About CIS Technology

A Contact Image Sensor (CIS) is an integrated module that comprises an illumination system, an optical system and a light-sensing system - all within a single compact component.

Cross section of a CIS scanner sensor

Cross section of a CIS scanner sensor.

The light-sensing system is the silicon surface which is divided up into (e.g. 5000) discrete square cells, the size of each being a fraction of an inch depending upon the required scanning resolution (a 400 dpi scanner will use a CIS whose cells are 1/400" across). The optical system is a rod lens array that directs light from the document surface onto the silicon surface. Such an optical system is 1:1, i.e. there is no reduction or enlargement; and this results in a scanner offering the best performance in terms of resolving power and geometric accuracy.

The illumination is provided by Light Emitting Diodes (LEDs) shining down a light guide which in turn directs the light onto the document. There are three LEDs (red, green, blue) which are strobed rapidly, thereby providing the color content in the scan. Note that there are no color filters in the optical or light-sensing system. The color gamut of a CIS scanner is therefore determined by the spectrum of each of the LEDs, rather than by color filters as is the case with CCDs. Because color filter technology is more developed than LED spectrum technology, a CIS scanner will not provide as wide a color gamut as a CCD scanner.

However, rapidly-strobed LEDs require no warm-up period. Therefore, a CIS scanner can be switched on and used immediately. Further, LEDs are very stable which means that a CIS scanner does not require frequent re-calibration.