How to choose measuring instruments to ensure print quality
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The quality of color image reproduction depends mainly on the tone level of the image and the degree of color reproduction. The tone and level of an image refer to the range of variation from light to dark in the image and the distribution of density data between light and dark. However, the tone and level are not the same concept. The tone refers to the density change of the overall brightness and darkness of the image. It is a collection of steps arranged in layers; the level is the density level from light to dark. The number of levels determines the color change and texture on the screen, which is the basic unit of the tone. Whether it is a tone or a hierarchy, they are functions of density and are external manifestations of density. In the color printing reproduction process, the reproduction of the tone level becomes essentially a reproduction of density.
We can control the density of the ink layer by using a densitometer to guide the printing and improve the printing quality. Color reproduction means that the print can faithfully restore the original color and ensure the color consistency of the same batch. Color reproduction is divided into two steps in the print copy process: color decomposition and color synthesis. The color decomposition is to decompose the original color into the three primary colors of the color to make each color separation plate. Color synthesis is the use of printing plates to imprint the three primary colors of ink on paper or other printing materials to reproduce the original color. In order to improve the degree of color reproduction, a colorimeter is usually used to judge and evaluate the color reproduction of a printed matter. This article gives a brief introduction to how to correctly select the measuring instrument for different purposes:
1. Printing ink layer density and dot enlargement
The printed solid density refers to the reflection density of a certain color ink printed on the printing material as a solid color block. The factors affecting the solid density are mainly the pigment of the ink, the absorption of light by the binder and the thickness of the ink layer of the ink and the ink film. Conjunctival state, etc. In the specific production process, once the ink is determined, the solid density is only related to the thickness of the ink layer, which indirectly reflects the thickness of the ink layer on the substrate. Therefore, the measurement and control of the density in the field is the basis for objectively evaluating the quality of printing.
The measurement principle of the density meter on the solid color block is that the light emitted by the internal light source of the densitometer is irradiated on the solid color block after passing through the filter color film, and the solid color block absorbs a part of the light, and also reflects a part of the light, and the reflected light is densitometer. The receiver receives, the electronic components inside the densitometer compare the difference between the two luminous fluxes, and calculate the density value, which is a physical quantity without a unit. There are a variety of densitometers on the market today, although they are structurally different, but in principle they are basically the same. The difference in the construction of the densitometer affects the measurement of the density, which means that measuring the same color block with a density meter produced by a different manufacturer may result in different density values, but we cannot say that a density meter is correctly measured. Another densitometer measures the error. In fact, there is no absolute density reference value in the field of offset printing. The reference values we have recommended before are 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.
The dot gain value is the percentage between the area of the color separation dot and the dot area on the corresponding printed matter. We can also calculate the dot gain value by means of a densitometer. The specific principle is as follows: The density meter first measures the solid density, then measures the dot density, calculates the dot coverage by the electronic components in the densitometer and displays the calculation result on the liquid crystal display, so that we can calculate the dot gain value. It is worth noting that before measuring the density, we first need to zero calibration on white paper. When measuring, the printed matter to be tested should be leveled to ensure that the measuring head is in close contact with the block to be tested, reducing measurement errors.
2, color measurement
In the printing process, we will encounter problems that cannot be solved by density meter, such as color calibration and measurement of computer monitors, calibration of digital proofing equipment, evaluation of digital proofing products, and evaluation of color reproduction of solid color patches. People often use the colorimeter to measure the color of the same batch of the same color block, and calculate the color difference ΔE of the same batch of the same color block according to the chromaticity value of the same color standard color block. If we control the ΔE value within the specified range Within, it is possible to ensure that the same product is printed before and after, or the same color deviation of the same sheet does not exceed the allowable range. The "Quality Product Conditions of the Press and Publication Administration" issued in December 1993 stipulated that the standard color difference ΔE of the same batch should be less than 4. In the colorimeter measurement process, one must pay attention: first adjust the zero calibration on the “standard whiteboard” instead of zero calibration on white paper. When measuring, the printed matter to be tested should be leveled to ensure that the measuring head is in close contact with the block to be tested, reducing measurement errors.
Of course, there are also occasions where the quality of print reproduction can be objectively reflected only by using both a densitometer and a colorimeter. For example: the reproduction of gray balance. There are also instruments in the market that combine a colorimeter and a densitometer. We call it a color density meter with a polarizer device. This device has its own unique flexibility. It uses a polarizer device when measuring density, in order to eliminate the reflective part of the surface of the ink layer (including specular and diffuse reflection of light), to ensure that the measured wet density is the subsequent dry-retraction density, so we use this The density density measured by the color densitometer is smaller than the value measured by other densitometers.