Most scanners start at about 300 x 300 dots per inch - in other words, the scanner reads 90,000 pixels per square inch for the image being scanned. When a scanner reads an image it makes two passes, first down the document and then back up it. Sometimes a scanner's resolution has two different figures, like 300 x 600, which indicates the scanner stops more often on its vertical pass than its horizontal run. Though this may improve the reproduction process slightly, the smallest number in the resolution description should always be considered the most reliable indicator of quality. Optical resolution measures the true number of pixels, with a larger resolution equaling a higher quality reproduction.
Interpolated resolution uses mathematical computations to increase the number of pixels scanned.
In plain terms, interpolated resolutions average two pixels side by side to create a third pixel that would theoretically equal the color of a pixel between the two dots. In effect, the scanner guesses at what pixels would look like if they were squeezed in between the pixels already being scanned. For most purposes, optical resoutions are a more reliable indicator of a scanner's true resolution capacity. There are generally accepted minimum resolutions for different scanning tasks. Most magazines have a linescreen between 130 and 150 lpi, so resolutions of 260 to 300 dpi should be sufficient for high end materials.