Discussion on Three Kinds of Evaluation Methods of Printing Quality

The evaluation methods of print quality include subjective visual inspection, density inspection and colorimetric detection.

Subjective visual inspection is a completely subjective and at the same time the simplest method. Directly conduct manual comparisons between the printed product and the standard proofs to determine the quality of the printed sheets. However, it is influenced by the experience, psychology and other factors of the observers. There are too many variable factors and it cannot be described quantitatively, 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 instruments used in this method are inexpensive and widely used. However, when it measures the color surface, it can only obtain the relative amount of a certain primary color ink in the printing, but cannot indicate the hue of the measured color, cannot be associated with the color system, and describes the color, and its use has certain limitations.

The colorimetric detection method first determines the spectral reflectance within the scope of the adjustment, and then according to the spectral reflectance or some chromaticity characteristic parameters deduced therefrom, the adjustment of the amount of ink is achieved, and the measurement result is directly related to the visual perception of the human eye. More intuitive to show the color of the print. The commonly used practice now is to extract some printed sheets by the press operator or quality inspector. The printed sheets are compared with the proofs by the visual comparison method. The quality control strips printed at the same time as the printed images are used to evaluate the quality of printed sheets. These control strips are generally placed on the tip of the print sheet. Although parameters such as solid density, overprint rate, dot gain, contrast, and the like of each color ink can be obtained, the overall print condition of the print screen cannot be completely displayed. Their judgment on the quality of the printed matter is subjective, and the customer's requirements are often compensated too much, which is not conducive to the quality control of the printed matter.

Recently, the on-line inspection of printing screens can detect the overall printing situation of printed sheets, but it is limited by computer image processing speed and printing format, and is now mainly used for small-format sheet printing. Based on this, using a spectrophotometer to measure the color of the printed sheet, the color of the detection point set on the printed sheet is obtained, which is more feasible and effective as a basis for printing control.

Most of the printing industry uses CIE1976L*a*b* uniform color space and its corresponding color difference formula. The advantage of this color space is that L*, a*, b* are consistent with the human eye's vision, and when the color difference of the color is greater than the visual recognition threshold and less than the color difference between adjacent two stages of the Munsell system, it can be better. Reflect the psychological effect of the color of the object. The use of color difference to evaluate the similarity of prints and the visual characteristics of the human eye is consistent, and good evaluation results can be obtained. Chromatic aberration can comprehensively reflect the color conditions and can effectively control the color quality of printed products. In general, >5 can clearly notice the difference in color between the test and standard proofs. (L, a, b are data of standard proofs, L, a, b are data of test proofs)