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By John D. Kohnke, C.G.A.E.

Today, high quality printing that is commercially acceptable can be achieved on modern lithographic printing presses with relative ease. However, pushing this equipment to their optimum performance levels is still in the realm of the craftsman and his or her repertoire of tools and techniques. One of these powerful tools, that is not widely used in the United States, is the measurement of relative print contrast on a printed sheet of paper.

The relative print contrast (K)% is essentially a measure of a printed screen quality in the three-quarter tone. By measuring relative print contrast, you can determine how much ink can be pumped without diminishing the quality of a printed screen and consequently maximizing ink film densities as well.

When printing, it is desirable to have as high a relative print contrast as possible. In English, this means that your solids have the highest possible ink film density while still having the screens print open. This is termed as "optimum halftone value difference." Essentially, the printed dots form a contrast to the solid.

Contrast is primarily affected by inking, assuming that you begin with a good set of film, properly imaged plates, and your press is set up to specification. When you increase the inking there is an accompanying increase in the ink film density and the relative print contrast as well. However, this increase in ink feed is desirable only up until a certain threshold. Should the ink film thickness be increased even further, an image's halftone and screen dots will tend to exhibit dot gain, and thus tend to fill in. This is particularly predominant in the three-quarter tone dot.

The result is a reduced proportion of white paper to ink and the relative print contrast decreases. Measuring contrast is a relatively simple process. Today many densitometer manufacturers have direct contrast display features in their products. However, you can also determine the relative print contrast manually with a formula.

The relative print contrast is calculated from the measured values of the solid ink density (DV) and the screen ink density (DR). For measuring relative print contrast, the DR value is measured in the three-quarter tone because of its sensitivity to inking shifts.

Mathematically, it is expressed:

K% = (Solid Ink Density) - (3/4 tone Ink Density) X 100
                          (Solid Ink Density)

or, K% =((DV -DR)/DV )x100

Let's take a look at an example of how ink density and relative print contrast react with each other. In the chart below, the X-axis represents the relative print contrast value, and the Y-axis represents, in descending format, the solid ink density value.

  DVDRK%
 An unprinted sheet of paper shows no contrast0.000.000%
 As ink densities rise, the contrast also rises1.000.6535%
 With a "standard ink density" the highest contrast is achieved1.500.8543%
 When overinking begins, the dots tend to gain, and the contrast decreases again2.001.2538%
In extreme cases, the screen fills in completely, and the contrast value is zero3.003.000%
So what value does this have for a pressman? First, it allows a pressman to know how much ink can be pumped onto the sheet in order to increase the image fidelity, color saturation, color gamut, and the intensity of the image without degrading the integrity of the screen values. In essence, it helps a pressman to push the limits of a high impacting visual image. Naturally, a higher impacting visual image is more competitive and thus easier to sell than a lower impacting one. Secondly, it provides essential screen quality control information. For example, if the contrast value deteriorates during a production run in spite of a constant solid ink density (DV), this may indicate that the blankets need to be washed. This information cannot be determined from a simple density measurement alone. The contrast value can also be used to assess other factors such as packing and cylinder pressures, blankets and underlays, dampener settings, and printing in and additive quality.

Used properly, relative print contrast is a powerful tool for optimizing your pressroom performance. If you aren't using it now you may wish to add it to your repertoire of quality control tools. You will be surprised at your conclusions and how you feel about your current benchmark measuring standards.

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