The color characteristics of the light source The influence of the light source on color perception is mainly analyzed from the three aspects of the spectral power distribution, color temperature, and color rendering of the light source.

1. Spectral power distribution Generally, the light emitted by the light source is a composite light composed of many different wavelengths of radiation, and the radiation power of each wavelength is also different. The distribution law of the spectral radiation power of the light source according to the wavelength is called the spectral power distribution. In chromatics, the relative spectral power distribution of the light source is mainly discussed, because the color of the light source depends on the relative energy ratio of the different wavelengths of light it emits. In the long evolutionary process experienced by human beings, due to long-term exposure to natural light, a special structure of human visual organs has been formed to adapt to changes in natural light intensity.

The biggest characteristic of natural light is that it has a continuous spectrum. In the early morning and dusk, the color temperature is about 2000K-4500K, and the noon color temperature is about 5000K-7000K. At night, people start to use torches, and then invented candles and oil lamps. These burning flares are continuous spectra with lower color temperature. The light source used in printing is an artificial light source, and its spectral power distribution curve is difficult to completely coincide with the spectral power distribution curve of daylight. The light color has a yellowish red color, which makes the printed matter more discolored. Therefore, the closeness of the spectral power distribution of the light source used for printing to sunlight determines the degree of color cast of the printed matter.

2. The color temperature black body (complete radiator) black body refers to an object that can be fully absorbed by incident electromagnetic waves, neither reflecting nor transmitting, its absorption capacity is the largest among all objects, so it is also the strongest emitting power among objects object. So you can use blackbody as a reference standard. A certain spectral power distribution shows a certain light color, and the spectral power distribution of black body radiation is determined by temperature. Therefore, people compare the relative spectral power distribution of the light source with the relative spectral power distribution of the blackbody radiation at a certain temperature. If the relative spectral power distribution of a light source and the blackbody at a certain temperature coincide, the light color of this light source It can be expressed by the temperature of the black body at this time, and the temperature of the black body is called the color temperature of the light source. There are many light sources whose chromaticity may not be exactly the same as the chromaticity of the black body when heated. The color temperature of the light source can only be determined by the color temperature of the temperature of the black body closest to it. This color temperature is called the correlated color temperature. Color temperature characterizes the spectral characteristics of the light source and is related to the spectral composition of the light source. The spectral composition of the light source determines the light color of the light source, so the color temperature expresses the light of the light source. A light source with a low color temperature has less blue light and more red light; a high color temperature is the opposite.

In the process of printing and copying, there are also high requirements for the color temperature of the light source. The color temperature of the light source for color separation should match the color sensitivity of the color separation film. Too high or too low color temperature will affect the color separation effect. For a light source with a color temperature between 5000K and 6500K, its spectral energy distribution is more in line with the requirements of photographic color separation. When printing, it is used as the illumination light source to see the sample. The color temperature is preferably 5000K or 6500K, otherwise the color will appear deviation.

3. Color rendering

(1) Definition of color rendering

Illumination light source not only requires high light efficiency and high luminous intensity, but also has a good color table and color rendering. The color table of the light source depends on the color temperature of the light source. The color temperature of the light source is low, reddish, and blue. The color rendering of the light source refers to the degree to which the light source presents the color of the object, that is, the degree of color fidelity. A variety of artificial light sources need to be used in printing. Because the spectral power of the artificial light source is different, the human eye will produce different color visions when the same object surface is illuminated by different light sources. People use sunlight as the most ideal light source, or choose a standard reference light source, and compare the artificial light source with it. The strength of displaying the same color is the color rendering of the artificial light source. The level of color rendering is expressed by the average color rendering index (Ra) or the characteristic color rendering index. Light sources with high color rendering have better color reproduction, and the closer we see colors to natural colors, light sources with low color rendering have poorer color reproduction, so the color deviation of the objects we see is also Will be larger. Generally, according to the general color rendering index, the color rendering of the light source can be divided into: excellent (R = 100-75), general (R = 75-5o), and poor (R <50) three quality levels, as the color rendering of the light source Qualitative evaluation. Light sources such as incandescent lamps, halogen tungsten lamps and cadmium lamps have a high color rendering index, generally around 85, and are often used in color film, color printing, textile industry and other occasions with high color reproduction requirements; the color rendering index of fluorescent lamps is 70 ~ 80, can be used for general lighting; the color rendering index of high-pressure mercury lamps and sodium lamps is low, usually less than 50, so it is not suitable for visual work such as color discrimination. index.

In some places, such as printing, photography, printing and dyeing, exhibition halls, etc., the color of the object is required to be truly displayed, so choose a light source with a higher color rendering coefficient to reflect the original color of the object to achieve good lighting effect. Now street lamps on the street have gradually adopted high-pressure mercury lamps, high-pressure sodium lamps and other gas discharge light sources. If viewed from a distance, the light emitted by the high-pressure mercury lamp is bright and white. However, when looking at the person's face illuminated by it, it looks as if a layer of cyan is smeared on the face, which shows that the color table of the high-pressure mercury lamp is not bad, but the color rendering is not good. The tungsten filament lamp is just the opposite. Although its light looks reddish or yellowish, the characteristics of the illuminated objects are rarely distorted, which means that the color table of the tungsten filament lamp is not very good, but the color rendering is very good. The light color of the low-pressure sodium lamp is very yellow. If a blue cloth is placed under the low-pressure sodium lamp, the blue cloth becomes black. This shows that the color table and color rendering of low-pressure sodium lamps are not good. The color table and color rendering of ammonia lamps are good. This means that in lighting, some light sources have poor color tables and color rendering, some are good, some have good color tables but poor color rendering, and some have poor color tables but good color rendering. The color table and color rendering of the light source are both different and related. The color of the light source is fundamentally determined by its spectral energy distribution. After the spectral energy distribution of the light source is determined, its color table and color rendering are also determined. However, it cannot be reversed that the spectral energy distribution of the light source can be determined by the color table of the light source. Light sources with very different spectral energy distributions can produce the same color table, which is the so-called "metamorphism" phenomenon. Although the light emitted by the high-pressure mercury lamp is close to daylight, its spectral energy distribution is very different from that of daylight. It has more blue light and blue light and lacks red light in its spectrum, so the face it shines will be grayish. s color. [next]

(2) Limitation of general color rendering index Ra

Although the general color rendering index is simple and practical, it shows serious shortcomings. First, color is a subjective feeling of people, not an inherent property of objects. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects, and viewing angles. There is no such thing as "true color". However, since the CIE system has been defined to reach a maximum value of 100 under the radiation of a black body, bulb manufacturers have consciously designed the bulb so that the color rendering when illuminating objects with it is as close as possible to the black body or sunlight. This means that when the spectral distribution of the light source deviates from the black body or sunlight, the color rendering index will decrease. For example, a white LED composed of three monochromatic LEDs of red, green, and blue, when its general color rendering index Ra is low, its color rendering is not necessarily bad. But in fact, many researchers Judd, Thorntou and Jerome have confirmed that people do not necessarily like the color of the reference light source prescribed by CIE. For example, using the incandescent lamp with a very low color temperature to irradiate green leaves, as mentioned earlier, is not necessarily the best choice. It is doubtful that the color rendering index is the optimal value Ra = 100 when irradiated by a black body or sunlight. The reference light source stipulated by CIE is the black body or sunlight which is closest to the correlated color temperature of the light source to be measured. They are all light sources with continuous spectrum of radiation and have spectral components of multiple colors. When the color temperature is at 6500 K, the spectral power distribution of its long and short waves is more balanced, and it should be more reasonable as a reference light source. However, when the color temperature is below 4000 K, the spectral power distribution is severely asymmetric. The short-wave spectral power of blue is much smaller than the long-wave spectral power of red, and its color is biased toward red. There is doubt as a reference light source. In the CIE color system, the 8 standard swatches are in medium lightness and color saturation, and are equally spaced in the u ~ v system. For indoor lighting, they can be considered to fully represent various commonly used colors. However, in outdoor lighting, there are often some colors with high color saturation, and these 8 standard color plates can no longer fully represent common colors. Many scholars believe that too few color plates are another deficiency of the general color rendering index. Although CIE has 6 color swatches with high color saturation of No. 9-14, they are not included in the general color rendering index n. In the practice of lighting, the well-known colors are skin, leaves, food, etc. Their colors are extremely important, but they are all excluded from the general color rendering index. Seim had proposed using the 20-speed standard swatch, but it was rejected because it made the calculation too complicated. Currently, computers are widely used, and it seems that this proposal has to be reconsidered. Since these two major problems exist in the evaluation of the color rendering of light sources, and many other evaluation methods have attracted wide interest, this article will briefly introduce what the author knows.

(3), Fleetley Index

Studies have shown that people tend to remember the colors of more familiar objects, but also remember their vivid and saturated colors. This memory color is often consistent with the favorite color, and tends to shift toward the direction of high saturation. For example, the memory color of people's skin color tends to shift toward red, and the color of leaves shift toward green. Obviously different from the R method in CIE. In fact, it is a correction to Ra. This correction includes two aspects: First, define Rf = 90 under the illumination of the reference light source, and only have Rf = 100 under the illumination of the hypothetical "perfect light source". Second, choose 10 standard color plates, that is, in addition to the original standard color plates No. 1-8, two color plates No. 13 and No. 4 are added, corresponding to skin color and leaf color. At this time, "perfect light source" refers to a light source that can shift the colors of the 10 standard color plates to the favorite direction under its illumination. It can be seen that for each standard color plate, the color coordinates of the corresponding "perfect light source" are different and can be determined by experiment. This also shows that such a "perfect light source" can only be hypothetical. The calculation method of Rf is similar to Ra, but there are two differences: (1) For each standard color plate, the color coordinates of the reference light source need to be adjusted, that is, the color coordinates of the "perfect light source" determined according to the experiment. Then, when the light source to be tested is illuminated, the color difference of each color plate is obtained by comparing with its corresponding "perfect light source". (2) When calculating Rf, take the average of the color difference of 10 swatches, but the weight of each swatch is different. L3 color swatch is skin color, weight is 35%, No. 2 is 15%, No. 14 is 15%, and the rest is 5% per block. The importance of skin color is particularly emphasized here. Therefore, the light source to be measured can be higher than the reference light source Rf = 90, but less than 100.

(4) Color preference index (CPI)

The color preference index (CPI) uses the concept of the favorite color proposed in the previous section to define the color preference index CPI = 100 under D65 light source illumination. Therefore, the CP1 of the light source to be measured can be obtained as follows: Under the illumination of the light source to be measured, calculate the difference between the color coordinates of the 8 standard color plates and the color coordinates of the favorite color, and find the average value of the vector sum (△ E) CPI = 156—7.18 (△ E) The above calculations are carried out in CIE's UV chromaticity system. Although CPI and Rf both use the concept of favorite colors, there is a big difference between them: (1) When calculating Rf, use 1-8 and l3, l4 a total of 10 standard color plates, while CP1 only uses 1-8 standard color plates. (2) When calculating Rf, the color difference (△ E) takes 1/5 of the experimental value, and CPI takes the original experimental value. (3) When calculating Rf, the weight of each color swatch is different, and the CPI takes the same weight. (4) By definition, the maximum value of Rf is 100, and the maximum value of CPI is 156.

Finally, it should be pointed out that the researchers with the two indexes of CPI use experiments to determine the favorite color, and the experiment uses sunlight lighting. There is now evidence that the favorite color is related to the correlated color temperature of the light source. Therefore, when using Rf and CPI for constant color rendering, only light sources with high color temperature are applicable.

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