Images are still occasionally printed using metal plates that are engraved or etched by hand. In the case of photoengraving, a similar process makes use of a camera. First, the image is photographed to produce a negative on a sheet of transparent film. The negative is then used to print the image on a sheet of zinc that is covered with a gelatin-like substance, or emulsion. Chemicals in the emulsion transfer the image to the zinc sheet. The zinc sheet is then treated with chemicals that etch the metal surface except where the image appears. The image remains elevated above the etched surface, and the plate is used to print the image on paper.
Black and white photographs with many shades of gray have been traditionally handled by a process called halftone engraving. With this technique, the original picture is first photographed. Then a screen in the camera is used to break up the picture into thousands of tiny squares. The negative consists of thousands of tiny dots, one for each square. The photoengraving from this negative has many tiny dots raised in relief above the eaten-away metal surface. Portions of the plate that will appear as dark areas in the finished picture are covered with relatively large dots. The portions of the plate that will appear gray are covered with smaller dots. And the portions that will print white are covered by dots that may appear invisible to the naked eye.
Ordinary newspaper pictures are produced with screens of about 5,000 dots per square inch (or about 70 dots per linear inch). A very fine-screened engraving, such as might appear in art books and magazines, might use up to 18,000 dots per square inch (or about 135 dots per linear inch).
Color printing requires plates for each color. Most color pictures can be printed using four plates, one for black and one each for red, blue, and yellow.
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