The evaluation methods of print quality include subjective visual method, density detection method and chromaticity detection method.
The subjective visual method is a completely subjective, but also the simplest method. Directly compare the printed matter with the standard sample sheet to make a judgment on the quality of the printed sheet. However, it is greatly influenced by the observer's experience, psychology and other factors. There are too many variable factors that cannot be quantitatively described, which affects the accuracy and reliability of the evaluation.
Density measurement is actually a measure of the amount of reflected or transmitted light, and is a reflection of the relative value of the same ink saturation. The instrument used in this method is cheap and widely used. However, when measuring the color surface, it can only obtain the relative amount of a certain primary color ink in printing, but cannot indicate the hue of the color being measured, and cannot contact the color metering system to describe the color. Its use has certain restrictions.
The chromaticity detection method first determines the spectral reflectance within the debugging range, and then realizes the adjustment and control of the amount of ink according to the spectral reflectance or some chromaticity characteristic parameters derived therefrom. The measurement result is directly related to the visual perception of the human eye. More intuitively shows the color of printing. At present, the commonly used method is that the printing press operator or quality inspector extracts some printed sheets, compares these printed sheets with the printed samples by visual comparison, and measures the quality control strip printed at the same time as the printed image to make an evaluation of the printed sheet quality. These control bars are generally placed on the trailing part of the printed sheet. Although parameters such as solid density, overprint rate, dot expansion, and contrast of various colors of ink can be obtained, they cannot completely display the overall printing situation of the printing screen. Their judgment on the printing quality is subjective, and the customer's requirements are often over compensated, which is not conducive to the control and management of the printing quality.
Although the online printing screen detection recently developed can detect the overall printing of printed sheets, it is limited by the speed of computer image processing and printing format, and is currently mainly used for small-format bill printing. Based on this, use a spectrophotometer to measure the chromaticity of the printed sheet to obtain the color development of the detection point set on the printed sheet, which is more feasible and effective as the basis for printing control.
Most of the printing industry uses the CIE1976L * a * b * uniform color space and its corresponding color difference formula. The advantages of this color space are: L *, a *, b * are consistent with the human eye's vision. When the color difference of the color is greater than the visual recognition threshold and smaller than the adjacent two-level color difference in the Munsell system, it can be better The psychological feeling effect of reflecting the object color. The use of chromatic aberration to evaluate the similarity of printed matter is consistent with the visual characteristics of the human eye, and a good evaluation effect can be obtained. The color difference can comprehensively reflect the color condition, and can effectively control the color quality condition of the printed product. Generally,> 5 can clearly detect the color difference between the test sample and the standard sample. (L, a, b are the data of standard sample, L, a, b are the data of test sample)
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