How to use measuring instruments to ensure printing quality

The reproduction quality of a color image mainly depends on the tone level of the image and the degree of color reproduction. The gradation and gradation of an image refer to the range of light to darkness in the image and the density data distribution between light and dark. However, tone and level are not the same concept. Tonality refers to the change in the density of the overall contrast of the image. It is a collection of ladders that are arranged together and the level is the density level from bright to dark. How much the level determines the color change and texture on the screen, it is the basic unit of composition tone. Regardless of the tone or the level, they are all functions of density and are external manifestations of density. In the process of color printing copying, the reproduction of the tone levels essentially becomes a reproduction of the density.

We can use the density meter to control the ink layer density to guide the printing and improve the printing quality. Color reproduction means that prints faithfully reproduce the colors of the originals and ensure the consistency of the colors of the same batch of products. The color reproduction is divided into two steps in the process of print copying: color decomposition and color synthesis. Color decomposition is to decompose the original color into the three primary colors of the color material and make each color separation version. Color synthesis is the use of printing plates to imprint three primary colors of ink on paper or other substrates to reproduce the colors of the originals. In order to improve the degree of color reduction, people usually use a colorimeter to judge and evaluate the color reproduction of prints. This article gives a brief introduction to how to use the measuring instrument correctly for different purposes:

1. Printing ink density and dot gain

Printing density refers to the density of a certain color ink printed on the printing material into a solid color block. The factors that affect the density of the actual field are ink pigment, the absorption of light by the binder, the thickness of the ink layer, and the thickness of the ink film. Conjunctival state and so on. In the specific production process, once the ink is determined, the solid density is only related to the thickness of the ink layer, which can indirectly reflect the ink layer thickness on the substrate. Therefore, the measurement and control of the density on the ground is the basis for an objective assessment of print quality.

The principle of the densitometer's measurement of solid patches is that the light emitted by the densitometer's internal light source passes through the filter patch and is then directed onto a solid patch. The solid patch absorbs a portion of the light and reflects some of the light. The reflected light is reflected by the densitometer. The receiver receives, and the electronic components inside the densitometer compare the difference in these two luminous fluxes and calculate the density value. The density value is a physical quantity with no unit. There are a variety of densitometers on the market today. Although they are structurally different, they are basically in communication. The difference in the density of the densitometer will affect the measurement of the density. This means that measuring the same color block with different manufacturers' density meters may give different density values, but we cannot say that a certain density meter measures correctly. Another densitometer measurement error. In fact, there is no absolute density reference value in the field of offset printing. The reference values ​​we have previously recommended are only a range. As long as the print density value is within this range, we all consider the print quality to be acceptable.

The dot enlargement value refers to the percentage between the dot separation area and the dot area on the corresponding print. We can also use the density meter to calculate the dot gain value. The specific principle is as follows: Densitometer firstly measures the density in the field, then measures the dot density, calculates the dot coverage by the electronic components in the density meter, and displays the calculation result on the liquid crystal display. In this way, we can calculate the dot gain value. It is worth noting that before measuring the density, we first need to zero the calibration on white paper. During the measurement, the printed matter to be tested should be laid flat to ensure that the measuring head is in close contact with the block to be measured and measurement errors are reduced.

2, color measurement

During the printing process, we will encounter problems that cannot be solved with densitometers, such as color calibration and measurement of computer monitors, calibration of digital proofing equipment, evaluation of digital proofing products, and evaluation of solid color reproduction. People often use the colorimeter to measure the color of the same batch of same-color blocks, and measure the color difference ΔE of the same batch of same-color blocks by using the color values ​​of the same-color standard blocks, if we control the ΔE value within the specified range. Inside, you can ensure that the same product before and after the print, or the same place with the same color deviation does not exceed the allowable range. The “Quality Product Conditions of the Press and Publication Administration” issued in December 1993 stipulated that the color difference ΔE of the same batch of the same color standard should be less than 4. In the colorimeter measurement process, one must pay attention to: first zero calibration on the "standard whiteboard", rather than zero calibration on white paper. During the measurement, the printed matter to be tested should be laid flat to ensure that the measuring head is in close contact with the block to be measured and measurement errors are reduced.

Of course, there are also occasions in which the quality of print reproduction can be objectively reflected only by using a density meter and a colorimeter at the same time. For example: the reproduction of gray balance. In the current market, there is also a device integrating a colorimeter and a density meter. We call it a color densitometer with a polarizer device. This device has its unique flexibility. It uses a polarizer device when measuring density. The purpose is to eliminate the part of the surface of the ink layer that reflects light (including specular reflection and diffuse reflection of light) and to ensure that the measured wet density is the future dry density, so we use this The color density meter measured the value of the wet density smaller than that measured with other densitometers.