Density measurement has always been the most commonly used measurement method in the printing industry, but density meters cannot provide psychophysical quantities related to human eye sensitivity, so their analytical measurement capabilities are limited. Therefore, in the detection and evaluation of printed matter, chromaticity measurement plays an increasingly important role. In the color measurement, in order to obtain the measurement data that can accurately reflect the quality of the print, the following points must be noted.
1. Whiteboard calibration
Because the density meter and the spectrophotometer are very precise instruments, the routine calibration of the measuring instrument must be performed before the measurement in order to ensure the accuracy of the measurement results. In general, each device is equipped with a standard whiteboard, such as DensiEye750 density measuring instrument, X-R ite500 series spectrophotometer, etc. According to the purpose of the measurement data, we choose whether to perform the zero adjustment on the whiteboard or on the white paper, set the calibration warning, and if the calibration time is exceeded, the whiteboard must be calibrated.
2. Substrate
The color measurement value will be different due to the choice of the substrate used in the measurement and get different color values. The effect of the black and white substrates on the color measurement value will be different depending on the transparency of the substrate, and the greater the transparency of the substrate, the greater the influence of the substrate. Therefore, in the process of high-quality printing production and color matching inspection, attention should be paid to placing and using the correct substrate on the inspection table.
It is generally recommended to use the following standards: When the opacity of the substrate is greater than or equal to 99, the measurement result will not be affected by the substrate. When the opacity is between 95 and 99, a black substrate should be used for color measurement, and the opacity is less than Use a white substrate at 95.
In actual production, people usually use a white substrate as a substrate for color measurement. For example, when performing color measurement and inspection on packaging printed matter such as plastic films with greater transparency, a white substrate that conforms 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 substrate, it is necessary to pay attention to the selection of the same substrate, that is, to pay attention to the consistency of the use of the substrate.
3. Measuring light source
A, B, C are simulated incandescent lamp, noon daylight, cloudy daylight or cloudy noonday light, and the radiation distribution of D65 light source is after many measurements of the sunlight spectrum at different times, different climates and different locations , After a complicated averaging process. The measurement results of A light source and other light sources are very different, and 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. D series light source has better display performance because its color temperature is 5000K-7500K, close to white. In the printing industry, D50 light source is recommended when observing transmission samples, and D65 light source is recommended when observing reflection samples. Therefore, when measuring the data, you must indicate under which light source the measurement is made.
4. Measure the angle of view
The resolution of the human eye to color is affected by the size of the field of view. Experiments show that when observing colors with a small field of view (<4 °), the human eye has a low ability to discern color differences. When the field of view is increased from 2 ° to 10 °, both the accuracy of color matching and the ability to distinguish chromatic aberrations 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 field angle selected during measurement must also be marked.
5. Color space
The standard colorimetric system has a variety of color spaces, that is, there are many different expressions to choose from. Choosing a color space is actually choosing the expression form of the measurement result.
6. Measurement of color difference
Color difference is an important parameter index for the detection and evaluation of printing quality. The size of color difference directly affects the quality and grade of the product. The color difference formula based on the CIE1976LAB uniform color space is commonly used in the printing industry, but the data expression of the color difference formula cannot match the visual perception of the human eye, so color research experts have successively launched the CMC (l: c) color difference formula, CIE94 Color difference formula and the latest CIEDE2000 color difference formula, among which CMC (l: c) color difference formula is listed as international and national standards in the textile industry.
The results measured using different color difference formulas vary greatly. Some people measured the tristimulus values ​​of 10 pairs of color samples through experiments. By comparing the color differences calculated using the three color difference formulas, it was found that there is △ ELAB> △ ECMC (2: 1)> △ E2000 (1: 1: 1) the trend of. Therefore, it is necessary to indicate which color difference formula is selected in the measured color difference data. [next]
7. Aperture size
When the color measurement is performed, the aperture of the color measuring instrument may be longer than the side of the measuring color block. At this time, another color block must be selected for measurement or another color measuring instrument. The aperture size specified by the user should not be greater than 5mm. Generally, a standard 3-8mm and a small 1-7mm aperture are used. The aperture of 3 ~ 8mm is used to measure the color mark (control strip) used in standard sheet printing, and the aperture of 1 ~ 7mm is used to measure the color mark used in web printing.
8. Response method
The response states are T, E, A, I and other states. State T and state E are the two most commonly used states. The commonly used wide-band response is the T state (American ANSI standard, widely used in the North American printing industry), the commonly used narrow-band response is the E state (European DIN standard), when the E state is selected, the density meter's Minor changes are more sensitive. In terms of printed matter measurement, when using state T or state E, for yellow, the value measured in state E is greater than the value measured in state T. Therefore, when comparing and evaluating the measured values, the response status needs to be adjusted to be consistent.
9. Observation conditions
The 0 / d condition means that the light source illuminates the sample substantially vertically, and the integrating sphere is used to receive the diffuse reflected light of the sample. In the case of regular reflection, the reflected energy of the sample is completely received, which is the "reflectance" in the true physical sense . The d / 0 condition means that the light illuminates the sample after being diffused by an integrating sphere, and is substantially perpendicular to the surface of the sample to receive reflected light. This condition is closer to the normal observation of the object by the human eye, that is, the sample is white light from all sides Illumination, the human eye is basically perpendicular to the sample for observation. The colorimetric value of the sample is different under different lighting / observation conditions.
The samples with good diffuse reflectivity are less affected by the geometric conditions, and the samples with poor diffuse reflectivity have higher gloss. Different geometric conditions will bring about differences in the luminous flux received by the detector, and the reflection factor is greatly affected by the geometric conditions. The measurement of the sample under different geometric conditions will not change the proportion of tristimulus values, that is, different geometric conditions have little effect on the chromaticity coordinates, but the 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 total reflectance of the tested sample, 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 its chromaticity value under different geometric conditions should be closer. The data shows that this is not the case. The reason for this is that fluorescent whitening material is added to the white paper, so its test is special.
Fluorescent materials have the characteristics of fluorescence excitation. When the fluorescent material is irradiated with light of a certain wavelength or a certain wavelength, the fluorescent material is excited to emit light longer than the irradiation wavelength, and the emitted light has both a reflective part of the irradiated light and excitation by the irradiated light. The fluorescence emission part, its simultaneous or separate reception will affect its measurement value. Therefore, the position of the monochromator in the measuring device will have a great influence on the measurement result. When measuring the chromaticity value of a fluorescent sample, a post-spectral colorimeter should be used, that is, the monochromator is located behind the sample and before the detector.
The non-fluorescent sample has no fluorescence excitation characteristics, the light incident on it follows the law of reflection, only the reflected flux and no radiated flux, regardless of monochromatic light illumination or monochromatic light reception, the detector receives the sample to the irradiation wavelength The reflected flux and the position of the monochromator have no effect on the measurement.
In principle, color measuring instruments with different lighting / observation conditions cannot be replaced with each other, especially for high-gloss samples and fluorescent materials. Therefore, when performing chromaticity testing, it should be clear what kind of lighting / observing conditions are adopted. When purchasing a color measuring instrument, it should be checked whether the lighting / observing conditions of the instrument are consistent with the corresponding product standards.
10. Summary
In the process of chromaticity measurement, we must pay attention to the choice of substrate, light source, color space, etc., and must be noted in the measurement result, otherwise the measurement result will likely be inconsistent with the customer's parameter index requirements, so it should be If a qualified product becomes an unqualified product, it is not conducive to the implementation of data and standardization of the production process.
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