The Colour Depth is a measure of how much colour data a scanner records. This figure is measured in bits. The greater the number, the more colour data the scanner records. Most consumer grade scanners have a colour depth of 24 bits, allowing them to distinguish about 16.7 million different colours.
As the name implies, grayscale depth is a measure of how many shades of grey a scanner records. Grayscale is a scanning mode where the scanner does not record any colour information and instead only records shades of grey. This yields a faster scan than full colour mode, and is useful for scanning newsprint, black and white photography, etc. A grayscale depth of 8 bits allows 256 shades of grey to be distinguished.
The connection/Interface type determines how the scanner connects to the host computer. Most scanners today, like most computer peripherals, connect to the computer via USB. Older scanners may use a Centronics style connector. Some scanner models may support connection via SCSI, or by IEEE 1394 (FireWire) ports.
A scanner's density range is a measure of how well it can cope with images that have a lot of contrast - that contain both very dark and very bright regions. A scanner with high density can discern and reproduce details in both the bright and dark portions of the image it is scanning.
The light source illuminates the image to be scanned. Ideally, the light source should produce a white light (for single pass scanners) that is as close to natural sunlight as possible. Scanners typically use fluorescent bulbs or LED illumination to provide the light for scanning.
Some scanners come with a Media/Document feeder, others support them as accessories that can be fitted to the scanner at a later date. They allow for the batch-scanning of documents, so that multi-page or multiple items can be scanned without user supervision.
The document size is the maximum size of paper that will fit in the scanning area of the scanner. As a general rule, all scanners support the standard A4 paper size, if you need to scan larger sizes than A4 then you will need an oversized scanner.
Some scanners are capable of supporting media types other than just paper. Scanners that can also scan transparencies and film will include features to aid this, such as a backlight to illuminate transparencies from behind.
Resolution is a measure of how much information a scanner records about the image it is scanning. It is measured in Dots Per Inch, Pixels Per Inch or Samples Per Inch (DPI, PPI and SPI respectively), though in all cases they refer to the same thing.
For many scanners the horizontal and vertical resolution are different. In these cases two figures are given in the format HORIZONTAL x VERTICAL, so a scanner with 1200 x 600 resolution scans 1200 DPI across the page, but only 600 DPI as it moves down the page.
This is the resolution at the scanning head, a measure of the scanner's true resolution. The optical resolution describes how many dots per inch the scanning head is capable of sampling. The higher the number, the more detail the scanner is capable of discerning, which in most cases leads to a better quality image.
The software that comes with scanners is capable of "upsampling" a scanned image, also known as Interpolation. This process uses a software method to make the scanned image seem larger than the amount of data actually scanned would allow. Interpolated resolution figures are often wildly exaggerated for marketing purposes, and the interpolation process itself can cause a significant loss of quality, so the interpolated resolution figure is generally fairly meaningless and shouldn't be used as a deciding factor when choosing a scanner.
Scan Element Type
The scan element, or scan head, is the device that converts the image being scanned into data that can be sent to a computer. The scan head contains a sensor that converts the image in front of it into data. Scanners use either Charge-Coupled Devices or Contact Image Sensors (CCD and CIS respectively) for this purpose.
CIS scanners have lower power requirements than CCD scanners, in some models the USB connection alone may be enough to meet the power needs of a CIS scanner. CIS scanners are also less bulky than CCD scanners. However, CCD scanners generally have superior image quality, especially when it comes to scanning something that is not perfectly flat.
A scanner can have either a single pass or a multi pass scanning mode. In the multi pass system, a colour image is built up by scanning the image in 3 stages, known as passes. Each pass is illuminated with either a red, green or blue light source and then the red, green and blue channels are combined into a single full-colour image. Single pass scanners have a single white light source and record red green and blue image data in a single operation. Single pass scanners are naturally faster than multi pass as they only need to scan the image once as opposed to three times as in the case with multi pass scanners, however multi pass scanners can produce higher fidelity colour reproduction and are still popular in professional circles for this reason.
TWAIN is a standard software protocol and applications programming interface (API) that regulates communication between software applications and imaging devices such as scanners and digital cameras. TWAIN compliant imaging devices will be able to import images directly into many popular graphics packages, such as Adobe Photoshop.