A scanner saves each image as a series of thousands of tiny elements called pixels. Optical resolution is the number of samples per horizontal and vertical inch that a scanner can scan. The second number quoted is usually the vertical resolution. The vertical resolution is determined by how far the carriage moves between the lines. Interpolated resolution is in reality a marketing technique to make you believe the scanner is really amazing.
Interpolated resolution adds data to the image that really isn t present.
This method averages colors which are near each other and attempts to decide what the in between dots should be. Whenever you add colors mathematically, the results won t be as accurate as the original. Our group rarely needs ppi higher than 300. This is sufficient for a normal photo and even most photo printers. Higher ppi would usually only be needed for high quality line art or scanning film.