What is LPI?
LPI stands for Lines Per Inch. In printers, dots are combined in patterns to approximate lighter and darker colors or lighter and darker grays from a black-only printer. The number of dots in the pattern dictates how many shades (steps) you can make between absolute saturation (on a black & white printer, all dots on would be black, all dots off would be white). So, if you take the number of steps you want between absolute saturation and no saturation, you can calculate the pattern size. Example: to get 256 steps between black and white (the number of steps the human eye can discern normally), I would need a 16 x 16 grid (the square root of 256 is 16). If I had a 600 dpi printer, I would effectively get (600 divided by 16 = 37.5) 37.5 LPI. The LPI represents the apparent crispness and conciseness of focus of the printed image. LPI is normally only a concern for prepress work and professional printers. Examples of various standard LPI’s to help comprehend the issue: Newsprint – 60 or 85
