Densitometric Color and Image Analysis

. Factors affecting trap values. Trapping capacity is directly related to the tack of the ink (as well as other factors). Tack is a measure of the resistance of the ink film to splitting. The Inkometer is a device used to measure the tack of ink. When asked, ink manufacturers will provide the printer with tack values. When printing four colors of ink in sequence on a sheetfed press, the tack of the inks should be graded to yield good trap percentages. The first ink film printed should have the highest tack value, followed by successively lower tack values. This assures that the ink films will adhere well. If the second ink film down had a higher tack value than the first, the ink on the blanket would "stick" more effectively to the blanket than it would to the already printed ink film, resulting in poor transfer.

Print contrast measurements. Print contrast is a densitometric measurement that indicates how well the three-quarter tone to shadow areas of an image are reproducing on press. To make the measurement, a 75% patch (three-quarter tone) and a solid for the color in question must be measured with the densitometer and the following formula applied:

where DS is the density of the solid and D75 is the density of the 75% tint patch.

. Interpreting print contrast. The print contrast value will increase when the difference between the solid ink film and the 75% halftone dot is highest. When the value is relatively high, better shadow detail will reproduce in the image. When the value is low, the image will produce with flat shadow detail. An ink film that is too light will yield a low print contrast value. The light solid will not contrast well with the 75% dot. Similarly, an ink film that is too thick will also cause print contrast to drop. This is because of excessive dot gain in the 75% dot area, making contrast between the 75% dot and the solid weak. Please see figure 3-19 in unit 12 for GRACoL target values for print contrast.

Dot gain. Dot percentages on the printing plate will grow when transferred to the printed sheet. This occurs because the pressure between the plate and blanket causese the ink film covering the dots on plate to spread outward. More spreading occurs as the dots are transferred to the paper. Dot gain, or tone value increase, is inherent in any printing process. Controlling the consistency and balance of dot gain for each ink color is more important than its actual value.

The densitometer is used to measure dot gain, usually from the 50% dot. The murray-Davies dot gain formula is as follows:

where DT is the density of the tint, and DS is the density of the corresponding solid of the same ink color.

Please see Figure 3-12 for GRACoL target values dot gain.