Halftone screen attributes include the screen frequency and dot shape for each screen used in the printing process. For color separations, the prepress person must also specify an angle for each of the color screens. Set the screens at different angles to ensure that the dots placed by the four screens blend into continuous color and do not produce moiré patterns.

Halftone screens consist of dots that control how much ink is deposited at a specific location on press. Varying their size and density creates the illusion of variations of gray or continuous color. For a process color image, four halftone screens are used: cyan, magenta, yellow, and black—one for each ink used in the printing process.

Halftone screen with black ink

Halftone screens with process ink at different screen angles; correctly registered dots form rosettes.

In traditional print production, a halftone is produced by placing a halftone screen between a piece of film and the image and then exposing the film. In Photoshop, you specify the halftone screen attributes just before producing the film or paper output. For best results, your output device (a PostScript imagesetter, for example) should be set to the correct density limit, and your processor should be properly calibrated; otherwise, results can be unpredictable.

Before creating your halftone screens, check with your print shop for preferred frequency, angle, and dot settings. (Use the default angle settings unless your print shop specifies changes.)