(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
In modern printing, the measurement of color density and chroma is widely used in plate making, proofing, printing, and began to pursue more color accuracy and descriptiveness. So what is density? What is chroma? What role does density play in printing? What is the role of chroma? What are their measuring instruments? What is the difference between their respective application areas? What are the disadvantages? These problems have plagued many of us, including many of us who often see similar issues on the forums that must win. With the above questions, we begin the discussion of this article. I believe that after reading, the reader will have a more comprehensive understanding and differentiation of chromaticity and density measurement.
Density and chroma
The so-called density is the logarithm of the reciprocal of the reflectance or transmittance measured on the reflective or transmissive manuscript (we will only discuss the case of the reflective manuscript for convenience of discussion below). It sounds like this concept is very abstract, and it is also reflectivity, reciprocal, and logarithmic, but with a little care, we will find that the most direct source of the measured density value is calculated by the measured reflectance owned. Reflectivity is also the only factor that can affect the density value. The stronger the object's ability to absorb light, the lower the reflectivity of the object, and the higher the reflectivity of the object. The relationship between these three is certain.
Let's take a look at what is chromaticity measurement.
As its name implies, chroma is a measure of color. This measure is an "objective" description of color. The reason why the objective is quoted is because this objective is based on the visual physiology of the human eye But it is based on the average visual perception of color by most people. This metric can finally be expressed in the form of a value. There are three types of normative chroma that we commonly use: CIEXYZ, CIELAB, and CIELUV. This is a bit like the length measurement of different units we use (for example: the relationship between inches and centimeters), the difference is that there is no absolute conversion relationship between them.
Through the above discussion, we know what density is, and what chroma is (at least there should be a rough impression), then we take a look at the different measurement tools used to measure density and chroma.
Density and color measurement tools
Obviously the density meter is used to measure density. There are two main types of densitometers that are commonly used to measure printed materials. One is the spectral narrowband color density, and the other is the spectral broadband color density. The narrow band and wide band of the spectrum are mainly realized by different filters. The density meter using a broadband filter, of course, the result of color measurement is of course the broadband density of the spectrum, and the narrow band is the opposite. The density meter we use will be different according to different situations. For example, narrowband measurements increase sensitivity to small changes in density. Compared with broadband filter measurements, they are less like human visual responses. Narrowband density measurement is mainly used to measure dot gain, overprint, ink layer thickness and ink strength. The broadband filter density measurement value does not depend on the absolute value of the spectral distribution, but on the relative spectral radiation distribution, that is, it is always related to the relative spectral density of the sensor used for density measurement and the spectral transmittance of the filter . Broadband measurement is mainly used to evaluate hue, grayscale, transparency and color correction.
Now let's put aside the problems of broadband and narrowband, and generally speaking of the problem of density meter. Density meter uses three different color filters in the measurement of printed samples, the most common is the complementary color filter using the hue of ink (generally standard color ink), such as the filter for yellow measurement The blue color is used (λ = 430nm), the green filter is used for magenta measurement (λ = 530nm), and the basic cyan measurement is red (λ = 620nm). Such measurement results are clearly directed to ink, not human eyes. This kind of measurement can only tell us the relative amount of certain ink on the printed and printed products, that is, whether the amount of certain ink at the measurement site is sufficient and whether the expected density is reached. At the same time, it can also make a certain range of color contrast, which has little to do with human vision.
In this way, we found that the density meter's ability to measure and indicate hue is really limited. The density meter is not a colorimeter. Although the readings of three color filters used at the same time, the hue can be approximately indicated. However, this indication of hue is quite inaccurate and cannot meet the needs of printing color measurement. For more needs (such as: analyzing the whiteness of paper, analyzing the color of the original, etc.), the measurement of chromaticity has begun to receive more widespread attention.
There are two main types of chromaticity measurement. The first method is to use the photoelectric colorimeter to measure color. The photoelectric colorimeter is very similar to the density meter in principle. Its appearance, operation method and even the purchase price are quite close. The photoelectric colorimeter directly displays the tristimulus values ​​x (—) (λ), y (—) (λ), z (—) (λ), and most of them also convert the tristimulus values ​​into color space scales, for example into CIELAB scale, but most of them have only one or two kinds of illumination, so the color measured with a colorimeter does not always express visual color. In addition, CIELAB is not a very ideal colorimetric system for printing because it cannot CIELUV also calculates the color saturation. The photoelectric colorimeter is sufficient in determining the color difference, so it can be used as a measurement of color difference comparison in the printing shop. The accuracy of many high-end photoelectric colorimeters is also high enough to measure absolute color and relative color difference, but generally speaking, people prefer to use spectrophotometers to complete the above tasks.
A colorimeter can be regarded as a reflectance meter, or a densitometer without a logarithmic converter but with a set of special color filters. Of course, this is a method that can complete color measurement. The purpose of the additional set of color filters is to weight each wavelength of the spectrum in each channel of the colorimeter based on the CIE spectral tristimulus value. But the colorimeter is different from the density meter. It mainly deals with the reflectance problem rather than a logarithm problem, but the reflectance is easily converted into density, and vice versa is also possible. The spectral component of the colorimeter is considered to have a good linear relationship with human visual acuity. But in fact this is impossible (involving the problem of Luther condition *), so there is an error in principle in the photoelectric colorimeter.
The second method is to use a spectrophotometer to measure color. Just as the three-color filter photoelectric colorimeter can be regarded as a special reflectance measuring instrument, the spectrophotometer can also be seen in this way, but it is different from the photoelectric colorimeter, the spectrophotometer measures the whole of an object Visible reflection spectrum, spectrophotometer is measured point by point in the visible spectrum domain, that is, measured at some discrete points, usually one point every 10 or 20nm, and 16 to 31 points in the range of 400 to 700nm. Some spectrophotometers continuously measure the spectrum, and the three-color photoelectric colorimeter only measures three points, so the spectrophotometer can provide much more information, at least 16 points. .
Spectrophotometers measure color as a physical phenomenon not controlled by the observer. In order to obtain the tristimulus value, it can integrate the reflection spectrum and can interpret the color as the visual response. It is the most flexible color measurement instrument.
Certain phenomena in the printing process, such as paper dot coverage, ink strength, etc., are essentially physical phenomena that occur within a narrow band. Of course, it is best to use narrow-band measurements for evaluation. However, it should be noted that narrow density measurement cannot be used to measure visual color, but spectrophotometric measurement can solve this problem. Because the measurement it makes is a narrow band measurement, it is sufficient for sampling the spectrum, so it can do color measurement consistent with vision. In order to make the desired type of measurement (narrowband or wideband), a calculation program can be written in advance for the spectrophotometer. Many new spectrophotometers include a computer. It is appropriate to complete standard print copy quality control and narrow-band measurement according to the program, but it is obviously more expensive than the densitometer.
As we all know, the most basic method of measuring color is the subjective visual method. This method matches the unknown color visually according to the color in the chromatogram. The color data measured by the spectrophotometer is finer than the resolving power of the human eye. This is useful for analyzing the concentration of the pigment, only according to some formulas After calculation, the amount of raw materials can be analyzed and controlled.
According to the measured value of the spectrophotometer, the density value and chromaticity value can be calculated (but the reverse calculation is incorrect); the metamerism phenomenon can be analyzed; the new spectrophotometer can also directly convert the spectrophotometric measurement data into other color The parameters of the system and the conversion method are the same as the colorimeter.
The application of density measurement and color measurement in printing:
1. In order to make the printed matter produce a stable and consistent hue, the printer operator seems to adjust the inking to compensate for the changes in printing parameters. In the middle of printing, when the dots increase and change, the overprint color will change significantly. The operator of the printing press will maintain or reconstruct the matching relationship between the printed matter and the standard proof by adjusting the amount of ink. The amount of ink printed on the printed sheet will affect the amount of dot increase. Conversely, the amount of dot increase can also be controlled by changing the solid density.
Automatic adjustment can obtain better process control. The printing press is controlled by the amount of inking. Based on this premise, quantify the expected mesh tone value (in the image, the observer sees the hue and color saturation), and measure the mesh tone density (not the solid density) It is possible. Due to the synthesis of colors, the observer sees the red, green, and blue-violet light reflected by the printed sheet, which reaches the eye to form a comprehensive color vision. Changing the dot increase amount and ink overprinting has a significant effect on the printing color effect. The amount of red, green, and blue-violet light that forms the printing color can be displayed on the fluorescent screen and compared with the standard printing sample, so that it is possible to control the printing color consistency.
The densitometer can effectively measure the reflectance of red, green, and blue-violet light on a given surface. Therefore, a new type of density meter can be used to measure the color adjustment surface specified on the color proofing or standard printing sheet, and use the measured value as the control value or target value when the surface is printed. When the printed matter passes through the printing machine, the corresponding part on the printed sheet is measured, and the measured value is compared with the target value, and the automatic control of the printing quality can be realized.
The density meter is to adjust the density on the standard printed sheet, then measure the density on the production printed sheet, and then compare it with the density reading of the same part on the standard printed sheet. The measured value can indicate whether the contents of yellow, magenta and cyan are equal. If the value of the production sheet deviates from the zero value, this indicates that the printed image no longer matches the standard sheet, and it may have to be corrected. The three density readings will show the necessary correction function. The densitometer reading does not indicate a change in printing conditions, but indicates a change in the thickness of the ink layer. Compensating for changes in printing conditions will return the measurement surface to the balance of red, green, and blue-violet colors.
The production sheet may have the correct hue without the correct saturation, in which case all three density readings will be wrong. According to the size and balance of the reading, the necessary correction can be pointed out. Maintaining color balance is more important than maintaining correct color saturation.
What is not known at this moment is the correct amount of change in qualified hue and color saturation. If these amounts are determined, the algorithm, programming, and adding them to the system can be determined. Past experience has pointed out that it is more accurate if the density meter is zeroed on paper instead of zeroing on pre-proofing. This needs to be determined by experiment.
The test items of most color control systems are the same, and each item is combined in different forms for different reasons. The test items that may be included are field, overprinted field, overprinted dot block, tri-color gray balance dot block, dot increase, ghosting, slide or plate exposure, etc.
2. For multi-color printing, density measurement has disadvantages. It does not match the color vision of the human eye, and people cannot exchange color information with customers clearly and effectively using density measurement language. However, such information exchange is becoming more and more important now. The specifications of the products must be explained in a way that customers can understand, and color measurement has become an indispensable research object for printers. Only color measurement can express what color the eye sees and what color difference is allowed.
The internationally recognized color classification system is the CIE color space developed by CIE in 1931. The CIE standard chromaticity diagram includes all the hue, and the saturation of the color gradually increases from the inside out.
CIE coordinates can be transformed into three-dimensional CIELAB and CIELUV color spaces through mathematical transformation. These two color spaces combine the accuracy of mathematical methods with the advantages of the isometric distribution of visual colors. These systems have been used in Heidelberg's CPC color control system, and its benefits have three main points:
First, it is sufficient to achieve an objective match between the copied color and the sample color, which has nothing to do with the changes in lighting conditions and human subjective perception of color;
Second, these systems are applicable in any color matching process in the industry without any restrictions;
Third, they are excellent tools for printers to ensure print quality.
Observing color is one thing, printing this color is another. Choosing colors is a subjective behavior, and determining tolerances for colors to be copied requires objective standards. How should printers exchange opinions with customers on color issues and at the same time make correct explanations of the colors they see? Usually the language used for density control in the printing process control is still limited to standard inks in printing, and it is not considered superficial. In fact, ink density measurement has a disadvantage: it does not evaluate colors like human eyes, but only provides conclusions about the thickness of the ink layer. For objective visual-based color matching, spectrophotometry is a prerequisite. Just as a fingerprint is a unique characteristic of a person, the characteristic of each color is determined by its wavelength position. With the help of chromaticity measurement, the spectral wavelength can be converted to a certain point in the CIELAB color space and the colors can be objectively compared.
In this system, the color difference is represented by the difference in color position, which is represented by ΔE. If the subjective evaluation has a large color difference, the ΔE value is also large (that is, the position deviation is also large), regardless of its color.
Transferring the hue value from the original to the printed matter requires the relevant personnel to have rich experience and familiarity with the various processes in between, so the various processes such as color separation, screening, proofing, printing and so on must be properly coordinated. However, due to the need to convert the RGB system in the prepress equipment to the CMYK system of the offset printing process, some special difficulties have arisen. If colorimetry is introduced into the printing process, the color can be determined directly in the printing workshop, and any reflected image, such as photographic originals, pre-proofing proofs, and proofs drawn on the printing machine can be measured (as long as these are measured Chroma values ​​are comparable). In this way, the inking control and adjustment system of the printing machine can be used for rapid adjustment, so that the color fluctuation in printing is kept within the tolerance range
In the printing industry, the study of chromaticity measurement is useful for understanding color processing, product and instrument design, and chromaticity measurement has some obvious advantages.
At present, the application of color measurement in the printing industry is mainly in the following aspects:
â‘ The quality control of raw materials, especially the control of ink and paper, has become a routine task in some printing houses. Spectrophotometric data is very valuable for the measurement of paper whiteness;
â‘¡Formulate precise specifications of ink and paper standards;
â‘¢Analysis and measurement of gray balance, best tone reproduction and color correction for different inks, paper and printing conditions;
â‘£Analyze the matching of the color of the proofing proofs and the printing paper, and analyze the chromaticity characteristics of the pigments used in the pre-proofing process;
⑤Analyze the difference between the color gamut of one set of inks and the color gamut of each set of inks;
â‘¥Analyze the relationship between the original and the copied image;
⑦ Adopt colorimetric measurement specifications to increase the degree of standardized production, so as to save materials, reduce errors and improve product quality;
⑧Quality control of printing colors;
⑨Analyze the composition of the pigment matching the spot color;
â‘©Precise color correction on color separation equipment, and control color reproduction on printing machine.
Keywords: chromaticity, density, densitometer, colorimeter, spectrophotometer
* Luther condition: the visual filter should be used to measure the visual density. The spectral transmittance τ (λ) of the filter and the relative spectral sensitivity S (λ) of the sensor must be combined to simulate the spectral sensitivity V (λ) of the human eye ), Which satisfies the following formula: τ (λ) ≈V (λ) / S (λ)
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