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During the scan process the light of a certain source of light is reflected on the scan document (reflected light) or shines through the document (transparent light) and is dissected by three filters in the components red, green and blue. These three components are met (either after each other or together) on photo electronic sensors where three values (RGB) with each pixel are registered.
The data depth of every channel is given in bit and normally it lies between 8 bit and 12 bit per channel. When the data is given to the PC it is converted by gamma tables to 8 bit. The maximum resolution of a scanner should be given in pixel per inch (ppi). Normally the number of dots though is given in dots per inch (dpi). Both notations give the number of pixels so they are identical. It is sensible to estimate the quality of a scanner by the amount of pixels in an image. This means that an image of 2,4 cm x 3,6 cm with 1.800 dpi has as much information as an image with 21 cm x 14 cm with 300 dpi. The maximum resolution of many scanner models with CCD sensors is higher in vertical direction than in horizontal. This is because in vertical direction the resolution depends on the mechanical resolution of the feed rate whereas in horizontal direction the number of CCD sensors are vital for the resolution. |
Sheet feed scanners
With sheet feed scanners the imaging sensor stays static, while the original is pulled over it. This is the reason why these scanners are superior when coming to large format originals of any length.
more Flatbed scanners
Flatbed scanners fix the original while the imaging sensor runs over the document line-by-line. Thanks to this multilateral technique, the originals vary from transparancy and A4 to bigger than A0 fromats.
more Drum scanners
Drum scanners have a transparent drum with precise guidance on which the document is possitioned. While the drum rotates with more and more speed, the image is scanned pointwise.
more Repro scanners
Camera and repro scanners use aereal or line imaging sensors. The sensor is possitioned above the document on a tripod and is similar to the setup of a classical repro camera.
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