Mechanical Preparation, Continued
Halftones
Some printing presses and machines are not capable of producing good
quality continuous tone images. Continuous tone images, such as
watercolors, drawings, oil paintings, photographs, and other works composed
of a series of tones tend to blend together or lose detail. To separate these
tones, the artwork must be shot through a screen that breaks the image up
and records it as a series of dots. This process is known as halftone
screening. Exposure, similar to that used in line photography, is determined
by the intensity of the light reflected from the original copy, distance between
the film and the screen, size and shape of the lens aperture, speed and
contrast of the film emulsion, and the duration of the exposure.
BLACK-AND-WHITE HALFTONE SCREENS: Halftone screens used for
black-and-white photography are generally rectangular and made of glass or
acetate. Glass halftone screens are made of two sheets of optical quality
glass, each etched with fine parallel lines filled with opaque pigment and
sealed together with the lines crossing each other at right angles. Halftone
screens are available in standard rulings from 50 to 400 lines per inch set at a
45-degree angle to the screen. The lines on the screen are the same width as
the space between them. Therefore, a 50-line screen has 50 lines and 50
spaces to each inch. Screens are grey or magenta in color. The 45-degree
angle makes the dot pattern less noticeable to the human eye. Position a
glass halftone screen a short distance in front of the film plane. Position an
acetate halftone screen in direct contact with the film.
COLOR HALFTONE SCREENS: Halftone screens used for color
separation work are circular. You must set the angle of the screen and
rotate it for each color shot. Halftone screens for color work are not in
color. Color halftone screen are available in standard rulings.
DIGITAL HALFTONE SCREENS: A computer with a desktop scanner or
video digitizer scans continuous tone artwork and photography transforming
them into a digital format. The scanner must be a grey scale scanner. If you
alter the image, do so before scanning. Limited image alteration is possible
with the appropriate image-editing software. Scan the imagery at a resolution
twice the final screen ruling. Save the image in a Tagged Image File Format
(TIFF) or Raster Image File Format (RIFF). You can output these digital
halftones to laser printers or high-resolution copier-printers.
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