Density meter is one of the most important quality control tools in the pre-press and printing production process. The accuracy and consistency of color reproduction and the quality control of most workflows all depend on the density meter measurement. Practice has proved that the effective use of density meters is a powerful tool for the implementation of standardization, standardization and data quality management of printing and copying projects. Its main advantages are:
(1) Color is greatly influenced by people's visual subjectivity. Everyone has different feelings about color. Density meter measurement can provide an objective analysis to overcome the ills that vary from person to person, thus unifying the standard of people's judgment on the shade of ink color.
(2) The light source and environment have a great influence on visual color measurement. The color viewing is different indoors and outdoors, the color is different in the printing workshop and the office, and the density meter is not affected by the environment.
(3) Guarantee the quality of proofing and printing. Density meter measurement is very important in proofing and printing. In the production process, the color density is affected by many factors and has a density standard. Through the measurement, the density change can be effectively controlled to ensure the consistency and stability of ink color Sex.
(4) Set quality standards for proofing and printing. The use of a density meter for data management can provide printing personnel from different regions and different manufacturers to monitor the production process and achieve the same effect of ink color depth.
(5) Color files. The realization of digitization has become a prerequisite for printing quality management. The standard color data developed can be saved in a record file for recalling during the next batch of printing. This prevents the saved sample from being faded due to the color of the sample after a period of time. The color of each batch of printing is inconsistent.
A kind of density meter and measuring principle
1. Types of density meters Density meters are divided into transmission and reflection. The transmission density meter (Figure 9-4) is mainly used to measure the density of the transmission original, the density and dot percentage of the output film, and is used for the linearization operation of the imagesetter. The reflection densitometer (Figure 9-5) is mainly used to measure the density of the original reflection and proofing, the solid density and dot area of ​​various colors of the printed sample, the dot increase, the overprint, the printing contrast, the color deviation and the measurement and calculation of the three major inks characteristic.
(Figure 9-4 Transmission Density Meter)
(Figure 9-5 reflection density meter)
D (M) -density meter T-transmission TD-transmission density meter R-reflection RD-reflection density meter O-dot area tester TR-transmission and reflection dual-use OT-dot area transmission densitometer [next]
The Arabic number behind the model of the density meter is the serial number of the density meter. There are two types of domestic density meters, one is CMF, which stands for color reflection density meter, and the other is CMT, which stands for transmission density meter.
The measuring principle of the density meter is very close to the principle of visual identification by the printing operator. The working principle of the photoelectric density meter is to form the optical path through the light source and the filter, and the receiver converts the transmitted or reflected light into a corresponding intensity photocurrent according to the intensity. After the analog-to-digital conversion, it is displayed on the digital display. Thereby obtaining the density value of the transparent film or reflective printed matter.
2. The measuring principle of reflection density meter. The stable light source is focused on the surface of the printed product by focusing the lens, and part of the light is absorbed. The absorption depends on the thickness of the ink layer and the density of the pigment (Figure 9-4). The unabsorbed light is reflected by the surface of the printed paper, and the reflected light at a 45 ° angle to the irradiated light is collected by transmission. And transmitted to the receiver, the receiver converts the amount of light received into electricity, and the electronic system compares this measured current with a reference value (absolute white reflection). Based on the comparison value, the absorption characteristic of the measured ink layer is calculated, and the density unit of the measurement result is displayed on the screen.
(Figure 9-6 Measuring principle of reflection densitometer)
The relationship between reflectivity and density has been described in detail in Chapter 1. But in order to understand the relationship between reflectivity and density more intuitively, please refer to Figure 9-7.
(Figure 9-7 Relationship between reflectivity and density)
2. Application of density meter
Before using the density meter, you must first calibrate the zero on the standard whiteboard provided with the device. The standard whiteboard is an ideal completely white magnesium sulfate reflective surface. Zeroing the density meter means adjusting the density meter to a standard low density value and then performing a full calibration. Make the high density value of the densitometer equal to the density of the dark space, so as to determine the range of the densitometer, such as 0 ~ 3.00D or 0 ~ 3.50D range.
In actual production, the use of density meters is many, but the most important ones are the following:
(1) Measuring ink characteristics, the meaning of measuring ink characteristics is to provide reliable ink characteristic data, so that the plate-making personnel can compensate when setting colors before performing image color processing.
(2) Manuscript scanning. When scanning a manuscript with a scanner, the density of the manuscript can be measured by a densitometer. The density of general printed matter is 0.06 ~ 2.2. The density range of the reversal film can reach 0.02 ~ 4.0. In addition, the black and white field calibration can also be determined with a densitometer. Normally, the high optical density is 0 ~ 0.80, and the dark tone density range is 1.50 ~ 3.90.
(3) The output of the imagesetter, in order to ensure that the PS version of qualified quality can be obtained, the film dots output by the imagesetter must have a sufficient density value. The density of a single dot must be above 3.0, the density on the ground must be above 4.0, and the initial fog density should be less than 0.07.
(4) Proofing and printing, generally in the proofing and printing, the density meter should be used well. When using art paper to proof, the density range of each color is usually:
Y: 1.10 ~ 1.15 M: 1.40 ~ 1.50
C: 1.50 ~ 1.60 K: 1.70 ~ 1.80
In printing production, reflection densitometer is mainly used to measure and calculate relevant parameters. [next]
Three parameters affecting printing quality
The main parameters that affect the quality of printing are solid density, dot gain, and relative contrast. These quality parameters are all related to the thickness of the ink film on the printed sheet. From another perspective, they are related to density. Therefore, determining the optimal ink layer thickness is of great significance for controlling the printing quality.
1. Field density
The solid density refers to the area coverage of the dots of the printed product is 100%, that is, the part of the printed product that is completely covered by the ink layer. The density is measured with a reflection densitometer, according to the definition of reflection density:
If R represents the reflectivity of the ink layer to light, then the thicker the ink layer, the more light energy it absorbs, the greater the reflectance R, and the smaller the density Dv. As the ink layer thickens, the density value also increases.
The field density does not increase infinitely with the increase of the ink layer. When the thickness of the ink layer increases to a certain value, continue to increase the thickness of the ink layer. The field density has reached the maximum value and no longer increases, as shown in Figure 9-8. Show
Figure 9-8 Relationship between solid density and ink layer thickness
See the dark tone density range of Offset Printing products recommended by China's printing industry standards.
Table 9-2 Dark tone density range of lithographic printed matter, fine printed matter by color, general printed matter
Fine prints by color General print yellow (Y) 0.85 ～ 1.15 0.8 ～ 1.10 Magenta (M) 1.25 ～ 1.55 1.15 ～ 1.45 Cyan (C) 1.3 ～ 1.6 1.25 ～ 1.55 Black (K) 1.4 ～ 1.8 1.20 ～ 1.60 Superimposed color 1.5 1.30 or more2. Increased value of outlets
The field density can only reflect the thickness of the ink, and cannot reflect the change of the dot size during printing. It is normal for dots to increase properly during proofing or printing, but it must be controlled within the allowable range, otherwise it will affect the printing. Tone reproducibility and color reproducibility.
Table 9-3 50% dot increase value color fine print (%) General print (%)
Fine prints by color (%) General prints (%) Yellow (Y) 8 ～ 20 10 ～ 25 Magenta (M) 8 ～ 20 10 ～ 25 Cyan (C) 8 ～ 20 10 ～ 25 Black (K) 8 ～ 25 10 ～ 253. Relative contrast
Relative contrast, also called printing contrast, is an important parameter that controls the tone of the image. Determine the solid density Dv and dot density Dr on the printed matter or on the control strip, and substitute the following formula to calculate the K value
K = 1-
The K value varies from 0 to 1. The larger the K value, the smaller the ratio of dot density to field density, and the smaller the dot increase value. There are many factors that affect the K value, such as the thickness of paper and ink layer. Generally, the K value of coated paper is larger than that of offset paper. Under stable printing pressure and good printing conditions, when the K value is maximum, the dot The increase is the smallest.
Table 9-4 Relative Contrast K Value Color Fine Print General Print
Four high magnification applications
The so-called high magnifying glass refers to a magnifying glass with a magnification of more than 25 times. In the detection of printing quality, this magnifying glass not only observes the fit of the printed matter, but more importantly observes the changes of the dots in order to make corresponding printing processes Adjustment. Mainly manifested in the following aspects:
(1) Pay attention to observe whether the dots are elongated, ghost halo, or regular expansion.
(2) Pay attention to observe whether the outlets are hollow, out of shape, and shrink regularly.
(3) The area to be observed mainly refers to the highlight area, such as the cheekbone and cheekbones of the figure, and the bright spots around the eyes. The middle tone is the situation where 50% of the outlets are cornered. In the dark tone area, 7 ~ 9% of the dots are not unclear.
(4) Detect the depth of Printing Plate printing.
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