Flatbed scanner
A flatbed scanner is a type of scanner in which the material is placed on a glass plate with the side to be scanned facing down . For scanning, a lighting and scanning unit moves through a "bed" under this glass plate, similar to the scanning unit in a digital photocopier . Different versions of flatbed scanners are, for example, the desktop scanner or the XY scanner with variable optics. Formats up to DIN A0 can be scanned with flatbed scanners . Since the scanned material is not drawn in or moved in any other way, any object can be scanned with flatbed scanners, from postage stamps to book pages.
CCD elements
The electronic components for image acquisition are lines of photodiodes . It is a series of around 8000 light sensors that generate an electric current when light hits them. The line-by-line scanning can be done once for each color , or once with a 3-fold line and one color filter each . The light sensors are color-blind, which means that only the intensity of the color is recorded, not the color itself.
The CCD elements can cause image errors due to the differing light sensitivity of the photocells . With the current state of the art, a drum scanner is therefore often qualitatively better than a flatbed scanner.
Cooling of the CCD is necessary for high quality. Energy-saving scan units with contact image sensors and LEDs are also increasingly being used.
Scanning of transparent materials
Transparent objects such as slides and negatives cannot be scanned like paper pictures and other reflective originals, since they have to be illuminated from the opposite side instead of being illuminated from the front. Special flatbed scanners solve this problem with a so-called transparency unit . See: film scanner
