InGaAs shortwave infrared emerges as key complement to IR for military imaging

InGaAs shortwave infrared emerges as key complement to IR for military imaging
http://www.advancedimagingpro.com/ (2005-04-13)

Although thermal imaging has been well established for military night vision, indium gallium arsenide (InGaAs)-based shortwave infrared imaging (SWIR) is quietly earning a growing place as a vital complement to that technology. It is poised to make night vision capabilities even better than they are today.

Jeff Paul, program manager for the Defense Advanced Research Project Agency (DARPA), says: "The shortwave region contains five to seven times more energy from starlight and the glow of the night sky -- even without moonlight -- than any other spectral waveband. A variety of very interesting applications starts to come to mind once you have an imager that provides good, passive imagery in the SWIR spectral region. We are very interested in this technology."

The reasons are simple. SWIR detectors and cameras can intrinsically see things that thermal detectors cannot, and they can see them through glass. They can transmit digital information more readily than current image-intensifier tubes and with greater sensitivity than intensified imagers attached to CCDs (I2CCDs). Also, unlike SWIR cameras based on mercury cadmium telluride (HgCdTe) or indium antimony (InSb) detectors, InGaAs SWIR cameras operate at 20C, require no cooling or mechanical shutters and never need field Non Uniformity Corrections (NUCs). InGaAs is a proven material, that has been used in high volumes by the telecommunication industry for more than 20 years.

As a result, InGaAs SWIR cameras are as compact, versatile and simple to use as a commercial digital video camcorder. They provide analog video to standard TV or any commercial frame grabber card. Because they deliver simultaneous analog and digital outputs, advanced InGaAs SWIR cameras enable such easy transmission of information that Command and Control can see exactly what the soldier in the field sees as he sees it.

They are also small and light. An InGaAs SWIR camera about the size of a 9-volt battery went into field testing last year on unmanned aerial vehicles, including the Special Forces Pointer made by Aeroenvironment Inc. (Simi Valley, CA)

Also, there is the eye-safe benefit. It so happens that the "hot" part of the InGaAs detector’s response characteristic is large and peaks in the eye-safe (greater than 1400 nanometers) laser range. This makes InGaAs the suitable platform for imaging universal eye-safe laser-target illuminators. Lasers and LEDs at 1550 nanometers, moreover, are invisible to the enemy eyes as well as to night-vision goggles (NVG) and silicon detectors. Older 1060-nanometer lasers, by contrast, blind friend and foe alike and reveal a user’s position to standard enemy imagers.

Covert eye-safe lasers based on InGaAs technology open up a range of military possibilities for the emerging "virtual army," particularly in training, which is where the bulk of military lasers are used. Further innovations now allow InGaAs to cover the entire 400-1700 nanometer waveband, permitting a single imager to detect visible and invisible laser target pointers and designators.

While thermal imagers are good at spotting temperature differentials, they have drawbacks that InGaAs technology covers neatly, making the SWIR a complement to this technology. Thermal imaging does not work well at dawn or dusk, which are known as the crossover points.

What surveillance experts discovered is that SWIR works well around the clock and especially well at those crossover points. SWIR also produces images similar to a black and white visible camera, requiring almost no training to interpret. With SWIR, the foot soldier or tank gunner can tell a tank from a school bus, even on a moonless night. Interpretation of thermal images, by contrast, requires intensive training and, even then, does not permit definitive identifications.

Another drawback of thermal imagers is that they require costly germanium or silicon lenses and cannot see through plain glass. While a soldier can see through a window into a building with a SWIR camera, he cannot with a thermal camera. By the same token, a soldier can drive a HUMVEE at night by looking through the windshield with a helmet mounted SWIR camera inside the vehicle. A thermal camera must be mounted outside the vehicle because it can not see through glass.