-----Highlights. If the RGB highlight (lightest tone in the picture) is in the 245-250 range, the desire is for the printing process to deliver the lightest printable "dot." That requirement means different things for each process. If the picture prints on an aging non-heatset web press on a poor grade of newsprint, the lightest printable dot (of each color) could be 7-12%, but if the picture is to be printed using a well maintained sheet-fed press on a cast-coated sheet (as close to glass as paper gets), the lightest printable dot could be as small as 3%. Any generic separation table/profile designed for one printing process simply will not reproduce accurately on another.

-----Mid-tones. Even more significant is the way different printing processes reproduce mid-tones. This phenomenon is known as dot gain. Dot gain is the measurable amount that a 50% dot enlarges during the production process.

-----This change is due to many factors, but is most affected by how much the ink absorbs (and spreads) into a particular paper stock.
-----Imagine an eyedropper containing black ink. Now picture dropping a single drop of this ink onto a paper towel. Picture the way the ink would soak into the paper towel and spread. Now picture dropping this same single drop onto a glass tabletop. The ink sits on the surface of the glass and holds its shape. This simplistic illustration demonstrates the basic principle of dot gain.
-----Dot gain is measured by the way an ink/paper combination alters the size of a dot. Dot gain is inevitable. The best defense is calculated compensation. ------------------------Different papers print a known-value dot (in this case, 50%) differently. The amount that a dot typically enlarges from its original shape during the printing process is the same amount that the dot should be diminished in the separation process, in order to compensate for the expected gain.
-----If we find that the 50% tones in an image will actually measure 65% when printed on an particular paper, then we know that the dot gain is 15%.

THUS, the 65% area of the tonal curve were adjusted back 15%, when the picture printed, the 50% dot gains 15% and prints visually correct at 65%.

-----Shadows. Why do some pictures show significant detail in the darkest tones, and others just close in to solid black? The key is “shadow placement.”
-----Without exception, the pictures that print well in the darkest areas of the picture were prepared to meet the printing characteristics of a specific process, ink, and paper. Much in the same way that midtones are affected by how papers absorb ink, shadows are similarly affected. ICC profiles must account for what the printing trade calls the "maximum printable dot."
-----This means that each paper must be tested for the biggest dot it can print without plugging up. A proper ICC profile developed for unique paper composition will accommodate minimum highlights, dot gain compensation, and maximum shadow dots.

On a related issue go to ICC Profiles

<----BACK