DPI

With a background in the printing industry and a future in the graphic design industry I've decided to try and work out the difference between DPI, PPI and what I used to know as "screen ruling" - LPI.

I know that DPI refers to dots per inch and the greater the number of dots, the finer the detail will be. But how does this relate to the physical size of an image? Wikipedia states that inkjet printers (the most common type of personal and small business printer) are mostly capable of up to 360 DPI.

PPI stands for pixels per inch and refers to the measurement of a video display, but is more commonly called DPI.

"The DPI measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colours for each dot typically available on a printer." Because of the differences in the RGB colour system of a monitor which can produce 16,777,216 colours and CMYK system of a printer which can produce 8 colours (CMYK, brown [c+y+m], blue, green and red); "most printers must therefore produce additional colours through a halftone or dithering process". (DPI is also used incorrectly when examining the scanning process. About.com and Wikipedia refer to the accurate measurement as SPI - samples per inch, but i'm not considering that here.)

LPI refers to lines per inch and measures the number of lines in a halftone grid. The higher the LPI, the better the image quality. Newspapers used to be 85 LPI, standard quality colour printing was 150 LPI.

The closest information I could find regarding the relationship of DPI and LPI was from My Design Primer "A bitmap image's resolution should be twice the linescreen." For example, "a color magazine would require an image be 300dpi for best reproduction at 150lpi." Desk Top Publishing has a chart to get an idea of the typical LPI needed based on method of printing and type of paper.

All in all, a very unsatisfactory investigation, I really needed someone with me who could explain in person, but at least I've bookmarked some sites that can help me to sort out individual scenarios when i come across them.