The idea of this document is to explain some basic principles of scanning hardware and software. It's not related to any specific scanners nor to any specific scanning software.
At the moment it's a template only. Feel free to add what you are interested in.
Flatbed scanners provide a glass pane where the physical image is placed face down. The image is scanned by a movable sensor below the glass while the original does not move. The size of the physical image is limited by the glass pane.
This type of scanner has a fixed sensor whereas the physical image (piece of paper) is moved during the scan. Most scanners of this type have an automatic document feeder which can carry more than one piece of paper.
These scanners are used to scan transparent material like negative films or slides.
Handheld scanners are moved manually on the physical image. Because of practical reasons the image width is limited but the length is theoretically endless. While handheld scanners were qwuite popular about 10 years ago they are seldomly used nowadays.
For most scanners one or more of the following accessories exist, either optionally or already included.
A transparency adpater is used to scan transparent images (negative films, slides) on a flatbed scanner. A fixed or movable lamp is mounted on top of the scanner. Therefore the physical image is between the light and the sensor. Most scanners turn off the internal lamp when using the TA. Often the scan area is smaller then the full flabed area and the images are mounted in special frames.
While sheetfed scanners always provide automatic document feeders, ADFs are also available for some flatbed scanners. Usually a stack of papers can be placed on the scanners. With an automatic document feeder, many pieces of paper can be scanned in one batch without manual intervention. The pages are either moved through the scanner at constant speed and fixed sensor or scanned by a moving sensor after beeing placed on the fladbed automatically.
The image sensor is that part of the scanner, which translates the physical image it "sees" to analogue electrical signals. The physical image which is to be scanned is illuminated by a lamp (or LEDs). For reflective scanning (e.g. papers), the lamp is usually part of the sensor mounting plate or slider. For negative or positive scanning, the negative film or slide is located between the lamp and the sensor. While several types of image sensors are used, these are the main categories:
A charged-coupled device (CCD) sensor is an integrated circuit based on a one-dimensional array of capacitors. The image is projected on the CCD by a lens and/or mirror system from the physical image. There is one sensor line with included filter for each color (RGB, sometimes infrared). Older three-pass scanners had only one sensor line and a movable filter, which required three passes two scann in a color image. Usually cold cathode fluorescent lamps (CCFL) are used to illuminate the physical image.
Keep in mind that some of the steps mentioned here can be done by the scanner itsself (its firmware) without engagement of the driver.