Controlling and Detecting the Color of Printed Images in Disc Copying (I)

【Abstract】:Replication of CD-ROM: Mainly includes copying of VCD/CDROM/CD discs, DVD5/DVDROM disc duplication, the development of CD-ROM copying methods is increasing with the development of society, VCD/CDROM/CD disc duplication and DVD5/DVDROM disc duplication are mainly With CD-side printing, CD burning is the most basic method, and the other one is the most basic one, that is, the mass suppression of CDs. This is mainly suitable for the production and reproduction of large-volume optical discs.

In print copying, there is a considerable proportion of the number of color printed images. The color print image is formed by superimposing and printing the three primary colors of cyan, magenta, and yellow to give various colors, so that the color original image can be reproduced. In the copying, due to various processes and production factors, the color of the printed image does not restore the original color well. In order to obtain a satisfactory print image quality, the image color must be detected in the print production, and the color reproduction quality is controlled by adjusting some of the print variables.

First, the main factors affecting the quality of color reproduction <br><br><br><br><br><br><br><br><br><br><br> When copying on a printing press, there are many factors that affect the color of the copied image, such as: dot enlargement, printing color sequence and overprinting, ink hue and solid density, ink temperature and viscosity , Water supply (offset), paper properties, printing plate depth, printing pressure, etc.

1. Dot expansion

In printing, outlets will always expand, but when the amount of expansion exceeds a certain range, many quality problems will arise. Such dot enlargement will reduce the contrast of the image and make the whole image darker, dark dots will be crushed, and the copy hue will change drastically. When the image dots of various colors in printing are expanded at the same time, the image will become darker. When only one of the dots is enlarged, the copied image will have a color cast. For example, when the magenta version dot coverage expands in the middle tone, and 50% of dots change to 55%, the color of the printed image becomes reddish and the flesh color becomes uneven. Red, neutral color becomes light red, green is dirty. Both the printing pressure and the density of the ink in the field affect dot gain. Small changes in printing pressure can cause significant changes in the entire printed image; when the density of the ink in the field increases, the dot gain increases, which has a great influence on the tone in the image.

2. Printing color sequence and overprint

In the printing of color images, the inks are overprinted in one color, and the overprinting may cause color deviation, color mixing, and gradation disorder. Printing color sequence has a great influence on the color effect of overprinting. For multi-color printing presses, the printing interval time of each color ink is short, and the ink after printing is overprinted on the surface of printing wet ink, which belongs to the printing state of “wet and wet”. The ink printed on the paper surface in the overprinting color is superior to the ink printed on the surface of the wet ink layer. For two colors of ink, as long as the color sequence is reversed, the hue, brightness, and saturation of the overprint color may be different. If the cyan and magenta inks are overprinted, the magenta is printed first, then the magenta is overprinted; the magenta is printed after the magenta, and the overprinting is reddish.

In order to obtain a good overprinting effect, the viscosity of each color ink should be properly arranged after the color sequence is scheduled.

3. Ink hue and solid density

The inks used in printing production all have different degrees of color shift, resulting in color cast images, and color inks with less color shift should be used for color printing. The density of the solid ink density on the surface of the printed image determines the print image tone and tone reproduction range. The higher the density in the field, the wider the tone and tone reproduction range. The small density in the field reduces the color saturation of the image and the overprinting color weakens.

4. Ink temperature and viscosity

In printing image reproduction, ink viscosity is a very important parameter. In general gravure printing uses liquid ink, and in the inking apparatus, there is no ink distribution and ink licking mechanism. The ink viscosity is controlled by adding an appropriate amount of solvent to the ink tank. Offset and embossing generally use viscous ink, which is very viscous. In order to transfer the ink uniformly from the printing plate to the paper surface, both the offset printing and the embossing machine have a uniform ink and a re-inking device. The ink is squeezed, sheared, and separated during the transfer and smoothing of the rollers between these devices. The roller works to overcome the internal friction of the ink, and the temperature of the surface of the ink roller is increased, and the roller is subjected to extrusion, shearing, and separation during the process of being transferred and being evened. The roller works in order to overcome the internal friction of the ink, so that the surface temperature of the ink roller increases, and the viscosity of the ink that is transferred decreases. After the ink becomes thin, the amount of ink carried on the surface of the roller is reduced, and the amount of ink transferred to the surface of the paper is reduced, which changes the tone and tone of the printed image and destroys the consistency of the printed image. Some studies have shown that: After the printer is turned on and printed, the color deviation of the printed image is 60% due to the temperature change of the ink roller.

5. Offset ink balance

Offset printing ink production directly affects the quality of image reproduction. The small amount of water will make the plate dirty, paste version; water ambassador ink emulsification, so that the color saturation of the printed image is reduced.

Second, the detection of color printing image

The color of the color printed image is formed by overlapping the yellow, magenta, and blue primary inks in different proportions. Instead of measuring the color on the screen when measuring the color of a printed image, the quality control strip printed at the same time as the printed image is measured. The control strip is generally placed at the tip of the printed sheet. With the measuring instrument, the corresponding color patches of the control strips can be used to obtain the printing quality information, such as the solid density, overprinting rate, dot gain, dot density, neutral ash reduction, contrast and other parameters of each primary color ink to judge the image tone and tone reproduction.

There are three measurement methods for measuring the color of printed images, namely densitometer measurements, colorimeter measurements, and spectrophotometer measurements.

1. Densitometer measurement

Densitometer is the main instrument in color separation, plate making and printing. This measurement method has always been the most commonly used form of objective evaluation of quality in the printing industry. Densitometers are inexpensive and widely used. When the densitometer measures the color surface, only the relative amount of a certain color ink in the printing can be obtained. It cannot indicate the hue of the measured color. Densitometer measurements are not associated with a variety of color systems and therefore cannot be described in color language. The density meter has certain limitations in color measurement and evaluation. It is not a standard color measurement instrument.

2. Colorimeter Color Measurement

The colorimeter is to obtain the visual response proportional to the color tristimulus values ​​X, Y and Z by directly measuring the surface of the measured color. After conversion, the X, Y and Z values ​​of the measured color can be obtained, and these values ​​can also be obtained. Convert color parameters into other color spaces. The colorimeter is a special density meter with three broadband filters. Due to the fact that the instrument itself and the principle are thick in certain errors, the absolute accuracy of the color measurement value is not good. However, due to its low price, it is still a widely used color measuring instrument.

3. Spectrophotometer (Spectrophotometer) Color Measurement

Spectrophotometers measure the reflectance of light at each wavelength of the visible spectrum of the color surface. The light of the visible spectrum is irradiated with the color surface at a certain step distance (5 nm, 10 nm, 20 nm), and the reflectance is measured point by point. The spectrophotometric curve of the surface of the measured color can be obtained by plotting the reflectance value of each wavelength of light with each wavelength. Each spectrophotometric curve uniquely expresses a color. The measured value can also be converted to other color system values. Spectrophotometer is a flexible, ideal color measurement instrument. At present, color measuring instruments for color quality inspection of some printing machines equipped abroad are using spectrophotometers.