Halftones
Design would indeed be monotonous if all continuous-tone copy had to be reproduced in exactly the same manner. Fortunately, there are a number of things the designer can do with the conventional halftone. The particular approach is usually dictated by both esthetic and practical considerations. Here are some of the more popular kinds of halftone reproduction.
Square. A halftone that has been squared up. That is, all the corners are right angles. Although called a "square" halftone, it does not necessarily have to be square; it is in fact more often rectangular. Although there are also round and oval halftones, square halftones are the most widely used.
Silhouette. Also called an outline halftone. A halftone showing only the important area. The printer removes all the halftone dots from the background by opaquing the negative with paint or by covering it with a mask. There is also a modified silhouette, in which only a portion of the halftone is silhouetted and the remaining portions are the square sides of the halftone. Silhouette halftones are usually opaqued by hand and can be expensive.
Dropout. Also called a highlight halftone. A halftone in which certain areas have been highlighted by dropping out (eliminating) the screened dots, so that all that remains in the actual printing is the white of the paper. The dots are removed from the negative by opaquing them with paint, or by a special camera technique.
Vignette. A halftone in which the image fades almost imperceptibly into the white of the paper. Vignetting is done by an artist who creates the blend by airbrushing directly on the original art or photograph. This can also be expensive.
Combination Line and Halftone. As the name implies, part of the art is line and part is halftone. The line copy is shot as line and the continuous-tone is shot as halftone, then the two are combined for reproduction. In this way the strength of the black in the line copy is held. (if line copy were to be shot as continuous-tone copy it would break up into dots, which when reproduced would appear gray rather than black.)
Halftone Screens
There are two kinds of screens: glass screens and contact screens. The glass screen consists of a finely ruled grid pattern and is placed between the camera's lens and the film. The contact screen consists of a pattern of vignetted dots and is placed in direct contact with the unexposed film. Both produce the same results: they reduce continuous-tone copy to thousands of tiny dots (like copy), varying in size, shape, and number. When printed, these dots give the illusion of the original tones.
After the continuous-tone copy is screened, the film is developed, producing a halftone negative. The printed image is called a halftone. (The term "halftone" comes from the idea that the screening eliminates half the original image, which could be considered a "fulltone," and that only half of this full tone remains.)
Screens are measured by the number of lines per inch. The most commonly used are 55, 65, 85, 100, 120, 133, and 150. The more lines per inch a screen has, the finer the dot pattern and the higher quality halftone it will produce. For example, a 55-line screen has 3,025 dots per square inch, while a 150-line screen has 225,000. The 150-line halftone will look more like the original continuous-tone copy than the 55-line halftone because the dots will be more numerous, finer, and closer together.
The screen used is determined by the surface of the paper, the type of printing plate, and the quality of the printing press and the printing inks. Newspapers, for example, use rather rough paper, stereotype printing plates, presses not designed for fine reproduction, and inks of watery consistency. Therefore, newspapers require a coarse, 85-line screen. If a finer screen were used, the paper would be unable to hold the detail and the spaces between the dots would fill in. Magazines, on the other hand, use a smoother paper and are able to use finer screens, normally up to 150-line, and thus obtain finer, more detailed halftones. Obviously, it is important to match the screen to the paper and the printing conditions.