Contact Image Sensor
A scanner technology where a full-width row of sensors sits close under the paper, without the folded lens system a CCD scanner needs.
The name is about geometry: the photodiodes sit almost in contact with the original, with a row of rod lenses (SELFOC) imaging a narrow strip straight up and LED lighting built into the sensor bar. No mirror path, no reducing lens and no warm-up — the whole optical assembly fits in a few millimetres, which makes the scanner flat, frugal enough to run off the USB cable, and cheap to build.
The price is a shallow depth of field. A CCD scanner, with its long optical path, copes with thick originals and open books; the contact sensor blurs the moment the paper lifts a millimetre off the glass. It dominates cheap flatbeds, sheet-fed scanners and fax machines all the same, where the original lies flat anyway.