nm

Chromatogram coordinates

The chromaticity coordinate xy represents the color light of a light source, and its calculation formula is as follows

Where k is the normalization coefficient such that the final calculated Y value is 100; P(λ) is the SPD of the light source, which is available through the relevant measuring equipment; x(λ), y(λ), z(λ) are CIE Standard Chroma Observer Color Matching Function (CMF), divided into 2° and 10° fields of view, which are standardized data, the specific values ​​can be found in the references. Friends who are familiar with X-Rite should be familiar with the three curves because it is the original US X-Rite logo. Since the CIExy chromaticity diagram is non-uniform, that is, the same color distance in different regions on the CIExy diagram does not represent the same color difference. In order to improve the non-uniformity of the CIE xy chromaticity diagram, in 1960 CIE officially recommended the CIE1960 Uniform Color Space (UCS), which is the uv chromaticity diagram, and the calculation formula is as follows;

To further improve the uniformity of the chromaticity diagram, CIE released the 1976 UCS diagram, the u'v' diagram, in 1976, calculated as follows:

CIE standard chromatic observer color matching curve

X-Rite logo

Connect the chromaticity coordinates of each black body at each color temperature into a line, which is the Blackbody Locus, which can be found on the CIE xy, CIE uv or CIE u'v' chromaticity diagram.

Black body radiation trajectory in CIE xy chromaticity diagram

Color temperature and color deviation

When the color of the light source at the temperature T is the same as the color of the black body at a certain temperature Tc, the temperature Tc of the black body is referred to as the color temperature of the light source, that is, the color temperature. For the heat radiation sources such as incandescent lamps, because their spectral power distribution is continuous spectrum and close to the black body, their chromaticity coordinate points are basically located on the black body locus. The concept of color temperature can well describe the light of incandescent lamps. color. For other light sources, such as fluorescent lamps, the spectral power distribution is far from the blackbody, and the chromaticity coordinates do not necessarily fall on the blackbody temperature trajectory. Correlated Color Temperature (CCT) is needed to characterize the color and light characteristics of the light source. It is defined that when the color of the light source is closest to the color of the black body at a certain temperature, that is, when the distance between the two is the smallest, the temperature of the black body is used to represent the color temperature of the light source; the minimum distance is defined as the color deviation. Duv, '+' indicates that the color of the light source is above the black body track on the chromaticity diagram, that is, yellowish green, and '-' indicates that the color of the light source is below the black body track on the chromaticity diagram, that is, purpleish red. ANSI C78.377-2008, Specifications for the Chromaticity of Solid State Lighting Products standard definition of LED light source color temperature, and the meaning of bins in many LED datasheets are also here.

The nominal color temperature definition of the ANSI C78.377-2008 standard in the CIE xy chromaticity diagram

If the different light sources of the same correlated color temperature have a large difference in Duv, the corresponding light source color difference is also large, see the following figure.

3500K Duv=0.02

3500K Duv=-0.02

Daily common source color temperature

The color temperature of the light source commonly seen in daily life is shown in the following figure. The candle light is 1800-2000K, which is reddish and warm; the incandescent lamp is roughly 2800K, yellowish; the daylight at sunset or sunrise is 3000K; the daylight at noon is roughly 5500K; The sky is 7500K and so on.

The SPD, color temperature, Duv and chromaticity coordinate parameters of several common light sources are given below. I hope everyone has a concept.

Daylight - color temperature 5988K, xy (0.3221, 0.3345), Duv (0.0014)

Office lighting - color temperature 6145K, xy (0.3188, 0.3367), Duv (0.0041)

Mall lighting - color temperature 2942K, xy (0.4430, 0.4094), Duv (0.0013)

Mall lighting - color temperature 5754K, xy (0.3261, 0.3631), Duv (0.0136)

Personal suggestion: furniture lighting general bedroom can choose low color temperature warmer light, color temperature is about 3000K; living room, corridor, restaurant, etc. can choose 4000-5000K neutral white; and need to concentrate on the office, conference room or table lamp It is recommended to use high color temperature illumination, such as 6500K. As for the reason, first bury a foreshadowing and wait until the next time.

Yes, I don’t know if everyone noticed? When you go to the mall to buy a lamp, all the lamps on the outer packaging have the parameter of color temperature, and you can choose the color temperature that suits you.