Printing 'Dot Gain'

Introduction

FIGURE 1

The amount of spread varies depending on the stages the image has gone from design to print, ink thickness, the pressure used in printing, paper quality, etc.

With ink spread 2 effects occur:
• Although the spread is uniform (the distance out is the same) the amount of ink in the spread increases as the dot size increases.  As the dot gets bigger its leading edge or circumference gets longer and so the greater the total ink spread.
• An odd effect occurs around the mid point when dots begin to merge. The ink spread now overlaps with other dots and so has less visual effect. As the dots get larger then the space between them gets smaller so it is only in these white spaces between the dots that the ink spread is seen. The graph shows ink spread increasing up to the mid point and then decreasing as the dots continue to grow and overlap.

The end result is an ink spread, called a dot gain, that is higher in the mid tones than in the highlights or shadows.  To counter this effect a Dot Gain level can be selected in the colour settings. Typical levels of 15% to 30% are used depending on the ink and paper quality.

Dot spread

FIGURE 2

Another problem occurs when two or more dots meet. Because the ink is a liquid there is a jump in dot size as the various dots merge.

To help explain this then think of water as an example. Take a group of water drops lined up evenly as if they were printed ink dots. When the drops become bigger then there comes a point where they will meet. Ideally the water drops will begin to overlap each other but experience shows that this looks unnatural. What actually happens is that the joined edges round-out rather than develop sharp points. Therefore with a small increase in size the original water drops surge as they begin to merge with one another. A by-product of this surge is that the drops cease to be round, but the spaces between them then take on a round shape as if they were the drops themselves. Visually it seems that the drops go from being positive, before joining, to negative afterwards. 

Dot shape

FIGURE 3

Using a gradient of printing dots from black to white a change of dot shape (and therefore an increase in dot size) takes place as the dots meet at the mid point. At the highlight end the space between the dots is not circular, only the black dots are. But after the dots begin to merge it is this surge effect that now causes the space between the black dots to appear circular. In the shadows it is as if white dots being printed, when in fact it is just the effect of black dots as they spread into one another.

FIGURE 4

So far the assumption has been that the printer dots are circular, but this need not be so as the dots can be set to any one of a number of shapes. To avoid the jump in the dot size at the mid point it is helpful to use an elliptical dot.  Now that the dot is distorted the two axis meet at different stages, with the longer axis meeting towards the highlight and the shorter axis meeting towards the shadow. The result is that there are now two smaller jumps giving a smoother gradient.

Dot gain

FIGURE 5

Dot gain not only effects the mid tones but also the highlight (where the dots become larger), and the shadow (where the dots begin to fill in).This change is usually worst in the shadows where a range of dark greys become black.
NOTE:  It may seem contradictory to talk of dot gain when the shadow detail suffers a dot loss. Remember that it is the black ink that forms the dots. As the black shadow dots gain in size, the white spaces between them (the apparent white dots) get squeezed out of existence.

FIGURE 6

Even though the highlights may have been helped in size by dot gain, there still comes a point where the dot is too small to register on paper. The coarser the paper, the more noticeable this becomes, with newsprint being one of the worst. To account for dot gain, the tonal range of the image is reduced from 0-255 to fit within the printer black & white points (28-243 in this example). The result is a print with full tonal range.