Goonetilleke
INFRARED IMAGING TECHNOLOGY
Acronyms:
MRTD - Minimum resolvable temp difference. (0.1 0C is generally adequate)
MCT - Mercury Cadmium Telluride
CCD - charge coupled device
CMOS - complementary metal-oxide semiconductor
MWIR - mid wave IR (2.5 - 7 m)
LWIR - Long wave IR (7 to 15 m)
MDT - minimum detectable temperature
NEDT - Noise equivalent temperature difference
FLIR - forward looking infrared systems
TEC- thermoelectric cooler
COOLING
Cooling can be through liquid nitrogen, Stirling cycle, or thermoelectric. LWIR photon
detectors have to be cooled below 100 K with 77K considered a typical temperature. These
temperatures can only be reached with a finite lifetime mechanical cooler or with liquid our
nitrogen. Many MWIR detectors can operate at 20K and this temperature can be easily
achieved with a thermoelectric cooler. TECs appear to have an infinite lifetime. Coolers add
cost, bulk, and consume power. Thermal detectors can operate at room temperature and
therefore are called uncooled devices even though a TEC is generally used.
"Uncooled" thermal imaging detectors refers to detector arrays that operate at or
above room temperature. The term "uncooled" is used to distinguish this technology from the
historical norm, which is to use detectors that only operate at cryogenic temperatures, e.g.
the temperature of liquid nitrogen (77oK) or lower.
PHOTON DETECTORS
The most popular is Platinum Silicide (PtSi) which is sensitive in the 1-5.5 m region.
PtSi has low quantum efficiency (less than 1%). Most IR detectors are photon counters; that
is they count IR photons over very short periods of time. Quantum efficiency refers to the
relative efficiency at which IR photons are collected and converted into electrical charges.
PtSi operates in the short wavelength region (1-5 m), has good sensitivity (as low as 0.05
C), and has excellent stability.
Two other detector materials (both need cooling) commonly used are
1. Indium antimonide (InSb) (responsivity 2.2 - 4.6 m, high quantum
efficiency of around 80-90%) and
2. Mercury Cadmium Telluride (HgCdTe) - (responsivity 8 - 12 m). By
varying the Hg, Cd, Te mixture, it can be tailored to the MWIR or LWIR.
The most popular is the LWIR.
Both can work with thermoelectric (Peltier effect) cooling in the 3 m band, but MCT
requires cryogenic temperatures from liquid Nitrogen or Stirling cycle coolers to work at 8 to
12 m. Others are microbolometers. The microbolometer detector is a thermal detector
rather than a photon counter. It actually heats up as a result of being exposed to IR energy,
which changes its electrical resistance proportionately. The most benefit from a
microbolometer is that cryogenic cooling devices are not necessary. They also operate in the
long-wave length region making them useful in outdoor and low-temperature applications.
Thermal detectors usually have much lower sensitivity than photon detectors. As a result,
they will not replace photon detectors in critical, low signal-to-noise applications. With
pyroelectric detectors, thermal changes alter the electrical polarization which appears as a
voltage difference.
1
Goonetilleke
Two basic uncooled detector types have emerged today, ferroelectric detectors and
microbolometers. Ferroelectrics have been developed by Texas Instruments and GEC
Marconi; microbolometer technology has been developed by Honeywell. Ferroelectric
detector technology (as is used in the PV-320) takes advantage of a ferroelectric phase
transition in certain dielectric materials. At and near this phase transition, the electric
polarization of the dielectric is a strong function of temperature; small fluctuations of
temperature in the material cause large changes in polarization. Then, if the sensor is
maintained at a temperature near the ferroelectric phase transition and if the optical signal is
modulated (with a synchronous chopper), then, an infrared image can be readout that
reflects the scene temperatures. Microbolometer arrays, on the other hand, consist of
detectors made from materials whose electrical resistivity changes with temperature. Each
detector is part of a readout circuit that measures the resistance of the element as a signal.
The ferroelectric BST array is a ceramic material consisting of barium, strontium and
titanium salts. The nominal desired composition is Ba0.66Sr0.34TiO3. Because ferrolectrics
retain their electric polarization after
application and removal of an electric field, their polarization depends on temperature.
Human skin is a near perfect blackbody in the 8 to 12 m bands. Human body
temperature has its blackbody emission peak in the 8 to 12 m range. At 3 to 5 m human
skin reflect incident radiation, making the detected radiation in this band not necessarily
related to body temperature.
FOCAL PLANE ARRAYS
There are a several distinct technologies of thermal imaging available today. Most of
the newer camera designs are based on a focal plane array (FPA) device, that is a twodimensional array of infrared detectors used to create an image. (Earlier systems used either
a single element detector or a small array of detectors and scanned the scene across the
detectors with rotating mirrors). Other popular FPA technologies include both uncooled
(microbolometer, proelectric vidicon) and cooled (platinum silicide, indium antimonide) FPA
systems. The advantage of uncooled systems is system lifetime and cost (cooled sensor
systems need to be chilled to temperatures as low as 77K which requires the use of an
expensive and highly intricate mechanical cryogenic system).
A focal-plane array (FPA) detector is any detector that has more than one row of
detectors. A typical infrared FPA system has 256*256 detectors (256 columns and 256
rows). Detector arrays of 256 * 256 are common with MRTD down to 0.03 0C.
There are two types of infrared FPAs: monolithic and hybrid. Monolithic FPAs have
lower performance compared to their hybrid counterparts because of a lower fill factor. A
higher fill factor results in a much higher sensitivity. When a CCD detector is used in a
measurement infrared FPA camera, the errors must be compensated for. Optimum battery
life is achieved by using a CMOS multiplexer detector readout and high-efficiency rotary
Stirling cooler.
2