DocuTech Family

Printing Halftones

Background Information

The number of dots per inch your printer will print is a constant. You have no control over it.  You can get that information from your printer model's specifications. Xerox DocuSP printer output is 600 dpi.

There is an inverse mathematical relationship between the number of lines per inch (lpi) you choose to create your halftones and the number of grays that your printer can print for that job. The higher the number of halftone cells you call for at a given printer dpi, the lower the number of gray tones the printer can reproduce.

The Xerox DocuSP printers' optimum halftone frequencies are 53 lpi, 85 lpi and 106 lpi. They can be set at the printer's user interface by logging on as System Administrator, selecting the Printer Manager icon and then by selecting the Image Quality tab, right-clicking on the PostScript/PDL line for 600 dpi resolution and left-clicking on Properties.

High end graphics applications allow you to control the halftone frequency that the PostScript driver will use to generate the grays. They should match the optimum settings of the printer on which they'll be printed or else you'll see distortion patterns (called moirés) in the images.

The formula for determining the number of grays you can print in a job is as follows:

How it Looks

This is a picture of a linear gradation from black to white for a 600 dpi image at 106 lines per inch halftone frequency.

106lpi.jpg (3424 bytes)

This halftone frequency is useful for business graphics because the steps between the adjacent available grays provide enough contrast between text and gray backgrounds. It's not as useful for photographs because the contrast between adjacent shades of gray causes posterization, banding or stepping.

This is the same picture at 85 lines per inch halftone frequency.

85lpi.jpg (3600 bytes)

At the risk of losing contrast between text and gray backgrounds, this halftone frequency smoothes out  gray-to-gray transitions in photographs.

This is the same picture at 53 lines per inch halftone frequency.

53lpi.jpg (3116 bytes)

If you have the option, this halftone frequency is ideal for print jobs containing a lot of quality photo work. The content provider may need to exaggerate the difference between text color and background gray fields in order to maintain text legibility.

Here's a calculator to do the math

    Enter Printer Resolution (dpi):
    Enter Halftone Frequency (lpi):
    Read # of Gray Shades:
     

Created by Norman A. Teck
March 8, 2004