Foundations in Graphic Communications: Checking Color Proofs
Stochastic Screening
  Another way to screen an image for printing is to break it into a pattern of dots that have the same tiny size but are distributed according to image density. This is called stochastic or FM (frequency modulated) screening. It is the opposite of conventional or AM (amplitude modulated) screening, in which dots of varying sizes are spaced at regular intervals on an invisible grid. In an image that has been stochastically screened, shadow areas contain more dots than highlight areas. The "clumping" of the dots, not their size, determines the density of the image in any given area. Compared  with AM screening, stochastic screening can produce smoother tone gradations, sharper detail, and printed color that looks closer to the color of the original. This makes it desirable for high-end product catalogs, art reproductions, and other work with strict quality requirements. However, stochastic screening is an expensive, technically challenging technique that not every printer can provide. Stochastically screened images also are more prone to dot gain than images that have been screened conventionally.

In the black-and-white image below, stochastic screening is seen on the right, and conventional screening is seen on the left. The color image shows FM screening on the left and AM screening on the right.