0 Introduction
The quality inspection technology of printed products accompanies the development of printing requirements, from the initial subjective visual evaluation to the objective evaluation based on a certain theory, from off-line detection to the current on-line detection, and the continuous development of relevant theoretical knowledge and technology in the development process. Progressive results to promote their own development.
1 Subjective visual inspection
It is a completely subjective evaluation method and it is also the simplest method. It compares prints with standard proofs manually, evaluates differences in prints and standard proofs, and uses magnifiers to finely observe the shape and overprinting status of each color dot, and to make a qualitative assessment of the adjustment of dots. Its essence is a kind of visual photometric method. The principle is to use additive color mixing law to add the unknown color of three components together to describe the unknown color. Although the most reliable way for color evaluation is to use the human eye, and it is simple and flexible, due to the experience of observers and the influence of psychological and physiological factors, this method has too many variable factors and cannot be quantitatively described. To assess the accuracy and reliability.
2 Density testing
A beam of light is projected onto an object, and part of the light will be reflected or transmitted. The remaining light will be absorbed by the surface of the object. The density detection method compares the light intensity of the surface reflection (or transmission) with the intensity of the light irradiated on the surface, and then calculates the density value by the logical relationship recognized by everyone. The essence of the density measurement is therefore a measure of the amount of reflected or transmitted light. In color printing, the printing ink coloring is actually that the ink is printed on a white paper with a high reflectance, and selectively absorbs a portion of the wavelength of light from the light irradiated thereon, while reflecting the remaining light. Density reflects the absorption of light waves by the ink. The "color density" as used conventionally means that the density of the yellow, magenta, and cyan inks is measured by the red, green, and blue color filters respectively. As "density", it is only a measure of the physical absorption characteristics and only indicates the degree of "black" or "gray". In this sense, "color density" measurement is only a measure of "blackness" and is a reflection of the relative value of the same ink saturation.
Densitometers used in density measurement are transmission and reflection. The transmission densitometer measures the amount of light or transmission through the film; the reflection densitometer measures the amount of light or reflectance reflected from the test surface, and its basic operating principle is shown in FIG. Since the intensity of the reflected light from the wet-to-dry process of the ink film on the print is not the same, there is a certain error in the measured density, and the densitometer with a polarizing filter can overcome the density change caused by the wet and dry of the ink film. . As early as 1977, X-Rite introduced the world's first fixed filter reflectance densitometer that can automatically read the density of various colors within one measurement. Today, color reflection densitometers have become an indispensable tool in the printing shop. It intuitively reflects the density of four-color prints of C, M, Y, and K, dot percentages, and ink overprint rates, and is widely used in color and ink layers. Thickness control.
3 Chromaticity detection method
In the past, density measurement has been the most commonly used form of measurement in the printing industry due to hardware constraints. With the gradual promotion of new measuring instruments, people have begun to turn their attention to color measurement technology. Colorimetry has been applied to the printing process control.
Colorimetry was founded in the 1930s. It is the science of researching people's color vision laws, color measurement principles, color measurement instruments and their applications. It has become the basis for the industry to describe colors, measure colors, and use colors. Instruments based on colorimetric measurement principles are specially designed for color measurement, so they can accurately define and describe colors. These measuring instruments are divided into two types: colorimeters and spectrophotometers, and their working principles are different.
The spectrophotometer measures the color as a physical phenomenon that is not subject to the observer. It is generally composed of a dispersion system that provides monochromatic light from an illumination source and a detector system that uses the instrument's optical radiation for measurement. It uses spectral spectroscopy devices (such as spectroscopic prisms, interference filters, etc.) to split the light emitted by the light source into wavelength-distributed equal-wavelength spacing (eg, å•è‰²Î» = 20 nm çš„ of a monochromatic aperture that measures the entire visible reflection spectrum of an object. It is The point-by-point measurement in the visible spectral region, ie, measurement at some discrete points, is usually performed at intervals of 10 or 20 nm, measuring 16 to 31 points in the range of 400 to 700 nm, and then using this wavelength spacing as Δλ. Monochromatic light is used to illuminate the test color and the standard white.The ratio of the monochromatic reflection light density of the test color to the monochromatic reflection light density of the standard white is the "spectral reflection factor" βλ 。. The colorimeter is considered to be a kind of A special density meter with three broadband filters. The tristimulus value it obtains is accomplished by the instrument's internal optical analog integration, which is to use a color filter to correct the spectral characteristics of the instrument's light source and detector elements, making the output The size of the electrical signal is proportional to the tristimulus value of the color, and the standard color value is obtained by "matching" in the calculation. Because of the device itself and the principle There is a certain error, so that the absolute accuracy of the color measurement value is not ideal.
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