Get Your Dots Right
If you want to be taken seriously by customers, you have to provide sharp, accurate images. Calibrating your imagesetter helps you do that.
Running an uncalibrated imagesetter is a sure way to lower the quality standards of your in-plant. With a little diligence and the right tools, though, you can be sure that when you ask your imagesetter to produce a 20-percent dot, it delivers exactly that—not a 16- or 23-percent dot. A calibrated imagesetter will prove your expertise and ensure your dependability.
In addition to providing accurate tints, a properly calibrated imagesetter helps avoid blown out highlight and clogged shadow dots in your halftone images. Accurate halftone screens become much more important as your shop moves from black-and-white work into duotones, tritones and process color, since PostScript imagesetters use the same halftone techniques to produce color separations as they do to produce grayscale output.
When you process film output from your imagesetting system, the exposed areas are black and the unexposed areas are clear (or white, in the case of RC paper). The word "density" is used to quantify the darkness of solid black areas. Density is expressed as a number derived from a logarithmic calculation; the higher the number, the darker the black. The maximum density of a medium is called its Dmax.
You'll need to use a densitometer to measure the opacity or darkness of the media. There are two basic types of densitometers: transmissive units for reading film and reflective units for reading RC paper. There are also hybrid densitometers that provide both capabilities. Your densitometer should always be calibrated before you attempt to calibrate your imagesetter.
The density of the final output depends largely on the image recorder exposure setting. Most image recorders have a laser beam intensity adjustment (called a density or exposure setting) that controls the exposure level. Higher exposure settings produce darker blacks (higher density readings) on your film or RC paper output.