Testing requirements and data usage of offset product quality (middle)
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Fourth, density determination
1. Measuring principle of reflective density meter
The stable illumination source is focused by the lens and irradiated onto the surface of the printed matter. A part of the light is absorbed. The absorption depends on the thickness of the ink layer. The unabsorbed light is reflected by the surface of the printed paper. The lens collects at a 45 degree angle to the illuminated light. The reflected light is transmitted to the receiver; the receiver converts the amount of received light into a quantity of electricity, and the electronic system compares the measured current with a reference value (absolute white reflection amount), and calculates the measured ink layer based on the comparison value. Absorption characteristics, the measurement results are displayed on the screen in density units.
2. Selection of color filters in density measurement
Color measured Filter name Filter color code Color filter color
Y Kodak Redden 47B B Blue Purple
M Kodak Reyden 58 G Green
C Kodak Redden 25 R Red
BK Kodak Redden 106 O Light Orange
A complementary color filter of the second color ink is used to measure the color density of the overlay.
3. Problems that must be paid attention to during density testing
1 test error of different density meter
Although the density instruments used in China are partly made in China, most of them are imported, and the sources of imported density meters are very wide, such as Japan, the United States, and Italy. For different types of densitometers, the measured data must be different. For example, one density meter uses a Rayden filter, and the other density meter uses an interference filter to measure the same sample. The test results are completely different. Therefore, in order to avoid test errors of different densitometers, it is best to use the same series of densitometers in the same unit. In addition, the density data provided to the customer is also converted according to the density meter used by the factory.
2 Densitometer calibration must be accurate
Densitometers are usually subjected to high and low density calibration before use. Only when the standard is met, the density data of the test will be accurate. The low-density calibration of the densitometer depends mainly on the paper. Because the whiteness of the paper used in different countries is different, the difference in the low-density calibration values of different densitometers is caused. If there is 0.06, some are 0.07 and so on. It is reasonable to say that the low-density calibration value is most reasonable according to the paper used, but in this case, each paper has a zero adjustment point, and the operation is too troublesome. Therefore, in actual use, as long as it is the same density meter, it is scaled according to the low density value of the test board. Although the absolute value is slightly wrong, the relative comparison is accurate. At the same time, the high density value should also be fixed. For example, the high density should be 2.0. If it is set to 1.8, the density measured will be low. On the contrary, it is set to 2.2, and the measured density is high. In particular, the calibration of the two tests is different, and the test means that it is obvious.
3 It is necessary to periodically change the color filter for color test. The color density of the color filters used by different densitometers is not equal. The balance between the color filters is affected for a certain period of time, so it is necessary to insist on regular replacement.
4 to test under stable conditions
The power supply should be stable, the operation should be serious, and there must be a certain settling time after starting the power. The lining paper should be uniform when measuring. When there is a print with a picture, it is necessary to line the white paper for measurement. If there are maps on both sides, lining paper is required for the measurement, and the actual test is repeated several times. The error of density value within 1.0 is controlled at 0.01, and the error of density value above 1.0 is controlled at 0.02. Exceeding the given range, the density meter is checked and adjusted.
5 When measuring density, it is also necessary to pay attention to the difference between wet and dry measurements, generally differing by 5%-10%. If the dry density is used as the benchmark, the wetness should be appropriately high when wet, that is, the wet and dry density difference is taken into consideration. Therefore, the interval between the measured densities is preferably fixed (the color reflection densitometer with polarizing optics has no such problem). Usually the wet density has to be measured within 30 seconds after printing and the dry density is measured 30 minutes after printing. When measuring the color density of wet, it should be done at the same time interval.
6 The color density of printing inks having different hues cannot be compared. Reflection density measurements are not color measurements. It only makes sense to compare color inks with the same hue to each other.
5, dark tone density control range (Table 2)
6. The tolerance of the field density of different sheets of the same batch of products is:
Measured color 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 is consistent with the print.