Image Types Halftones Screen Frequency Resolution Exercise
This tutorial contains:
- definitions
and examples of image types:
- line art
- continuous tone
- halftone
- standard halftone screen frequencies
- Image resolution required for halftones
Image Types
There are two basic types of images:
Line art images contain only black and white (no shades of gray).
Line art image.
Continuous tone images contain shades of gray, or a range of colors and shades of color.
Continuous
tone black and white image.
Continuous
tone color image.
Line art and continuous tone images can be in different forms:
Reflective
- Color or black and white photographs
- Drawings, illustrations or paintings
Transparent
- Film negatives
- Transparencies (35mm, 2-1/4, 4 x 5, 8 x 10)
Digital
- grayscale image files
- rgb image files
- cmyk image files
Image Types Halftones Screen Frequency Resolution Exercise
Halftones
In order to print a continuous tone image on a conventional printing press, it must be converted to a halftone: a screen pattern of variable size dots which create an impression of tone values with one ink color.
Enalrged detail of halftone
dot pattern for a grayscale image.
Enlarged detail of halftone
dot pattern for a color image. Note that a separate halftone is generated
for cyan, magenta, yellow and black. Each halftone screen is rotated to
form a pattern called a "rosette".
Halftones are generated by the output device when a file is printed.
- Desktop laser printers have default settings for halftone frequencies based on printer resolution.
- Inkjet printers do not use halftone screens, but use a spray of ink.
- Imagesetters produce industry-standard halftones on negative film for use on commercial printing plates.
Note: Some imagesetters and 600-dpi laser printers use screening technologies other than halftoning. If you are printing an image on a nonhalftone printer, consult your service provider or your printer documentation for the recommended image resolutions.
Image Types Halftones Screen Frequency Resolution Exercise
Screen Frequency
Halftone screens are measured in lpi (lines per inch). This refers to how many rows, or lines, of dots fit in a linear inch.
The number of lpi in a halftone screen is called the screen frequency. It is also referred to as screen ruling or line screen.
The appropriate halftone frequency depends on the paper stock and type of press used for printing. Newspapers commonly use an 85-line screen. Magazines use higher resolution screens, such as 133 lpi and 150 lpi.
Note: Always check with your print shop for correct screen frequencies.
Image Types Halftones Screen Frequency Resolution Exercise
Image Resolution for Halftones
The relationship between image resolution and screen frequency determines the quality of detail in the printed image. To produce a halftone image of the highest quality, you generally use an image resolution that is from 1.5 to at most 2 times the screen frequency. But with some images and output devices, a lower resolution can produce good results.
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Formula for standard quality halftones (image fine detail of nominal importance): Image resolution = screen frequency x 1.5
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Formula for high quality halftones (image fine detail of great importance): Image resolution = screen frequency x 2 Example using 150 lpi halftone screen: 150 x 2 = 300 The correct image resolution is 300 ppi. |
To determine your printer's screen frequency, check your printer documentation or consult your service provider.
Image Types Halftones Screen Frequency Resolution Exercise