Application of Density and Color Measurement in Printing (Part 2)

The application of density measurement and colorimetry in printing:
First, in order to produce a consistent and consistent hue, the press operator appears to compensate for changes in printing parameters by adjusting the ink transfer. In the middle of the printing, when the dots increase and change, the overprint color will change significantly. The press operator will maintain or reconstruct the matching relationship between the print 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 gain. Conversely, the dot gain can also be controlled by changing the density on the ground.
Automatic adjustment can get better process control. The press is controlled by the amount of ink delivered. Based on this premise, the expected halftone value is quantified (in the image, the observer sees the hue and color saturation) and the mesh density is measured (rather than the density in the field). It is possible. Due to the color synthesis, the viewer sees the red, green, and blue light reflected by the printed sheets, and these lights reach the eyes to form a comprehensive color vision. Changing the dot gain and ink overprinting have a significant effect on the effect of printing color. The amount of red, green and blue violet that forms the printing color can be displayed on the fluorescent screen and compared with the standard printing sample, thus it is possible to control the printing color. consistency.
Densitometers can effectively measure the reflectance of red, green and blue violet light on a given surface. Therefore, a new type of densitometer can be used to measure the color tone or the specified mesh surface on the standard sheet, and the measured value is used as the control value or target value when the surface is printed. When the printed matter passes through the printing press, the corresponding part on the printed sheet is measured, and the measured value is compared with the target value to realize the automatic control of the quality of the printed matter.
Densitometers are zero-adjusted on a standard sheet and then measure the density on the production sheet and compare it with the densitometric reading on the same part 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 show the necessary corrections. Densitometer readings do not indicate a change in printing conditions, but indicate a change in the thickness of the ink layer. Compensation for changes in printing conditions will return the measurement surface back to the red, green, and blue color balance.
There may be a correct hue on the printed sheet without the correct saturation, in which case all three density readings will be wrong. Depending on the size and balance of the readings, the necessary corrections can be indicated. It is more important to maintain the color balance than to maintain the correct color saturation.
What is not known at this moment is the correct amount of qualified hue and color saturation changes. If these quantities are determined, algorithms can be determined, programs can be created, and they can be added to the system. Past experience has shown that if the densitometer is zeroing on paper rather than zeroing on pre- proofing, it will be more accurate, and this needs to be determined experimentally.
The test items for most color control systems are the same, and the various items are combined in different forms for different reasons. Test items that may be included are field, overprinted, overprinted dot patches, tricolor gray balance dot patches, dot gain, ghosting, slick, or plate exposures.
Second, density measurement is disadvantageous for multicolor printing. It does not coincide with the color vision of human eyes, 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 specification of the product must be explained in a way 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 hues, and the color saturation gradually increases from the inside to the outside.
The CIE coordinates can be transformed into 3D CIELAB and CIELUV color spaces through mathematical transformations. These two color spaces combine the precision of mathematical methods with the advantages of the equidistant distribution of visual colors. These systems have been used in the CPC color control system in Heidelberg. The main benefits of the system are threefold:
First, it is enough to make the copied color match the sample color objectively, which is not related to the change of lighting conditions and subjective perception of color.
Second, these systems are suitable for use 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. Determining the tolerance for the color to be copied requires an objective standard. How should the printer exchange opinions with the customer about the color problem and correctly explain the color they see? The density measurement language is usually used in the control of the printing process and is still limited to standard inks in printing, and it is not considered superficial. In fact, the ink density measurement has a disadvantage: it does not evaluate the color as human eyes, but only provides the thickness of the ink layer. Spectral colorimetry is a prerequisite for objective visual-based color matching. Just as fingerprints are unique to a person, the characteristics of each color are determined by its wavelength position. With the aid of colorimetry, the spectral wavelength can be converted to a defined point in the CIELAB color space and an objective comparison of colors can be made.
In this system, the color difference is expressed as the difference in color position, and is represented by ΔE. If there is a large color difference in the subjective evaluation, then the ΔE value is large (that is, the positional deviation is also large) regardless of the color.
Passing the hue value from the manuscript to the printed matter requires the relevant personnel to have a wealth of experience and familiarity with the various processes. Therefore, the processes of color separation, screening, proofing, and printing must be properly coordinated. However, due to the need to convert the RGB system in the prepress equipment to the CMYK system in offset printing, some special difficulties arise. If colorimetric measurements are introduced into the printing process, the color can be determined directly at the pressroom, and any reflected image, such as photographic originals, pre-proof proofs, and proofs extracted on the press, can be measured (as long as these are measured. Chroma values ​​are comparable). In this way, the printer's ink control and adjustment system can be used to make quick adjustments to keep color fluctuations within the print range within tolerances. In the printing industry, color measurement studies are designed to understand color processing, products, and instrument design. It is useful, chroma measurement has some obvious advantages.
At present, the application of colorimetric measurement in the printing industry is mainly in the following areas:
1 The quality control of raw materials, especially the control of ink and paper, has become routine in some printing plants. 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, paper, and printing conditions;
4 Analyze the color matching of proofs and matching of printing paper, analyze the color characteristics of the pigments used in the pre-proofing process;
5 analysis of a set of ink reproduction of the color gamut and each set of ink reproduction gamut;
6 analysis of the relationship between the original and the copied image;
7Using color measurement specifications to increase the degree of standardized production to achieve the purpose of saving material, reducing errors, and improving product quality;
8 Print color quality control;
9 analysis of the composition of the pigment matching the spot color;
10 Perform accurate color correction on the separation equipment and control color reproduction on the press.
Luther conditions: The visual density should be measured with a visual filter, which must be combined with the spectral transmittance of the filter, τ(λ), and the relative spectral sensitivity of the sensor, S(λ), to simulate the human eye's spectral sensitivity, V(λ). ), which satisfies the following formula: τ(λ)≈V(λ)/S(λ)