Flatbed scanners use a beam of light, mirrors, a focusing lens, filters and a sensor called a charge-coupled device (CCD) array to capture the image of a physical photo or other paper document and convert it into a digital file for use on your computer.
The lamp and mirrors are part of the scan head, which is a sliding mechanism that slowly passes beneath the document to be digitized. Once you have placed the paper face down on the flat glass bed, closed the lid and pressed ‘scan’, the scan head will begin to pass underneath the page. As it does so, the lamp casts a bright beam of light onto the document above to illuminate it.
The first mirror on the scan head – positioned just behind the lamp as it passes by the document – is angled so that it reflects the now-illuminated part of the page onto a second mirror. The second mirror then reflects the image onto a third.
At this point the significantly scaled-down picture is reflected from the last mirror through a lens to focus the light through colour filters (usually red, green and blue) and onto the sensor.
This sensor (the CCD array) consists of numerous light-sensitive diodes, which convert light energy (photons) into electrical energy (electrons). This produces an electronic, or digital, version of the image, which you can then port to your computer where you can edit, reprint and/or distribute the file through various means.