- Infrared vision
Infrared vision can be defined as the capability of biological or artificial systems to detect
infrared radiation. The terms thermal visionand thermal imaging["thermal imaging" Encarta® World English Dictionary [North American Edition] © & (P)2007 Microsoft Corporation. 17 Apr. 2008, [http://encarta.msn.com/dictionary_/thermal%2520imaging.html Encarta] .] , ["thermal imaging" Cambridge University Press 2008. 17 Apr. 2008, [http://dictionary.cambridge.org/define.asp?key=82411&dict=CALD Cambridge] .] are also commonly used in this context since infrared emissions from a body are directly related to their temperature: hotter objects emit more energy in the infrared spectrum than colder ones.
The human body, as well as many moving or static objects of military or civil interest, is normally warmer than the surrounding environment. Since hotter objects emit more infrared energy than colder ones, it is relatively easy to identify them with an
infrared detector, day or night. Hence, the term night visionis also used (sometimes "misused") in the place of "infrared vision", since one of the original purposes in developing his kind of systems was to locate enemy targets at night ["tank." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 17 Apr. 2008, [http://www.britannica.com/eb/article-57394 Britannica] .] . However, night visionconcerns the ability to see in the dark although not necessarily in the infrared spectrum. In fact, night vision equipment can be manufactured using one of two technologies ["How Night Vision Works" Howstuffworks. 17 Apr. 2008, [http://www.howstuffworks.com/nightvision.htm HowStuffWorks] .] : light intensifiers or infrared vision. The former technology uses a photocathode to convert light (in the visible or near infrared portions of the electromagnetic spectrum) to electrons, amplify the signal and transform it back to photons. Infrared vision on the other hand, uses an infrared detector working at mid or long wavelengths (invisible to the human eye) to capture the heat emitted by an object.
The infrared spectrum
electromagnetic spectrumhighlighting the infrared part located between the visible and the radio waves, is depicted in the figure. The IR spectrum can be subdivided into 5 regions, although this definition is somehow arbitrary and it differs from one author to another [Hudson R. D. 1969, “Infrared System Engineering”, John Wiley & Sons Inc., USA.] , [Piotrowski J. and Rogalski A. 2004, “Uncooled Long Wavelength Infrared Photon Detectors”, Infrared Phys. Technol., 46:115-131.] , [Rogalski A. and Chrzanowski K. 2002, “Infrared Devices and Techniques”, Contributed Paper: Opto-electronics Review, 10(2):111-136.] , [Ruddock W. 2004, “Infrared Imaging and Open Heart Surgery,” from InfraredThermography.com by Advanced Infrared Resources [online] : , accessed on June 28, 2004.] . The subdivision presented here is based on a combination of the atmospheric transmittance windows, i.e. the wavelengths regions in which infrared radiationis better transmitted through the atmosphere, the detector materials used to build the infrared sensors and the main applications. In this way, the Near Infrared (NIR) band is mostly used in fiber optic telecommunication systems since silica (SiO2) provides a low attenuation losses medium for the infrared, whilst the Short Wave Infrared (SWIR) band allows to work on long-distance telecommunications (remote sensing) using a combinations of detector materials. The Medium Wavelength Infrared (MWIR) and the Long Wavelength Infrared (LWIR) bands find applications in Infrared Thermography for military or civil applications, e.g. target signature identification, surveillance, NonDestructive Evaluation, etc. The Very Long Wavelength Infrared (VLIR) band is used in spectroscopy and astronomy.
The MWIR band is preferred when inspecting high temperature objects and the LWIR band when working with near room temperature objects. Other important criteria for band selection are [Maldague X. P. 2001, Theory and Practice of Infrared Technology for Nondestructive Testing, John Wiley & Sons, N. Y.] : the operating distance, indoor-outdoor operation, temperature and emissivity of the bodies of interest. For instance, long wavelengths (LWIR) are preferred for outdoor operation since they are less affected by radiation from the Sun. LWIR cameras are typically uncooled systems using a microbolometer Focal Plane Arrays commonly used in industrial IR applications, although cooled LWIR cameras using Mercury Cadmium Tellurium (MCT) detectors exists as well. On the contrary, the majority of the MWIR cameras require cooling, using either liquid nitrogen or a Stirling cycle cooler ["How Stirling Engines Work" Howstuffworks. 17 Apr. 2008, [http://travel.howstuffworks.com/stirling-engine1.htm HowStuffWorks] .] . Cooling to approximately -196oC (77 K), offer excellent thermal resolution, but it might restrict the span of applications to controlled environments.
Infrared vision is used extensively by the military for
night vision, navigation, surveillanceand targeting. For years, it developed slowly due to the high cost of the equipment and the low quality of available images. Since the development of the first commercial infrared camerasin the second half of 1960’s, however, the availability of new generations of infrared cameras coupled with growing computer power is providing exciting new civilian (and military) applications, to name only a few [ [http://www.ndt.net ndt.net] ] : Buildings and Infrastructure [Garziera R., Amabili M. and Collini L. “Structural health monitoring techniques for historical buildings”, Proc. IV Pan American Conference in NDE, [CD-ROM] , Buenos Aires, Argentina October 22-27, 2007 [available online:http://www.ndt.net/article/panndt2007/papers/141.pdf] ] , Works of Art [Grinzato E. “Temperature monitors works of art health as human beings,” 16th WCNDT - World Conference on Nondestructive Testing, [CD-rom] , Montreal (Quebec), August 30 - September 3, 2004 [available online: http://www.ndt.net/article/wcndt2004/pdf/thermography_thermal_techniques/34_grinzato.pdf] ] , Aerospace Components [Shepard S. M. “Flash Thermography of Aerospace Composites”, Proc. IV Pan American Conference in NDE, [CD-ROM] , Buenos Aires, Argentina October 22-27, 2007 [available online:http://www.ndt.net/article/panndt2007/papers/132.pdf] ] and Processes, Maintenance [Avdelidis N.P., Delegou E.T. and Moropoulou A. “A thermographic survey for the monitoring porous stone,” 16th WCNDT - World Conference on Nondestructive Testing, [CD-rom] , Montreal (Quebec), August 30 - September 3, 2004 [available online: http://www.ndt.net/article/wcndt2004/pdf/thermography_thermal_techniques/804_avde.pdf] ] , Defect detection and characterization, Law Enforcement, Surveillance and Public Services, Medical and veterinary, etc. The electronic technique that uses Infrared Vision to “see” thermal energy, to monitor temperatures and thermal patterns is called Infrared Thermography and is described next.
* [http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/what_is_ir.html What is IR ?]
* [http://mivim.gel.ulaval.ca/index.php Canada Research Chair in Multipolar Infrared Vision - MiViM]
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