The light in the photo is short-wave infrared. It reveals a person who had been hiding in the shadow of the trees.

The decision-making process of threat detection, assessment and response is only as good as the information provided by a security system, which must function with speed and accuracy.

The backbone of most security systems are video cameras, which can be categorized by the various bands of the electromagnetic spectrum that they use for night vision. These are:

• The visible bands (wavelengths 0.4 - 0.7 mm), where human eyes and CMOS or CCD silicon image sensors can see;
  
  
• Near-infrared (NIR, 0.7 - 1.1 mm), where CMOS or CCD cameras can image, but the distance and field-of-view they can see depends on the power of the illuminators;
  
  
• Mid-wave infrared (MWIR 2.0 - 5.0 mm) and long-wave infrared (LWIR 5.0 - 20 mm) or thermal imaging, whose cameras have sensors that detect the emissive heat properties of objects; and
  
  
• Short-wave infrared (SWIR 1.1 - 2.0 mm).

The night sky actually contains a considerable amount of light or night glow in the SWIR part of the spectrum. It emanates from a source of OH (Hydroxyl) ions, which exist at a height of 90 km in the earth’s atmosphere.

These ions absorb energy from the sun during the day, which makes them vibrate. As they vibrate, they emit energy as SWIR light, which during the night provides a natural and significant source of free illumination that can be used for excellent night vision.

Cameras that could image in the SWIR band previously have required image sensors made out of complex and expensive semiconductor materials.

Enlarge this picture The detail in the short-wave infrared (SWIR) illumination in the photo in the middle allows it to be used for facial recognition in the dark.

Corporations limited by their security budgets may compromise external perimeter surveillance coverage in favor of locations where lighting can be provided or less expensive short-range NIR cameras can be used.

In applications where budgets are larger, expensive thermal cameras can be used. Yet for security, a number of drawbacks should be considered. The first is that sensing heat emitted from a person versus reflected light loses significant image detail. Facial features required for identification are lost in a thermal image.

Second, objects such as vehicle license plates or ID badges cannot be read for identification purposes. The last significant drawback is that thermal imagers cannot image through glass. As such, specialized expensive optics need to be developed, adding to camera cost, and traditional protective glass or plastic enclosures for camera systems cannot be used.

The SWIR light emitted by the earth’s atmosphere offers a natural and zero-cost illumination source. A significant advantage is that it has no shadows, because it is not emitted from a single point.

The issue with utilizing this night glow for security cameras has been cost. TriWave technology from NoblePeak Vision Corp., Wakefield, Mass., has solved this problem by taking a standard CMOS image sensor process, and by using a germanium enhancement technique, has extended its wavelength sensitivity into the SWIR band.

Cameras featuring this technology will be available from various camera OEMs for field trials in the second quarter of 2008. For more information, visit www.noblepeak.com.