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Color is the most critical element of printing and copying. In the printing production process, how to ensure the true reproduction of color is a worldwide problem. At present, digital color management is used to describe the color characteristics of each production link in the production process, which is the best method to achieve accurate color reproduction, and its premise and basis is the accuracy of color measurement data.
At this stage, density measurement has shown superiority in some printing companies. The density measurement value can reflect the information of the thickness of the ink layer, and guide the control of the expansion of the dot, and control the printing quality through the data of the density measurement, which is convenient and reliable, and has been trusted by people. However, the density value does not directly reflect the stimulation of the image color on the human eye. It is basically used to monitor the printing color difference, but there are certain errors, the characterization of the color is not accurate enough, and it is not conducive to data exchange with customers. , Therefore derived from the measurement of brightness. Chromaticity measurement can better reflect the psychological and physiological laws of color vision, provide more detailed color information, and play a very good role in promoting color management in printing.
1. Principle and application of density meter
Density meter is composed of light source, lens group, polarizer (optional), color filter, sensor and electronic system, display and other parts.
The density meter uses the built-in red, green, and blue optical filters to measure the light reflection or transmittance of yellow, magenta, and cyan colors, and calculates the density value. This principle is based on the principle of the three-color filter, which makes its structure Very simple and widely used, but due to the defects of the color filter itself, it also constitutes an insurmountable limitation of the density meter: it can only measure the density value of the printed matter, and does not include the expression of hue, so it can not truly reflect the actual color. Visual effect is a kind of "color blindness" measurement equipment.
In addition, density meter measurement has other limitations. For example, the application field is limited to the four-color printing process. Although it is often used to assist in monitoring the ink layer thickness, there is no direct relationship between density and ink layer thickness. Therefore, the purpose of the density meter is that the user can perform pre-press compensation and correction on the film or the printing plate according to the maximum / small density, dot expansion and printing contrast provided by it, and guide the production management personnel to correctly screen and determine the ink volume, Control parameters such as exposure and water-ink balance are not good at measuring and controlling the accuracy of color reproduction in color management.
Second, the principle and application of photoelectric colorimeter
The photoelectric colorimeter can be regarded as a reflectance meter, which has a set of special three filters, which is different from the red, green and blue filters of the densitometer. This set of filters is based on the CIE spectrum tristimulus value. Each channel of the meter weights the wavelengths of the spectrum. It mainly deals with reflectivity rather than logarithm.
Relative to density measurement, photoelectric colorimeters can specifically describe color information through tristimulus values, rather than just limited to brightness information. Because it still uses the principle of three color filters, the spectral range of sampling is limited, resulting in low accuracy. , Not suitable for color measurement and control in high-precision color management.
3. Principle and application of spectrophotometer
Spectrophotometry is to take the entire visible spectrum at equal intervals to measure the spectral reflection. Compared with photoelectric colorimeter, spectrophotometry can be regarded as a continuous measurement of the spectrum, which provides much more and richer color information. Much more.
There are three basic principles of spectrophotometer: rotating filter spectroscopy, scattering prism spectroscopy, and diffraction grating spectroscopy, as shown in Figure 3. The first method is to install 20 to 30 narrow-band color filters on the disc, and realize the beam splitting by rotating the disc. The latter two methods use the dispersion of light to decompose the composite radiation of the light source into monochromatic radiation of different wavelengths and arrange them in a certain order. The dispersive element used is a prism or a diffraction grating.
For example, the scanning spectrophotometer on CPC2 of Heidelberg is based on the principle of diffraction grating.
The wavelength interval of a commonly used spectrophotometer is 10 nm or 20 nm, and the recorded visible spectrum is divided into about 30 bands. In some high-precision systems, the measurement interval can also be smaller (up to 1nm). Then, under the prescribed illuminator and observation field of view, the light quantity of each wavelength is measured one by one by a photodetector, and then the chromaticity value of the product to be measured is calculated according to the reflection spectrum or the transmission spectrum.
The CIE coordinates of different illuminants and observation fields can also be calculated from the data measured by the spectrophotometer. Specifically, when converting from the spectral value to the CIE color tristimulus value, the illuminant is directly used as a parameter, so if you switch from one lighting condition to other illuminant conditions, the mathematical approximation calculation method must be used.
The spectrophotometer is closer to the human eye's visual response than the densitometer and the colorimeter, because it measures the amount of reflected light in the entire visible spectrum, but it is different from the human eye. The eye evaluates light based on the perception of all wavelengths at the same time, and the measurement of the reflection spectrum of the printed matter must be carried out wavelength by wavelength. This must decompose the spectrum at each wavelength before the spectrum enters the photoelectric receiver. Photometers all use splitting on the reflected light path, and they are not true splitting. They just measure the predetermined wavelength and perform an accumulation rather than an integration.
The color defined by the spectral data is more complete, the measurement accuracy is very high, and the spot color can be measured. The spectral data can also obtain the density value and the chromaticity value after calculation. It is suitable for the evaluation of spot colors, spectrum analysis and color evaluation in the process of color management, and the production of color characteristic files of equipment.
In theory, all inks, whether four-color inks or spot color inks, can be measured with a spectrophotometer. The system automatically compares the measured data with the target color value and displays the comparison result on the screen. If the density value is selected, the quality can be judged by traditional methods; if the Lab value is selected, the color deviation can be intuitively judged by the â–³ E value, and the amount of deviation will be displayed in a chart . The operator can determine which area's color is correct and which area's ink is larger or smaller based on the data on the chart. If the operator decides to adjust the color, the spectrophotometer will also be controlled online, and the recommended adjustment data can be sent to the ink zone setting with the push of a button.
If the density value cannot be used to represent a spot color, the same is true for four-color inks. This is because density measurement has nothing to do with the content of ink pigments, and the expression of hue is not included in the density value at all, which is a problem well known to all printing companies. If the type of ink is changed, although it has the same printing density, it does not necessarily ensure that the same print color can be obtained.
The spectrophotometer also has the function of a density meter, and density measurement is a technology that cannot be completely abandoned overnight. Although some printers have turned to new technology, they still use the old technology as a tool for judging quality, which is very suitable for the diversified needs of users.
In addition to controlling the amount of ink in the ink area during printing, the spectrophotometric method has other advantages. In the process of implementing color management, in order to ensure the accuracy and consistency of colors in the various links of input to output, we need to use the ICC Profile feature description file to perform color management for each link. The realization of the production of the profile description file provides the premise and guarantee: because the Profile only supports Lab or XYZ values, not density values.
4. Actual use
At present, the most popular on the market is X-Rite's 500 series spectrodensitometer, which covers density measurement, dot gain, overprint rate, printing contrast, gray balance value, color tone error, color comparison and other density functions and Chroma function, of which 530 can also be used with color quality control software; X-Rite's DTP41 automatic scanning spectrophotometer is also used by many users, with a high degree of automation, making ICCProfile feature description files very convenient and fast.
GretagMacbeth's SpectroEye portable spectrophotometer provides all the chromaticity measurement functions required to accurately measure and control spot colors, and has all the color density measurement functions for monitoring colors in the printing process. SpectroEye provides a wide range of color space and color difference formula arrangement, including CMC, FMCⅡ or ΔE * 94 color difference formula of CIELAB color system, to ensure perfect match with the standard procedure of instrumented color quality control, and even as an ink color matching system. One part is used for ink ratio.
In addition, EFI's ES-1000 spectrophotometer is very convenient to measure the display screen or printed matter, and can be supported by general color management software, and users can choose to measure according to a single color block or a single color bar according to need. Some models can also support offline battery-powered mode, in this mode EFI spectrophotometer can measure up to 512 color block readings, and then upload to the computer when convenient, suitable for offline operation.
V. Conclusion
In general, colorimeters are used relatively little now. Density meters and spectrophotometers have shown their own advantages. Although the former does not have a colorimeter function, it reflects the thickness of the ink layer and monitors the printing status throughout the printing process. Both can be used, and the price is relatively cheap; the spectrophotometer has both density and chromaticity functions, and high accuracy, which is the trend of color management for color reproduction in the future, but at the same time, the price is also high, and it has not yet been widely popularized.
However, no matter what kind of measuring equipment is used, correct equipment calibration and parameter setting are very important. Only in this way can the normal use of the equipment and the reference value of the measured value be guaranteed. The general measurement parameters that should be considered are standard light source, measurement aperture, field angle, response method, calculation formula and coefficient, etc .; for density measurement, the selection of three-color filters and polarized light filters must also be considered; It is best to use the same measuring instrument and the same color management software. For example, the two processes of digital proofing linearization and ICC profile use the same ES-1000 spectrophotometer, which can ensure that the color descriptions are comparable And thereby reduce the error.
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