According to the knowledge of chromology, color has three properties, namely hue, lightness, and saturation. The ever-changing colors are presented by the arrangement and combination of different hue, different brightness and different saturation. Therefore, if the introduction of halftone dots can cause the printed matter to produce color changes in hue, lightness, and saturation, it can produce kaleidoscopic colors corresponding to the original.
As shown in Figure 1-8, the same yellow is printed in the same unit area. If the dot coverage rate accounts for 10% of the unit area, the color seen by the human eye is like a mixture of 90% white ink and 10% yellow ink. The color presented is a very light yellow, that is, the brightness is very high, and the saturation is very low. If the dot coverage rate accounts for 90% of the unit area, the color seen by the human eye is like the color of 10% white ink and 90% yellow ink mixed, which is a darker yellow. Compared with the former, the brightness decreases and the saturation increases. It can be seen that after the introduction of the dot, the change in the brightness and saturation of the color is generated by the change of the dot coverage.
(Figure 1-8 Changes in color brightness and saturation.)
The changes in the lightness and saturation of the colors only reflect the changes in the vertical and horizontal directions in the three-dimensional color, and there is no change in the color-color association on the circumference. In other words, there are still only eight hues on paper. It's just that each hue has a change in lightness and saturation, but how is the hue change on the printed matter realized? This is because the color light reflected on the printed matter produces a color addition effect (color light addition method) when it reaches the human eye, causing a change in hue.
The additive color effect is the result of dots appearing side by side. Because the distance between dots is extremely small, for a net line of 60 lines / cm, the number of dots per square millimeter area is 36. There are so many outlets in such a small area that the human eye cannot tell. Therefore, several colors of light reflected from the printed matter to the human eye hit the retina almost in the same place. According to the additive color mixing of different colors, a new hue is produced. If the dot coverage of the two colors is the same, the hue of juxtaposed colors is close to the hue of superimposed colors. Taking Figure 1-9 (a) as an example, if the dot coverage of yellow and magenta is close, the color hue of the side-by-side color is close to the superimposed hue of yellow and magenta, and the (b) diagram shows that because the dot coverage of yellow is greater than Magenta, after juxtaposition, produces a new hue with magenta close to yellow. Figure (c) is a hue containing yellow close to magenta. It can be seen from this that if the dot coverages of the two colors are different, the hue of the parallel colors tends to be the one with the larger coverage. Of course, when the dots are juxtaposed to form a color, in addition to the reflected color light of the dot layer, the reflected light of the white paper part also participates in the additive color mixing. The amount of shade in the white paper depends on the dot coverage.
(Picture 1-9 dotted color light addition method)
Therefore, the mechanism of dot color rendering is the use of pigment subtractive method and color light additive method, and through the change and combination of dot coverage, the printed color has changed in hue, lightness and saturation, thus showing a kaleidoscopic s color.
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