Mar

11

Unsharp Masking Origin

Filed Under Color Management, Digital Imaging, Herbiology, Images in Print


What does unsharp masking have to do with image sharpening? Well… very little in one respect, but a lot quite a lot in another. The term originates from a very unlikely source. Check it out—

In the lithographic pre-digital world of color separations, continuous tone CMY negatives were produced by placing red (#25), blue (#47B), and green (#58) Wratten filters in front of a process camera lens as individual CT films were exposed, one at a time, from a single color picture. Typically, the resulting CMY color negatives contained way too much contrast for the halftone contact screens used to transpose optical information into halftone dots. When too much contrast was delivered to these contact screens, they failed to faithfully deliver the full range of tones, rendering highlights blown out or shadows plugged.

To remedy this situation it was necessary to reduce the contrast of the separation films. This contrast reduction was accomplished by producing shallow, soft-focused film positive masks from the CMY negatives. The masks were generated by placing raw continuous-tone film on registration pins in a vacuum frame, placing a sheet of frosted acetate on top of the raw film, and then mounting one of the color negatives on top of the acetate. This film sandwich was drawn down under significant pressure and exposed via a carefully-controlled pin light source,

The net result of this “unsharp mask” procedure indeed reduced the overall contrast of each of the cyan, magenta, and yellow separation films. The resulting corrected contrast range enabled each of the color negatives to produce full range tonality. Interestingly, the byproduct of this process provided an additional benefit to the image quality. The soft nature of the positive mask sandwiched against the crisp edges of the original negatives produced a natural edge emphasis on all contrasting areas of the films. This emphasis delivered the net optical effect of detail sharpening. Now you know the truth about USM and what old-school litho gave to the new digital imaging movement!

Think about it!

©copyright 2008 Herb Paynter

http://www.imageprep.net 

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