4. Optical Dot Gain
Another component of total dot-gain is optical
dot-gain.
This is an optical illusion created by the scattering
of light around the dot and diffusion within the paper.
Some of the reflected light is trapped below the printed dot and this creates the perception of a dark halo around it making the dot appear darker as illustrated in Fig #7.
The combined effects of both mechanical and optical
dot-gains produce total dot-gain.
The above is a simplified depiction of a curve control dialog box. The actual process of obtaining and building up the curve profile to accommodate the press’es reproduction characteristics is much more complex and beyond the scope of this article.
In the first graph ,the red line shows the reproduction curve in the printed copy compared to the diagonal blue line which represents the original.
The second graph shows how the reproduction curve should be pulled down at the prepress stage to accommodate the previously known level of dot gain inherent in the press.
After printing , this compensation will then build up back to more closely match the original colour proof.
SWOP which stands for Specifications Web Offset Publications is an American based standard developed for the printing industry and these specify that acceptable dot-gain compensation levels for the four process colours in the 50% mid tone area are to be as follows;
