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Maintaining Dot Predictability

(August 2002) posted on Fri Aug 30, 2002

Coudray discusses the concept of linearization and explains how to apply it in digital prepress.

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By Mark A. Coudray

No matter how expensive your imagesetting device is, it must be measured and calibrated. Once you've done this, the daily output must be routinely monitored and corrected for drift from the ideal settings. This is a normal and expected part of any prepress operation. Even if you receive film from a service bureau or other provider, you still need some way of measuring to determine if the film is acceptable.


Taking measurements


To measure halftone dot gain or loss, you need two different instruments. The first is a transmission densitometer. This device measures the amount of transmitted light that comes through your negatives or positives. It is most often used in "Dot Area" mode so that the reading you see will be the actual percentage of area covered by the halftone dot.


You also use the transmission densitometer to measure the Dmax (maximum density) of the black, imaged areas of the film, checking that the film is dark enough to avoid burn through during screen exposure. The reading the device provides is a logarithm of light transmittance. A reading of 4.0 means that 1/10,000 of the light hitting the film is being transmitted through the black background. Films with values below 3.0 run the risk of burn through during screen exposure, which can change the halftone dot size and tonal values.


The second instrument you need is a reflection densitometer, which measures the light reflected from the surface of the printed image. It also measures direct dot area, calculates absolute dot gain, and reports the Dmax of the ink you are printing. The reflection densitometer is filtered to give direct readings of yellow, magenta, cyan, and black ink. This helps you control the dot area and understand how the ink density affects dot appearance. (To learn more about densitometers, see "Densitometry: Your Guide to Print Quality," by Frank Basch, <I>Screen Printing</I>, Sept. 2000, page 18.)


You want each halftone dot to represent a tone between white and black. In other words, a 50% black dot should represent a visual gray. As simple as this sounds, many things can keep this from happening. That's why you need to measure the process.



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