Infrared Detectors Image the Future
Smaller, flexible, lower-cost and more versatile detectors satisfy new application demands.
Anne Fischer Lent
The ability to see beyond the capabilities
of normal human vision is vital in areas such as emergency services, defense, science,
maintenance and surveillance, and all of these areas stand to gain by the latest
advances in infrared imaging technology.
Objects at room temperature glow brightest in
the wavelength range from 8 to 10 μm, so cameras that can see within that range
are targeted for applications in security, surveillance, navigation and flight
control, fire fighting and early-warning systems. Cameras with a wavelength range
of more than 10 μm are used in ground-based telescopes, which can survey the
Earth’s atmosphere, image distant stars and galaxies, and search for objects
such as planets in orbit.
These images were captured with Raytheon’s low-power IR camera with a
160 x 120 array.
Today’s thermal infrared imaging
cameras look like video cameras. They fit neatly in the palm of your hand or can
be mounted on a helmet or pole for security, fire fighting and surveillance.
Close-up of BAE Systems’ 28-μm vanadium microbolometer pixel.
At the heart of an IR camera is the
focal plane array, which is a two-dimensional matrix of pixels. Each pixel absorbs
radiation from the object being viewed, generating heat, and converts that heat
to an electrical signal. One recent technological advance that has increased the
applicability of imaging cameras is the drive to larger arrays. The more pixels,
the more discriminating the image will be. Arrays of 320 x 240 pixels were standard
until a few years back when manufacturers came out with 160 x 120 so they could
offer lower-cost products. Today some manufacturers, such as BAE Systems of Lexington,
Mass., offer 640 x 480 microbolometer-based detectors.
As the technology behind the manufacture
of detectors improves, resolution will continue to improve. According to Glen Francisco,
product line manager for amorphous silicon bolometers, fire and security at Raytheon
in Dallas, the technology is there to continue to shrink or grow the resolution
of the array. “It just takes experience, knowledge and a little time,”
Raytheon’s Control IR 2000 AS uses an uncooled, amorphous silicon bolometer.
The other components of an infrared
imaging camera are the optics, which zoom and focus the scene, and the electronics,
which produce the video signal. In the past, most thermal detectors used thermoelectric
coolers to stabilize the temperature, although uncooled detectors are now populating
the market. The drawbacks to using a cooler are that adding a device increases cost
and size, and that it consumes power from the battery.
Commercially available IR detectors
vary in thermal sensitivity and spectral responsivity, with requirements based on
specific applications. Thermal sensitivity is determined by the noise equivalent
temperature difference, which is equal to the temperature difference required in
a scene for the camera to produce a signal-to-noise ratio of 1. Of course, this
value depends on other things as well, such as different lenses or widely varying
BAE Systems’ camera with a 640 x 480 uncooled microbolometer focal
plane captured this thermal image
of a mall.
Spectral responsivity is the detector’s
approximate wavelength range. In general, short-wave IR covers from 1 to 3 μm,
mid-wave IR, from 3 to 5 μm, and long-wave IR, from 8 to 14 μm. Short-wave
IR uses natural reflection and emission, and is targeted for night-vision applications
that make use of available low light levels and sky glow. Mid-wave IR, which provides
good thermal imaging from room-temperature to hot objects, is frequently used in
predictive maintenance and fire-detection applications. Long-wave IR is suitable
for colder conditions and where there is no solar reflection, and is used in surveillance,
military and other applications where cold and solar glare are considerations.
A microbolometer is a micromachined bridge that
often is coated with vanadium oxide or amorphous silicon. These materials absorb
the infrared radiation, and the change in temperature changes its resistance, which
is sensed electronically. There are advantages and disadvantages to each, but the
choice for the end user often boils down to finding a camera that will offer the
resolution, size, flexibility, cost and other features needed for a particular application.
Infrared cameras are used in commercial,
military and scientific applications; the demands of military applications drove
much of the research over the past 10 years. The result is a lower-cost product
that meets the needs of many commercial uses, such as predictive maintenance, process
control, security, night vision in cars, fire fighting, testing and law enforcement.
To meet the needs of commercial markets,
keeping costs down is paramount, Francisco said. He said that Raytheon uses amorphous
silicon in its detectors for just this reason. Detectors manufactured with vanadium
oxide have to be packaged one at a time and take more care to ensure the best performance,
while silicon detectors are designed for easy setup and can be produced more quickly.
High-volume production translates to lower cost and not necessarily to decreased
sensitivity, he said.
The company’s new Control IR
2000 AS is a long-wave-IR video camera core targeted at OEMs supplying the commercial
IR industry. The same camera core can be used in a variety of commercial applications
simply by changing its housing or mount. Among the main users of such cameras are
utility companies that perform preventive maintenance by monitoring lines and transformers
for hot spots. Another increasingly popular use is as a fire fighting tool, although
manufacturers must continue to drop prices below the current cost of about $20,000
before fire departments can put one of the cameras into the hands of each firefighter.
The cantilever approach
Sarcon Microsystems Inc. in Knoxville, Tenn.,
has taken a unique approach to infrared sensing that is based on microcantilever
technology. The new technology results in an uncooled IR detector that approaches
the theoretical limits of infrared sensitivity, said David J. Smith, vice president
of sales and marketing. He said that the company’s new detectors are 10 times
more sensitive than microbolometer-based detectors.
The difference lies in the fact that
the microcantilever technology uses capacitance rather than resistance. The microcantilever
arm is attached at one end to a fixed support structure, while the other end is
allowed to bend. The surface of the detector is coated with an absorbent material,
and when the infrared radiation converts to heat, the temperature of the arm rises.
When the cantilever is heated, it bends, generating an electrical signal in the
detector that is proportional to the intensity of the radiation absorbed by the
sensor. These detectors can be stacked in rows and columns to produce a two-dimensional
array of up to 320 x 240 pixels.
The new IR detectors will be sold to
OEMs along with the supporting electronics. Smith said that they will be used in
the same applications as other types of detectors, but will provide sensitivity
10 times greater than other technologies will, and at a lower cost.
Extended-wavelength cameras are a new
trend for spectroscopy and military applications, and may soon trickle down to the
commercial side. The cameras are built on standard sensor technology with additional
processing of the detector material at the fabrication level. They operate over
a wider spectral range than sensors built the standard way. One example of such
a material is VisMir, developed by Santa Barbara Research Center in Goleta, Calif.
This indium-antimonide sensor material is processed in such a way that it extends
spectral response from the mid-IR (5.5 μm) down into the visible band.
Indigo Systems Corp., also in Goleta,
has developed a material that extends the short-wave response of its standard InGaAs
detectors, offering simultaneous wavelength capability for the visible and near-IR.
Called VisGaAs, for visible indium gallium arsenide, the detectors are responsive
down into the ultraviolet at 350 nm while maintaining response out to 1700 nm.
The benefits of this material, said
Austin Richards, senior applications engineer, is that it provides a broad-spectrum
capability that other sensors can’t achieve. The advantage is that VisGaAs
is uncooled and uses readily available InGaAs as the starting material.
The trend continues
The trend toward smaller size, lower power and
decreased cost will continue while performance improves and opens up new applications.
At the same time, new demands, such as the implementation of devices for the homeland
security initiative, will push manufacturers to continue to innovate.
Gabor F. Fulop of Maxtech International
in Fairfield, Conn., said that it’s an exciting time for this market, which
will continue to grow at a rate of about 20 percent a year. Coupled with evolutionary
advances in electronics, such as more-powerful digital signal processors, infrared
imaging will make important inroads into a variety of application areas.
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