Print is a visually judged product and artwork. Due to factors such as printing materials and process technology, the color of the printed image is often not reproduced well. This involves quality detection and control problems in the printing process.

Currently, by measuring the print quality control strips printed simultaneously with the printed images, real-time print quality information can be obtained, such as solid density, overprint rate, dot gain, dot density, neutral ash reduction, contrast, etc., of each primary color ink. The tone and tone reproduction of the image. Based on the introduction of density measurement and colorimetric measurement, this article focuses on the functions of the widely used Bruner second-generation test strip and the trend of quality inspection and control in the printing industry.

1 Density measurement

Densitometers are available in many types including transmission densitometers, reflection densitometers, transmissive/reflective dual density meters, black and white densitometers, color densitometers, densitometers with polarization filter devices, and densities without polarization filter devices. Meters and so on.

Densitometer itself has its unique advantages and is an important instrument in color separation, plate making and printing. Density measurement has always been the most commonly used method for objectively evaluating quality in the printing industry. Density values ​​can be used to easily indicate the amount of ink. The basic principle of densitometer measurement is that within a certain range, the ink layer thickness is proportional to the density value.

The important thing in the printing image reproduction is color matching, not the matching of the ink quantity, and the hue matching is more important than the matching of the color saturation. This is the theoretical basis for the transition from density measurement and control to colorimetric measurement and control of print quality control.

2 color measurement

In the printing industry, the use of new types of colorimeters and spectrophotometers has made printer operators aware of the importance of colorimetric measurements because they are closely related to the spectral sensitivity of the human eye and provide CIE color system parameters. .

There are two main methods of colorimetry, one is photoelectric colorimeter, and the other is spectrophotometer colorimetry.

1) Photoelectric colorimeter. In principle, an opto-electronic colorimeter is very similar to a densitometer. It can be considered that a photoelectric colorimeter is a special density meter with three broadband filters, and its appearance, operation method, and price are similar to that of a density meter. The photoelectric colorimeter directly displays the tristimulus value. It can also obtain the visual response proportional to the color tristimulus value by directly measuring the surface of the measured color. After conversion, the X, Y, and Z values ​​of the measured color can also be obtained. These values ​​are converted into color parameters of other color space, for example converted to CIELAB scale.

2) Spectrophotometer. Spectrophotometers are the most flexible colorimetric instruments that measure color as a physical phenomenon that is not subject to the observer. Spectrophotometers measure the spectrum continuously, while the opto-electronic colorimeter measures only three points, so the spectrophotometer can provide more information (at least 16 points).

At present, the color measuring instruments used for color quality inspection of some foreign printers are mostly spectrophotometers.

3 Bruner second generation test strip

In practice, we have found that the quality of printed matter can only be controlled based on production experience and subjective judgments, and that it cannot meet the requirements for precise copying. Therefore, the printing control bar has emerged.
The printing control strip consists of solid blocks, different mesh points, and signal elements for visual inspection. It is mainly used to measure color separation, plate exposure, color reproduction, proofing, and printing effects. There are many types of measurement and control strips used in the printing industry today, such as Bruner, FOGRA, and GATF. Their performance is basically the same.

At present, the Bruner test strip system is widely used. So far, the Brunel control strip has been developed to the third generation and its functions are basically similar. In the following, Bruner's second-generation test strip is taken as an example to introduce its functional characteristics.

Bruner's second-generation test strip has a width of 6×190mm. Based on the first-generation three-segment measurement and control strip, a 75% thick and thin network segment is added to form a five-segment structure (as shown in Figure 1). , namely, on-site, 50% coarse network, 50% fine network, 75% coarse network, and 75% fine network.

According to Bruner's study, the difference between the density values ​​of the fine mesh and the coarse mesh is the dot gain value when measured at the 50% dot point; if the thickness is measured at the 75% dot, the difference between the coarse and fine mesh density is divided by 2 Is the dot expansion value. In the printing process, the most sensitive part of dot gain is 75% dots, and the largest dot gain is 50% dots. Therefore, the dot gain value is usually measured using 75% dot segments and 50% dot segments.

In the actual production process, the Bruner second generation test strip and the screen fine mesh control section, the neutral gray balance section, the overprint and the color mark detection section, and the black density three-color reduction section are combined to form a multi-function. Test strip (as shown in Figure 2). (to be continued)