Halftone
The reproduction of continuous tone by a grid of dots that vary in size, creating the impression of shades from a single ink.
Measured in
lpi
In depth
Presses can only put ink down or not, so tone must be simulated. Larger dots in a fixed grid read as darker areas from normal viewing distance. Each colour is screened at a different angle to avoid the interference pattern called moiré, and the resulting rosette is visible under a loupe on any printed page.
Examples in practice
Examining a newspaper photograph with a magnifier reveals the varying dot sizes that produce its grey tones.