Introduction to Prepress Knowledge of Flexo Printing Process (I)

Understanding (electronic) pre-press for flexo-An introduction for the pre-press novice (Part 6) Color is a visual effect produced by the human eye's stimulation of light. The formation of color depends on the interaction of the three elements. Role - light, color body and human eyes. From a strictly physics point of view, "color" (not actual, specific ink) is light of different wavelengths. The human eye and the brain respond differently to the stimulating effect of different wavelengths of light, and thus get different colors in the human brain. Since the human feeling is actually not measurable, the "color phenomenon" is also unmeasurable. However, light stimulates the human brain and produces a corresponding sense of color. This physical process is measurable. First, the color system and color measurement For the printed color, it is usually measured by means of a reflection densitometer. However, these reflection densitometers capture only a small part of the spectrum of color, that is, because of the use of color filters in the densitometer, only certain colors in a specific range can be truly read by the instrument. Take and measure, the rest of the colors are excluded to varying degrees. It is like seeing the world through a narrow slit on the wall. Within the window formed by the slits, the human eye can clearly observe the outside landscape, and the parts beyond it are invisible. If this narrow gap is facing the sky, then it is very accurate to determine the weather conditions within the field of vision, but the overall weather conditions may be very different from this. Reflective densitometers used in printing are similar. There are three "observed slits" in the reflectance densitometer, corresponding to the yellow, magenta, and cyan after printing, and black does not require a color filter and is always displayed. Each color filter has an opening as an "observation hole" that has been adjusted according to the European color standards of the three primary colors Y, M, and C. The "observation hole" can be slightly wider or narrower. The narrow observation hole can accurately describe the observed color, while the slightly wider observation hole makes the observation scope wider and more open. This will have an effect on accuracy, but because the densitometer aggregates the total measurement results into an "average", it is generally called "optical density." Therefore, in the densitometer, it is better to use a narrower "observation hole" filter, this filter is called "narrowband filter." Second, the disadvantages of the density meter Although the narrow-band filter color measurement of three primary colors is very effective, but it still has the drawbacks that can not be overcome. In fact, this kind of narrow-band color filter only observes the very narrow three segments in the whole spectrum, but can't accurately describe the characteristics of many colors other than the three primary colors. However, we know that in the packaging and printing, these non-primary colors account for a large part, first of all, a variety of spot colors are used in the printing; secondly, because the so-called "three primary colors" are generally associated with European standards used in offset printing, Certain deviations (especially in flexo printing), and densitometers are usually calibrated according to European standards. In addition, halftone images are often printed in spot colors during the copying process. Just as meteorologists have to be able to make a very accurate assessment of the overall condition of the weather after observing the entire sky, packaging printers also need to observe the entire spectral range to be able to accurately describe the characteristics of a particular color. The only thing the density meter can accurately measure is the three primary colors that comply with European standards; but even so, because there are certain differences between different types of density meters, it may also have a certain impact on the results. Spectrophotometers As already mentioned, from a strictly physics perspective, colors consist of light of different wavelengths. More precisely, colors are formed by a combination of different wavelengths. Each chromophore reflects light of different wavelengths of incident light (no light without a light source), and may be totally absorbed or partially absorbed. Color is formed by the "visible light" part of the human eye that can stimulate the human brain. Therefore, in fact, the color (including the printing of three primary colors, of course) is the effect of mixing light of different wavelengths, which makes people once again doubt the efficacy of the density meter. Only the spectrophotometer can accurately measure the characteristics of all colors, which is the only reliable means and approach. The principle used by this spectrophotometer is different from the principle of observing the world through the slits on the wall. It is observed through a window with shutters, although the blinds also divide the window into slits. However, all the slits can be combined to make all the scenery visible. When measuring with a spectrophotometer, the entire spectrum of visible light is divided into very small segments, scanned one by one, and then the results of each step are summed to obtain a numerical value, and the characteristics of the color are thus formed. In the future, the evaluation of color characteristics must be completed with the help of a spectrophotometer. In fact, the use of spectrophotometers is not very common at present, because there are certain difficulties in understanding and using them, and the price of spectrophotometers is higher than that of densitometers. At present, the price-performance spectrophotometer is available on the market. The processing of measurement data has been gradually replaced and completed by the corresponding software program. Fourth, the color model of the human eye (in healthy people under normal visual conditions) there are three different types of receptors to capture light, the "color sensitivity" of these three types of receptors is different, and each type of receptor is only for themselves Corresponding nerve signals produce stimuli. Therefore, under the joint action of the three neural signals, a certain sense of color is formed in the human brain. At least three "coordinates" are needed in all models that describe colors. The color represented by these coordinates is determined by the basic system used. Scientific color models, such as CIELuv or CIELab, are based on certain assumptions: it is assumed that each individual color is based on a red-green vision, a yellow-blue vision, and a brightness. Consisting of. The RGB color model is applied to the light color range and describes the color characteristics based on the amount of red, green, and blue in the color. The color model CMY, which is mainly used in printing, describes the characteristics of the color with the contents of cyan, magenta, and yellow. In printing, we often describe colors in the CMY color system with symbols such as C50%, M30%, and Y10%. However, this is only a special case and is usually expressed in terms of CMYK, where the letter “K” stands for black ink. We know that in the case of color printing, if only yellow, magenta, and cyan are used for printing, a certain color cast will appear in the dark portion, so it is necessary to add a certain amount of black ink.