Sidebar: Halftoning and Halftone Cells

Halftones are nothing new, and in fact, have been in use since the late 1800’s. Traditionally, halftones were created via a photomechanical process, which is still sometimes used today: Light passes through a negative image and a halftone screen, exposing a piece of film with a pattern of dots. The dots are evenly spaced but vary in size: Dots in dark areas of a picture are large, while the dots in light areas of the picture are small. It is this variation in size that we perceive as variations in tone. The dots, when printed with black ink on white paper, optically blend to create shades of gray. Color halftones work the same way: The cyan, magenta, yellow, and black dots produce a wide range of colors when they optically mix with each other and the paper color.

Most of the halftones printed today are not produced photomechanically. Instead, software programs and digital printers are used to convert a grayscale or color image into a halftoned image. Digital halftones differ from traditional halftones in several important respects. The first, and perhaps most crucial, concerns the size of the dot. Traditional halftoning produces variable-size dots. A few high-end digital printers can pull off this trick, but most printers on the market produce dots of one, fixed size only.

Digital printers compensate for a fixed-size dot by clumping dots together to form super dots — called halftone cells. A halftone cell is simply a grid of dots. Remember that the dots on a digital printer are small — really small. High-resolution imagesetters print 1,270 or 2,540 dots per inch (that’s about .00039 of an inch per dot). Even a 600-dpi desktop printer produces dots that measure only .00167 of an inch. So even when printer dots are grouped together into a halftone cell, they aren’t noticeable to the naked eye.

The figure below illustrates the relationship between printer dots and halftone cells. The halftone cell is composed of a grid — or matrix — of printer dots. The matrix of a halftone cell is the same size throughout a halftoned picture. For this example, the matrix is only 8 dots by 8 dots. However, the apparent size of the halftone cell will change depending on which dots within the matrix print as black and which are left white. When only a few dots within a halftone cell are filled with black, a light gray tone results. The halftone cell appears to grow larger, and the gray values become progressively darker, as more dots within the matrix are filled with black.

Printers simulate halftoning by using halftone cells. Rather than printing bigger dots to produce darker colors, printers fill more of the dots of their halftone cells.

Halftone cells are themselves arranged into rows, which are called lines. The true measure of halftone resolution is lines-per-inch, or lpi. So you’ll often see halftone resolution defined as Line Screen Frequency, which simply notes the number (or frequency) of halftone lines in a particular unit of measure (such as an inch). All this terminology is a holdover from traditional pre-press techniques. Originally, lines per inch and line screen frequency referred to the number of ruled lines etched into the halftone screen.

>