Densitometric Color and Image Analysis

Densitometry provides the critical data required to maintain image quality throughout a pressrun by measuring the thickness of the ink film applied to paper. An ink film that is too thin produces a washed-out image, while an excessively thick film can lead to poor drying, halftone plugging, and scumming or toning. Operators use a densitometer to obtain numerical feedback via optical density readings, typically taken from solid color patches printed in the trim area. These readings correlate directly with ink film thickness: a thicker film yields a higher optical density. Industry standards, such as the General Requirements for Applications in Commercial Offset Lithography (GRACoL), provide specific target values and tolerances for solid ink density.

Hue error and grayness measurements. A densitometer also evaluates the character of process colors (cyan, magenta, yellow) used in full-color printing. Inks vary between manufacturers; a cyan from one source may not match a cyan from another. This inconsistency challenges accurate color image preparation and editing in prepress, making objective measurement essential.

Determining hue error. Process colors are not perfectly pure. A theoretically pure magenta, for instance, would reflect no cyan or yellow when measured. In reality, impurities cause it to reflect small amounts of other colors. Hue error, expressed as a percentage, quantifies this deviation from a pure state. A magenta might have a 40% hue error, while a yellow might have only 3%, indicating yellow is purer. To calculate hue error, each process color is measured through the densitometer's red, green, and blue filters. The formula applied is:

where DH is the highest density value, DM is the middle density value, and DL is the lowest density value.

Determining grayness. Equal amounts of yellow, magenta, and cyan should produce gray. However, each process ink behaves as if contaminated with slight amounts of the other two, giving it a grayish quality. Grayness, expressed as a percentage, is determined by measuring each color through the three filters and applying the formula:

where DH is the highest density value, and DL is the lowest density value.

Trapping measurements. Trapping values indicate how effectively a wet ink film adheres to a previously printed ink film compared to plain paper. Since secondary colors like reds, greens, and blues are created by overprinting process inks, trapping measurements offer insight into color effectiveness. To calculate trapping, each process color and each secondary overlap color must be measured through each densitometer filter, and the printing order must be known. The formula is:

where DOP is the density of the overprint, D1 is the density of the first color printed, and D2 is the density of the second color printed. All densities are measured using the color filter for the second color printed.

Interpreting trapping values. A trap value of 100% is considered perfect, indicating the second ink adheres with the same thickness as it does on paper. This is rarely achieved. Typical values range from 80% to 90%, indicating some adhesion loss. A very low value, such as 60%, signifies a weak secondary color, resulting in less rich corresponding colors in the final image.

These objective measurements are increasingly integrated with automated closed-loop systems that adjust press settings in real time. Furthermore, a continued industry emphasis on sustainability drives the precise application of materials, reducing waste while maintaining color fidelity through data-driven processes.

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.