Halftones are little dots used to recreate shading. Since screens can only let ink through or block it, shading is produced by little spots of full color separated by space to give it an illusion of grayscale.

Halftones (Wikipedia)

http://www.ted.photographer.org.uk/photoscience_halftones.htm

 

from wikipedia:

Halftone is the reproduction technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing.

Where continuous tone imagery contains an infinite range of colors or greys, the halftone process reduces visual reproductions to a binary image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. At a microscopic level, developed black-and-white photographic film also consists of only two colors, and not an infinite range of continuous tones.

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