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Film Positives and Accurate Image Translation

(December 2002) posted on Sun Jan 05, 2003

Coudray examine the different types of imagesetters and offers advice for getting consistent, high-quality output.

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By Mark A. Coudray

Drum-based systems Only certain models of silver-based film imagesetters use drums. These systems provide both a fixed imaging width and depth of image. They also have the ability to prepunch film before it is imaged. This makes for extremely accurate images than are very easy to register on screens and on the press. The drum-based systems tend to be offered in smaller formats that capstan-drive systems and are usually more expensive. Drum-based imagesetters typically provide a registration tolerance of ±0.0003 in. per 16 x 20-in. sheet of film.

Examine the different types of imagesetters and advice for getting consistent, high-quality output.Mark Coudray

The accuracy of film positives is determined by the resolution of the imagesetter and the optical density or dynamic range it delivers. The higher the resolution, the better formed the dots and the sharper the image.

The resolution needed is determined by the applications that will be printed. Most textile printers will only need 600 dpi. Graphics printers may need 1200 dpi or higher. For graphics work, the minimum resolution should be 16 x the halftone line count that will be printed. So, for a 65 line/in. halftone, the minimum film positive resolution would be 1040 dpi. Anything less than this, and you will lose tonal steps.

Be careful of interpolated resolution, which you'll find in devices with reported resolutions of 400 x 800, 600 x 1200, 720 x 1440, and so on. This means that the resolution of the head is fixed in the "X" direction, and half stepped in the "Y." In other words, the resolution is artificially increased by overlapping the dots in one direction. The resulting positive may be fine, but this quality depends largely on the RIP software that drives the imagesetter. Every RIP is different, and you need to do the verification, rather than rely on a salesman's promise.

Image resolution also depends on the minimum "spot" size the device images on the film (multiple printed "spots" comprise a single halftone dot). Most systems have a fixed spot size and change the size of imaged halftone dots by adjusting the spacing and overlapping of spots.

Optical characteristics of the positive

The optical properties of the film carrier and imaged area also determine the quality of the film positive. The points to consider here include Dmin, Dmax, and dynamic range.


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