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The color characteristic of the light source The influence of the light source on the 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 The light emitted by a general 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 radiant power of the light source according to the wavelength is called the spectral power distribution. In chromatics, the relative spectral power distribution of a light source is primarily 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 that human beings have experienced, due to the long-term natural light environment, a special structure of human visual organs has been formed to adapt to changes in natural light intensity.
The most important feature of natural light is its continuous spectrum. In the early morning and dusk, the color temperature is about 2000K~4500K, and the midday 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 of lower color temperatures. 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. For example, the incandescent light source has lower power in the short-wave blue band, so the incandescent light source The light color has a yellow-red color to make the prints color cast. Therefore, the spectral power distribution of the light source used for printing and the degree of closeness to sunlight determine the degree of color cast of the printed matter.
2, color temperature black body (complete radiator) black body refers to the object that the incident electromagnetic wave can absorb, neither reflection nor transmission, its absorption capacity is the largest among all objects, so it is also the strongest emission in the object. object. Therefore, the black body can be used as a reference standard. A certain spectral power distribution exhibits a certain color of light, and the spectral power distribution of blackbody radiation is determined by temperature. Therefore, the relative spectral power distribution of the light source is compared with the relative spectral power distribution of the black body radiation at a certain temperature. If the relative spectral power distribution of a certain light source and the black body at a certain temperature coincides, the light color of the light source is 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 does not necessarily match 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 black body closest to it. This color temperature is called the correlated color temperature. The color temperature characterizes the spectral properties of the source and is related to the spectral composition of the 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 components and more red light components; the color temperature is higher.
In the process of printing and copying, there is also a high requirement for the color temperature of the light source. The color temperature of the color separation light source should match the color sensitivity of the color separation film. If the color temperature is too high or too low, the color separation effect will be affected. For a light source with a color temperature between 5000K and 6500K, the spectral energy distribution is more in line with the requirements of photographic color separation. It is used as a lighting source for reading when printing. The color temperature is preferably 5000K or 6500K, otherwise the color presented will be biased.
3, color rendering
(1), definition of color rendering
The illumination source not only requires high light efficiency, high luminous intensity, but also good color table and color rendering. The color table of the light source depends on the color temperature of the light source, and the color temperature of the light source is low, reddish, and vice versa. The color rendering of a light source refers to the extent to which the light source appears to the color of the object, that is, the degree of color fidelity. A variety of artificial light sources need to be applied in printing. Since the spectral power of the artificial light source is different, the surface of the same object is illuminated by different light sources, and the human eye produces different color perceptions. 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 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. A light source with high color rendering is better for color reproduction. The color we see is closer to the natural color, and the light source with low color rendering is less likely to reproduce the color. Therefore, the color deviation of the object we see is also It will be bigger. 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 inferior (R < 50) three quality grades, as color rendering for the light source. Qualitative evaluation. Light sources such as incandescent lamps, tungsten halogen lamps and cadmium lamps have a high color rendering index, generally around 85, which is often used in applications such as color film, color printing, and textile industry where color reproduction is demanding; the color rendering index of fluorescent lamps is 70. ～80, can be used for general lighting; high color mercury lamp and sodium lamp, etc. have a low color rendering index, usually less than 50, so it is not suitable for color vision and other visual work. The color rendering index is important for quantitatively evaluating the quality of light source color reduction performance. index.
In some places, such as printing, photography, printing and dyeing, exhibition halls, etc., the color of the object is required to be vividly displayed. Therefore, selecting a light source with a high color rendering coefficient can truly reflect the original color of the object to achieve good illumination. effect. At present, street lamps on the streets have gradually adopted gas discharge sources such as high-pressure mercury lamps and high-pressure sodium lamps. If viewed from a distance, the light from the high-pressure mercury lamp is bright and white. However, when I saw the face of the person being illuminated by it, it seemed to have a layer of gray on the face, which indicates that the color chart of the high-pressure mercury lamp is not bad, but the color rendering is not good. The tungsten filament lamp is exactly the opposite. Its light looks reddish and yellowish, but the characteristics of the illuminated object are rarely distorted. That is to say, the color spectrum 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 the low pressure sodium lamp are not good. The color table and color rendering of the ammonia lamp are good. That is to say, in the lighting, some light sources have poor color table and color rendering, some are very good, some color charts are good but the color rendering is not good, some color charts are not good but the color rendering is good. 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 distinct spectral energy distributions can produce the same color table. This is the so-called "homologous" phenomenon. The light emitted by the high-pressure mercury lamp is close to the color meter and the daylight, but its spectral energy distribution is very different from that of sunlight. Its spectrum is mostly blue, blue and lacks red light, so the face it is illuminated will be grayish. s color. [next]
(2) Limitations of general color rendering index Ra
Although the general color rendering index is simple and practical, it shows a serious deficiency in terms of it. First of all, color is the subjective feeling of people, not the inherent property of the object. It is related to lighting conditions, observers, irradiance, illuminance, surrounding objects and observation angles. There is no such thing as "real color". However, since in the CIE system, the highest value of 100 is defined in the approximate blackbody radiation, the bulb manufacturer has consciously designed the bulb so that the color rendering when irradiating the object 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 is lowered. 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 sometimes 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 source illumination specified by the CIE. For example, the use of incandescent lamps with very low color temperatures to illuminate green leaves, as mentioned above, is not necessarily the best option. It is stipulated that the color rendering index is the best value Ra=100 in blackbody or sunlight. The reference source specified by the CIE is the black body or daylight closest to the correlated color temperature of the source to be tested. They are all sources of radiation continuous spectrum with spectral components of multiple colors. When the color temperature is 6500 K, the spectral power distribution of the long and short waves is relatively balanced, which should be considered reasonable as a reference light source. However, when the color temperature is below 4000 K, the spectral power distribution is seriously asymmetrical, and the blue short-wave spectral power is much smaller than the red long-wave spectral power, and its color is biased toward red, which is questionable as a reference light source. In the CIE color system, eight standard swatches are in medium brightness and color saturation, and are equidistantly spaced in the u-v system. They are considered to be adequately representative of various common colors for indoor lighting. However, in outdoor lighting, there are often some colors with higher color saturation. These 8 standard swatches are not enough to represent the common colors. Many scholars believe that the number of color plates is too small, which is another deficiency of the general color rendering index. Although CIE also has 6 swatches with higher color saturation of 9-14, they are not included in the general color rendering index n. In lighting practice, the well-known colors are skin, leaves, food, etc., their color is extremely important, but they are excluded from the general color rendering index. Seim had proposed using a 20-speed standard swatch, but it was rejected because it made the calculation too complicated. Currently, computers are commonly used, and it seems that this proposal has to be reconsidered. Since the color rendering evaluation of the light source has two major problems, and many other evaluation methods have attracted wide interest, this article will give a brief introduction to the author's knowledge.
(3), the Fletcher index
Studies have shown that people tend to remember the color of a more familiar object, and remember its vivid, saturated color. This memory color tends to be consistent with the favorite color and tends to shift toward a high saturation. For example, the memory color of people's skin color tends to shift toward the red direction, and the color of the leaves shifts toward the green direction. Obviously different from the R method in CIE. In fact, it is a correction to Ra. This correction includes two aspects: First, Rf=90 is defined under the illumination of the reference light source, and Rf=100 only under the imaginary “perfect light source†illumination. Second, select 10 standard swatches, that is, in addition to the original 1 to 8 standard swatches, plus l3 l4 two swatches, corresponding to skin color and leaf color. At this time, the "perfect light source" refers to a light source that can shift the color of 10 standard swatches in the preferred direction under the illumination thereof. It can be seen that for each standard swatch, the color coordinates of the corresponding "perfect light source" are different and can be determined experimentally. This also shows that such a "perfect light source" can only be imaginary. The method of Rf is similar to Ra, but there are two differences: (1) For each standard swatch, the color coordinates of the reference source need to be adjusted, that is, the "perfect light source" color coordinates determined according to the experiment. Then, when the light source to be tested is illuminated, the chromatic aberration of each swatch is obtained by comparing it with its corresponding "perfect light source". (2) When calculating Rf, the average of the color differences of the l0 swatches is taken, but the weight of each swatch is different. The l3 swatch is the skin color, the weight is 35%, the number 2 is 15%, the 14th is 15%, and the rest is 5% per piece. The importance of skin color is especially emphasized here. Therefore, the light source to be tested may be higher than the reference light source Rf=90, but less than 100.
(4), color preference index (CPI)
The color preference index CPI (color preference index) uses the concept of the favorite color proposed in the previous section, defined under the D65 light source illumination, the color preference index CPI=100. Therefore, the CP1 of the light source to be tested can be obtained by calculating the difference between the color coordinates of the eight standard swatches and the color coordinates of the favorite color under the illumination of the light source to be tested, and obtaining the average value of the vector sum (ΔE) CPI= 156—7.18 (ΔE) The above calculations are all performed in the CIE UV color system. Although both CPI and Rf take advantage of the concept of favorite color, there is a big difference between them: (1) When calculating Rf, use 1-8 and l3, l4 for a total of l0 standard swatches, while CP1 only uses 1-8 standard swatches. (2) When calculating Rf, the color difference (ΔE) is taken as 1/5 of the experimental value, and the CPI is taken as the original experimental value. (3) When calculating Rf, the weights of the swatches are different, and the CPI takes the same weight. (4) The maximum value of Rf is 100 according to the definition, and the maximum value of CPI is 156.
Finally, it should be pointed out that researchers who have two indexes with CPI use experiments to determine the color of love, while in the experiment, daylight illumination is used. There is now evidence that the preferred color is related to the correlated color temperature of the light source. Therefore, when Rf and CPI are used for constant color rendering, only a light source of high color temperature is applied.
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