Thermographic camera
Theory of operation
Infrared energy is just one part of the electromagnetic spectrum that encompasses radiation from gamma rays, x-rays, ultra violet, a thin region of visible light, infrared, terahertz waves, microwaves, and radio waves. These are all related and differentiated in the length of their wave (wavelength). All objects emit a certain amount of black body radiation as a function of their temperatures. Generally speaking, the higher an object’s temperature is, the more infrared radiation as black-body radiation it emits. A special camera can detect this radiation in a way similar to an ordinary camera does visible light. It works even in total darkness because ambient light level does not matter. This makes it useful for rescue operations in smoke-filled buildings and underground.
Images from infrared cameras tend to be monochromatic because the cameras are generally designed with only a single type of sensor responding to single wavelength range of infrared radiation. Color cameras require a more complex construction to differentiate wavelength 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 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. This technique is called density slicing.
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 blue. A scale should be shown next to a false color image to relate colors to temperatures. Their resolution is considerably lower than of optical cameras, mostly only 160×120 or 320×240 pixels. Thermographic cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often deemed as dual-use and export-restricted.
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.
Thermal imaging photography finds many other uses. For example, firefighters use it to see through smoke, find persons, 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. Thermal imaging cameras are also installed in some luxury cars to aid the driver, the first being the 2000 Cadillac DeVille. Some physiological activities, particularly responses, in human beings and other warm-blooded animals can also be monitored with thermographic imaging. Cooled infrared cameras can also be found at most major astronomy research telescopes.
Thermographic image of a ringtailed lemur
Thermographic image of a snake around an arm
Thermographic image of several lizards
Types
Thermographic cameras can be broadly divided into two types: those with cooled infrared image detectors and those with uncooled detectors.
Cooled infrared detectors
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 to just below room temperature, depending on the detector technology. Most modern cooled detectors operate in the 60 K to 100 K range, depending on type and performance level. 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 power-hungry and time-consuming. The camera may need several minutes to cool down before it can begin working. The most commonly used cooling systems are rotary Stirling engine cryocoolers. Although the cooling apparatus is comparatively bulky and expensive, cooled infrared cameras provide superior image quality compared to uncooled ones. 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 Joulehomson 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 m)
indium arsenide
mercury cadmium telluride (MCT) (1-2 m, 3-5 m, 8-12 m)
lead sulfide
lead selenide
Infrared photodetectors can be created with structures of high band gap 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. Their wide range use is difficult 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 regualr use outside of scientific research.
Uncooled infrared detectors
Thermal imaging camera & screen, photographed in an airport terminal in Greece. Thermal imaging can detect elevated body temperature, one of the signs of the virus H1N1 (Swine influenza).
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 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 difference in their fabrication processes, limited by currently available technology.
Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. The material are used to form pixels with highly temperature-dependent properties, which are thermally insulated from the environment and read electronically.
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 320×240 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 thin film vanadium pentoxide 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.
Some of the materials used for the sensor arrays are eg.:
vanadium(V) oxide (metal insulator phase change material, for microbolometer arrays)
lanthanum barium manganite (LBMO, metal insulator phase change material)
amorphous silicon
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)
polyvinylidene difluoride (PVDF)
Thermographer training and certification
Aside from test equipment, training is the most important investment a company will make in an infrared inspection program. Advances in technology have provided infrared equipment that is user-friendly; however, infrared thermography is not a “simply point and shoot” technology. In addition to understanding the object or system being inspected, thermographers must also understand common error sources that can influence observed thermal data. Typically, infrared training courses should cover the topics of infrared theory, heat transfer concepts, equipment selection and operation, how to eliminate or overcome common error sources, and specific applications. Home and business owners who attempt thermal imaging themselves, in an effort the improve home energy efficiency, often misinterpret the images and may be prompted to spend on unnecessary improvements. Structural thermal imaging professionals can properly interpret readings and recommend cost-effective measures to improve building efficiency.
Certification is written proof of qualification, and a well trained thermographer will not only be trained, but properly certified. ISO 18434 lays down the criteria for certification. This allows for three levels of thermographers:
A Level 1 qualified thermographer is “certified to perform industrial thermographic measurements and basic IR thermography according to established and recognized procedures .
A Level 2 qualified thermographer is “qualified to perform and/or direct IR thermography according to established and recognized procedures.”
A Level 3 qualified thermographer is “qualified to perform and/or direct all types of thermographic measurements and analysis.”
To conform to these requirements fully, a Level 1 must:
Complete at least 40 hours training under an approved program and pass an exam with at least 75%.
Work for at least 12 months as an active thermographer.
Have at least 400 hours of cumulative experience in thermography.
Submit proof of this.
A Level 2 must:
Meet all the requirements of a Level 1 thermographer.
Complete a further 40 hours training under an approved program and pass an exam with at least 75%.
Work for at least 24 months as an active thermographer.
Have at least 1200 hours of cumulative experience in thermography.
Submit proof of this.
A Level 3 must:
Meet all the requirements for a Level 2 thermographer.
Complete a further 40 hours training under an approved program and pass an exam with at least 75%.
Work for at least 48 months as an active thermographer.
Have at least 1920 hours of cumulative experience in thermography.
Submit proof of this.
There is also a Certification in Building Science Thermography. A CBST has special training in building diagnostics and the interpretation of the patterning in thermal images used while investigating building failures.
It is important when hiring thermographers to check their Certification level, and to ensure their knowledge of your application is sufficient. It is also recommended that thermography companies have either their own Level 3 thermographer, or that they hire in the services of a Level 3.
In the USA ASNT certification is mainly used. Similarly, it follows three levels. It must be pointed out that under the ASNT system, certification is by the employer. This means that thermographers cannot bring their certification from job to job with them, and it also means that many one-person consultancy businesses may practice “self certification”, which is a major downfall of this system.
Applications
Originally developed for military use during the Korean War, thermographic cameras have slowly migrated into other fields as varied as medicine and archeology. More recently, the lowering of prices have helped fuel the adoption of infrared viewing technology. Advanced optics and sophisticated software interfaces continue to enhance the versatility of IR cameras.
Astronomy, in devices such as the Spitzer Space Telescope
Night vision
Firefighting operations
Military and police target detection & acquisition
Law enforcement and anti-terrorism
Predictive maintenance (early failure warning) on mechanical & electrical equipment
Process monitoring
Condition monitoring & surveillance
Automotive applications
Energy auditing of building insulation and detection of refrigerant leaks
Roof inspection
Auditing of acoustic insulation for sound reduction
Masonry wall structural analysis
Moisture detection in walls & roofs (and thus in turn often part of mold remediation)
Chemical imaging
Medical testing for diagnosis
Nondestructive testing
Quality control in production environments
Research & development of new products
Pollution effluent detection
Locating unmarked graves
Aerial archaeology
Paranormal investigation
Search and rescue operations
Technical surveillance counter-measures
Quarantine monitoring of visitors to a country
Flame detector
Specifications
Some specification parameters of an infrared camera system are:
Number of pixels
Noise Equivalent Temperature Difference (NETD)
Spectral band
Sensor lifetime
Minimum resolvable temperature difference (MRTD)
Field of view
Dynamic range
Input power
Mass and volume
See also
Wikimedia Commons has media related to: Thermal images
Wikimedia Commons has media related to: Thermography
Digital infrared thermal imaging in health care
Thermography
Infrared photography
Thermal imaging camera
Categories: Cameras by type | Astronomical imaging | Surveillance | Infrared imagingHidden categories: Articles lacking sources from January 2009 | All articles lacking sources
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