Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 104

2.4.2 How Scanners Work
Flatbed and slide scanners are popular with many home computer users.
Following is a brief summary on how these scanners work and their most
important technical features. Flatbed and slide scanners are sometimes
used in professional environments in addition to the higher-resolution drum
scanners.
The decisive factor when choosing a good scanner is it´s physical
resolution, i.e. not interpolated. Flatbed scanners have a physical resolution
of 300/600/1200/2400/4800 dpi and higher. Keep in mind that the higher
values boasted by scanners utilizing so-called interpolated resolutions are
calculated by “adding” image dots. This type of interpolation (supersampling)
doesn't actually increase the accuracy or quality of an image; also, you can
interpolate your image by using the image editing program.
When using a flatbed scanner , place the original face down on the glass
plate. Underneath that plate is a sled that carries the charge-coupled device
(CCD) on two rails. The CCD consists of light-emitting components and
sensors that measure the light values reflected from the original document
and subsequently relay them to the computer as image data. The physical
resolution achieved is dependent on the number of light elements and
sensors as well as the sled's speed.
There are two types of flatbed scanners: single-pass scanners and three-
pass scanners.
In a single-pass scanner, the sled passes only once underneath the
original document, capturing the color values for the three primary colors at
once. Three-pass scanners use three passes, one pass for each of the primary
colors—red, green and blue.
Some flatbed scanners provide additional features, such as automatic
page feeders, mounts you can use to hold small photos, transparencies, and
negatives in place. These negatives and slides are scanned like regular original
documents. However, they are exposed to an external light source.
Slide scanners use either the same technical principle as flatbed
scanners or a sensor chip that is similar to the capture chip in a digital camera.
The maximum resolution depends on the arrangement and density of the
sensors on the chip. The highest resolution that can be achieved is dependent
on the alignment and density of the sensors on the CCD line or on the chip.
Most chips can support up to 4800 dpi, and professional devices provide even
higher resolutions. However, many of the scanners in the home user segment
accept only framed slides or film negatives. Professional equipment, on the
other hand, can work with medium formats as well as larger film formats.