Samples per inch (SPI) is a measurement of the resolution of an

image scanner An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting or an object and converts it to a digital image. Commonly used in offices are variations of the desktop ''flatbed scanner'' ...

, in particular the number of individual samples that are taken in the space of one linear

inch Measuring tape with inches The inch (symbol: in or ″) is a unit of length in the British imperial and the United States customary systems of measurement. It is equal to yard or of a foot. Derived from the Roman uncia ("twelfth ...

. It is sometimes misreferred to as

dots per inch Dots per inch (DPI, or dpiThe acronym appears in sources as either "DPI" or lowercase "dpi". See "Print Resolution Understanding 4-bit depth – Xerox" (PDF). Xerox.com. September 2012.) is a measure of spatial printing, video or image scanner ...

, though that term more accurately refers to

printing Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...

resolution. Generally, the 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 Optical resolution describes the ability of an imaging system to resolve detail, in the object that is being imaged. An imaging system may have many individual components, including one or more lenses, and/or recording and display components. ...

ranging from 100 to 2400 SPI; high-end scanners may have an optical resolution of 4800 SPI or more. Many scanners use

interpolation In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has ...

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 pixels in the scanned image. Horizontal and vertical SPI ratings may differ for a given scanner; typical flatbed scanners use a horizontal array of

sensors A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

that are passed across the bed using an electric

stepper motor A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any posi ...

. The density of the array of sensors determines the horizontal scanning resolution, while the minimum step size of the motor determines the vertical resolution. 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 :

\tfrac

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 Tag Image File Format, abbreviated TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is widely supported by scanning, faxing, word process ...

) or lossy image formats (like

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...

See also