Light source is an extremely important factor affecting color perception. Different light sources will cause different color perceptions of the same object. For example, the color perception of clothes in the store and in the sun is different.
Light is essentially an electromagnetic wave that includes many bands. The sensitization range of the human eye only accounts for a small portion of the entire electromagnetic spectrum. It is generally considered that the visible light wavelength range is 380-780 nm, and some textbooks are written as 400-700 nm. In essence, the difference between the two is small because the human eye is at 380. The response at -400nm and 700-780nm is low and has minimal impact on color perception.
The spectral power distribution is the "identity" of the light source
The core parameter determining the color characteristics of the light source is Spectral Power Distribution (SPD), which is the radiant power distribution of the monochromatic light of the light source at different wavelengths. Many other light quality parameters are calculated from the spectral power distribution, such as color temperature, Color rendering index, chromaticity coordinates, etc., the calculation of these parameters will be explained in the following article. The spectral power distribution is usually expressed in a Cartesian coordinate system. The abscissa is the wavelength and the ordinate is the radiated power per unit wavelength interval. The spectral power distribution of the tungsten halogen lamp is higher in the long wavelength band, and its color light is reddish. The spectral power distribution of the standard 6500K daylight is higher in the short band, and the color light is blue. The SPD of the light source can be measured by a spectroradiometer. Famous manufacturers include PhotoResearch in the United States, Jeti in Germany, Konica Minolta in Japan, and Topcon. The measurement of light will be detailed later in the article.
CIE lighting body
In order to unify the standards of color evaluation and measurement, the International Lighting Commission CIE recommended several illuminants, including standard illuminator A, standard illuminator D65, daylight D series illuminator, illuminator E; illuminating body refers to a group The specific spectral power distribution data, the values ​​of which have been standardized, the specific values ​​or calculation methods can be found in the references.
The standard illuminator A is a Planck radiator (also referred to as "black body radiation") having an absolute temperature of 2856K. Planck radiators, which are heat radiation, 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. As long as the temperature is constant, the spectral distribution of the black body can be calculated, as shown below. 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.
Planck radiator spectrum
Illuminant D represents the relative spectral power distribution of daylight in each phase, also known as typical daylight or recombination daylight, such as the widely used D65 and D50 in the industry. It is well known that the spectral power distribution of daylight in different regions and at different times is different. In 1963, some color scientists observed and measured the spectral power distribution, color temperature, etc. of different regions and at different times, and accumulated a large amount of daylight data. In 1964, Judd and his collaborators found that daylight SPD of any phase can be represented by three basic functions. In 1967, CIE defined and calculated the daylight SPD for each phase based on Judd's research.
Solar phase spectral power distribution for each phase CIE
The most commonly used CIE in the D series of illuminators are D50, D55, D65, and D75, which represent the correlated color temperatures of 5000K, 5500K, 6500K, and 7500K, respectively.
The illuminating body E represents an illuminating body whose spectral power in the visible light band is a constant value, and is also called an equal energy spectrum or an equal energy white light. This is an artificially defined relative spectral power distribution that does not actually exist.
In addition, CIE also announced the F series of illuminators, from F1 to F12, representing fluorescent lamps; F1-F6 is a standard fluorescent lamp, F7-F9 is a broadband high color rendering index fluorescent lamp, and F10-F12 is a narrowband three-primary fluorescent lamp. Fluorescent illuminators commonly found in color science are F2, F7, F8, F11 and F12. F2 is a Cool White Fluorescent (CWF), F7 is a fluorescent lamp that simulates a D65 standard illuminator, F8 is a fluorescent lamp that simulates a D50 standard illuminator, F11 is TL84 or U40, and F12 is TL83 or U30.
CIE standard light source
The standard light source is a real physical radiator that can emit light, and the illumination body is a set of spectral power distribution values ​​recommended by CIE. The CIE of the standard light source is recommended as follows:
The standard light source A is a standard illuminator A realized by a transparent glass gas-filled tungsten filament lamp having a color temperature of 2856K as an A light source.
For D series illuminators, CIE has not recommended the corresponding standard light source, so the simulation of D series illuminators has become one of the important topics in current light source research. The artificial light sources of the currently developed analog D series illuminators include: fluorescent lamps, incandescent lamps with filter, high-voltage xenon lamps, and LED spectrum-tunable simulators; among them, the LED spectrum can be adjusted to simulate the D series of illuminants with the best effect. The quality parameters are optimal, and in future articles, the indicators and methods for evaluating the standard light box source will be specifically mentioned.
For the F series of illuminators, although the CIE does not recommend the corresponding standard light source, the color industry practitioners must be familiar with the corresponding commercial light source name, which will be mentioned in the table below.
Standard light box common light source
The peers engaged in the color industry should be familiar with standard light boxes. The role of standard light boxes is to simulate various common lighting scenes for visually judging whether the sample colors meet the requirements. The common lighting bodies and corresponding commercial light source names in the light boxes are summarized as follows:
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There are a few points to note: 1. The SPD of F7, F8 and D65, D50 is quite different; for example, the SPD of F7 and D65 is compared, for example, the difference between all D65 fluorescent lamps on the market and the standard D65 SPD is large, even if the D65 fluorescent lamp perfectly reproduces the SPD of F7. It also causes a large difference between the observation of the color sample under the D65 fluorescent lamp and the CIELAB value measured by the standard D65 observation or measurement equipment. In addition, all daylight simulators in standard light boxes should be reproducible with standard daylight SPD instead of F7 or F8 SPD. |
2. The color temperature, color rendering index, and chromaticity coordinate xy in the above table are theoretical standard values, which do not represent the actual data of commercial light sources; and the measured data of commercial light sources differs greatly from the theoretical standard values; D65 fluorescent lamps in standard light boxes As an example, in general, the measured color temperature difference is between 200-300K from the standard value, and the measured color rendering index is between 90-95.
3. The color evaluation should be observed under the standard unified lighting environment; if it is directly observed under real sunlight, the consistency of the lighting conditions cannot be guaranteed; because the real daylight changes with time and with the environment.
4. With the continuous development of LED technology, it has been widely used in people's daily work and life. It is believed that in the near future, fluorescent light boxes will be replaced by LED standard light boxes, and LED light sources are needed to simulate LED lighting scenes in daily work and life. And spectrally tunable LED technology can better simulate standard daylight SPD.
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