The Principle of Color Measuring Instrument Based on Color Reproduction and Its Application

Color is the most critical element of print reproduction. How to ensure the true reproduction of color is a worldwide problem in the printing production process. At present, the use of digital color management, the description of the color characteristics of production processes in the production process, is the best way to achieve accurate color reproduction, and its premise and foundation 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 ink layer thickness, and guide the control of the dot expansion. The density measurement data is used to control the printing quality, which is convenient and reliable, and has already been trusted by people. However, the density value cannot directly reflect the stimulation of the human eye with the color of the image. It can basically monitor the color difference in the process of monitoring printing, but there is a certain error. The color representation is not accurate enough, and it is not conducive to data exchange with customers. , so derived from the chromaticity measurement. Chromaticity measurement can better reflect the color vision psychology and physiological law, provide more detailed color information, and play a very good role in promoting the printing color management.

First, the principle and application of density meter

The densitometer consists of a light source, a lens group, a polarizer (optional), a color filter, a sensor, an electronic system, and a display. The measurement principle of the reflection densitometer (with color filter) is shown in Figure 1:

Densitometers use built-in red, green, and blue optical filters to measure the light reflection or transmittance of yellow, magenta, and cyan colors, and calculate density values. This principle based on three-color filters makes its structure It is very simple and widely used, but due to the defect of the color filter itself, it also constitutes a limit that the density meter can't exceed: It can only measure the density value of the print, but does not include the expression of the hue, and thus cannot truly reflect the actual color The visual effect is, visually speaking, a "color blind" measurement device.

In addition, densitometer measurements have Other limitations, such as: the application of the field is limited to four-color printing process, although often used to help monitor the ink layer thickness, but there is no direct relationship between density and ink layer thickness. Therefore, the purpose of the densitometer lies in that the user can perform pre-print compensation and correction on the film or printing plate according to the maximum/minimum density, dot gain and print contrast provided by the user, so as to guide the production manager in correct screening and determine the ink volume. The control parameters such as exposure amount and 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

Photoelectric colorimeter can be seen as a reflectometer, it has a special set of three filter, which is different from the red, green and blue filters of the densitometer, this set of filters according to the CIE spectrum tristimulus value in color The wavelength of each wavelength of the spectrum is weighted in each channel of the meter. It deals with the reflectivity problem rather than the logarithmic problem. Photoelectric colorimeter measurement principle shown in Figure 2:

Compared to the density measurement, the photoelectric colorimeter can specifically describe the color information through the tristimulus values ​​instead of being limited to the luminance information. Since the principle of the three-color filter is still used, the spectral range of the sampling is limited, and thus the accuracy is not high. It is not suitable for high-precision color measurement and control in color management.

Third, spectrophotometer principle and application

Spectrophotometry is the measurement of spectral reflectance across the entire visible spectrum at evenly spaced points. Compared to a photoelectric colorimeter, spectrophotometry can be viewed as a continuous measurement of the spectrum. It provides much more color information and is richer. Much more.

There are basically three principles of spectrophotometer spectrophotometry: rotating filter spectrophotometry, scattering prism spectroscopy, diffraction grating spectroscopy, as shown in Figure 3. The first method is to install 20 to 30 narrowband filters on the disc, and split the light by rotating the disc. The latter two methods utilize 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 in Heidelberg is based on the principle of diffraction gratings.

The wavelength separation of commonly used spectrophotometers is 10 nm or 20 nm, and the recorded visible spectrum is divided into approximately 30 segments. In some high-precision systems, the measurement interval can also be smaller (to 1 nm). Then, under the specified illumination body and observation field of view, the light quantity of each wavelength is measured one by one by the photodetector, and then the color value of the product to be tested is calculated according to the reflection spectrum or transmission spectrum.