Color temperature related color temperature from the spectral energy distribution and color of the light source can be introduced into the color temperature, which represents the color of the light source. When the color of light emitted by the light source is the same as the color radiated by the black body at a certain temperature, the temperature of the black body is called the color temperature of the light source Tc, referred to as the color temperature (CT), expressed by the thermodynamic temperature scale open (K).
For some light sources (mainly the gas discharge light source with strong line spectrum), the color of the light emitted by it and the color of the blackbody radiation at various temperatures are not exactly the same (the color coordinates are different), then the general concept of color temperature can not be used to describe its color. In order to facilitate comparison, the concept of correlation color temperature is used. If the color of the light emitted by the light source is the closest to that radiated by the blackbody at a certain temperature, that is, the color distance in the uniform chromaticity map is the smallest, then the temperature of the blackbody is called the relevant color temperature (CCT) of the light source, and the symbol is Tcp. Obviously, the expression of color in terms of relative color temperature is rather crude, but it expresses color to a certain extent.
Within the region enclosed by the tongue curve of the X-Y chromaticity map is a curved line representing the trajectory of the blackbody radiation. If the measured and calculated X and Y values of the light emitted from a light source are exactly consistent with the X and y values of a point on the trajectory, then the corresponding blackbody temperature at that point is the color temperature of the light source. However, the X and Y values of the light emitted by most gas discharge sources are not on this trajectory, but some distance away from it. At this time, according to the definition of the relevant color temperature, the comparison of the chromaticity point of the light source and some adjacent blackbody points between the “color distance”. Because the linear distance in the X-Y chromaticity map is not proportional to the “color distance”, it is best to use the U-V chromaticity map. Figure 7.19 shows the comparison of blackbody radiant color temperature and related color temperature. In the figure, at many points of the trajectory in boldface, there are many straight lines that intersect and are perpendicular to the trajectory. The color distance between each point on the vertical line and the intersection of the vertical line and the blackbody trajectory is the smallest, so the relevant color temperature of each point on the vertical line is the blackbody temperature at the intersection point. The relative color temperature of each point on the vertical line is equal, so it is called equal color temperature line.
For any light source, after the spectral power distribution of the light source is measured, the trichromatic stimulus value X, Y, Z can be calculated (or X, Y, Z can be measured directly with a photoelectric colorimeter), so as to calculate the color coordinates X, Y, U, V of the light source, and then the relevant color temperature of the light source can be found out from FIG. 7.19.
Color rendering as illumination source, in addition to the requirements of high light efficiency, but also requires that the light emitted by it has a good color. The color of the light source has two meanings; Color chart and color rendering. The colors that the human eye sees when looking directly at the light source are called the color table of the light source. Color coordinates, color temperature and so on is the description of the quantity of the color table. Color rendering refers to the objective effect of the light from the light source on the object. If the color effect of the colored objects after irradiation is the same as that of the standard light source, the light source is considered to have good color rendering; On the other hand, if the color of the object is distorted after being illuminated, the color rendering of the light source is poor. The effect of illumination source on object color table is called color rendering.
In the light source, low pressure sodium lamp color is very poor, while incandescent lamp color is very good. Incandescent lamps reproduce the color of objects, while low-pressure sodium lamps magically turn blue paper black. Why does the blue paper turn black in low pressure sodium light? To get to the bottom of this question, we must first analyze daylight. It turns out that sunlight is a mixture of red, orange, yellow, green, green, blue, purple and other colors in a certain proportion. When sunlight hits an object of one color, the object absorbs light of other colors and reflects light of that color back. For example, when the sun hits blue paper, it reflects blue light and absorbs other light, so that the paper looks blue to the human eye. Low pressure sodium lamps do not. The light emitted by low-pressure sodium lamps is predominantly yellow. When the yellow light hits the blue paper, the blue paper absorbs all the yellow light. Blue paper can reflect blue light, but because there is basically no blue light in the light of low-pressure sodium lamp, it cannot reflect blue light. So the blue paper turns black in the low pressure sodium light. The spectral energy distribution of incandescent lamp is continuous, all kinds of colors of light have, so the general color can be reflected, have better color.
In the solar energy system, the general color development index Ra is such a compromise product: it is relatively simple, only a value within 100 is needed to express the color development performance of the light source, and Ra=100 is considered as the most ideal color development. But sometimes it doesn’t feel that way. Leaves, for example, do not look very bright under an incandescent light bulb. What’s the problem? What is the general color index is discussed below.
For simplicity, only the general color development index R is discussed here. Without discussing its specific calculation method. In fact, in daily life, the color of the light source is often tested. Many people have the experience that when a careful woman is shopping for clothes in the market, she often goes outside to take a look at the color in the sunlight. In doing so, we are actually testing the color rendering of the store light source: to see how the same dress will differ in color when it is illuminated by the store light source and when it is illuminated by the sun. So to describe the color of a light source, two additional elements are needed: daylight (reference light source) and clothing (colored objects). In CIE color system, in order to determine the color rendering of the light source to be measured, the reference light source should be selected first, and it is believed that the color of the illuminated object can be displayed most perfectly under the illumination of the reference light source. CIE color system stipulates that when the relevant color temperature of the light source to be measured is lower than 5000K, the black body with the closest color temperature is used as the reference light source. When the relevant color temperature of the photometry source is greater than 5000K, the D light source with the closest color temperature is used as the reference light source. Here the D light source is a sequence of daylight whose chromatic coordinates can be represented digitally and which is related to the color temperature.
After selecting the reference light source, you also need to select the colored object. Because of the variety of colors, it is necessary to select a standard set of colors that adequately represent the colors commonly used. The CIE color system selects eight colors that combine a variety of shades with moderate lightness and chroma. In the U-V color system, the special color rendering index Ri of each standard color plate can be obtained by measuring the difference between the color coordinates under the light source to be measured and the reference light source, that is, the color displacement δEI.
The general color rendering index Ra is obtained by arithmetically averaging the special color rendering index Ri measured by eight standard color swatches. The maximum value of the general color rendering index Ra of the visible light source is 100, and it is considered that the color rendering of the light source is the best.
Light from different light sources, due to the difference in spectral power distribution, takes on a variety of different colors. The light emitted by tungsten filament lamp is yellow in white, which is due to the more red and yellow light components in the spectrum of tungsten filament lamp. High-pressure mercury lamps emit bluish light because they contain more blue and violet light in the spectrum. The light of the low-pressure sodium lamp was a yellow color. The color of the xenon lamp is closer to that of daylight. The color temperature values of various common light sources are shown in Table 7.6. The color parameters of several light sources are shown in Table 7.7.