4. Density determination
1. The measuring principle of reflective density meter
A stable light source is focused on the surface of the printed product by focusing the lens. Part of the light is absorbed. The absorption depends on the thickness of the ink layer. The density of the colorant. The unabsorbed light is reflected by the surface of the printing paper. The reflected light is transmitted to the receiver; the receiver converts the amount of light received into electricity, and the electronic system compares the measured current with the reference value (absolute white reflection), and calculates the measured ink layer based on this comparison value. Absorption characteristics, measurement results are displayed on the screen in density units.
2. Selection of color filters in density determination
Color to be measured Color filter name Color filter code name Color filter color
Y Kodak Leyden 47B B Blue Purple
M Kodak Leighton 58 G green
C Kodak Leyden 25 R Red
BK Kodak Leighton 106 O Light Orange
The complementary color filter of the second color ink is always used to measure the stacked color density.
3. Problems that must be paid attention to during the density test
â‘ Test error of different density meters
Although some of the density meters used in China are domestically produced, most of them are imported, and the sources of imported density meters are very wide, including Japan, the United States, and Italy. Different types of density meters must have different measured data. For example, one densitometer uses a Raden filter and the other densitometer uses an interference filter to measure the same sample. The test results of the two are completely different. Therefore, in order to avoid test errors of different density meters, it is better to use the same series of density meters in the same unit. In addition, the density data provided by the customer should be converted according to the density meter used by the factory.
â‘¡The calibration of density meter should be accurate
Before using the density meter, high and low density calibration are usually performed. Only when the standard is set, the density data of the test will be accurate. The low density calibration of the density meter mainly depends on the paper. Due to the different whiteness of the paper used in various countries, the difference in the low density calibration value of different density meters is different. If it is 0.06, some is 0.07 and so on. It stands to reason that the low-density calibration value is the most reasonable based on the paper used, but this requires that each paper has a zero point, which is too troublesome to operate. Therefore, in actual use, as long as it is the same density meter, the calibration is performed according to the low density value of the test board. Although the absolute value has a slight error, the relative comparison is accurate. At the same time, the high density value should also be calibrated. For example, the high density should be 2.0. If it is set to 1.8, the measured density will be low. On the contrary, it is set as 2.2, and the measured density is higher. In particular, the calibration of the two tests is different, and the test means that it is obvious.
â‘¢The color filters should be replaced regularly for color testing. The color density of the color filters used in different density meters is not equal. The balance between the color filters used for a certain period of time will be affected. Therefore, it is necessary to insist on regular replacement.
â‘£ Test under stable conditions
The power supply should be stable, the operation should be conscientious, and there must be a certain stabilization time after the power is turned on. The backing paper should be unified during measurement. In case of a printed sample with a picture on it, line it with white paper for measurement. If there are pictures on both sides, liner paper is needed for the measurement, and the actual measurement must be repeated several times. The error of the density value within 1.0 is controlled at 0.01, and the error of the density value above 1.0 is controlled at 0.02, beyond the given range, the density meter should be checked and adjusted.
⑤ When measuring the density, it is necessary to pay attention to the difference between the wet measurement and the dry measurement, which generally differs by 5% -10%. If the dry density is used as the benchmark, the density should be properly high during wet measurement, that is, the difference between wet and dry density should be taken into account. Therefore, it is better to fix the interval time for density measurement (color reflection densitometers with polarization optics no longer have this problem). Usually the wet density must be measured within 30 seconds after printing, and the dry density is measured 30 minutes after printing. When measuring the wet color density, it should be carried out at the same time interval.
â‘¥ The color density of printing inks with different hues cannot be compared. Reflection density measurement is not color measurement. It only makes sense to compare color inks with the same hue with each other.
5. Dark tone density control range (Table 2)
6. The allowable error of the field density of different printed sheets of the same batch of products is:
Color to be measured Fine print Ds General print Ds
Y 0.85-1.10 0.80-1.05
M 1.25-1.50 1.15-1.40
C 1.30-1.55 1.25-1.50
BK 1.40-1.70 1.20-1.50
C, M ≤ 0.15, BK ≤ 0.20, Y ≤ 0.10, the color conforms to the printed sample.
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