Section 4 | Page 5

Screening Process and  Tone Compression

In 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 halftone screen or, more commonly, a digital scanner. The original color image is separated into four separate halftone imagesù one for each of the three process colors and one for black.

Historically, reproduction of continuous-tone images has relied on halftone screening methods that produce 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 proportionally to the tonal value of the original image.

See Figure