Screening Process and Tone Compression

I n offset printing, to be ink-printable, a continuous-tone image such as a photograph is converted into small dots of varying sizes using a camera and a halftone screen or, more commonly, a digital scanner. The original color image is separated into four separate halftone images – one for each of the three process colors and one for black. Historically, reproduction of continuous-ton images has relied on halftone screening methods that produced dots of different sizes in a fixed grid pattern. To be reproducible on press, each area of the original image is converted to a certain dot size to give the same visual appearance as the original image. When printed, areas with larger dots appear darker than areas with smaller dots. The size of each halftone dot is measured in terms of dot area percentage, from 1% to 100%.. In a conventional halftone image, the dot size changes proportionately to the tonal value of the original image.

The coarseness of the grid, or screen ruling, determines the distance form the center of one dot to another. Newer digital screening methods produce very small, similar-sized dots randomly placed, not on a fixed grid. In these screened images, the number of small dots in a given area changes proportionately to the tonal value of the original image. Regardless of the screening method, a continuous-tone image must be converted into small dots to be reproducible on press.

The goal of four-color process printing is to create the illusion of continuous-ton color. Reproducing good tone is considered the first and foremost objective in achieving good color reproduction. The primary factor that limits color reproduction with subtractive color systems is tone compression.

What is tone? Tone is actually the lightness/darkness value of an image. The tonal range of an image is the transition from the highlight (or minimum density) tot he shadow (or maximum density) areas. On a printed sheet, the highlight areas have minimum ink coverage and the shadow areas have maximum ink coverage.

The density range between the highlight and shadow areas can vary from one image to another. One image may have a narrow tonal range while another image can show a wide tonal range. Regardless of the tonal range, the number of density levels in a screened image. In other words., the number of density levels of an original is usual far greater than what is achievable on press. This means the tonal range of an original image must be compressed during the image reproduction process. The result is tone compression which requires that certain parts of the tonal range must be emphasized at the expense of others. Because of this inevitable compromise, a decision must be made as to what parts of an original image are the most important to reproduce accurately. The entire tonal range of an original image is usually difficult to reproduce on press. Detail in the highlight ares may have to be sacrificed to hold the detail in the shadow ares or vice versa.

Tone compression is more manageable if the original image is produced using special photographic techniques. An experienced photographer can adjust the lighting of a subject to change the contrast, or reduce the tonal range, of the original image to match the capabilities of the reproduction process. A low-contrast image requires less tone compression than a high contrast image and is easier to reproduce on press.