How to choose measuring instruments to ensure prin

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How to choose measuring instruments to ensure the printing quality

the reproduction quality of color images mainly depends on the gradation of images and the degree of color reproduction. The gradient and level of the image refers to the change range from light to dark in the image and the density data distribution between light and dark due to the accurate measurement of light. However, tone and level are not the same concept. Tone refers to the density change of the overall light and dark contrast of the image. It is a set composed of various ladder levels arranged; The level is the density level from light to dark. Generally, the number of mineral oil levels with medium viscosity determines the color change and texture on the picture. It is the basic unit of the level. Whether it is tone or level, they are all functions of density and the external expression of density. In the process of color printing reproduction, the reproduction of tone level essentially becomes the reproduction of density

we can control the density of ink layer by using densimeter to guide printing and improve printing quality. Color reproduction means that the printed matter can faithfully restore the original color and ensure the color consistency of the same batch of products. Color reproduction is divided into two steps in the process of printing reproduction: color decomposition and color synthesis. Color decomposition is to decompose the original color into three primary colors of pigment, and make each color separation plate. Color synthesis is to use a printing plate to imprint three primary colors of ink on paper or other printing materials to reproduce the color of the original. In order to improve the degree of color restoration, people usually use colorimeter to judge and evaluate the color reproduction of printed matter. This paper briefly introduces how to correctly select measuring instruments for different purposes:

1. Printing ink layer density and dot expansion

printing field density refers to the reflection density of a certain kind of color ink printed on the substrate as a field color block. The factors that affect the field density mainly include the absorption of light by ink pigments and binders, the thickness of ink layer and the conjunctive state of ink film. In the specific production process, once the ink is determined, the field density is only related to the thickness of the ink layer, which can indirectly reflect the thickness of the ink layer on the substrate. Therefore, the measurement and control of field density is the basis of objectively evaluating printing quality

the measuring principle of the densimeter for the field color block is that the light emitted by the internal light source of the densimeter passes through the color filter and irradiates on the field color block. The field color block absorbs part of the development strategic light of the high-efficiency and green plastic granulator, and also reflects part of the light. The reflected light is received by the receiver of the densimeter. The electronic components inside the densimeter compare the difference of the two luminous flux to calculate the density value, Density value is a physical quantity without unit. At present, there are many kinds of densitometers on the market. Although they are different in structure, they are basically the same in principle. The difference in the structure of the densitometer will affect the density measurement results, that is to say, measuring the same color block with the densitometer produced by different manufacturers may get different density values, but we cannot say that one densitometer measures correctly while the other measures incorrectly. In fact, there is no absolute density reference value in the field of offset printing. The reference value we recommended before is only a range. As long as the density value of the printed matter is within this range, we all think that the printed matter is qualified

dot enlargement value refers to the percentage between the dot area on the color separation sheet and the dot area on the corresponding print. We can also calculate the point expansion value with the help of the densimeter. The specific principle is as follows: the densimeter first measures the field density, then measures the point density, calculates the point coverage through the electronic components in the densimeter, and displays the calculation results on the LCD, so that we can calculate the point expansion value. It is worth noting that before measuring the density, we must first adjust the zero calibration on the white paper. During measurement, the printed matter to be measured should be placed flat to ensure that the measuring head is in close contact with the block to be measured and reduce the measurement error

2. Chromaticity measurement

in the printing process, we will encounter some problems that cannot be solved by densitometer, such as the color calibration and measurement of computer display, the calibration of digital proofing equipment, the evaluation of digital proofing products, and the evaluation of stable numerical color reproduction of field color block color difference within ± 2. People often use a colorimeter to measure the chromaticity of the same batch of the same color block, and calculate the color difference of the same batch of the same color block based on the chromaticity value of the same color standard block Δ E. If we control Δ If the E value is within the specified range, it can ensure that the color deviation of the front and rear printed sheets of the same product or the same sample sheet does not exceed the allowable range. The standard color difference of the same batch of the same color is stipulated in the "conditions for high quality products of the publication administration" issued in December 1993 Δ E should be less than 4. In the measurement process of colorimeter, people must pay attention to: first, adjust the zero calibration on the "standard white board", rather than on the white paper. During measurement, the printed matter to be measured should be placed flat to ensure that the measuring head is in close contact with the block to be measured and reduce the measurement error

of course, there are also such occasions: only when densitometer and colorimeter are used at the same time, can the printing reproduction quality be objectively reflected. For example, the reproduction of grey balance. At present, there is also an instrument integrating colorimeter and densitometer in the market. We call it a color densitometer with polarizer device, which has its unique flexibility. When measuring density, it will use a polarizer device to eliminate the reflective parts of the ink layer surface (including specular reflection and diffuse reflection of light) and ensure that the measured wet density is the future dry density. Therefore, the wet density measured by this color densitometer is smaller than that measured by other densitometers. (end)

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