Halftones - 14066

Figure 1-37. Vignetted head-and-shoulders portrait. - 14066_59

Illustrator Draftsman 1 & C - Volume 4 Presentation Graphics
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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. Continued on next page 1-50

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