Color measurement method and its limitations
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 determined; for the measurement of the object color, the spectral luminosity characteristic of the object is measured. For example, the spectral radiance factor β(λ) of the reflective object and the spectral reflectance P(λ), the spectral transmittance τ(λ) of the transmissive object, and the like. 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 colorimetry, and the Y value of the selected standard illuminant is adjusted. To 100.
Color measurement includes two categories: the measurement of the color of the light source and the measurement of the color of the object. Object color measurement is further divided into fluorescent object measurement and non-fluorescent object measurement. In actual production and daily life, the methods involved in measuring color measurement of a large number of non-fluorescent objects are divided into two categories: visual color measurement and instrument color measurement. Among them, instrument color measurement includes density method, photoelectric integration method and spectrophotometry.
First, the visual method
Visual inspection is a traditional method of color measurement. It is a completely subjective evaluation method and the simplest one. It directly compares the printed matter with the standard proofs, evaluates the color difference between the printed matter and the standard proofs, and also uses the magnifying glass to observe the shape and overprinting status of each color dot finely, and qualitatively evaluates the value of the dot. The essence is a visual photometry, the principle is to use the 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, the method has too many variables and cannot be quantitatively described, thus affecting To the accuracy and reliability of the assessment.
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 absorbed, 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 reflectance spectrum, essentially evaluating the brightness factor of each color of the printed surface, regardless of the hue. In color printing, the color of the printing ink is actually that the ink is printed on the white paper with higher reflectivity, and selectively absorbs a part of the wavelength light from the light irradiated thereon, and reflects the remaining light. The density reflects the absorption characteristics of the ink on the light wave. The "color density" referred to in customary means that the density of yellow, magenta, and cyan inks is measured by three kinds of color filters of red, green, and blue, respectively. Density is only a measure of physical absorption characteristics and only represents the degree of black or gray. In this sense, 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 density measurement 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 on the printed film from the wet to the dry process is different, the density of the measurement has a certain error, and the density meter with the polarizing filter can overcome the density change caused by the wet and dry of the ink film. . Color reflection densitometer has become an indispensable tool in the printing shop. It intuitively reflects the density of C, M, Y, K four-color printing, dot percentage, ink overprinting rate, etc., and is widely used for color and ink layer thickness control. among.
Third, photoelectric integration method
For a long time, the density method has occupied a high position in color measurement, but with the application of CIE1976L*, a*, b* has become more common, and the entire workflow from prepress to printing has been used, and density measurement is insufficient. Meeting the needs of printing or other industries, people are increasingly aware of the importance of chromaticity, and the rapid development of modern colorimetry has laid the foundation for the objective evaluation of color by photoelectric integration instruments.
Photoelectric integration is a common method used in instrument color measurement in the 1960s. It is not measuring the color stimuli value of a certain wavelength, but measuring the tristimulus values ​​X, Y, and Z of the sample by integral measurement over the entire measurement wavelength interval, and then calculating the chromaticity coordinates and the like of the sample. The filter is usually covered on the detector to correct the relative spectral sensitivity S(λ) of the detector to the spectral tristimulus values ​​x(λ), y(λ), z(λ) recommended by CIE. When the light stimulus is received by such three photodetectors, the tristimulus values ​​X, Y, and Z of the sample can be measured with one integral. The filter needs to meet Luther conditions to precisely match the photodetector.
The colorimetric accuracy of this type of instrument is directly related to the extent to which the instrument meets the Luther condition, and it is difficult to fully comply with the above conditions. In the actual color filter correction, due to the limited variety of color glass, the instrument cannot fully meet the Luther condition, and can only approximate the matched 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%.
Photoelectric integral instruments cannot accurately measure the tristimulus values ​​and chromaticity coordinates of excellent sources, but can accurately measure the color difference between two color sources, which is also called color difference meter. Foreign color difference meters have been mass-produced since the 1960s, such as the Japanese Minolta desktop colorimeter CR-400/410 and the color color difference meter CR-321. China has been developing such instruments since the early 1980s. Now it has used the TG-PIIG automatic colorimetric colorimeter produced by Beijing Optical Instrument Factory. However, compared with foreign countries, the different color difference meters developed in China are consistent. Not enough sex. The color luminance meter is also a photoelectric integral instrument that performs color parameter measurement on long-distance targets through a telescope system.
Fourth, spectrophotometry
Spectrophotometry, also known as colorimetric spectrophotometer, is the spectral reflectance of a sample at each wavelength by comparing the light energy reflected (transmitted) by the sample with the light energy of standard reflection (transmission) under the same conditions. The standard observer and standard light source provided by CIE are calculated according to the following formula, so that the tristimulus values ​​X, Y, Z are obtained, and the chromaticity coordinates x are calculated by X, Y, Z according to CIE Yxy, CIE Lab and other formulas. y, CIELAB colorimetric parameters, etc.
It determines the color parameter by detecting the spectral composition of the sample, not only can give the absolute value of X, Y, Z and the color difference value â–³E, but also can give the spectral reflectance value of the object, and can draw the color spectrum of the object color. Reflectance curve. Therefore, it is widely used in color matching and color analysis. It can realize high-accuracy color measurement by using such instruments. It can calibrate photoelectric integrated color measuring instruments and establish color standard. Therefore, the splitting instrument is color. The authoritative instrument in the measurement. [next]
V. Limitations of current color measurement methods
There are many academic reports on color measurement methods, but many people just repeatedly introduce the advantages of colorimeter, the convenience of use, the consistency of measurement results with the human eye, and few scholars talk about the defects of color measurement instruments. Therefore, the last simple example of this article introduces the shortcomings of the currently used measuring instruments, hoping to draw the attention of the people concerned, in order to promote the further development of color measurement.
1. Defects in working principle
The color measurement accuracy of the photoelectric integral instrument is directly related to the degree to which the instrument meets the Luther condition, but the absolute absolute match is not achieved, and the measurement result may cause an error. Moreover, the colorimeter of different models and different manufacturers will have different differences in the use of Luther condition simulation, so the comparability is not strong.
Under the spectrophotometric method, the spectrophotometer of the spectrophotometer is used to directly obtain the r(λ) of the sample at each wavelength, and then the standard observers x(λ), y(λ), z(λ) provided by CIE are used. Calculated with the standard light source S(λ) to obtain X, Y, Z. Under this method, the value of the reflectance r(λ) of each wavelength is obtained. The spectroscopic part of the instrument is relatively expensive, and the operation and maintenance are very inconvenient. On-site management that is not conducive to print quality. In addition, since such instruments are mainly performed by calculation, the data calculated under some light sources (such as D65) may not match the actual observed data. Because the D65 light source is not actually put into use.
2. Measurement defects
1 The situation when considering the background.
In the process of quality management of printed matter, sometimes the influence of background on color has to be considered. However, in the case of considering the background, it is currently impossible to correctly calibrate the colors. For example, if a red sample is placed on a green background and a white background, if the measurement is performed with a spectrophotometer (or a colorimeter) at this time, it should be concluded that the red stimulus values ​​of the red samples in the two backgrounds are equal, that is, the color. They match each other. However, in reality this is completely two different colors. Therefore, current color measuring instruments cannot quantitatively estimate the influence of the background color, which hinders its application range.
2 pairs of prints printed with UV inks.
UV inks are also widely used in the printing industry. The UV content of this ink is rich, and the results measured by different light sources vary greatly. How to standardize the measurement of such colors is not good internationally. The problem with the method is that there is no ideal source of light. CIE recommends D65 as a calibration for UV ink prints because the UV portion of the source is rich. However, since the spectral energy curve of the light source is very complicated, it is difficult to use artificial simulation.
3 For prints printed with particulate inks.
Particulate inks are also widely used in the packaging and printing industry. The biggest feature of such prints is that they get different colors when you look at the samples from different angles. Obviously, it is not objective to calibrate such samples with current color measuring instruments (which can only be measured from one direction). The best solution is to install a light receiver in all directions of the color measuring instrument to calibrate all colors from all directions. Such instruments must be made very large and the price must be particularly expensive.
4 prints for transparent media.
When light is applied to such prints, so-called edge loss occurs due to the transmission of light and the effect of reflection. At this point, proper measurement of such samples requires a special illumination and receiving system, that is, the illumination area needs to be much larger than the receiving area, but the existing color measuring instruments are not equipped with such optical systems.
5 in other aspects of the shortcomings.
Color is an important factor in evaluating the quality of prints, but it is not the only factor. When objectively evaluating prints, it is necessary to make a comprehensive evaluation of the sense of color, hand touch, texture, and uniformity of color depth. However, there is currently no such intelligent color measuring instrument.
3. Comparison with density measuring instruments
Recently, some domestic scholars believe that color instruments 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 densitometer to measure the values ​​of the yellow, magenta, and cyan inks, respectively, but this value has a completely different meaning from the value given by the colorimeter. The density directly reflects the amount of light reflected from the printed matter and the film. Therefore, the value can directly judge the depth of the color, the thickness of the ink, etc., which guides the production manager to correctly screen the net, determine the amount of ink, and the amount of exposure. Ink balance and so on are crucial. On the contrary, any color measuring instrument cannot do this. Therefore, it can be said that the color measuring instrument and the density meter play an important role in two different stages in the printing production, and cannot replace each other. That is, the role of the density meter runs through the actual production process, and the color measurement instrument plays a significant role in the management of the printed product.
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, their application in the field of printing will have a great leap. The future is limitless.
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