A line consists of halftones that is built up by physical ink dots made by the printer device to create different tones.
Specifically LPI is a measure of how close together the lines in a halftone grid are.
In order to effectively utilize the entire range of available LPI in a halftone system, an image selected for printing generally must have 1.5 to 2 times as many samples per inch (SPI).
For instance, if the target output device is capable of printing at 100 LPI, an optimal range for a source image would be 150 to 200 PI.
Conversion of the LPI to the DPI is done by simple multiplication:[1] Countries using the metric system tend to use lines per centimeter (L/cm).