Some points to note in color measurement

Density measurement has always been the most commonly used measurement method in the printing industry, but densitometers cannot provide psychophysical quantities that are related to human eye sensitivity, so their analytical measurement capabilities are limited. Therefore, color measurement plays an increasingly important role in the detection and evaluation of prints. In color measurement, in order to obtain measurement data that can correctly reflect the quality of printed matter, the following points must be noted.

1. Whiteboard correction

Because densitometers and spectrophotometers are very sophisticated instruments, routine calibration of the measurement instrument must be performed before measurement to ensure the accuracy of the measurement results. In general, each device is equipped with a standard white board, such as DensiEye750 density meter, X-R ite500 series spectrophotometer and so on. According to the purpose of the measurement data, we choose to zero on the whiteboard or white paper, set a correction warning, if the correction time is exceeded, then the whiteboard correction.

2. Substrate

The color measurement values ​​will be different depending on the substrate chosen for the measurement. The effect of the black and white substrates on the color measurement will vary with the transparency of the substrate, and the substrate with greater transparency will be affected by the substrate. Therefore, in the high-quality printing production and color matching inspection process, care should be taken to place and use the correct substrate on the inspection table.

It is generally recommended to use the following criteria: When the opacity of the substrate is greater than or equal to 99, the measurement results will not be affected by the substrate, when the opacity is between 95 and 99, the color measurement should use a black substrate, the opacity is less than 95 white substrate.

In actual production, people usually use a white substrate as a substrate for color measurement. For example, when measuring and verifying the color of packaged printed matters such as plastic films with large transparency, a white substrate conforming to the ISO standard should be placed under the printed matter to avoid unnecessary errors. At the same time, when measuring the color information on the same printing material, it is necessary to pay attention to the same substrate, that is, to pay attention to the consistency of the substrate.

3. Measuring light source

A, B, and C are simulated incandescent, noon daylight, cloudy daylight, or cloudy noon daylight, and the D65 light source's radiation distribution is made after many measurements of sunlight spectra at different times, in different climates, and at different locations. After a complex averaging process comes out. The results of measuring A light source and other light sources are very different. Now it is rarely used. F series light sources are generally used to measure fluorescent products. We can say that C light source and D65 light source are the most useful for the printing industry. Due to its color temperature of 5000K-7500K, D series light sources are close to white, so its display performance is good. In the printing industry, D50 light source is recommended for observation of transmission samples, and D65 light source is recommended for observation of reflection samples. Therefore, when measuring data, it must be noted under which light source to measure.

4. Measure the angle of view

The resolution of the color of the human eye is affected by the size of the field of view. Experiments show that the ability of the human eye to discriminate color differences is low when viewing colors with a small field of view (<4°). When the viewing field of view is increased from 2° to 10°, the accuracy of color matching and the ability to discriminate color differences are both There is an increase; but when the field of view is further increased, the accuracy of color matching is not improved. In the 2° field of view and the 10° field of view, the colors presented by the same color are also different. Therefore, in the color measurement, the angle of view selected for measurement must also be marked.

5. Color space

The standard colorimetric system has a variety of color spaces, that is, there are many different forms of expression to choose from. Selecting the color space is actually the selection of the expression of the measurement results.

6. Measurement of color difference

Chromatic aberration is an important parameter indicator for quality inspection and evaluation of printed matter. The size of color difference directly affects the quality of the product and its level. In the printing industry, the color difference formula based on the CIE1976LAB uniform color space is commonly used, but the data expression of the color difference formula cannot match the visual perception of the human eye. Therefore, experts in color science have successively introduced the CMC (l:c) color difference formula, CIE94. The color difference formula and the latest CIEDE2000 color difference formula, in which the CMC (l:c) color difference formula is listed as an international and national standard in the textile industry.

The results of measurements using different color difference formulas vary widely. Some people experimentally measured the tristimulus values ​​of 10 pairs of color samples. By comparing the color differences calculated by using the three color difference formulas respectively, it was found that there exists ΔELAB> △ECMC (2:1)> △E2000 (1:1:1) overall. the trend of. Therefore, in the measured color difference data must also indicate which color difference formula is selected.

7. Aperture size

When performing color measurement, it may happen that the aperture of the colorimetric instrument is larger than the side length of the measurement color patch. At this time, other color patches are selected for measurement or other color measurement instruments are selected. The user-specified aperture size should be no more than 5mm, generally using a standard 3 to 8mm and a small 1 to 7mm aperture. The 3 to 8 mm aperture is used to measure the color standard (control strip) used in standard sheetfed printing, and the 1 to 7 mm aperture is used to measure the color scale used in roll printing.

8. How to respond

The response states are T, E, A, I, etc. The states T and E are the two most commonly used states. The commonly used wide-band response is T-state (US ANSI standard, widely used in North American printing industry), the commonly used narrow-band response is E-state (European DIN standard), when the E-state is selected, the densitometer to the print density value Minor changes are more sensitive. In print measurement, when using the state T or the state E, for yellow, the value measured in the E state is larger than the value measured in the T state. Therefore, in the comparative evaluation of the measured values, the response state must be adjusted to be consistent.

9. Observation conditions

The 0/d condition means that the light source illuminates the sample substantially vertically, and the diffuse reflection light of the sample is received by the integrating sphere. With regular reflection, the reflected energy of the sample is completely received. It is a “physical reflectance” in the real physical sense. . The d/0 condition means that the light illuminates the sample after being diffused by the integrating sphere, and the reflected light is substantially perpendicular to the surface of the sample. This condition is closer to the observation of the object by the human eye in the normal case, that is, the sample is white light from all sides. Lighting, the human eye is basically perpendicular to the sample for observation. Samples with different illumination/observation conditions have different chroma values.

The samples with good diffuse reflectivity are affected little by the geometric conditions, and the samples with poor diffuse reflectivity have high gloss and different geometric conditions will bring differences in the light flux received by the detector. Its reflection factor is greatly influenced by the geometric conditions. Samples measured under different geometric conditions do not change the ratio of tristimulus values, ie, different geometric conditions have little effect on chromaticity coordinates, but spectral reflectance factors are different, resulting in different tristimulus values. Therefore, as long as it is not an ideal Lambertian body, the difference in geometric conditions will affect the spectral reflectance and the total reflectance of the sample under test, and the degree of influence is related to the gloss of the sample surface.

The diffusivity of the white paper surface is better, the gloss is weaker, and the reflection factor is also higher. It is inferred from the above that the chromaticity values ​​under different geometric conditions should be relatively close. The data shows that the actual situation is not the case. The reason for this is that the fluorescent whitening material is added to the white paper, so its testing is special.

The fluorescent material has a fluorescent excitation characteristic. When a certain or a certain wavelength of light is used to irradiate the fluorescent material, the fluorescent material is excited to emit light longer than the irradiation wavelength, and the emitted light includes both the reflected part of the irradiated light and the excited light. The fluorescence emission part, which receives or receives separately, affects the measurement value thereof. Therefore, the position of the monochromator in the measuring device has a great influence on the measurement result. When measuring the colorimetric value of a fluorescent sample, a post-spectrometer colorimeter should be used, ie the monochromator is located after the sample and before the detector.

The non-fluorescent sample has no fluorescence excitation characteristics. The light incident on it follows the reflection law. Only the reflection flux and no radiation flux, regardless of the monochromatic light illumination or the monochromatic light reception, the detector receives the sample wavelength. The reflected flux, how the position of the monochromator has no effect on the measurement.

In principle, colorimetric instruments with different illumination/observation conditions cannot replace each other, especially for high-gloss samples and fluorescent materials. Therefore, when performing a colorimetric test, what lighting/observation conditions should be specified. When purchasing a colorimetric instrument, it should be checked whether the instrument's illumination/observation conditions are consistent with the corresponding product standards.

10. Summary

In the process of colorimetric measurement, we must pay attention to the choice of substrate, light source, color space, etc., and must be noted in the measurement results, otherwise the measurement results may be inconsistent with the customer's parameter index requirements, so that it should be Qualified products become unqualified products, which is also not conducive to the enterprise's implementation of the digitization and standardization of production processes.

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