The effect of ink on color reproduction
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What is smog? Hydrating is almost just a failure of coated paper and paperboard. It usually occurs on four-color sheetfed offset and web offset presses, as well as on flexographic presses, with many complex physical and chemical reasons. The occurrence of the ink phenomenon is the difference in the ink absorption of the substrate, the problem of ink fixation and drying (counter overprinting) or the difference in the absorption of the substrate by the printing medium.
Since the light transmitted or reflected from the printed surface or through the translucent sample is unevenly transmitted, the brightness of the printed surface and the portion of the printed surface are changed. When there is such a change, the eye can be easily perceived by the eye. The phenomenon of ink. The frequency of these changes in space may vary, and the change between one brightness and the other determines the degree of ink. With the aid of any device, changes in the 12 mm portion and the other portion can be perceived with the eye.
In recent years, a variety of instruments have been developed to quantify ink: image analyzers based on benchtop scanning or CCD technology, and even densitometers. For the most part, however, the user must determine any threshold with a pixel value of R degrees, which determines what is the target of interest in the target. As a result, most of the ink measurement results 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 print quality by the eye and the ink measurement of the instrument. Sex. Another limitation that is commonly encountered is that only a few instruments can make parameter adjustments to measure visible and less visible inks.
First, some theoretical views on measuring ink
The usual practice is to classify the ink as a general sign of inhomogeneity. According to this, the technique for quantifying the unevenness can be easily applied. The unevenness of the print is a measure of the change in the image point of the ROI (region of interest) within an image or within the range of an image. It is within a pre-set pixel luminance value (LV) range, typically 0 to 255, where 0 is the darkest (no reflected light) and 255 is the brightest (complete reflection).
The inhomogeneity is then calculated as the arithmetic difference of the average image point LV, and this arithmetic difference belongs to those intermediate LV values above and below all values in the image. In terms of frequency distribution, the intermediate value determines the LV in which the 1/2 pixel LVS is higher than this value and the 1/2 pixel LV is lower than this value.
Thus, LV: pixels are brightness values, pixel reflection values, pixel brightness, hue;
NU: unevenness;
Lx: average value of LV ≤ intermediate LV;
Ux: average value of LV> intermediate LV;
Then NU=Ux-Lx,
With the aid of an image analyzer, the area of the inspection site is determined by the required image resolution and sensitivity. In general, the area of the inspection site is between 1 mm 2 and 3 mm 2 for the ink that is perceived within the visible range, and between 100 μm 2 and 1 mm 2 within the less visible range. The inspection site moves over the image, called the non-moving window (MW), and the MV moves in its height and width increments throughout the ROI, so no part is ignored. At each location, 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 image in the range of MWs (Wsd) and the average value (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 measurements
Color perception is the basic aspect of vision. The feeling of changing color space is very different from our feeling of brightness change. In order to simplify the quantitative color change, a spherical spectrophotometer with a 9 mm aperture was used for this experiment, which is consistent with the perceived 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]. △Z quantify color change
Second, the experimental method
In order to determine the final correlation between the ink smear index and the color change, a large number of experiments were conducted. Make a print on the web offset press. Day International 9500 is the benchmark for printing blankets. Lightweight coated paper is tested with #2~#5 coated paper. All printed samples were processed by Verity IA, the ink analysis software in a high-speed computer. This computer 256MB extended memory enhancement processed images captured by the AGFAT1200 platform scanner, 550ppi imaging resolution for all tests, optical resolution of the scanner The rate is up to 1200ppi. Quantifying the ink with standard deviation is a good way to determine the frequency of change in luminance values occurring between two adjacent parts of the image. There is a good correlation between the quantification of the ink index and the visual assessment of print uniformity. With appropriate parameters, not only can the visible ink be quantified but also the less visible ink can be quantified.
Third, the conclusion
With a desktop scanner you can quantify visible inks and less visible inks. It is interesting to note that the color change caused by the ink is closely related to the ink index. If the market demand is clearly defined in terms of the ink specification, then the subjective ink evaluation can be turned into a specification for developing ink defects in the development of paperboard. In order to predict the aging strength of the base paper before printing, further work is being carried out in order to predict the color change and the ink absorbing intensity caused by the smear.