DPI is the maximum number of dots per inch a printer can access per inch. And since computers/printers are all binary, these dots are either ON or OFF. They make up a grid pattern as shown on the left.</div>

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LPI is lines per inch - or a screen - the number of Rounded dots (made of the smaller square DPI dots) per inch.(right diagram)

Since the printer's dpi dots are either ON or OFF they cannot print shades of colours like grey. It is limited to black or white. In order to simulate shades of grey - the printer uses round dots of differing sizes that, when places beside on another at high resolution, trick the eyes into assuming that the area is grey. </div>

In order to make round dots - the printer must use a pattern of SQUARE dots - as shown on the left. The higher resolution (Dots per inch) the printer, the rounder the dots look.

It is called Lines per inch because each dot has a center point and is created at a different size depending on the shade of grey to be used and these center points are assigned by lines per inch. See the right of the diagram.</div>

Look closely at a photograph in the newspaper. It is made of little dots of differing sizes.</div>

A newspaper photo graph is typically 85 lpi. So if you took a magnifying glass, you could count approximately 85 little round dots of various sizes in an inch. It would take a minimum of a 600 dpi printer to achieve dots that look round.</div>

A glossy magazine is typically 150 or 200 lpi. Much higher resolutions in order of 2400 DPI and above are required to achive this.</div>

LPI usually has an angle as well. Typically black is placed at a 45 degree angle so that the eyes do not notice that it is really a grid. Tip a newpaper at 45 degrees so that the grid is straight up and down - it gets annoying.</div>

Control over LPI is only truly available on Postscript capable printers. Inkjets and non-postscript laser printers use different dithering principles to achieve grey shades.</div>


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