The fundamental task of color measurement is to measure the color stimulus function φ(λ). For the measurement of the light source, the relative spectral power distribution P(λ) of the light source is actually measured. For the measurement of the object color, the spectral photometric characteristics of the object are measured. For example, the spectral radiance factor β(λ) and spectral reflectance P(λ) of a reflective object, and the spectral transmittance τ(λ) of a transmissive object. After the color stimulating 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 colorimetry, and the Y value of the selected standard illumination body can be adjusted. To 100.

Color measurement includes the measurement of the color of the light source and the measurement of the color of the object. Object color measurement is divided into fluorescent object measurement and non-fluorescent object measurement. In actual production and daily life, a large number of methods for color measurement of non-fluorescent objects are classified into visual color measurement and instrument color measurement. Among them, the instrument color measurement includes density method, photoelectric integration method and spectrophotometry.

First, visual method

Visual method is a traditional method of color measurement. It is a completely subjective evaluation method and it is also the simplest method. It compares prints with standard proofs manually, evaluates differences in prints and standard proofs, and uses magnifiers to finely observe the shape and overprinting status of each color dot, and to make a qualitative assessment of the adjustment of dots. Its essence is a kind of visual photometric method. The principle is to use additive color mixing law to add the unknown colors of each component together to describe the unknown color. Although the most reliable way for color evaluation is to use the human eye, and it is simple and flexible, due to the experience of observers and the influence of psychological and physiological factors, this method has too many variable factors and cannot be quantitatively described. To assess the accuracy and reliability.

Second, the density detection method

Density measurement does not actually directly measure the density value, but measures the amount of reflected light and the amount of incident light, assuming that the difference between the reflected light and the light provided by the densitometer is the amount of light absorbed, that is, the absorption of the ink layer on the printing surface. The amount of light. Density measurement takes into account the overall light quantity characteristics of the entire reflection spectrum, essentially evaluating the brightness factor of each color of the printed surface, regardless of the hue. In color printing, the printing ink coloring is actually that the ink is printed on a white paper with a high reflectance, and selectively absorbs a portion of the wavelength of light from the light irradiated thereon, while reflecting the remaining light. Density reflects the absorption of light waves by the ink. The term "color density" as used conventionally means that the density of the yellow, magenta, and cyan inks is measured by the red, green, and blue color filters, respectively, during measurement. Density is only a measure of the physical absorption characteristics and only indicates the degree of blackness or grayness. In this sense, the color density measurement is also only a measure of blackness, which is a reflection of the relative value of the same ink saturation. Densitometers used in densitometry have both transmission and reflection. The transmission densitometer measures the amount of light or transmission through the film. The reflection densitometer measures the amount of light or reflectance reflected from the test surface. Since the intensity of the reflected light from the wet-to-dry process of the ink film on the print is not the same, there is a certain error in the measured density, and the densitometer with a polarizing filter can overcome the density change caused by the wet and dry of the ink film. . The color reflection densitometer has become an indispensable tool in the printing shop. It intuitively reflects the density of four-color printing of C, M, Y and K, the dot percentage of the dots, and the ink overprinting rate. It is widely used for color and ink layer thickness control. among.

Third, the photoelectric integration method

For a long time, the density method occupies a very high position in color measurement, but with the widespread use of CIE1976L*, a*, b*, and has spread throughout the workflow from prepress 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 colorimetry, and the rapid development of modern colorimetry also lays the foundation for the objective evaluation of color by optoelectronic integration instruments.

Photoelectric integration is a common method used in instrument color measurement in the 1960s. It is not to measure the color stimulus value of a certain wavelength, but to measure the tristimulus values ​​of the sample, X, Y and Z, through the integral measurement within the entire measurement wavelength interval, and then calculate the parameters such as the chromaticity coordinates of the sample. Usually covered with a filter on the detector, the detector's relative spectral sensitivity S (λ) is modified to CIE recommended spectral tristimulus values ​​x (λ), y (λ), z (λ). When these three photodetectors receive light stimulus, the tristimulus values ​​X, Y, and Z of the sample can be measured with one integral. The filter needs to satisfy the Luther condition in order to precisely match the photodetector.

The colorimetric accuracy of this type of instrument is directly related to the instrument's compliance with Luther's conditions. It is very difficult to fully comply with the above conditions. In the actual color correction, due to the limited variety of color glass, the instrument cannot fully comply with the Luther condition, and can only approximate the matching integral error of the x(λ) and z(λ) curves in accordance with the applied filter method. Less than 2%, the matching integral error of the y(λ) curve is less than 0.5%.

The photoelectric integral type instrument cannot accurately measure tristimulus values ​​and chromaticity coordinates of an excellent source, but can accurately measure the color difference of two color sources, and is therefore also called a color difference meter. Abroad colorimeter began mass production in the 1960s, such as the Japan Minolta desk colorimeter CR-400/410, color and color difference meter CR-321. China began to develop this kind of instrument from the early 80s of the last century, and now has used a better TG-PIIG automatic colorimeter color difference meter produced by Beijing Optical Instrument Factory, but compared with foreign countries, different colorimeters developed in China are consistent. Not enough sex. The color luminance meter is also a kind of photoelectric integration type instrument, which performs color parameter measurement on a long-distance target through a telescope system.

Fourth, spectrophotometry

Spectrophotometry is also called colorimetry spectrophotometer. It uses the reflected (transmitted) light energy of a sample to be compared with the standard reflected (transmitted) light energy under the same conditions to obtain the spectral reflectance of the sample at each wavelength. The standard observer and standard light source provided by CIE are calculated according to the following formula to obtain the tristimulus values ​​X, Y and Z, and the X-, Y- and Z-formulas are calculated according to the formulae CIE Yxy, CIE Lab and so on. y, CIELAB color parameters, etc.

It determines the color parameters by detecting the spectral composition of the sample. It can not only give the absolute value and color difference value ΔE of X, Y, Z, but also can give the spectral reflectance value of the object, and can draw the spectroscopic color of the object. Reflectance curve. Therefore, it is widely used in color matching and color analysis. With such an instrument, high-accuracy color measurement can be achieved. The photoelectric integration colorimetric instrument can be calibrated to establish a colorimetric standard. Therefore, the spectroscopic instrument is a color. The authoritative instrument in the measurement.