What is a halftone screen?

Halftone screens are rows of dots used for printing. Halftone screens are required to print photographs. If you look at any commercially printed piece under a magnifying glass, all the images are made up of dots. If the image is in color, you will most likely see dots in the four "process" colors (cyan, magenta, yellow, black):

Since halftone screens are a graphic "representation" of the original image, dot size matters. The smaller the dot, the more detail is visible and the printed piece is closer to the original. Dots sizes have been standardized based on printing technology and paper quality:
  • Newspaper/Laser = 85-120 LPI
  • Professional offset printing = 133-175 LPI

When scanning for print, the standard formula is to double the number halftone dots to arrive at the number of pixels needed. However, most designers have standardized with these two resolutions:
  • Newspaper/Laser = 150 PPI (2 x 75 LPI)
  • Professional offset printing = 300 PPI (2 x 150 LPI)

PPI = Pixels Per Inch (this is used to describe dots per inch or DPI)
LPI = Lines Per Inch (this used to describe halftone screens)
(PPI and DPI mean exactly the same thing and can be used interchangeably)

Note: Inkjet printers do not use halftone screens. Instead they spray random color dots of the same size (more dots for darker colors, less dots for lighter colors). Scans for inkjet printers typically do not need to be more than 300 PPI, even for the highest quality photo printers that spray ink at 2800 DPI.

Also see this Wikipedia article