Samples per inch
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Samples per inch (SPI) is a measurement of the resolution of an image scanner, in particular the number of individual samples that are taken in the space of one inch. It is sometimes referred to as dots per inch, though that term more accurately refers to printing resolution. Generally, greater the SPI of a scanner, the more detailed its reproduction of the scanned object.
Typical consumer-level flatbed scanners are capable of optical resolution ranging from 100 to 2400 SPI; high-end scanners may have an optical resolution of 4800 SPI or more. Many scanners use interpolation techniques to achieve a higher effective SPI rating, with some manufacturers offering nearly one million SPI, though the quality is primarily limited by the optical resolution; interpolated SPI does not provide additional detail to the scanned image.
Horizontal and vertical SPI ratings may differ for a given scanner; typical flatbed scanners use a horizontal array of sensors that are passed across the bed using an electric stepper motor. The density of the array of sensors determines the horizontal scanning resolution, while the minimum step size of the motor determines the vertical resolution.
See also: dots per inch, pixels per inch.