The principle and application of color measurement instrument based on color reproduction
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Color is the most important element of printing and reproduction. In the printing production process, how to ensure the true reproduction of color is a worldwide problem. At present, the use of digital color management to describe the color characteristics of each production link in the production process is the best way to achieve accurate color reproduction, and the 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 can reflect the information of the thickness of the ink layer, and guides the control of the dot enlargement. The data of the density measurement is used to control the printing quality, which is convenient and reliable, and has been trusted by people. However, the density value can not directly reflect the image color stimulation to the human eye. It can basically monitor the printing color difference, but there is a certain error, the color is not accurate enough, and it is not conducive to data exchange with customers. Therefore, derived brilliance measurements. Chromaticity measurement can better reflect the color vision psychology and physiological laws, provide more detailed color information, and play a very good role in promoting print color management.
First, the principle and application of the density meter
The density meter consists of a light source, a lens group, a polarizer (optional), a color filter, a sensor and an electronic system, a display, etc., and the measurement principle of the reflection densitometer (with a color filter) is as shown in FIG.
The densitometer uses built-in red, green, and blue optical filters to measure the light reflection or transmittance of the yellow, magenta, and cyan colors, and calculates the density value. This principle based on the three-color filter makes the structure It is very simple and widely used, but due to the defects of the color filter itself, it also constitutes a limitation that the density meter cannot overcome: it can only measure the density value of the printed matter, and does not include the expression of the hue, so it cannot truly reflect the actual color. The visual effect is visually a "color blind" measuring device.
In addition, densitometer measurements have other limitations. For example, the field of application is limited to the four-color printing process. Although it is often used to assist in monitoring the thickness of the ink layer, there is no direct relationship between density and ink layer thickness. Therefore, the purpose of the density meter is that the user can pre-compensate and correct the film or the printing plate according to the maximum/small density, dot gain and printing contrast provided by the user, and guide the production manager to correctly screen the net to determine the amount of ink. Control parameters such as exposure, ink balance, etc., and 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 thought of as a reflectometer with a special set of three color filters, which is different from the red, green and blue color filters of the densitometer. This set of color filters is based on the CIE spectral tristimulus value. Each channel of the meter weights each wavelength of the spectrum, which involves primarily reflectivity issues, not logarithmic problems. The principle of photoelectric colorimeter measurement is shown in Figure 2:
Compared to density measurement, photoelectric colorimeter can describe color information by tristimulus value instead of just brightness information. Because it still uses the principle of three color filters, the spectral range of sampling is limited, resulting in low precision. It is not suitable for color measurement and control in high-precision color management.
Third, the principle and application of spectrophotometer
Spectrophotometry measures the spectral reflectance of the entire visible spectrum at equal intervals. Compared with the photoelectric colorimeter, spectrophotometry can be regarded as continuous measurement of the spectrum. It provides much more color information and is richer. Much more.
There are basically three kinds of spectroscopic principles of the spectrophotometer: rotary color 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 to achieve the 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 elements used are prisms or diffraction gratings. For example, the scanning spectrophotometer on Heidelberg CPC2 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 segments. In some high precision systems, the measurement interval can also be smaller (to 1 nm). Then, under the prescribed illumination body and the observation field of view, the amount of light of each wavelength is measured one by one by a photodetector, and the chromaticity value of the product to be tested is calculated according to the reflection spectrum or the transmission spectrum.
The CIE coordinates of the different illuminants and the field of view can also be calculated from the data measured by spectrophotometer spectroscopy. Specifically, when converting from spectral values to CIE color tristimulus values, the illuminating body is directly used as a parameter, so if it is converted from one lighting condition to another, the mathematical approximation calculation method is used.
Spectrophotometers are closer to the human eye's visual response than densitometers and colorimeters because they measure the amount of reflected light across the visible spectrum, but are different from the human eye. The eye evaluates the light on the basis of sensing all the wavelengths at the same time, and the measurement of the reflection spectrum of the printed matter must be performed one by one. This requires the spectrum to be decomposed at each wavelength before the spectrum enters the photoreceptor. The photometer uses splitting on the reflected light path, and it is not true splitting. It only measures the predetermined wavelength for a summation rather than an integral summation, as shown in the following formula:
Spectral spectral data defines a more complete color, high measurement accuracy, and can measure spot colors. Spectral data can also be calculated to obtain density and chromatic values. Applicable to the evaluation of spot colors, spectral analysis and color evaluation in the color management process, and the production of equipment color characteristics files.
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 result of the comparison on the screen. If the density value is selected, the quality can be judged by the traditional method; if the Lab value is selected, the deviation of the color can be visually judged by the ΔE value, and the amount of the deviation will be displayed in a chart. . The operator can judge which area is the correct color based on the data on the chart, and which area has a large ink or a small ink. If the operator decides to adjust the color, the spectrophotometer will also be online controlled to send the recommended adjustment data to the ink zone settings with the push of a button.
If a spot color cannot be represented at all by the density value, the same is true for the four-color ink. This is because the density measurement has nothing to do with the content of the ink pigment. The density value does not contain the expression of the hue at all, which is a problem familiar to all printing companies. If the type of ink is changed, although there is the same printing density, it is not necessarily ensured that the same print color can be obtained.
Spectrophotometers also have the function of a densitometer, which is a technique that cannot be completely abandoned overnight. Although some printers have turned to new technologies, they still use old technology as a tool for judging quality, which is suitable for the diverse needs of users.
In addition to controlling the amount of ink in the ink zone during printing, spectrophotometry has other advantages. In the process of color management implementation, in order to ensure the accuracy and consistency of color in all aspects of input and output, we need to use ICC Profile to describe the color management of each link. The measurement method of spectrophotometry is The implementation of the profile characterization file provides the premise and guarantee: because the profile only supports Lab or XYZ values, and does not support density values.
Fourth, the actual use
At present, the most popular in the market is X-Rite's 500 series spectrodensitometer, which covers density measurement, dot gain, overprint ratio, print contrast, gray balance value, hue error, color comparison and other density functions. Chroma function, 530 can also be used with color quality control software; X-Rite DTP41 automatic scanning spectrophotometer is also used by many users, high degree of automation, making ICC Profile feature files very convenient and fast.
GretagMacbeth's SpectroEye portable spectrophotometer provides all the colorimetric measurements required to accurately measure and control spot colors, as well as all color density measurements for monitoring color during printing. SpectroEye offers a wide range of color space and color difference formulas, including the CIELAB color system's CMC, FMCII or ΔE*94 color difference formulas to ensure perfect alignment with the standard procedures for instrumental color quality control, even as an ink color matching system. Part of it is used to carry out the ink ratio.
In addition, EFI's ES-1000 spectrophotometer is easy to measure whether it is a monitor screen or a printed matter. General color management software can support it, and users can choose to measure according to a single color block or a single color bar. Some models can also support the offline battery power mode. In this mode, the EFI spectrophotometer can measure up to 512 color block readings, and upload them to the computer when convenient, suitable for offline operation.
V. Conclusion
In general, colorimeter is used relatively now. Densitometers and spectrophotometers have each shown their own advantages. Although the former has no chroma 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 chromatic functions, and high precision, which is the trend of color reproduction color management in the future, but at the same time the price is also high, and it has not been widely popularized at present.
However, no matter which measuring device is used, it is very important to correctly correct the device and set the parameters. Only in this way can the normal use of the device and the reference value of the measured value be guaranteed. Generally, the measurement parameters that should be considered include standard light source, measurement aperture, field of view angle, response mode, calculation formula and coefficient; for density measurement, consider the selection of three-color filter and polarized filter; color management process It is best to use the same measuring instrument and the same color management software, such as digital proofing linearization and ICC Profile. Both processes use the same ES-1000 spectrophotometer to ensure that the color description is comparable. And thereby narrowing the error.