Application of Reflection Density Measurement and Colorimetric Measurement in Printing (7)

5.5 Ghost/Transformation

Select the appropriate color channel and measure the density of lines in different directions on the control block. Their reflection density difference is the relative measure of ghost/deformation.

5.6 Change in colour on the same printed sheet

In order to control the uniformity of the ink in the entire printing press, it is usually necessary to ascertain the change in ink flow in the printing direction or in the lateral direction, for example, to quantify the degree of mechanical shadowing and other printing failures in the printing. Density measurements are usually performed at a solid spot to accurately determine the cause of the failure.

Because colorimetry can more accurately represent chromatic aberrations, chromaticity measurements are used when it is necessary to determine the color change of a solid patch or a halftone patch to determine the print result.

5.6.1 Density Measurement

The instrument is placed on the color channel that yields the highest density reading and the solid field density change is measured as a percentage. The calculation method is:

[(Dmax/Dmin)-1]×100%

Among them: Dmax is the maximum density value of the solid spot on the printed product;

Dmin is the minimum density value for monochrome solids on the print.

5.6.2 Color Measurement

The CIE LAB color difference ΔE*ab of color blocks having the same structure on the print (for example, "K12, C60, Y100", "K100%" (balanced color blocks C75, M70, Y70") is measured, and the color difference at the corresponding measurement position is reported.

5.7 Change of Solid Color of Printing Products

Choose at least 15 representative samples from the bulk product. If the number of prints exceeds 150,000, increase the number of test samples proportionally. Measure and control the on-the-spot color patch, compare the measurement result with the error stipulated in GB/T 17934.

5.7.1 Color Measurement

Measure the L*, a*, b* data of the CIE LAB for each printed sheet and calculate the ΔL*, Δa*, and Δb* differences relative to the average of the batch product, and calculate the average of ΔL*, Δa*, and Δb*. The values ​​and the standard deviation are then multiplied by 1.96 to determine the 95% confidence range (that is, the 95% range of samples). Finally calculate the CIE LAB color difference (ΔE*ab) for each average and confidence range. The color difference obtained from the average value indicates the color deviation between the printed product and the unprinted sample, and the color difference obtained from the degree of confidence indicates the color change between the batch prints.

5.7.2 Density Measurement

Place the instrument in the color channel giving the highest reading of the basic color density tested. Determine the density difference between the sample and the average position of the batch print. Calculate the "average" and "standard deviation" statistical values ​​for each location. These two values ​​represent the actual color deviation and direction and magnitude of the change in the measured location. Repeat the measurement at other locations.

Appendix A (Appendix of Tips)

Gravimetric test for ink overprinting rate

The following is a laboratory method for determining the exact comparison of the ink layer weight of the printed ink with the unprinted substrate weight. This method does not serve as a process control step. The preparation of the sample complies with ISO 5737 "Printing technology - Preparation of standard prints for optical tests".

On the printability meter, the first color ink sample is printed with a solid printing plate to achieve a desired color value or density value. Then, a solid ground ink of the second color is printed on the unprinted substrate with the same solid printing plate and brought to the specified colorimetric value or density value. The ink thickness transferred to the printing substrate is determined by measuring the amount of ink reduced on the plate or the amount of ink added to the substrate (if an accurate analytical balance is used, the reduced weight of the plate is generally measured in comparison with the increment of the measured substrate. More accurate, because the sensitivity of the substrate weight test is affected by environmental conditions). The value obtained is the thickness SS of the separately printed second color ink.

The second ink is printed on the first ink layer with the same printing plate. The thickness of the transferred second color ink layer is measured, and the thickness of the ink layer is the thickness S0 of the second color ink superimposed on the first color ink layer.

The ink overprinting rate is calculated as follows:

Ig(%)=100×(S0/SS)

Among them, Ig is the weight law overprint rate;

S0 is the thickness of the second ink overprint;

SS is the thickness of the second color ink printed separately.

The color sequence used should be stated in the report. Note: If the ink absorption of the first color ink is higher than the ink absorbency of the substrate, the trapping rate calculated by the gravimetric method is usually close to 100%, and sometimes even higher.

Appendix B (Appendix of Tips)

Polarization in density measurement

The light reflected from a surface generally includes two parts: the first surface reflects and the backing reflection from below the surface, wherein the first part of the reflected light is related to the "gloss" of the sample, and only the second part reflects light depending on the color . In printing process control, the basic purpose of density measurement is to provide a method for measuring the amount of toner per unit area. Some densitometer designers try to eliminate the first part of the measured light reflected.

For high gloss specimens, the standard 0°/45° or 45°/0° densitometer geometry eliminates the spectral effects of spectral reflections. However, for a low gloss surface, these geometric conditions allow an appropriate amount of light to enter the detector. Some manufacturers provide densitometers with polarizers to eliminate light reflected from the first surface.

Obviously, although the polarization device attenuates the consistency between the visual and density values, it is beneficial for process control, including:

- For higher densities, the linear difference between the density and the amount of paint per unit area also applies.

- Reduce the additive failure of density.

- The dependence of the reflection density on the ink drying time is reduced.

It is important to consider the spectral characteristics and other optical properties of the polarizer when designing the instrument. Densitometers without a polarizer that comply with ISO 5-3 and ISO 5-4 regulations will not comply with ISO 5-3 and ISO 5-4 regulations after the polarizer is installed. Similarly, densitometers with a polarizing film that comply with ISO 5-3 and ISO 5-4 may not comply with ISO 5-3 and ISO 5-4 regulations after removal of the filter.

When calibrating a densitometer with a biased plate, the specimen is required to have high gloss properties. It can also be used for calibration without polarimetric densitometers. See ISO 14981.

Appendix C (Appendix of Tips)

Plate tone value

Under certain conditions, that is, in the absence of a more precise measuring instrument, the densitometer is used to measure the gradation value of the printing plate. The gradation value measured by this method can be used as the coverage area of ​​the printing dot area. The following conditions must be met in the measurement:

- The density difference between printed and non-printed areas on the plate is at least 0.7;

- There should be no change in the density of printed and non-printed parts of the plate;

- Demand density meter is a precision instrument that guarantees at least three digits or can directly read out the tone value;

- Densitometer effectively measures the diameter of the light hole not less than 15 times the width of the cable.

Measurement methods:

Non-printing sites, solids, and dot density were measured on the same plate using a reflection densitometer. If the plate has developed and is sensitive to light, it needs to be measured immediately after development. If the density difference between the printed and non-printed parts is less than 0.7, the contrast must be improved with more feasible methods (such as using ink). For solid patches, maximum density values ​​can be obtained using a densitometer with a color filter. (When reading data using a spectral colorimeter, the tone value calculation can select a narrow spectral band, and the absorption spectrum of the plate coating The area should be included in this narrow spectral band and this light can be measured.) If the reading of the densitometer is related to the direction, at least 10 measurements should be averaged, of which 5 are taken in a direction parallel to the plate cylinder diameter and the other 5 are taken perpendicular to the plate cylinder diameter.

The gradation value of the printing plate internet point control block is calculated using the 3.31 definition method.