Halftones and Line Screens

In offset lithography, the density of CMYK inks can not be varied in continuous fashion across an image, so a range is produced by means of halftoning. In halftoning, tiny translucent CMYK ink dots of variable sizes are printed in overlapping grids. As the dots get smaller, so does the shade of color that is reproduced. Grids are placed at different angles for each of the ink colors. Smaller halftone dots absorb less light; thus, as a result of an increase in the amount of reflected light, apparent density is decreased and the object appears lighter.

halftoning screen angles (133lpi 40% screen enlarged)

A Line Screen is the measure of how many halftone lines are printed in a linear inch. The value is expressed as Lines Per Inch (LPI). This important measurement related to the way printers reproduce photographic images also defines the necessary resolution of an image. The LPI is dependent on the output device and the type of paper. Countries using the metric system may use lines per centimeter (L/cm).

To simulate shades of gray using only black ink a printer prints varying sizes and patterns of halftone spots (spots are made up of many dots of ink/toner). Small halftone spots (fewer dots) create the visual illusion of a light gray while larger halftone spots (more dots) appear darker, blacker.

The lower the LPI the more obvious the halftone dots are in the printed image. Look at these typical halftone ranges:

• A 300-600 DPI laser printer can usually only print at an LPI of 50-65, resulting in coarse images

• Because of the absorbency of newsprint, newspapers typically use 85 LPI (you can see this with your naked eye!)

• Imagesetters and platesetters print at much higher resolutions and can print up to 200 LPI (you need a magnifying glass to see these)

• Most all of TSG's print products are printed at 133 lpi (according to SWOP Specifications for Web Offset Printing)

• The main decision to use certain Line Screen versus another mostly depends on the paper being used for printing (uncoated stock generally uses a lower LPI, and coated stock generally uses a higher LPI)

The simple formulas below will help you determine if your image has sufficient resolution for your print needs. The general rule of thumb is to have images with a resolution of 2 times the line screen.

133 lpi requires images at 266dpi (133 lpi x 2 = 266dpi)

150 lpi requires images at 300dpi (150 lpi x 2 = 300dpi)