Cameras
[edit]
Image of a Pomeranian taken in mid-infrared ("thermal") light
(false-color)
A thermographic camera (also called an infrared camera or thermal imaging
camera, thermal camera or thermal imager) is a device that creates an image
using infrared (IR) radiation, similar to a normal camera that forms an image
using visible light. Instead of the 400–700 nanometre (nm) range of the visible light
camera, infrared cameras are sensitive to wavelengths from about 1,000 nm
(1 micrometre or μm) to about 14,000 nm (14 μm). The practice of capturing and
analyzing the data they provide is called thermography.
Types
[edit]
Thermographic cameras can be broadly divided into two types: those with cooled
infrared image detectors and those with uncooled detectors.
Cooled infrared detectors
[edit]
A thermographic image of several lizards
Thermal imaging camera & screen, in an airport terminal in
Greece. Thermal imaging can detect fever, one of the signs of infection.
Cooled detectors are typically contained in a vacuum-sealed case
or Dewar and cryogenically cooled. The cooling is necessary for the operation of the
semiconductor materials used. Typical operating temperatures range from 4 K
(−269 °C) to just below room temperature, depending on the detector technology.
Most modern cooled detectors operate in the 60 Kelvin (K) to 100 K range (-213 to -
173 °C), depending on type and performance level.[6]
Without cooling, these sensors (which detect and convert light in much the same
way as common digital cameras, but are made of different materials) would be
'blinded' or flooded by their own radiation. The drawbacks of cooled infrared cameras
are that they are expensive both to produce and to run. Cooling is both energy-
intensive and time-consuming.
The camera may need several minutes to cool down before it can begin working.
The most commonly used cooling systems are peltier coolers which, although
inefficient and limited in cooling capacity, are relatively simple and compact. To
obtain better image quality or for imaging low temperature objects Stirling engine
cryocoolers are needed. Although the cooling apparatus may be comparatively bulky
and expensive, cooled infrared cameras provide greatly superior image quality
compared to uncooled ones, particularly of objects near or below room temperature.
Additionally, the greater sensitivity of cooled cameras also allow the use of higher F-
number lenses, making high performance long focal length lenses both smaller and
cheaper for cooled detectors.
An alternative to Stirling engine coolers is to use gases bottled at high pressure,
nitrogen being a common choice. The pressurised gas is expanded via a micro-sized
orifice and passed over a miniature heat exchanger resulting in regenerative cooling
via the Joule–Thomson effect. For such systems the supply of pressurized gas is a
logistical concern for field use.
Materials used for cooled infrared detection include photodetectors based on a wide
range of narrow gap semiconductors including indium antimonide (3-5 μm), indium
arsenide, mercury cadmium telluride (MCT) (1-2 μm, 3-5 μm, 8-12 μm), lead sulfide,
and lead selenide.
Infrared photodetectors can be created with structures of high bandgap
semiconductors such as in quantum well infrared photodetectors.
A number of superconducting and non-superconducting cooled bolometer
technologies exist.
In principle, superconducting tunneling junction devices could be used as infrared
sensors because of their very narrow gap. Small arrays have been demonstrated.
They have not been broadly adopted for use because their high sensitivity requires
careful shielding from the background radiation.
Superconducting detectors offer extreme sensitivity, with some able to register
individual photons. For example, ESA's Superconducting camera (SCAM). However,
they are not in regular use outside of scientific research.
Uncooled infrared detectors
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Uncooled thermal cameras use a sensor operating at ambient temperature, or a
sensor stabilized at a temperature close to ambient using small temperature control
elements. Modern uncooled detectors all use sensors that work by the change
of resistance, voltage or current when heated by infrared radiation. These changes
are then measured and compared to the values at the operating temperature of the
sensor.
Uncooled infrared sensors can be stabilized to an operating temperature to reduce
image noise, but they are not cooled to low temperatures and do not require bulky,
expensive, energy consuming cryogenic coolers. This makes infrared cameras
smaller and less costly. However, their resolution and image quality tend to be lower
than cooled detectors. This is due to differences in their fabrication processes,
limited by currently available technology. An uncooled thermal camera also needs to
deal with its own heat signature.
Uncooled detectors are mostly based on pyroelectric and ferroelectric materials
or microbolometer technology.[7] The material are used to form pixels with highly
temperature-dependent properties, which are thermally insulated from the
environment and read electronically.
Thermal image of steam locomotive
Ferroelectric detectors operate close to phase transition temperature of the sensor
material; the pixel temperature is read as the highly temperature-dependent
polarization charge. The achieved NETD of ferroelectric detectors with f/1 optics and
320x240 sensors is 70-80 mK. A possible sensor assembly consists of barium
strontium titanate bump-bonded by polyimide thermally insulated connection.
Silicon microbolometers can reach NETD down to 20 mK. They consist of a layer
of amorphous silicon, or a thin film vanadium(V) oxide sensing element suspended
on silicon nitride bridge above the silicon-based scanning electronics. The electric
resistance of the sensing element is measured once per frame.
Current improvements of uncooled focal plane arrays (UFPA) are focused primarily
on higher sensitivity and pixel density. In 2013 DARPA announced a five-micron
LWIR camera that uses a 1280 x 720 focal plane array (FPA).[8] Some of the
materials used for the sensor arrays are amorphous silicon (a-Si), vanadium(V)
oxide (VOx),[9] lanthanum barium manganite (LBMO), lead zirconate
titanate (PZT), lanthanum doped lead zirconate titanate (PLZT), lead scandium
tantalate (PST), lead lanthanum titanate (PLT), lead titanate (PT), lead zinc niobate
(PZN), lead strontium titanate (PSrT), barium strontium titanate (BST), barium
titanate (BT), antimony sulfoiodide (SbSI), and polyvinylidene difluoride (PVDF).
Specifications
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Some specification parameters of an infrared camera system are number
of pixels, frame rate, responsivity, noise-equivalent power, noise-equivalent
temperature difference (NETD), spectral band, distance-to-spot ratio (D:S), minimum
focus distance, sensor lifetime, minimum resolvable temperature
difference (MRTD), field of view, dynamic range, input power, and mass and volume.
Difference from infrared film
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IR film is sensitive to infrared (IR) radiation in the 250 to 500 °C (482 to 932 °F)
range, while the range of thermography is approximately −50 to 2,000 °C (−58 to
3,632 °F). So, for an IR film to work thermographically, the measured object must be
over 250 °C (482 °F) or be reflecting infrared radiation from something that is at least
that hot.
Night vision infrared devices image in the near-infrared, just beyond the visual
spectrum, and can see emitted or reflected near-infrared in complete visual
darkness. However, again, these are not usually used for thermography due to the
high temperature requirements, but are instead used with active near-IR sources.
Starlight-type night vision devices generally only magnify ambient light.
Passive vs. active thermography
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All objects above the absolute zero temperature (0 K) emit infrared radiation. Hence,
an excellent way to measure thermal variations is to use an infrared vision device,
usually a focal plane array (FPA) infrared camera capable of detecting radiation in
the mid (3 to 5 μm) and long (7 to 14 μm) wave infrared bands, denoted as MWIR
and LWIR, corresponding to two of the high transmittance infrared windows.
Abnormal temperature profiles at the surface of an object are an indication of a
potential problem.[10]
In passive thermography, the features of interest are naturally at a higher or lower
temperature than the background. Passive thermography has many applications
such as surveillance of people on a scene and medical
diagnosis (specifically thermology).
In active thermography, an energy source is required to produce a thermal contrast
between the feature of interest and the background. The active approach is
necessary in many cases given that the inspected parts are usually in equilibrium
with the surroundings. Given the super-linearities of the black-body radiation, active
thermography can also be used to enhance the resolution of imaging systems
beyond their diffraction limit or to achieve super-resolution microscopy.[11]
Advantages
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Thermography shows a visual picture so temperatures over a large area can be
compared.[12][13][14] It is capable of catching moving targets in real time.[12][13][14] It is able to
find deterioration, i.e., higher temperature components prior to their failure. It can be
used to measure or observe in areas inaccessible or hazardous for other methods. It
is a non-destructive test method. It can be used to find defects in shafts, pipes, and
other metal or plastic parts.[15] It can be used to detect objects in dark areas. It has
some medical application, essentially in physiotherapy.
Limitations and disadvantages
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There are various cameras cheaper and more expensive. Quality cameras often
have a high price range (often US$3,000 or more) due to the expense of the larger
pixel array (state of the art 1280 x 1024), while less expensive models (with pixel
arrays of 40x40 up to 160x120 pixels) are also available. Fewer pixels reduce the
image quality making it more difficult to distinguish proximate targets within the same
field of view.
There is also a difference in refresh rate. Some cameras may only have a refreshing
value of 5 –15 Hz, other (e.g. FLIR X8500sc[3]) 180 Hz or even more in no full window
mode.
Also the lens can be integrated or not.
Many models do not provide the irradiance measurements used to construct the
output image; the loss of this information without a correct calibration for emissivity,
distance, and ambient temperature and relative humidity entails that the resultant
images are inherently incorrect measurements of temperature.[16]
Images can be difficult to interpret accurately when based upon certain objects,
specifically objects with erratic temperatures, although this problem is reduced in
active thermal imaging.[17]
Thermographic cameras create thermal images based on the radiant heat energy it
receives.[18] As radiation levels are influenced by the emissivity and reflection of
radiation such as sunlight from the surface being measured this causes errors in the
measurements.[19]
Most cameras have ±2% accuracy or worse in measurement of temperature and
are not as accurate as contact methods.[12][13][14]
Methods and instruments are limited to directly detecting surface temperatures.
Applications
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Kite aerial thermogram revealing features on/under a grassed
playing field. Thermal inertia and differential transpiration/evaporation are involved
UAS thermal imagery of a solar panel array in Switzerland
AN/PAS-13 thermal rifle scope mounted on an AR-15 rifle
Thermographic image of a ring-tailed lemur
Images from infrared cameras tend to be monochrome because the cameras
generally use an image sensor that does not distinguish different wavelengths of
infrared radiation. Color image sensors require a complex construction to
differentiate wavelengths, and color has less meaning outside of the normal visible
spectrum because the differing wavelengths do not map uniformly into the system
of color vision used by humans.
Sometimes these monochromatic images are displayed in pseudo-color, where
changes in color are used rather than changes in intensity to display changes in the
signal. This technique, called density slicing, is useful because although humans
have much greater dynamic range in intensity detection than color overall, the ability
to see fine intensity differences in bright areas is fairly limited.
For use in temperature measurement the brightest (warmest) parts of the image are
customarily colored white, intermediate temperatures reds and yellows, and the
dimmest (coolest) parts black. A scale should be shown next to a false color image
to relate colors to temperatures. Their resolution is considerably lower than that of
optical cameras, mostly only 160 x 120 or 320 x 240 pixels, although more
expensive cameras can achieve a resolution of 1280 x 1024 pixels. Thermographic
cameras are much more expensive than their visible-spectrum counterparts, though
low-performance add-on thermal cameras for smartphones became available for
hundreds of dollars in 2014.[20] Higher-end models are often deemed dual-use military
grade equipment, and are export-restricted, particularly if the resolution is 640 x 480
or greater, unless the refresh rate is 9 Hz or less. The export from the USA of
thermal cameras is regulated by International Traffic in Arms Regulations. A thermal
camera was first built into a smartphone in 2016, into the Cat S60.
In uncooled detectors the temperature differences at the sensor pixels are minute; a
1 °C difference at the scene induces just a 0.03 °C difference at the sensor. The
pixel response time is also fairly slow, at the range of tens of milliseconds.
Thermography finds many other uses. For example, firefighters use it to see
through smoke, find people, and localize hotspots of fires. With thermal
imaging, power line maintenance technicians locate overheating joints and parts, a
telltale sign of their failure, to eliminate potential hazards. Where thermal
insulation becomes faulty, building construction technicians can see heat leaks to
improve the efficiencies of cooling or heating air-conditioning.
Hot hooves indicate a sick cow.
Thermal imaging cameras are also installed in some luxury cars to aid the driver
(automotive night vision), the first being the 2000 Cadillac DeVille.
Some physiological activities, particularly responses such as fever, in human beings
and other warm-blooded animals can also be monitored with thermographic imaging.
Cooled infrared cameras can be found at major astronomy research telescopes,
even those that are not infrared telescopes.
