What is nuisance? Ink is almost exclusively a problem with coated paper and paperboard. It usually occurs on four-color sheetfed and web offset presses, and on flexographic printing presses, causing a variety of complex physical and chemical causes. The occurrence of the ink phenomenon is due to the difference in the absorption of the ink by the substrate, the problem of ink fixation and drying (reverse printing), or the difference in the absorption of the fountain solution by the substrate.
Since the light transmitted or reflected from the printed surface or transmitted through the translucent sample is inhomogeneous, the brightness of this part of the printed surface changes with that of the other part. When such a change exists, the ocular effect can be easily perceived by the eyes. The phenomenon of ink. The frequency with which these changes occur in space may vary, and the change between one brightness and the other determines the degree of ink. In the absence of any equipment, the change from the 12mm site to the other site is noticeable by the eye.
In recent years, there have been a variety of instruments used to quantify ink bleed: image analyzers based on benchtop scanning or CCD technology, and even densitometers. But for the most part, the user must determine any threshold with a pixel value of R degrees, which determines what is the part of interest in the object. As a result, most of the ink measurement results are not reproducible between two different operators, and only a few instruments can verify the clear correlation between practical eye evaluation print quality and the instrument's measured ink pointer. Sex. Another limitation usually encountered is that only a few instruments can make parameter adjustments to measure visible and less visible blemishes.
First, a number of theoretical viewpoints on measuring ink
The usual practice is to classify ink as a general symptom of inhomogeneity. According to this, the technique for quantizing non-uniformity can be easily applied. Print inhomogeneity is a measure of the change in image point of an ROI (Region of Interest) within an image or within the range of an image. It is within a predetermined range of pixel luminance values ​​(LV), typically 0 to 255, where 0 is the darkest (no reflected light) and 255 is the brightest (full reflection).
The non-uniformity is then calculated as the arithmetic difference of the average image point LV, and this arithmetic difference belongs to the values ​​of the intermediate LV above and below all the values ​​in the image. In terms of frequency distribution, the median determines the LV where 1/2 pixel LVS is above this value and 1/2 pixel LV is below this value.
Thus, LV: pixels are luminance values, pixel reflection values, pixel brightness, hue;
NU: Inhomogeneity;
Lx: the average value of LV? middle LV;
Ux: average LV> middle LV;
Then NU=Ux-Lx,
With an image analyzer, the area of ​​the test site is determined by the required image resolution and sensitivity. In general, the area of ​​the test site is between 1 mm 2 and 3 mm 2 for omnivores perceived within the visible range, and between 100 μm 2 and 1 mm 2 in the less visible range. The test part moves over the image, called the non-moving window (MW), and the MV moves in its entire height and width increments, so no part is ignored. At various locations, standard deviation (SD) and average (A) are recorded in memory to generate vectors each containing all of the individual MV measurements.
When the entire image is examined by the MV and two vectors are extracted, the standard deviation (SD) of the luminance values ​​of the images in the range of MWs (Wsd) and the average (A) of the luminance values ​​in the range of MWs (Wa) are calculated:
Standard deviation of SDwsd=Wsd`S
Awsd=Wsd`S average
Standard deviation of Sdwa=Wa`S
Theoretical perspective on color change measurement
Color perception is the basic aspect of vision. The feeling of color space change is very different from our sense of brightness change. In order to simplify the quantification of color changes, a spherical spectrophotometer with a 9 mm aperture was used for this experiment, which is consistent with the perceptual frequency cutoff of the color change.
Each ROI was measured with a 3 x 3 mm grid and Δa* and Δb* were measured at each point.
The quantity ΔZ= calculates the square root [(Δa*)2 + (Δb*)2 ]. ΔZ quantifies color change
Second, the experimental method
In order to determine the final correlation between the ink pointer and the color change, a large number of experiments were conducted. A print was made on a web offset press (Heidelbergm-110). The printer was equipped with a CTP Thermal830 Kodak Poly-chrome plate, Sun Chemical supplied the ink, and Day International 9500 was the benchmark for printing blankets. Lightweight coated paper was tested using #2 to #5 coated paper. All printed samples were processed by Verity IA, a high-speed calculator in the Moisture Analysis software. This calculator has 256MB of extended memory to enhance the processing of images captured by the AGFAT1200 platform scanner. The image resolution of 550ppi was used for all tests. Optical resolution up to 1200ppi. Using the standard deviation to quantify ink is a good way to determine how often the luminance value changes between two adjacent parts of an image. There is a good correlation between the quantification of the ink pointer and the visual assessment of print uniformity. With proper parameters, not only can the visible ink be quantified but also the less visible ink can be quantified.
Third, the conclusion
With a benchtop scanner, visible ink and less visible inks can be quantified. It is interesting to note that the color change caused by the ink is closely related to the ink pointer. If the market needs are clearly defined in terms of the AIDS regulations, the subjective valuing of the ink can be turned into a specification for displaying blemishes in the development of paperboard. In order to predict the ink strength of the base paper before printing, further work is being carried out to predict the color change and ink strength caused by the ink. (Text / Xiao Jin)