Soldier at 11:52, 10 September 2023

2023-09-10T11:52:03Z

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{{Short description|Appearance of objects to infrared sensors}}
'''Infrared signature''', as used by [[Military science|defense scientists]] and the [[military]], is the appearance of objects to [[infrared sensors]].<ref>The Handbook Of The SAS And Elite Forces. How The Professionals Fight And Win. Edited by Jon E. Lewis. p.330-Tactics And Techniques, Personal Skills And Techniques. Robinson Publishing Ltd 1997. ISBN 1-85487-675-9</ref> An infrared signature depends on many factors, including the shape and size of the object,<ref>Mahulikar, S.P., Potnuru, S.K., & Kolhe, P.S.: (2007) "Analytical estimation of solid angle subtended by complex well-resolved surfaces for infrared detection studies", ''Applied Optics'', v. '''46(22)''', pp. 4991-4998.</ref> [[temperature]],<ref>Mahulikar, S.P., Sane, S.K., Gaitonde, U.N., & Marathe A.G.: (2001) "Numerical studies of infrared signature levels of complete aircraft", ''Aeronautical Journal'', v. '''105(1046)''', pp. 185-192.</ref> and [[emissivity]], reflection of external sources ([[earthshine]], [[sunshine]], [[skyshine]]) from the object's surface,<ref>Mahulikar, S.P., Potnuru, S.K., & Rao, G.A.: (2009) Study of sunshine, skyshine, and earthshine for aircraft infrared detection, ''Journal of Optics A: Pure & Applied Optics'', v. '''11(4)''', no. 045703.</ref> the background against which it is viewed<ref>Rao, G.A., & Mahulikar, S.P.: (2005) "Effect of atmospheric transmission and radiance on aircraft infrared signatures", ''AIAA Journal of Aircraft'', v. '''42(4)''', pp. 1046-1054.</ref> and the waveband of the detecting sensor. As such there is no all-encompassing definition of infrared signature nor any trivial means of measuring it. For example, the infrared signature of a truck viewed against a field will vary significantly with changing weather, time of day and engine loading.

Two fairly successful examples of defining the infrared signature of an object are the '''apparent temperature difference''' at the sensor and the '''contrast radiant intensity''' (CRI) definitions.

==Apparent temperature difference==
The apparent temperature difference method of defining infrared signature gives the physical temperature difference (e.g. in [[kelvin]]s) between the object of interest and the immediate background if the recorded radiance values had been measured from perfect [[blackbody]] sources. Problems with this method include differences in radiance across the object or the immediate background and the finite size of the detector's pixels. The value is a complex function of range, time, aspect, etc.

==Contrast radiant intensity==
The contrast radiant intensity method of defining infrared signature is to take the difference in average radiance of the object and that of the immediate background and multiply this by the projected area of the object. Again the CRI value will depend on many factors.

==Commercial software==
In the design phase, it is often desirable to employ a computer to predict what the infrared signature will be before fabricating an actual object. Many iterations of this prediction process can be performed in a short time at low cost, whereas use of a measurement range is often time-consuming, expensive and error-prone.

A number of software houses have built infrared signature prediction software packages. These generally require a CAD model of interest plus a large set of parameters to describe a specific thermal environment and the internal temperatures of the platform and thermal properties of the construction materials. The software then solves a set of thermal equations across the boundaries and for electromagnetic propagation in a specified infrared waveband. The primary output is a measure of infrared signature, though usually surface temperatures can be given (since this usually has to be calculated to obtain the infrared signature prediction) and also visual representations of how the scene may appear to various imaging infrared detectors.

Infrared signature prediction models are very difficult to validate except for simple cases because of the difficulty in modelling a complex environment. Both sensitivity analysis of this type of software and experimental measurements has shown that small variations in weather can have a significant impact on the results. As such, there are limitations on what can be achieved from modelling the infrared problem, and sometimes experimentation is necessary to achieve accurate knowledge of the nature of an object's physical existence in the infrared wavebands.

== Infrared stealth ==
[[Infrared stealth]] is an area of [[stealth technology]] aimed at reducing infrared signatures.<ref>Mahulikar, S.P., Sonawane, H.R., & Rao, G.A.: (2007) "Infrared signature studies of aerospace vehicles", ''[[Progress in Aerospace Sciences]]'', v. '''43(7-8)''', pp. 218-245.</ref> This reduces a platform's susceptibility to infrared guided weapons and infrared surveillance sensors,<ref>Rao, G.A., & Mahulikar, S.P.: (2005) "New criterion for aircraft susceptibility to infrared homing missiles", ''Aerospace Science & Technology'', v. '''9(8)''', pp. 701-712.</ref> and thus increases the platform's overall survivability. Infrared stealth is particularly applicable to military jets because of the detectable engines<ref>Mahulikar, S.P., Kolhe, P.S., & Rao, G.A.: (2005) "Skin temperature prediction of aircraft rear fuselage with multi-mode thermal model", ''AIAA Journal of Thermophysics & Heat Transfer'', v. '''19(1)''', pp. 114-124.</ref> and plumes<ref>Mahulikar, S.P., Rao, G.A., Sane, S.K., & Marathe, A.G.: (2005) "Aircraft plume infrared signature in nonafterburning mode", ''AIAA Journal of Thermophysics & Heat Transfer'', v. '''19(3)''', pp. 413-415.</ref> from non-stealth aircraft, but it also applies to military helicopters,<ref>Mahulikar, S.P., Prasad, H.S.S., & Potnuru, S.K.: (2008) "Infrared signature suppression of helicopter engine duct based on `conceal and camouflage`", ''AIAA Journal of Propulsion & Power'', v. '''24(3)''', pp. 613-618.</ref> warships, land vehicles and dismounted soldiers.

A military aim in studying infrared signatures is to understand the likely infrared signature of threats (and develop the equipment required to detect them) and to reduce the infrared signature of their own assets to threat sensors. In practice this might mean equipping a warship with sensors to detect the exhaust plumes of incoming [[anti-ship missile]]s while also having an infrared signature below the detection threshold of the [[infrared homing|infrared sensor guiding]] the missile.

An exhaust plume contributes a significant infrared signature. One means to reduce IR signature is to have a non-circular [[exhaust system|tail pipe]] (a slit shape) to minimize the exhaust cross-sectional volume and maximize the mixing of hot exhaust with cool ambient air (see Lockheed F-117 Nighthawk). Often, cool air is deliberately injected into the exhaust flow to boost this process (see [[Ryan AQM-91 Firefly]] and [[Northrop Grumman B-2 Spirit]]). Sometimes, the jet exhaust is vented above the wing surface to shield it from observers below, as in the [[Lockheed F-117 Nighthawk]], and the unstealthy [[Fairchild Republic A-10 Thunderbolt II]]. To achieve [[infrared stealth]], the [[exhaust gas]] is cooled to the temperatures where the brightest wavelengths it [[Wien's displacement law|radiates]] are [[infrared window|absorbed by atmospheric carbon dioxide and water vapor]], dramatically reducing the infrared visibility of the exhaust plume.<ref>[http://www.ausairpower.net/optical-warfare.html] Optical Warfare - The New Frontier</ref> Another way to reduce the exhaust temperature is to circulate [[coolant]] fluids such as fuel inside the exhaust pipe, where the fuel tanks serve as [[heat sink]]s cooled by the flow of air along the wings.{{Citation needed|date=July 2010}}

Ground combat includes the use of both active and passive infrared sensors and so the [[USMC]] ground combat uniform requirements document specifies infrared reflective quality standards.<ref>[https://web.archive.org/web/20170816104956/http://www.gao.gov/assets/100/96770.pdf GAO-10-669R Warfighter Support]</ref>

==See also==
* [[Infrared]]
* [[Electromagnetic modeling]]
* [[Radar cross section]]
* [[Infrared detector]]
* [[Infrared countermeasures]]
* [[Survivability]]

==References==
<references/>

{{DEFAULTSORT:Infrared Signature}}
[[Category:Infrared imaging]]
[[Category:Camouflage]]
[[Category:Stealth technology]]

[[ja:赤外線#静脈認証]]

Infrared signature - Revision history

Soldier at 11:52, 10 September 2023