Samples per inch
Encyclopedia
Samples per inch 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 linear inch
. 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.
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 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
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. 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
).
Image scanner
In computing, 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. Common examples found in offices are variations of the desktop scanner where the document is placed on a glass...
, in particular the number of individual samples that are taken in the space of one linear inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...
. It is sometimes misreferred to as dots per inch
Dots per inch
Dots per inch is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch . The DPI value tends to correlate with image resolution, but is related only indirectly.- DPI measurement in monitor...
, though that term more accurately refers to printing
Printing
Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....
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
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 a lens and 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
Interpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
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
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
that are passed across the bed using an electric stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
. 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 :
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
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
).
See also
- dots per inchDots per inchDots per inch is a measure of spatial printing or video dot density, in particular the number of individual dots that can be placed in a line within the span of 1 inch . The DPI value tends to correlate with image resolution, but is related only indirectly.- DPI measurement in monitor...
- pixels per inchPixels per inchPixels per inch or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners, and digital camera image sensors....
- lines per inchLines per inchLines per inch is a measurement of printing resolution in systems that use a halftone screen. Specifically, it is a measure of how close together the lines in a halftone grid are. Higher LPI indicates greater detail and sharpness....