Black & White Halftone


In 1887 the Ives process was improved upon with the invention of the crossline screen. This device is produced from two sheets of glass into which a fine series of evenly spaced parallel lines have been etched then filled in with an opaque material. The first sheet of glass is then rotated ninety degrees to the second sheet and both are sealed face to face with a transparent cement to form a crossline grid. The space between the intersecting lines on this combined grid creates a field of tiny square apertures that will break up the tones of the image projected through it. Halftone negatives were made by photographing an image through a crossline grid placed at a calculated distance from its surface. Apertures that receive a lot of light cast a large dot, while apertures exposed to small amounts of light will cast a small dot. As the number of lines increase on a screen by placing them closer together, the halftone dots become smaller and more numerous creating a tighter frequency measured in dots per inch (dpi). More dots will increase sharpness and render greater detail, but at the expense of decreasing the optical tonal range a plate can print. This new halftone negative would then be contact printed onto a photosensitized substrate. The halftone dots that results from this process only create the illusion of the full tonal range carried by the original image, in actuality its components are completely rendered in black or white. Many black & white postcards in both lithography and line block printing were cheaply produced in this manner throughout the 20th century.

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