| Uncooled thermal imaging works round the clock |  Thermal Image produced by a QinetiQ designed Thermal Imaging system |
| Lawrence Mayes |
Thermal imaging relies on the infrared radiation given out by all bodies and because it doesn’t rely on external illumination is a 24-hour capable technology.
Work started at QinetiQ's Malvern site on uncooled thermal sensors in the mid 1960s and in 1969 the first product: a single element detector for use as a burglar alarm (PIR) sensor emerged. The first thermal imaging (TI) detector was the pyroelectric vidicon (PEV) - a type of tv pickup tube - which was developed in the early 1970s in collaboration with English Electric Valve company; this product was used in the production of 11,000 fire-fighting cameras until focal plane arrays of pyroelectric detectors became available; even now PEV tubes are still in production in the former Soviet Union and China. In the 1980s, it became clear that focal plane array technology offered a cheap and flexible alternative to photon detectors, which was, at that time, the only technology capable of producing high quality thermal images.
Initially, 1D arrays were made which needed a mirror scanner to produce an image; however, 2D arrays (starting with a 16x16 element device in 1983) followed.
Image resolution of uncooled sensors has risen steadily: in 1990 an array with 100x100 individual detector elements was made, resolutions of 258x128 were achieved in 1994 rising to 384x288 in 1996, which, at the time, was the world’s largest uncooled TI array.
Current developmentCurrent work on detector technology has two strands - one which, using the current detector material technology, will improve the sensitivity of detectors by using a new detector element structure - microbridge technology. The second strand concentrates on lower resolution arrays that will be physically small, low power and extremely cheap to produce.
The microbridge structure (see figure 1) achieves high sensitivity by thermally isolating the detector material from the readout chip by placing it on table-like structures. The legs mechanically support and provide electrical contact to the detectors.
Underlying technology
Photon detectors require cooling to
temperatures around -100ºC to -200ºC and manufacture is
difficult and expensive. Thermal detectors produce an
electrical signal in a different way - a change in
temperature of a material gives rise to a change in some
property which is measured electronically, e.g., resistance. As any
energy absorbed by the detector will change its temperature
the thermal detector is a potentially wavelength independent
(although its response is usually tailored for specific
applications by choice of an optical window material). Since
a temperature change in the detector is measured,
thermal detectors are able to operate at room temperature.
Imaging a scene on an array of such detectors produces a
pattern of minute temperature variations in the detector
plane. Objects at room temperature have a peak emission is in
the infrared at around 10 microns and as the atmosphere has
an low attenuation window between 8 - 14 microns; this band is
usually used for uncooled TI. Atmospheric obscurants such as
mist or smoke are more easily penetrated by infrared
radiation than visible light and so make TI useful on the
battlefield and in civilian applications such as
fire-fighting.
At QinetiQ pyroelectric (ferroelectric) bolometers are used; this is the same material technology use in PIR detectors - the feature that changes with temperature is the electrical polarisation - for a change in the material’s temperature there is a corresponding change in charge distribution which will produce a measurable voltage. A complete detector comprises the ceramic detector material itself ‘diced’ into individual detector elements and each connected to a silicon readout chip. The element size is around 50 micron square with a typical array around 10 x 20 mm.
Recent breakthroughs
Advanced digital signal processing is
required to produce defect-free images from uncooled arrays -
each detector element has a slightly different sensitivity
and signal processing corrects for these variations. This is
achieved by automatically measuring the responsivities of all
the elements in a detector, storing the correction factors
and applying these to the detectors’ outputs in real
time.
Market applications
Commercial applications include
predictive maintenance, process control, non-destructive
testing, law enforcement, medical, driver vision enhancement,
piloting aids, fire-fighting, and security. Fire-fighting is
the one major commercial area where the QinetiQ-developed
100x100 element sensors have seen commercial exploitation in
products marketed by BAE (in the UK) and Cairns (in the US).
These have replaced the former PEV-based imagers.
Low-cost, low-resolution detectors have both military applications and civilian ones. Potential applications include security (both ‘burglar alarm’ type applications and security/surveillance), identification of individuals (as a biometric sensor for ATM machines) and energy conservation applications.
