Halftone

Halftoning is a method of printing shades using a single colour ink. By varying the size or density of the dots, the eye can see a shade somewhere between the solid colour and the colour of the background paper. The effect has its limits, when the dots get too small or spaced too far apart, the eye starts seeing dots again.

A pattern of dots that is used to represent a particular shade is known as a halftone screen. The number of dots measured horizontally in one inch is the typical way of measuring a screen, known as the screen ruling and represented with a hash mark. For instance, a screen with 150 dots per inch would be referred to as "150#".

When different screens meet, a number of distracting visual effects can occur, including the edges being overly emphasized. This problem can be reduced by rotating the screens in relation to each other. This screen angle is another common measurement used in printing, measured in degrees clockwise from a line running to the left (9 o'clock is zero degrees).

Halftoning is also commonly used for printing colour pictures. The general idea is the same, by varying the density of the four primary printing colours, cyan, magenta, yellow and black, any particular shade can be reproduced.

In this case there is an additional problem that can occur. In the simple case, one could create a halftone using the same techniques used for printing shades of grey, but in this case the different printing colours have to remain physically close to each other to fool the eye into thinking they are a single colour. To do this the industry has standardized on a set of known angles, which result in the dots forming into small circles or rosettes.

Links:

Halftone Screens and Dots (http://www.dtp-aus.com/hlftone.htm)