About Posterization and Gray Levels

The number of dots in a halftone spot determine the number of available gray levels. For example, a four by four dot halftone cell will yield 17 gray levels (multiply 4 x 4 = 16, + 1 for either all dots on [black] or all dots off [white]). A 16 x 16 dot halftone cell will yield 257 gray levels (multiply 16 x 16 = 256, + 1 for either all dots on or all dots off).

However, there is an inverse relationship between the halftone screen frequency and printer resolution. When you have a halftone with a high screen frequency, you are asking the printer to create more "spots" per inch. In order to create more spots, the spots must become smaller. In other words, a finer halftone screen slices the printer dot grid into smaller cells. When halftone cells shrink in size, the number of dots in each cell is reduced. This results in fewer available gray levels, and leads to images becoming posterized.

Therefore, ways to maintain gray level availability are:

• Use a lower screen frequency when scanning at high resolution
• Scan at lower output resolution when using fine halftone screens

NOTE: This information is important when scanning below 600 dpi. It does not apply to copy or during reduction.

NOTE: For more information on how halftone screen frequency and printer resolution are related, consult Real World Scanning and Halftones, 2nd Edition by Blatner, Fleishman, and Roth.

TIP: If you have programmed and run a job, and received unexpected results, ensure that the system-wide settings are all set back to their defaults by clicking <Clear All> or <Reset> in the user interface. Then proceed to do job programming pertinent to your job.