2. Dot Gain
In printing, dot-gain ( also known as TVI or Tonal
Value Increase) refers to the fact that on the printed copy, the size of the dot
is slightly larger in diameter than the size of the same dot in the original
copy.
This changes the values of the screen tones and
causes colours to shift causing colour variations.
A schematic of this is drawn in figure # 1 below. It is exaggerated for effect.
Taking a larger portion of a 50% screen area, the effect of a 20% dot-gain in the mid-tone area would cause the results seen in Fig #2 even though ink film thickness has not been changed .
Figure
#3 shows the effect of dot gain on a colour picture at various simulated dot
gain
levels.
The picture is that of the Royal Pavillion in Brighton , UK. Observe that as the level of dot-gain increases , image details and contrast are being lost, especially in the trees and the lower parts of the building. Generally the image tends to get darker.
