Density and colorimetric measurement in printing applications
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In modern printing, the measurement of color density and chromaticity is widely used in plate making, proofing, printing, and more pursuit of color accuracy and descriptiveness. So what is density? What is color? What is the role of density in printing? What is the role of chromaticity? What are their measuring instruments? What is the difference between their respective application areas? What are the shortcomings? These problems have plagued many of us, including the fact that we often see many problems like this in the winning forum. With the above questions, we begin the discussion of this article. I believe that readers will have a more comprehensive understanding and distinction between chromaticity and density measurements.
Density and chromaticity
The so-called density is the logarithm of the reflectance or transmittance reciprocal measured on a reflective or transmissive manuscript (for the sake of discussion, we will only discuss the case of a reflective manuscript). It sounds like this concept is very abstract, it is reflectivity, it is reciprocal, and it is logarithmic, but with a little care we will find that the most direct source of measured density values is calculated by measuring the reflectivity. owned. Reflectance 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 the three is certain.
Let's take a look at what is a colorimetric measurement.
Chromaticity, as its name suggests, is a measure of color. This measure is an "objective" description of color. The reason why it is objectively quoted is because it is based on the visual physiology of the human eye. But it is the average visual perception of color for most people. This metric can be expressed in the form of a value. There are three commonly used metric forms of the norm: CIEXYZ, CIELAB, CIELUV. This is a bit like the length weighting of the different units we use (eg, the relationship between inches and centimeters), except that there is no absolute conversion relationship between them.
From the above discussion we know what density is, and what chromaticity is (at least there should be a rough impression), then let's take a look at the different measurement tools used to measure density and chromaticity.
Density and colorimetric measurement tools
It is obvious that the densitometer is used to measure density. There are two main types of measurement values for printed matter using a densitometer, one is the spectral narrow band 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 wide-band filter is of course the spectral broadband density, and the narrow band is the opposite. The densitometer we use will vary depending on the situation. For example, narrow-band measurements add sensitivity to small changes in density, and they are less like human visual response than broadband filter measurements. Narrowband density measurement is mainly used to measure dot gain, overprint, ink layer thickness and ink strength. The broadband filter density measurement does not depend on the absolute value of the spectral distribution, but on the relative spectral radiation distribution, which is always related to the relative spectral density of the sensor used for density measurement and the spectral transmittance of the filter. . Broadband measurements are primarily used to evaluate hue, grayscale, transparency, and color correction.
Now let's put aside the problem of broadband and narrowband, and generally talk about the problem of density meter. In the measurement of printed proofs, the density meter uses three different color filters, the most common one is the complementary color filter using ink (usually standard color ink), such as the filter for yellow measurement. Blue (λ = 430 nm) was used, green color was measured for magenta (λ = 530 nm), and red (λ = 620 nm) was used for basic cyan measurement. Such measurements are clearly directed at the ink, not the human eye. This measurement can only tell us the relative amount of certain ink on the printed and printed print, that is, whether a certain amount of ink at the measurement site is sufficient and whether the desired density is reached. At the same time, a certain range of color contrast can be made, and this contrast has little to do with the human eye's vision.
In this way, we found that the density meter's ability to measure and indicate hue is limited. The density meter is not a colorimeter. Although the readings of the three color filters used simultaneously can be used to indicate the hue. However, this indication of hue is quite inaccurate and cannot meet the needs of printing color measurement. For more needs (eg, analysis of paper whiteness, analysis of the color of the original, etc.) the measurement of chromaticity has begun to receive more attention.
There are two main types of chromaticity measurement. The first method is to use the photoelectric colorimeter to measure the color. The photoelectric colorimeter is very similar to the density meter in principle, and 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 also converts the tristimulus values into color space scales, for example, converted to CIELAB scales, but large Most have only one or two kinds of illumination, so the color measured with the colorimeter does not always show visual color. In addition, CIELAB is not an ideal color system for printing because it cannot calculate the color like CIELUV. saturation. Photoelectric colorimeters are sufficient in determining chromatic aberration and can therefore be used in the printing shop for measurement of color difference comparison. Many high-end photoelectric colorimeters are also accurate enough to measure absolute color and relative color difference, but in general, people prefer to use a spectrophotometer to accomplish the above tasks.
A colorimeter can be thought of as a reflectometer or a densitometer without a logarithmic transducer but with a special set of color filters. Of course, this is a way to perform colorimetric measurements. The purpose of the additional set of color filters is to weight the individual wavelengths of the spectrum in each channel of the colorimeter based on the CIE spectral tristimulus values. But a colorimeter is different from a densitometer. It involves a reflectivity problem rather than a logarithm problem, but the reflectivity is easily converted to density, and vice versa. The spectral components of the colorimeter are considered to have a good linear relationship with human visual sensitivity. But in fact this is impossible (involving the Luther condition* problem), so the photoelectric colorimeter has errors in principle.
The second method is a method of measuring color using a spectrophotometer. Just like the three-color filter photoelectric colorimeter can be regarded as a special reflectivity measuring instrument, the spectrophotometer can also be seen like this, but unlike the photoelectric colorimeter, the spectrophotometer measures the whole of an object. Visible reflectance spectra, spectrophotometers are measured point by point in the visible spectral domain, ie at some discrete points, typically measuring one point every 10 or 20 nm, measuring 16 to 31 points in the range of 400-700 nm. Some spectrophotometers measure the spectrum continuously, while the three-color filter photometer measures only three points, so the spectrophotometer can provide much more information, at least for 16 points. .
Spectrophotometers measure color as a physical phenomenon that is not dominated by observers. In order to obtain the tristimulus value, it can integrate the reflection spectrum and explain the color as a visual response. It is the most flexible color measuring instrument.
Some phenomena in the printing process, such as paper dot coverage, ink intensity, etc., are essentially physical phenomena occurring in a narrow band, and it is of course better to use narrowband measurements for evaluation. However, it should be noted that narrow density measurements cannot be used to measure visual color, but spectrophotometric measurements can solve this problem. Because the measurements it makes are narrow-band measurements, it is sufficient to sample the spectrum, so color measurements consistent with the vision can be made. To perform the expected type of measurement (narrowband or wideband), a calculation program can be pre-programmed for the spectrophotometer. Many new spectrophotometers include a computer that performs standard print copy quality control and narrowband measurements according to the program, but it is significantly more expensive than a densitometer.
It is well known that the most basic method of measuring color is subjective visual method. This method is to visually match the unknown color according to the color in the chromatogram. The color data measured by the spectrophotometer is finer than the resolution of the human eye. This is useful for analyzing the concentration of the pigment. It only needs to be based on some formulas. By performing calculations, the amount of raw materials can be analyzed and controlled.
According to the measured value of the spectrophotometer, the density value and the 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 data into other colors. The parameters of the system are the same as the colorimeter.
Density measurement and colorimetric measurement in printing applications:
First, in order to produce a consistent and consistent hue of the printed matter, the printer operator seems to compensate for the change in the printing parameters by adjusting the ink. In the middle of printing, when the dot gain changes, the overprint color will change significantly. The press operator will maintain or reconstruct the match between the print and the standard proof by adjusting the amount of ink. The amount of ink printed onto the printed sheet affects the amount of dot gain, and conversely, the dot gain can also be controlled by changing the density of the solid.
Automatic adjustment for better process control. The press is controlled by the amount of ink delivered. Based on this premise, the expected mesh value is quantified (in the image, the observer sees the hue and color saturation), and the mesh density is measured (instead of the solid density). It is possible. Due to the color synthesis, the observer sees the red, green, and blue-violet light reflected by the print, which reaches the eye to form a comprehensive color vision. Changing the dot gain and ink overprinting have a significant effect on the color of the printing. The amount of red, green and blue violet light that forms the printing color can be displayed on the screen and compared with the standard printing sample, which makes it possible to control the printing color. consistency.
The densitometer can effectively measure the reflectivity of red, green, and blue-violet light on a given surface. Therefore, a new densitometer can be used to measure the specified mesh surface on the color proof or standard sheet, and the measured value is used as the control value or target value when printing on the surface. 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, so that the automatic control of the printed matter quality can be realized.
The densitometer measures the density on the production sheet after zeroing on the standard sheet and compares it to the density reading on the same portion of the standard 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 zero, this indicates that the printed image no longer matches the standard sheet and may have to be corrected. The three density readings will display the necessary corrections. The densitometer reading does not indicate a change in printing conditions, but rather indicates a change in the thickness of the ink layer. Compensating for changes in printing conditions will cause the measuring surface to return 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. The necessary corrections can be indicated based on the size and balance of the readings, and maintaining color balance is more important than maintaining proper color saturation.
What is not known at this time is the correct amount of acceptable hue and color saturation changes. If these quantities are determined, the algorithm can be determined, programmed, and added to the system. Past experience has pointed out that it is more accurate if the densitometer is zeroed on paper without zeroing on the pre-sample. This need is determined experimentally.
The test items for most color control systems are the same, and the items are combined in different forms for different reasons. Test items that may be included are field, overprinted, overprinted dot blocks, three-color gray balance dot blocks, dot gain, ghosting, slippage or plate exposure.
Second, for multi-color printing, density measurement is disadvantageous. It does not match the color vision of the human eye, and people cannot use the density measurement language to clearly and effectively exchange color information with customers. However, such information exchange is becoming more and more important at present. The specifications of the product must be explained by the method that the customer can understand. Color measurement has become an indispensable research object for the printer. Only color measurement can express what color the eye sees and what color difference is acceptable.
The internationally recognized color classification system is the CIE color space developed by CIE in 1931. The CIE standard chromaticity diagram includes all the hues, and the saturation of the color gradually increases from the inside to the outside.
The CIE coordinates can be transformed into a three-dimensional CIELAB and CIELUV color space by mathematical transformation. These two color spaces combine the precision of mathematical methods with the advantages of visual color equidistant distribution. These systems have been used in the Heidelberg CPC color control system, and its benefits are mainly three:
First, it is enough to achieve an objective match between the copied color and the sample color, regardless of changes in lighting conditions and subjective perceptions 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 matter. Choosing color is a subjective behavior, and determining the tolerances for the colors to be copied requires objective criteria. How should the printer exchange ideas with customers about color issues and at the same time make a correct description of the colors they see? Often used in printing process control is the density measurement language, which is still limited to standard inks in printing and is not considered superficial. In fact, ink density measurement has one drawback: it is not the result of evaluating the color like the human eye, but only the thickness of the ink layer. For objective visual-based color matching, spectroscopic color measurement is a prerequisite. Just as fingerprints are a unique feature of a person, the characteristics of each color are determined by its wavelength position. By means of chromaticity measurement, the spectral wavelength can be converted into a certain point in the CIELAB color space and the color can be objectively compared.
In this system, the chromatic aberration is expressed as the difference in color position, expressed by ΔE, and if there is a large chromatic aberration in the subjective evaluation, the ΔE value is also large (i.e., the positional deviation is also large) regardless of the color thereof.
The transfer of hue values from the original to the printed matter requires extensive experience and familiarity with the various processes involved, so the processes of color separation, screening, proofing, and printing must be properly matched. However, due to the need to convert the RGB system in the prepress equipment to the CMYK system of the offset process, some special difficulties arise. If the colorimetric measurement is introduced into the printing process, the color can be determined directly in the printing shop, and any reflected image, such as a photographic original, a pre-sample, and a sample taken on the printing press, can be measured (as long as these are measured). Chromaticity values are comparable). In this way, the inking control and adjustment system of the press can be used for rapid adjustment, keeping the color fluctuations in the printing within the tolerance range.
In the printing industry, colorimetric measurements are useful for understanding color processing, product and instrument design, and colorimetric measurements have some distinct advantages.
At present, the application of colorimetric measurement in the printing industry is mainly as follows:
1 Quality control of raw materials, especially ink and paper control, in some printing plants, this has become a routine work. Spectrophotometric data is valuable for measuring whiteness of paper;
2 to develop precise specifications for ink and paper standards;
3 Analytical measurement of gray balance, best tone reproduction and color correction for different inks, papers and printing conditions;
4 analyzing the color of the proof proof and the matching of the printing paper, and analyzing the chromaticity characteristics of the pigment used in the pre-sampling process;
5 analyzing the difference between the color gamut of a set of ink reproduction and the color gamut of each set of inks;
6 analyzing the relationship between the original and the copied image;
7 Adopt chroma measurement specifications to improve the degree of standardized production to save materials, reduce errors, and improve product quality;
8 quality control of printed colors;
9 analysis of the composition of the pigment matching the spot color;
10 Accurate color correction on the color separation device to control color reproduction on the press.
Keywords: chromaticity, density, density meter, colorimeter, spectrophotometer
*Luther condition: The visual density should be measured with a visual filter. 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 of the human eye V(λ). ), that is, satisfy the following formula: τ(λ)≈V(λ)/S(λ)