Contact Image Sensors (CIS) are a relatively recent technological innovation in the field of optical flatbed scanners that are rapidly replacing CCDs in low power and portable applications. Contact Image Sensor is an integrated module that comprises an illumination system, an optical system and a light-sensing system – all within a single compact component. Like a CCD, a CIS senses light that is incident on its silicon surface and like a CCD, the silicon surface is divided into square cells. However unlike a CCD, the size of each cell on the silicon surface is the same as the size of the information being captured (e.g. in a 600 dpi scanner each cell is 1/600″ across) and information is captured at 1:1. As no reduction or enlargement is required, the distance between the document being scanned and the sensor is very short (~13 mm – hence, contact image sensor).

Advantages and Applications of Contact Image Sensors (CIS)

Contact Image Sensors can save a lot of time and efforts in assembly and maintenance because a CIS contact image sensor is modularized. All the necessary optical elements, such as mirrors, light source and a photoelectric converting device, are included in a compact module. Thus, a CIS module can help to simplify the inner structure of a scanner. Moreover, a CIS contact sensor is smaller and lighter than a CCD line sensor. With a CIS contact sensor, the scanner can be portable with height around 30 mm.

Contact Image Sensor is widely used for fax machines, optical flatbed scanners and portable applications (e.g. portable scanner), electrograph, bar code reader and optical identification technology industry.

CCD Vs. CIS (Contact Image Sensor) Comparison

In many inexpensive flatbed scanners, a contact image sensor replaces the charge-coupled device (CCD) array, lamp, lens, mirrors and filters with rows of green, red and blue light-emitting diodes (LEDs). These CIS mechanisms have 300 to 600 sensors arranged across the scanning area and close to the glass plate. During the scanning process, the LEDs create the white light that illuminates the image, which is then captured by the sensors. Scanners that use contact image sensors are cheaper than those with CCD arrays, but they produce lower-quality images.

Because the optical path is short and simple, quality CIS wide format scanners cost less to manufacture than quality CCD scanners, are lighter and more compact, robust and portable and require little or no calibration.

Because LEDs do not need warm up time (unlike fluorescent tube illumination), CIS wide format scanners can be used as soon as they are turned on and can be turned off when not in use. LEDs also require less power than fluorescent tubes, only come on while a scan is being made, last indefinitely (no consumables) and are environmentally friendly – LEDs contain recyclable components and are mercury free.

As the name implies, Contact Image Sensor systems place the image sensor in near direct contact with the object to be scanned in contrast to using mirrors to bounce light to a stationary sensor, as is the case in conventional CCD scanners. A CIS typically consists of a linear array of detectors, covered by a focusing lens and flanked by red, green, and blue LEDs for illumination. The use of LEDs allows the CIS to be highly power efficient, allowing scanners to be powered through the minimal line voltage supplied via a USB connection. CIS devices typically produce lower image quality compared to CCD devices; in particular, the depth of field is greatly limited, which poses a problem for material that is not perfectly flat. However, a CIS contact sensor is smaller and lighter than a CCD line sensor, and allows all the necessary optical elements to be included in a compact module, thus helping to simplify the inner structure of the scanner.