By John Keller
The ability to detect and identify objects in nighttime darkness can come from a wide variety of sensors, but accepted methods involve those that sense light either in the longwave infrared, midwave infrared, or near infrared spectra.
Longwave and midwave infrared (IR) typically are the realm of thermal, or heat-seeking sensors, while near infrared is where light intensifiers come into play. Heat seekers and light intensifiers both have their advantages and disadvantages, yet both kinds are getting smaller, lighter, and of better quality.
Many experts agree that longwave and midwave IR sensors are best at quick detection of objects of interest, such as vehicles or humans hiding in foliage or complex urban environments. These sensors are particularly good at contrasting objects with heat signatures substantially different from their surroundings.
In a general sense, longwave IR sensors have been the standard for ground-based applications, such as thermal viewers on the U.S. Army M-1 main battle tank and M2 Bradley Fighting Vehicle. This sensor, which detects energy in the 8-to-12-micron spectrum, is the basis of so-called second-generation IR sensors that use scanning linear arrays of detectors.
Midwave IR sensors, which became popular in the 1990s, typically have been used for airborne applications to detect targets against the relatively cool background of the sky. These sensors, which detect energy in the 3-to-5-micron spectrum, have been implemented in steered 2D arrays, and have evolved from resolutions of 320x240 pixels to 640x480 pixels.
These days, however, the performance of longwave and midwave IR sensors has become so good that there is no clear difference in their quality, says Mike Scholten, vice president of business development for electro-optical components and technologies at IR sensor designer DRS Technologies Reconnaissance, Surveillance, and Target Acquisition segment in Dallas.
Moreover, IR sensor manufacturers are moving toward implementing detectors in 2D steered arrays in the midwave as well as the longwave spectra, Scholten says.
For night-vision use, the military places a premium on small size, low power consumption, and light weight for easy deployment with infantry soldiers or light vehicles, and today’s infrared sensors are starting to fit the bill. Reducing size and weight typically requires elimination of any external coolers, and longwave and midwave IR sensors both can operate at room temperature.
Key to room-temperature operation of IR sensors has been microbolometer technology that uses vanadium oxide (VOx) and amorphous silicon materials. DRS is producing 640x480-pixel microbolometer IR sensor technology at 25-micron pitch, and is working on even smaller pitches with larger formats, Scholten says.
The primary advantage of the near-infrared light intensifiers, in contrast with infrared sensors, is their ability to identify targets once they are detected because they excel in the detailed images they produce of nighttime scenes.
Near-infrared night-vision sensors detect light in the 0.6-to-0.9-micron spectrum, and are the basis of today’s advanced third-generation night-vision goggles, monoculars, and weapons sights.
“Generation-3 technology is based on the photo cathode, which is a solid-state gallium arsenide compound that is much more sensitive to light energy than were the generation-2 sensors,” says Don Morello, director of U.S. government marketing at ITT Night Vision in Roanoke, Va.
Second-generation light intensifiers combined a microchannel plate with the photo cathode tube to multiply electronics and produce an image. The gallium arsenide-based photo cathode, conversely, “converts the photons to electronics in a much more efficient manner to get more signal out,” Morello says.
“We chose gallium arsenide for its sensitivity to the near infrared, because that is where most of the energy in the night sky is,” Morello continues. “Generation-2 loses sensitivity in the near infrared, while Gen. 3 peaks in the near infrared.”
Fusing sensor data
The next steps for improving thermal sensors and light intensifiers for night-vision devices will involve combining information from several kinds of sensors, as well as operating the devices in pure digital mode without converting to or from analog.
Today’s light-intensifying night-vision goggles, for example, must convert digital data to analog for display in the goggles. Future all-digital night-vision devices will show sensor imagery on small digital displays.
On the one hand, digitized night-vision sensors will offer many opportunities for digital image enhancement and networking, but on the other hand will require small displays that are rugged, power-efficient, and inexpensive enough for widespread military use.
The future Enhanced Night Vision Goggle (ENVG) from ITT will overlay imagery from a longwave infrared sensor, as well as a gallium arsenide-based photo cathode light intensifier. “The beauty of overlaying the technologies in one field of view is to get the best of detection and identification,” Morello explains.
Morello warns, however, that future ENVG devices will be substantially more expensive than today’s third-generation night-vision optics. Moreover, not all military situations will call for ENVG capability; third-generation devices often will be just fine.
“There will always be a mix in the field with the Gen.-3 image intensifiers,” Morello says. “We see the future of Gen.-3 image intensifiers as fairly bright. Our emphasis now is to make Gen. 3 a high-volume, high-quality product. It’s not fair to say we are pushing that technology. The Army has said to industry to keep giving them more of the same.”