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.
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.
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.
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, so as 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 of CIE Yxy and CIE Lab. 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. [next]
5. Limitations of current color measurement methods
At present, there are many academic reports on color measurement methods, but many people only repeatedly introduce the advantages of colorimeters, the ease of use, and the consistency of the measurement results with the human eye. Few scholars talk about the defects of color measurement instruments. Therefore, at the end of this article, we will briefly introduce the shortcomings of the measuring instruments currently in use, hoping to attract the attention of the relevant personnel, so as to promote the further development of color measurement.
1. Defects in working principle
The accuracy of the color measurement of the photoelectric integration instrument is directly related to the degree of the instrument's compliance with Luther's condition, but complete absolute compliance is not achieved, so the measurement results will produce errors. In addition, different models and manufacturers of colorimeters have differences in the use of Luther's condition simulation, so the comparability is not strong.
Under spectrophotometry, use the spectrophotometer's spectrophotometer to directly obtain the r (λ) of the sample at each wavelength, and then use the standard observers x (λ), y (λ), z (λ) provided by CIE Calculate with standard light source S (λ) to get X, Y, Z. Under this method, the value of the reflectance r (λ) for each wavelength must be obtained. The spectroscopic part of the instrument is relatively expensive, which is very inconvenient for operation and maintenance. Not conducive to on-site management of printing quality. In addition, because such instruments are mainly calculated, the data calculated under certain light sources (such as D65) may not match the actual observed data. Because the D65 light source is not actually put into use.
2. Defects in measurement
â‘ It is necessary to consider the situation in the background.
In the process of quality management of printed matter, sometimes it is necessary to consider the effect of background on color. However, in consideration of the background, it is currently impossible to make a correct calibration of colors. For example, a red sample is placed under a green background and a white background, if measured with a spectrophotometer (or colorimeter) at this time, it should be concluded that the tristimulus value of the red sample under the two backgrounds is equal, that is, the color Are matched to each other. However, in reality these are completely two different colors. Therefore, current color measuring instruments cannot yet quantitatively estimate the impact of the background color, which hinders its application range.
â‘¡For printed matter printed with UV ink.
UV ink is also widely used in the printing industry at present. The UV part of this ink is rich. The results measured with different light sources are very different. How to standardize the measurement of such colors is not yet good in the world. The problem is that no ideal light source exists. CIE recommends that D65 can be used as a calibration for UV ink prints, because the ultraviolet part of the light source is rich. But because the spectral energy curve of the light source is very complicated, it is difficult to simulate manually.
â‘¢ For printed matter printed with particulate ink.
Particle ink is also widely used in the packaging and printing industry. The biggest feature of this type of printed matter is: when you look at the sample from different angles, you get different colors. Obviously, it is not objective to calibrate such samples with current color measuring instruments (which can only measure from one direction). The best solution is to install light receivers in all directions of the color measuring instrument to calibrate all colors from all directions. In this way, the instrument must be very large, and the price must be particularly expensive.
â‘£For printed materials of transparent media.
When light is irradiated on such printed products, so-called edge loss will occur due to the effects of light transmission and reflection. At this time, the correct measurement of this type of sample requires a special lighting and receiving system, that is, the lighting area needs to be much larger than the receiving area, but the existing color measuring instruments are not equipped with such optical systems.
⑤ Deficiencies in other aspects.
Color is an important factor in evaluating print quality, but it is not the only factor. When objectively evaluating prints, a comprehensive evaluation of color, touch, texture, uniformity of shades, etc. is also required. But there is no such intelligent color measuring instrument.
3. Comparison with density measuring instruments
Recently, some domestic scholars believe that the colorimeter can be used to completely replace the density meter, which actually confuses the different properties of the two types of instruments. Three color filters are also used on the density meter to measure the values ​​of yellow, magenta, and cyan inks, but this value has a completely different meaning from the value given by the colorimeter. The size of the density directly reflects the amount of light reflected from the printed matter and the film. Therefore, from this value, it is possible to directly judge the shade of the color, the thickness of the ink, etc. The balance of water and ink is crucial. In contrast, any color measuring instrument cannot do this. Therefore, it can be said that color measuring instruments and densitometers play important roles in two different stages of printing production, and cannot replace each other. That is, the function of the density meter runs through the actual production link, and the color measuring instrument plays an important role in the management of printed products.
Through the above discussion, it can be found that although color measuring instruments have been widely used and developed rapidly, there are still many defects. If these defects can be completely overcome, then their application in the printing field will have a great leap , The future is unlimited.
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