This number is called the “frequency” or “Lines-per-inch – LPI.” Yes, lines-per-inch doesn’t seem right but in the old camera days you would refer to the halftone screen “ruling” and this is where it came from. Halftone dots are referred to by a number based on the number of dots per linear inch. While this article is more about halftone dots, the problem of exposing and printing a small dot is still the same whether it is a halftone or random square dot. Index color separations are done using random square dots that are all the same size. Since halftone dots are not actually dots, but small ellipses, they have a definite pattern and angle to them (figure 2).ĭon’t confuse halftone dots with stochastic random dots (figure 3).

The halftone process does not actually happen until you press the print button and the image is “rastorized” and a program called PostScript converts the gray areas into a series of dots. Images that have shading or tints of a color are made into a halftone as they are output to a printer. This is where the halftone comes in.Ī halftone is simply a group of large and small dots that when viewed at a distance, have the appearance of continuous shades of gray or color in an image. Since a photograph has various tones ranging from solid black all the way to solid white, a method has to be used whereby you can reproduce the photograph in these “continuous tones” and still print a “solid” area of ink. In screen printing and the general printing industry, the screen (or plate in lithography) is only capable of printing solid areas. Note: Click on images to see larger version. I am sure that the other writers, seminar lecturers, and developers of separation software will agree that now that the separations are easy, teaching printers how to work with halftones is the next step. And with low priced color separation software and lots of Photoshop training articles, creating halftone separations isn’t that hard. Even though the average printer’s bread-and-butter is still “spot color”, printers want to do more photorealistic images on light and dark shirts. Over the years a constant source of discuss is about halftone dots.

Photoshop halftone screen frequency