LPI and How It Relates to Other Measures of ResolutionLPI (lines per inch) is an important measurment related to the way printers reproduce photographic images. 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 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.
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