Artwork Resolution
One of the most confusing aspects of designing and printing is resolution and the measurement of resolution. You may see other measurement acronyms (DPI, LPI and SPI) and wonder how they correlate. To clarify the terminology, here are definitions and explanations for each term.

PPI
PPI (pixels per inch) is how a monitor displays an image. How the image looks on the screen is determined by the resolution of the monitor, it is the display resolution, not the image resolution.

DPI
DPI (dots per inch) is probably the most familiar and most misused measure of resolution. It is not the resolution of scanned images, or monitor image. DPI is the measure of how many dots of ink or toner a printer can place within an inch. Basically a 600 dpi printer prints 600 tiny dots horizontally and vertically up one inch.

SPI
SPI (samples per inch) is a measurement of image resolution when doing a scan, or using an image from a CD or digital camera. Some scanning manufacturers use DPI in place of SPI in listing their scanners resolution capabilities. This is misleading because there are no dots in the image until it is actually printed.

When you scan images keep in mind the final printing output. Scan too low a resolution and the image will end up with a low quality image. Scan too high and it will use unnecessary disc space. Around 300 dpi is a good resolution to scan.

LPI
LPI is a standard offset printing term, and is dependent on the output device and the type of paper used for printing.

A greyscale, going from 1 shade of grey to another shade of grey, uses a halftone screen divided into cells, which contain spots. The lines per inch is calculated to how many spots there are in 1 inch. It can also be regarded as Line Screen.

When offset printed, the colours of a CMYK job is seperated into each colour, the screen is placed at different angles to produce a better ink mix of seamless, smooth imagery. The lower the LPI the more visible the spots are, 300 to 600 dpi will only produce 50 - 65 LPI, producing a coarser image.