Influence of Zimo on color reproduction
What is Zimo? Zimo is almost just a failure of coated paper and cardboard. It usually occurs on four-color sheetfed offset and web offset presses, as well as on flexographic printing presses. Many complex physical and chemical causes The occurrence of the ink phenomenon is due to the difference in the absorption of ink by the substrate, the problem of ink fixation and drying (reverse overprinting) or the difference in the absorption of fountain solution by the substrate.
Due to the uneven transmission or reflection of light from the printed surface or through the translucent sample, the brightness of this part and the other part of the printed surface changes. When there is such a change, the eye can easily perceive the nourishment. Ink phenomenon. The frequency of these changes in space may be different, and the change between one brightness and the other determines the degree of nourishment. Without the aid of any equipment, you can perceive the changes between the 12mm part and another part with your eyes.
In recent years, there have been a variety of instruments used to quantify ink: image analyzers based on desktop scanning technology or CCD technology, and even density meters. But for most of them, the user must determine an arbitrary threshold with a pixel value of R degrees. This threshold determines what is the part of interest in the object. As a result, most of the measurement results of the nourishing ink are not reproducible between two different operators, and only a few instruments can confirm that there is a clear correlation between the evaluation of the printing quality with the eye and the nourishing ink measured by the instrument Sex. Another limitation usually encountered is that only a few instruments can adjust parameters to measure visible and less visible ink.
1. Several theoretical points of view of measuring ink
The usual practice is to classify the mousse as a general symptom of unevenness. According to this, the technique for quantifying non-uniformity can be easily applied. The unevenness of the printed matter is a measure of the change in the image points of the ROI (region of interest) in an image or within the range of an image. It is within a preset pixel brightness value (LV) range, usually 0 to 255, where 0 is the darkest (no reflected light) and 255 is the brightest (fully reflected).
The non-uniformity is then calculated as the arithmetic difference of the average image point LV, and this arithmetic difference belongs to those intermediate LV values ​​that are higher and lower than all values ​​in the image. In terms of frequency distribution, the median value determines the LV in the case where 1/2 pixel LVS is higher than this value and 1/2 pixel LV is lower than this value.
Therefore, LV: pixel is the brightness value, pixel reflection value, pixel brightness, hue;
NU: unevenness;
Lx: the average value of LV ≤ middle LV;
Ux: the average value of LV> intermediate LV;
Then NU = Ux-Lx,
With the help of an image analyzer, the area of ​​the inspection site is determined by the required image resolution and sensitivity. In general, for inks that are perceived within the visible range, the area of ​​the inspection site is between 1 mm2 and 3 mm2, while within the less visible range is between 100 μm2 and 1 mm2. The inspection part moves on the image, called the non-moving window (MW). The MV moves in increments of its height and width over the entire ROI, so no part is ignored. At each location, the standard deviation (SD) and average value (A) are recorded in memory to generate vectors containing all the individual MV measurements.
When checking the entire image by MV and extracting two vectors, the standard deviation (SD) of the image brightness value in the range of MWs (Wsd) and the average value (A) of the brightness value in the range of MWs (Wa) are used to calculate
SDwsd = Wsd'S standard deviation
Awsd = average of Wsd'S
Sdwa = Wa'S standard deviation
Theoretical views on color change measurement
Color perception is a basic aspect of vision. The perception of changes in color space is very different from our perception of changes in brightness. In order to simplify the quantitative color change, a spherical spectrophotometer with a 9mm aperture was used for this experiment. This aperture coincides with the perceived frequency cutoff of the color change.
Measure each ROI with a 3 × 3mm grid, and measure △ a * and △ b * at each point.
Quantity â–³ Z = Calculate the square root [(â–³ a *) 2 + (â–³ b *) 2]. â–³ Z quantifies color change
2. Experimental method
In order to determine the final correlation between the ink index and the color change, a large number of experiments were conducted. Make prints on a web offset press (Heidelbergm-110). This printing machine is equipped with CTP Thermal830 Kodak Poly-chrome printing plate, Sun Chemical provides ink, Day International 9500 is the benchmark for printing blanket, and # 2 ～ # 5 coated paper is used to test lightweight coated paper. All printed samples are processed by Verity IA and ink analysis software in a high-speed computer. This computer 256MB extended memory enhances the processing of images captured by the AGFAT1200 platform scanner. The imaging resolution of 550ppi is used for all tests. The rate is as high as 1200ppi. Using standard deviation to quantify moistening is a good way to determine the frequency of changes in brightness between two adjacent parts of an image. There is a good correlation between the quantification of the ink index and the visual assessment of the uniformity of the printed product. With appropriate parameters, not only visible ink but also less visible ink can be quantified.
3. Conclusion
With the help of a desktop scanner, it is possible to quantify visible ink and less visible ink. It is interesting to note this: the color change caused by the ink is closely related to the ink index. If the market demand is clearly determined in terms of the nourishing specifications, then the subjective nourishing evaluation can be turned into a specification that shows nourishing defects in developing paperboard. In order to predict the ink strength of the base paper before printing, further work is underway to predict the color change and ink strength caused by the ink.
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