Color measurement includes the measurement of light source color and the measurement of object color. Object color measurement is divided into fluorescent object measurement and non-fluorescent object measurement. In actual production and daily life, a large number of non-fluorescent objects are used for color measurement and color measurement are divided into visual color measurement and instrument color measurement. Among them, instrument color measurement includes density method, photoelectric integration method and spectrophotometry.
The fundamental task of color measurement is to determine the color stimulation function φ (λ). For the measurement of the light source, it is actually to determine the relative spectral power distribution P (λ) of the light source; for the measurement of the color of the object, it is to determine the spectral photometric characteristics of the object , Such as the spectral radiance factor β (λ) and spectral reflectance P (λ) of reflective objects, the spectral transmittance τ (λ) of transmissive objects, etc. After the color stimulus function φ (λ) is measured, the CIE tristimulus values ​​X, Y, and Z of the measured color can be obtained according to the three basic equations of chromatics, and the Y value of the selected standard illuminant can be adjusted To 100.
1. Visual method
The visual method is a traditional color measurement method. It is a completely subjective evaluation method, and it is also the simplest method. It compares the printed matter with the standard sample directly to evaluate the color difference between the printed matter and the standard sample. At the same time, it also uses a magnifying glass to carefully observe the shape and overprinting of the dots of each color, and makes a qualitative assessment of the dot adjustment value. Its essence is a kind of visual photometric method. The principle is to use the additive color mixing law to add the unknown colors of various components together to describe the resulting unknown colors. Although the most reliable method for color evaluation is to use the human eye, it is simple and flexible, but due to the influence of the observer's experience and psychological and physiological factors, the method has too many variable factors and cannot be quantitatively described, which affects To assess the accuracy and reliability.
Second, the density detection method
Density measurement does not actually measure the density value directly, but only measures the amount of reflected light and the amount of incident light. It is assumed that the difference between the reflected light and the light provided by the densitometer is the amount of light absorption, that is, the absorption of the ink layer on the printed surface. The amount of light. Density measurement considers the overall light quantity characteristics of the entire reflection spectrum, and is essentially an evaluation of the brightness factor of each color on the printed surface, regardless of the hue. In color printing, the color of the printing ink is actually that the ink is printed on a white paper with a high reflectivity, and selectively absorbs part of the wavelength of light from the light irradiating it, and reflects the remaining light. Density reflects the absorption characteristics of ink to light waves. The traditional "color density" refers to the measurement of the density of yellow, magenta, and cyan inks through three color filters of red, green, and blue, respectively. Density is only a measure of physical absorption characteristics and only indicates the degree of black or gray. In this sense, color density measurement is also a measurement of blackness, and is a reflection of the relative value of the same ink saturation. There are two types of density meters used in the density measurement method: transmission and reflection. The transmission density meter measures the amount of light or transmittance through the film, and the reflection density meter measures the amount of light or reflectance reflected from the test surface. Because the intensity of the reflected light of the ink film layer on the printed matter is different from wet to dry, there is a certain error in the density measurement, and the density meter with polarizing filter can overcome the density change caused by the dry and wet ink film . The color reflection densitometer has become an indispensable tool in the printing workshop. It intuitively reflects the density, dot percentage, and ink overprint rate of C, M, Y, K four-color printing, and is widely used for color and ink layer thickness control. among.
Three, photoelectric integration method
For a long time, the density method occupies a high position in the color measurement, but with the application of CIE1976L *, a *, b *, it has gradually become common, and has covered the entire workflow from pre-press to printing, and density measurement is not enough To meet the needs of printing or other industries, people are increasingly aware of the importance of chromaticity, and the rapid development of modern chromatics has also laid the foundation for the objective evaluation of color by photoelectric integration instruments.
The photoelectric integration method is a common method used in instrument color measurement in the 1960s. It does not measure the color stimulus value of a certain wavelength, but measures the tristimulus values ​​X, Y, Z of the sample through integral measurement in the entire measurement wavelength interval, and then calculates the chromaticity coordinates and other parameters of the sample. Usually the filter is used to cover the detector, and the relative spectral sensitivity S (λ) of the detector is corrected to the spectral tristimulus values ​​x (λ), y (λ), z (λ) recommended by CIE. When such three photodetectors are used to receive light stimuli, the tristimulus values ​​X, Y, Z of the sample can be measured with one integration. The filter needs to meet the Luther condition to accurately match the light detector. Luther's conditions are as follows:
The accuracy of the color measurement of this type of instrument is directly related to the degree to which the instrument meets the Luther condition, and it is very difficult to fully meet the above conditions. In the actual color filter correction, due to the limited variety of color glass, the instrument cannot completely meet the Luther condition, and can only be approximated by applying the partial filter method to make the matching integral error of the x (λ) and z (λ) curves. Less than 2%, the matching integral error of the y (λ) curve is less than 0.5%.
The photoelectric integration instrument cannot accurately measure the tristimulus value and chromaticity coordinates of the excellent source, but it can accurately measure the color difference of the two color sources, so it is also called a color difference meter. Foreign color difference meters have been mass-produced since the 1960s, such as Japan Minolta desktop color difference meter CR-400 / 410 and color color difference meter CR-321. China began to develop such instruments in the early 1980s, and now it has used a better TG-PIIG automatic color measurement colorimeter produced by Beijing Optical Instrument Factory, but compared with foreign countries, the different colorimeters developed in China are consistent. Sex is not enough. The color brightness meter is also a photoelectric integration instrument, which measures color parameters of long-distance targets through a telescope system.
4. Spectrophotometry
Spectrophotometry is also called a color measuring spectrophotometer. It compares the light energy reflected (transmitted) with the standard reflection (transmitted) light energy under the same conditions to obtain the spectral reflectance of the sample at each wavelength, and then uses The standard observer and standard light source provided by CIE are calculated according to the following formulas to obtain the tristimulus values ​​X, Y, and Z, and then the chromaticity coordinates x are calculated by X, Y, and Z according to the formulas such as CIEYxy and CIELab. y, CIELAB chromaticity parameters, etc.
It determines the color parameters by detecting the spectral composition of the sample. It can not only give the absolute value of X, Y, Z and color difference value â–³ E, but also give the spectral reflectance value of the object, and can draw the color of the object. Reflectance curve. Therefore, it is widely used in color matching and color analysis. The use of such instruments can achieve high-accuracy color measurement, calibration of photoelectric integral color measurement instruments, and establishment of chromaticity standards. Therefore, spectroscopic instruments are colors. The authoritative instrument in measurement.
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