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. The most common type of scanner used in the home and the office is the flatbe ...

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

inch The inch (symbol: in or prime (symbol), ) is a Units of measurement, unit of length in the imperial units, British Imperial and the United States customary units, United States customary System of measurement, systems of measurement. It is eq ...

. 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. E ...

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 mathematics, 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 ...

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 often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

that are passed across the bed using an electric

stepper motor A stepper motor, also known as step motor or stepping motor,Clarence W. de Silva. Mechatronics: An Integrated Approach (2005). CRC Press. p. 675. "The terms ''stepper motor'', ''stepping motor'', and ''step motor'' are synonymous and are often u ...

. 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 A sensor array is a group of sensors, usually deployed in a certain geometry pattern, used for collecting and processing electromagnetic or acoustic signals. The advantage of using a sensor array over using a single sensor lies in the fact that an ...

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 or Tagged Image File Format, commonly known by the abbreviations TIFF or TIF, is an image file format for storing raster graphics images, popular among graphic artists, the publishing industry, and photographers. TIFF is w ...

) or lossy image formats (like

JPEG JPEG ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...

References

{{DEFAULTSORT:Samples Per Inch Computing input devices Units of density

See also

References