How Does a Flatbed Scanner Work?

By Edward Mercer

Flatbed scanners work much like digital cameras.
i Thomas Northcut/Photodisc/Getty Images

A common item in many offices, a flatbed scanner helps digitize physical documents for easier sending and storage. Using a flatbed scanner is very simple – requiring no more than loading a document and starting the device – yet the technology behind scanning is fairly advanced. To put it rather simply, a flatbed scanner is something like a digital camera, using mechanical and optic processes to create a digital image of the loaded document.

Loading a Document

A scanner needs a source document from which to begin – anything from a photograph to a letter. In the case of a flatbed scanner, this document is placed against the top glass surface of the scanner and the lid is closed over the document to prevent light from entering the device. The transparent glass keeps the document in place while allowing the sensors to detect document details. As in a camera, the dark conditions allow the scanner to control the amount of light reflected on the document surface.


Once the document is in place and the lid is closed, a moving belt slides a light source across the entire surface of the document. The motor in a scanner is finely tuned to make sure this light source travels at the same speed and creates the same amount of light across the whole document, ensuring a uniform exposure. The light shined on the document then reflects back into the machine and is reflected onto the lens by a series of mirrors.


Much like a digital camera lens, a scanner lens includes a photosensitive element that detects differences in reflected light as different shades of color. Most scanners use an electronic light-sensitive circuit known as a charged couple device, although some high and low end scanners use different technologies like photomultiplier tubes or contact image sensors. All of these devices turn shades of light reflected off the document into digital information on shades of color to be located at different parts of the final document.

Sending a Document

The optic information from a scanner sensor is turned into digital pixel information in order to be saved as a digital file, such as a JPG, PNG or TIFF. A scanner relays this information to a computer through a wireless or connecting cable connection to save the digital file. After the transfer, the image file of the document is accessible on the computer and can be opened, saved, edited or deleted just like any other digital picture file.