It is sometimes misreferred to as dots per inch, though that term more accurately refers to printing resolution.
Generally, the greater the SPI of a scanner, the more detailed its reproduction of the scanned object.
Similar characteristics are present in drum scanners, which continuously spin the item being scanned past the sensor array for numerous imaging passes.
To calculate the number of raw data bytes that a scanned image will take up, you can use the follow formula :
Where : vSPI is the vertical SPI hSPI is the horizontal SPI (can be considered the same as vSPI if not specified specifically) area is the squared area of the scanned document in inches² color depth encoding is the number of bits used to encode a given amount of color information (256 colors=8; 65'536 colors=16; 16million colors=24) Remember that this will give raw data bytes, images are almost always compressed when saved to disk using lossless (like PNG, TIFF) or lossy image formats (like JPEG).