QMS 860 Guide de l'utilisateur télécharger pdf (Page 190)

using 300 dpi is 53 lpi. This typically allows 33 different levels of

gray. You may change screen frequency through your application

or by using the PostScript setscreen operator. The QMS Crown

Technical Reference Manual, which is an optional purchase

available from your QMS vendor, contains information on

setscreen.

The extra gray levels available through higher resolution provide

a smoother shift from the darkest black to very light grays while

higher resolution provides sharper image outlines.

Screen Angle is the angle at which a halftone screen prints. The

default screen angle for your QMS 860 Print System is 45°. This is

the normal angle for a black and white printer. You can change the

screen angle through the PostScript setscreen operator. The QMS

Crown Technical Reference Manual, which is an optional purchase

available from your QMS vendor, contains information on

setscreen.

Gamma Correction Concept

Often in scanned images, gray scaling may be too light or dark to

show details clearly. Using gamma correction on a gray-scale

image is similar to using a graphic equalizer on a home stereo; the

range of gray levels is adjusted for the best possible image. This

sharpens overall midrange contrast while blacks (shadows) and

whites (highlights) keep their original values. Gamma correction

sets gray-scale values specified through the setgray PostScript

operator to your printer’s gray-scale values.

The gamma correction value provided through the printer control

panel refers to a constant value applied to the scan. Increasing the

gamma value lightens the image and decreasing the value darkens

the image.

Chapter 6

Print Quality 6-5

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QMS 860 Guide de l'utilisateur Page 190