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 printer uses a halftone grid divided into cells. The cells contain the halftone spots. How close together the cells in the grid are is measured in lines per inch. This is the LPI or line screen.
How SPI, PPI, DPI, and LPI relate
The lower the LPI the more obvious the halftone dots are in the printed image. A 300-600 DPI laser printer can usually only print at an LPI of 50-65, resulting in coarser images. Higher resolution imagesetters can print a much higher LPI resulting in smoother, almost-like-continuous tone photographs.
Some types of paper can only reproduce lower LPI images because of how they absorb ink and how the dots of ink spread out on the paper. Newsprint typically uses 85 LPI. The halftone cells must be further apart because the ink spreads more (dot gain). High quality coated paper can accept more closely spaced rows of dots at 150 LPI or more because there is less dot gain.
LPI and Scanning: When scanning photographic images you need to know the final output LPI in order to insure that you scan at the proper resolution (SPI). Insufficent resolution will result in lower quality printed images. Too much resolution results in wasted information and unnecessarily large file sizes.
LPI and Digital Images: As with scans, images acquired through digital photography, from the Web, or from CD collections require the right amount of resolution for the final printed LPI. Use the LPI formulas to determine if an acquired image has sufficient resolution for your needs.
LPI and Printing: When printing photographic images you need to know the final output LPI in order to insure that your digital images (scans, digital photos, images from CD, etc.) have sufficient resolution (SPI).
LPI and the Web: On the Web, LPI is not a factor because images display on-screen in pixels (PPI).
The Typical LPI for offset printing ranges from 85-133 lines per inch. The figures are much lower for screen printing and laser printing. High quality offset or gravure printing such as for glossy color magazines may go as high as 300 LPI.
In calculating the required resolution for an image, LPI (based on type of paper and printing method) X 2 is the most commonly used formula (i.e. 133 LPI X 2 = 266 required SPI).
See the LPI Chart for a list of more common paper choices/printing methods and their typical LPI range. The included LPI Formulas show examples for calculating required image resolution under various conditions.
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