Densitometric Color and Image Analysis
Solid ink film measurement. To maintain image quality during the pressrun, the operator is concerned with the ink film thickness being printed on the paper. When ink films are too thin, the images will appear to be washed out. When ink films are printed too thickly, many problems can result including poor drying, halftone plugging, and scumming or toning. The densitometer is used to provide numerical feedback on ink film thickness. These optical density readings are taken from solid color patches printed in the trim area of the form. Generally, the thicker the ink film, the higher the optical density reading. Published standards for the printing industry, like the General Requirements for Applications in Commercial Offset Lithography (GRACoL), specify density values and tolerances for solid ink film (Figure 3-11).
Hue error and grayness measurements. The densitometer can be used to evaluate the used to evaluate the character of each of the process colors used to print full-color images. Ink color quality may vary from manufacturer to manufacturer. For example, cyan ink from one manufacturer may not match the cyan ink from another manufacturer. Without consistency of process ink color, preparation and editing of color images in the prepress stage cannot be accurate.
. Determining hue error. The process colors of yellow, magenta, and cyan are not completely pure. For example, a theoretically pure magenta would reflect only its own color and no cyan or yellow when measured with a densitometer. Because magenta is impure, it acts as though it is reflecting small amounts of yellow and cyan. Hue error, expressed as a percentage, describes the deviation of the process color from a pure state. For example, a magenta ink film might have a hue error of 40%, while a yellow might have a hue error of 3%. These values indicate that yellow is the purer of the two colors and that magenta has a higher hue error. To measure hue error, each process color is measured with the densitometer through each of the three filters: red, green, and blue. For example, when magenta is measured through a blue filter, the density reading shows how much yellow is reflecting from the magenta. When read through a red filter, the density of the cyan contamination in the magenta is determined. Once all readings have been taken, hue error values are determined by applying the following formula:
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 will produce gray. Process color inks are slightly grayish, each color behaving as though it were contaminated with slight amounts of the other two. The grayness of the ink film, expressed as a percentage, can be easily determined with a densitometer. Each process color must be measured through each of the three filters. Once all measurements have been recorded, grayness is determined by applying the following formula:
where DH is the highest density value, and DL is the lowest density value.
Trapping measurements. Trapping values express how well one process color ink film is adhering to the previously printed ink film. Because the adhesion characteristics of plain paper are diffferent from those of a wet ink film, not all of the ink film density that prints on paper may also be printed on a previously printed ink film. Because secondary colors are created by printing overlapping cyan, magenta, and yellow ink films, trapping values will give insight into the relative effectiveness of the blues, reds, and greens. Before applying the trapping formula, each process color ink film and each secondary overlap color must be measured through each filter of the densitometer. Also, the order that the ink films were printed must be known.
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. Note: All three densities are measured using the color filter for the second color printed.
. Interpreting trapping values. A trap of 100% is considered to be perfect. This means that the second ink film down is adhering with the same ink film thickness on the paper as ite is on previously printed ink film. Normally, this does not happen. Rather, typical trapping values range from 80%~90%, meaning that some adhesion is lost on the second color down. Very low values, like 60% trap, indicates that the secondary color is very weak and the colors in the printed image corresponding with that color will nto be rich.
. 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.
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