Spectrum measurement and chromaticity adjustment o

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Spectral measurement and chromaticity adjustment of offset printing

it is no longer incredible to use spectral measurement and chromaticity adjustment in offset printing. This method is not only suitable for four-color printing, but also has great advantages for spot color printing. In principle, the traditional densimeter is a kind of "color blind" measuring instrument, which can only measure the density on the print. It is possible to measure cyan, magenta and yellow inks by using red, green and blue optical filters. Theoretically, one limitation of density measurement is that the optical color filter must be accurately adjusted according to the printed color, which is no problem in standard four-color printing, but completely impossible in spot color printing

limitations of density measurement:

① what human eyes see is chromaticity measurement

② density can't explain the hue at all

③ the polarization filter will distort the measurement results

④ the scope of application is limited to the printing process

⑤ there is no direct relationship between density and ink layer thickness

in 1990, Heidelberg company proposed spectral measurement, but this method once aroused doubts of some people in the printing industry. The problem is that in the papers on spectral measurement and chromaticity adjustment, there is often a lack of reference values for adjusting the printer. It is not easy for printers to see how spectral chromaticity measurement can deduce the recommended standards for adjusting the ink bucket keys of the printer from these data

Heidelberg company and a famous manufacturer of chromaticity measuring instrument unified the line, gave up the measurement of color filter, moved towards spectral measurement, and displayed the calculated density value on the display. The advantage of this method is that the calculated density value is very accurate, which is exactly the same as that of the automatic measuring instrument used in the printing machine. In addition to obtaining the CIE LAB value of chromaticity measurement, the printer can also obtain the density value display calculated by the color spectrum

spectrum measurement and chromaticity adjustment work

in principle, all printing colors, whether standard four colors or spot colors, can be measured by spectrum. The measured values can be automatically compared with the rated values given by printing, and the printer can see the results on the display screen. If the density value is taken as the given value, the measured value and the rated value can be compared as before; If lab is taken as the given value, the printer can clearly see the deviation of de (delta-e) value. In addition, the printer can also obtain the percentage deviation of the rated ink supply on the graph. At a glance, you can see which ink bucket keys have normal ink supply and which ones have insufficient or excessive ink supply. If the ink color needs to be tracked, the measuring instrument can transmit the recommended value for adjusting the position of the ink bucket key, which can be adjusted through the button of the printer. After a short time of adaptation, many printers feel that the de deviation value is more convenient than the density display through comparison

prinectimage control measurement method: irradiate the print with white light, use a spectrophotometer to decompose the radiated light into spectral color, and use computer software to analyze the excellence measurement results. The value obtained from it is displayed on the display screen and continues to be transmitted to the printer as a standard for tracking ink color. As shown in Figure 1

Figure 1 Schematic diagram of working mode of primectimage control

density cannot express hue

it is impossible to give a fixed density value in advance when printing spot color, and other measures need to be taken when encountering spot color. Because the density measurement will ignore the pigment of the ink, the density cannot express the tone. Human eyes can compare color vision, but not density. If the ink type is changed in the printing process, it cannot be guaranteed that the same printing density can produce the same color vision. However, some printers who are used to density measurement do not accept it, which is also a topic often quoted as "standard four-color printing"

because the spectral measuring instrument can complete the task of the densitometer, Heidelberg decided long ago to retain the density display while using spectral measurement and chromaticity adjustment. Because some printers have always been accustomed to the connection between electromechanical and oil pump by adopting elastic coupling, they cannot ignore the density value, but the density value will lose its significance as time goes by. The Prinect measuring instrument in Heidelberg takes this into account

but one deficiency of chromaticity adjustment is the lack of polarization filter, and the polarization filter in the densitometer optical path can balance the gloss effect produced by wet printing. People are used to comparing the dry print with the wet print, hoping to get the same result. Because the customers of the printing factory only pay attention to the effect after the printing sheet is dried, people often ignore the change of the theme tone of the picture after the printing sheet is dried. Although the density measured by polarizer may not change much, it has an effect on color vision. At this time, the reference value can be measured by using the old print. But printers always use wet sheets to evaluate the ink color, and then adjust the amount of ink. The polarizer balanced gloss effect is only a numerical value, and the actual color difference between dry and wet still exists, although it may not be reliable

other advantages of spectral colorimetry measurement

apart from adjusting the ink bucket key during printing, the spectrophotometric measurement performance of the measuring instrument can also be used in other aspects

when color data appears in the form of more and more media neutral (GB), it is quite important for printing houses to respond accordingly. Producers and media designers want to see the proofing effect that restricts the color as much as possible before offset printing. The solution is that ICC gold free spraying team adopts special effect powder and unique powder surface treatment process color documents. If you want to generate such a file, you cannot avoid the spectral measurement method, but the file only recognizes the lab value, not the density value

image control measuring instrument builds a bridge from printing to prepress. The measuring instrument can be equipped with a color interface module, such as measuring 70cm × The test version of it8/7.3 with 928 measuring color blocks on a 100cm printing sheet can be completed in only 25 seconds, and the corresponding test version is attached. After the measured values are stored in the file, a program is used to make ICC Color files for the larger the surface roughness, and continue to process them, providing a full set of solutions together with the molding software print open

achieve personalized matching

suppliers use print color management services to support customers in the actual transformation. The printing and prepress engineers in Heidelberg are responsible for the correct adjustment of the color workflow, including the establishment and implementation of ICC Color documents for digital proofing. Before using the display to evaluate the images subject to color constraints, the display must be calibrated first. The same is true of the workflow applied in printing plants. Therefore, Heidelberg provides customers with personalized matching with print C, namely, Vickers hardness value (HV) olor management software

in short, spectral measurement and chromaticity adjustment are neither magic nor scientific secrets. To be exact, it is a method of great significance. Only by spectral measurement and chromaticity evaluation can the color be consistent with international standards, and the printers can meet the market demand in the future. For the contents of spectral measurement and chromaticity adjustment, please refer to the offset process standard manual and ISO international standard of the German printing and Media Association

source: keyin media - "printing technology"

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