Application of Reflection Density Measurement and Chromaticity Measurement in Printing (6)

4.4 Measurement of Chroma Parameters

4.4.1 Instrument Regulations

The colorimeter, measurement conditions, and calculations should conform to ISO 13655. If a tristimulus colorimeter is used, the measurement conditions (such as backing, geometric requirements, etc.) shall be in accordance with ISO 13655.

Note: Some colorimeters can also calculate density values ​​from spectral data. These instruments may be equipped with polarizing filters. Polarization filters are not recommended in the chroma measurement mode of these instruments.

4.4.2 Measurement Result Report

The report should meet the requirements of ISO 13655 on color measurement data and indicate the exact conditions of use. The report content should include the following parameters:

- Colorimeter manufacturer and model name (if tristimulus value photometric type, it should be noted),

- Specimen backing (if not black),

- sampling light hole (mm),

- Uncertainty of mixed standards (ISO 15790)

5 Measurement methods

5.1 Deviations between on-site color samples and sample proofs

Measure the tristimulus value of each printed basic color (C, M, Y, K) solid color block on the print control strip, calculate the color difference CIELAB ΔE*ab of the corresponding spot on the proof sample and the proof sample, and compare the result with GB/T. The deviations in the relevant parts of 17934 are compared.

5.2 Basic density or relative density of printing

Select the color channel that gives the highest density reading of the basic color to be measured, and measure its density in the field and density in the blank. In the report of the density measurement results, the field density measurement is directly obtained and the relative density is calculated by the following formula:

Dr=Ds-D0

Among them: Dr is the relative density;

Ds is the density of the field;

D0 is the density of blank parts.

5.3 The tone value of printed products

Choose the color channel that gives the maximum reading of the basic color to be measured. Measure the density of the blank substrate, the density of the field, and the density of the specified basic color mesh toning block. If the densitometer cannot directly display the tone value, the tone value can be calculated from the density value using the formula in 3.31.

Note: There is some relationship between the 1st tone value (percentage of halftone area coverage) and the condition of the instrument, especially the difference between the midtone adjustment value of the yellow, polarizer narrowband densitometer and the non-polarizer broadband densitometer is 2%. the above.

The increase in the second-order adjustment is obtained by calculating the difference between the corresponding gradation values ​​on the print and the control strip film or digital file. All gradation increments must be determined by the gradation values ​​of at least 3 mesh patches on the test strip, and preference is given to selecting controls on the film or digital file that include changes from 10% to 90% in 10% increments. Piece. The smooth curve is drawn by plotting the difference from the tone value on the film or digital file. This curve represents the tone value increase function and also represents the dot gain function.

5.4 Overprinting rate

The density of the first solid ground, the second solid ground, and the overprinted solid ground are measured with the color channel measuring the highest value of the second color, and the apparent trap rate is calculated from the Preucil formula:

Ip=(D0-D1)/D2

Where: Ip is the ink's overprint rate;

D0 is the overprint density;

D1 is the first color density alone;

D2 is the second color density alone.

Note: 1 The trap rate obtained by using the Preucil formula is not the absolute value of the amount of ink applied to the first color ink by the second color ink. The calculated trap rate depends on the printing color sequence. Although the amount of ink transferred is the same, the rate of overprinting is different because of the difference in the opacity of the ink and, more importantly, the difference in the choice of color channels. It can be clearly seen that using different color channels will produce different results.

2 Actual (weight percentage) The ink overprint rate (Ig) can be tested using the gravimetric method described in the appendix. Overprinted microscopic inspection and image analysis techniques can provide additional information about how the ink is attached to the substrate and ink, and the overprint rate does not provide this information.

3 The apparent trap rate can be used in the process to monitor changes in the production process.

4 In addition to the Preucil formula, there are many other formulas available. Most formulas are derived from the Preucil formula but contain correction coefficients. They all have the disadvantages mentioned in Note 1.

5 Density Meters with Polarizer and Narrowband Spectroscopy Products Ip is more similar to Ig than densitometers without polarizers.