Infrared signature

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Infrared signature, as used by defense scientists and the military, is the appearance of objects to infrared sensors. [1] An infrared signature depends on many factors, including the shape and size of the object, [2] temperature, [3] and emissivity, reflection of external sources (earthshine, sunshine, skyshine) from the object's surface, [4] the background against which it is viewed [5] 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.

Contents

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 kelvins) 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. [6] This reduces a platform's susceptibility to infrared guided weapons and infrared surveillance sensors, [7] and thus increases the platform's overall survivability. Infrared stealth is particularly applicable to military jets because of the detectable engines [8] and plumes [9] from non-stealth aircraft, but it also applies to military helicopters, [10] 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 missiles while also having an infrared signature below the detection threshold of the 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 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 radiates are absorbed by atmospheric carbon dioxide and water vapor, dramatically reducing the infrared visibility of the exhaust plume. [11] 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 sinks cooled by the flow of air along the wings.[ citation needed ]

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. [12]

See also

Related Research Articles

<span class="mw-page-title-main">Infrared</span> Form of electromagnetic radiation

Infrared is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with waves that are just longer than those of red light, the longest waves in the visible spectrum, so IR is invisible to the human eye. IR is generally understood to include wavelengths from around 750 nm to 1000 μm. IR is commonly divided between longer-wavelength thermal IR, emitted from terrestrial sources, and shorter-wavelength IR or near-IR, part of the solar spectrum. Longer IR wavelengths (30–100 μm) are sometimes included as part of the terahertz radiation band. Almost all black-body radiation from objects near room temperature is in the IR band. As a form of electromagnetic radiation, IR carries energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon.

<span class="mw-page-title-main">Thermographic camera</span> Imaging device using infrared radiation

A thermographic camera 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 to about 14,000 nm (14 μm). The practice of capturing and analyzing the data they provide is called thermography.

<span class="mw-page-title-main">Stealth technology</span> Military technology to make personnel and material less visible

Stealth technology, also termed low observable technology, is a sub-discipline of military tactics and passive and active electronic countermeasures, which covers a range of methods used to make personnel, aircraft, ships, submarines, missiles, satellites, and ground vehicles less visible to radar, infrared, sonar and other detection methods. It corresponds to military camouflage for these parts of the electromagnetic spectrum.

<span class="mw-page-title-main">Thermography</span> Infrared imaging used to reveal temperature

Infrared thermography (IRT), thermal video and/or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object in a process, which are examples of infrared imaging science. Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects with a temperature above absolute zero according to the black body radiation law, thermography makes it possible to see one's environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and other users of surveillance cameras.

<span class="mw-page-title-main">Stealth aircraft</span> Aircraft which use stealth technology to avoid detection

Stealth aircraft are designed to avoid detection using a variety of technologies that reduce reflection/emission of radar, infrared, visible light, radio frequency (RF) spectrum, and audio, all collectively known as stealth technology. The F-117 Nighthawk was the first operational aircraft explicitly designed around stealth technology. Other examples of stealth aircraft include the B-2 Spirit, the B-21 Raider, the F-22 Raptor, the F-35 Lightning II, the Chengdu J-20, and the Sukhoi Su-57.

Plasma stealth is a proposed process to use ionized gas (plasma) to reduce the radar cross-section (RCS) of an aircraft. Interactions between electromagnetic radiation and ionized gas have been extensively studied for many purposes, including concealing aircraft from radar as stealth technology. Various methods might plausibly be able to form a layer or cloud of plasma around a vehicle to deflect or absorb radar, from simpler electrostatic or radio frequency discharges to more complex laser discharges. It is theoretically possible to reduce RCS in this way, but it may be very difficult to do so in practice. Some Russian missiles e.g. the 3M22 Zircon (SS-N-33) and Kh-47M2 Kinzhal missiles have been reported to make use of plasma stealth.

<span class="mw-page-title-main">Infrared search and track</span> Method for detecting and tracking objects which give off infrared radiation

An infrared search and track (IRST) system is a method for detecting and tracking objects which give off infrared radiation, such as the infrared signatures of jet aircraft and helicopters.

<span class="mw-page-title-main">Microbolometer</span> Type of bolometer

A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Infrared radiation with wavelengths between 7.5–14 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.

<span class="mw-page-title-main">Motion detector</span> Electrical device which utilizes a sensor to detect nearby motion

A motion detector is an electrical device that utilizes a sensor to detect nearby motion. Such a device is often integrated as a component of a system that automatically performs a task or alerts a user of motion in an area. They form a vital component of security, automated lighting control, home control, energy efficiency, and other useful systems.

Lead selenide (PbSe), or lead(II) selenide, a selenide of lead, is a semiconductor material. It forms cubic crystals of the NaCl structure; it has a direct bandgap of 0.27 eV at room temperature. A grey solid, it is used for manufacture of infrared detectors for thermal imaging. The mineral clausthalite is a naturally occurring lead selenide.

<span class="mw-page-title-main">Advanced very-high-resolution radiometer</span>

The Advanced Very-High-Resolution Radiometer (AVHRR) instrument is a space-borne sensor that measures the reflectance of the Earth in five spectral bands that are relatively wide by today's standards. AVHRR instruments are or have been carried by the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms (POES) and European MetOp satellites. The instrument scans several channels; two are centered on the red (0.6 micrometres) and near-infrared (0.9 micrometres) regions, a third one is located around 3.5 micrometres, and another two the thermal radiation emitted by the planet, around 11 and 12 micrometres.

<span class="mw-page-title-main">Proximity sensor</span> About proximity sensor

A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact.

<span class="mw-page-title-main">Flare (countermeasure)</span> Aerial defence against heat-seeking missiles

A flare or decoy flare is an aerial infrared countermeasure used by an aircraft to counter an infrared homing ("heat-seeking") surface-to-air missile or air-to-air missile. Flares are commonly composed of a pyrotechnic composition based on magnesium or another hot-burning metal, with burning temperature equal to or hotter than engine exhaust. The aim is to make the infrared-guided missile seek out the heat signature from the flare rather than the aircraft's engines.

A gas detector is a device that detects the presence of gases in an area, often as part of a safety system. A gas detector can sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.

Electro-optical MASINT is a subdiscipline of Measurement and Signature Intelligence, (MASINT) and refers to intelligence gathering activities which bring together disparate elements that do not fit within the definitions of Signals Intelligence (SIGINT), Imagery Intelligence (IMINT), or Human Intelligence (HUMINT).

Geophysical MASINT is a branch of Measurement and Signature Intelligence (MASINT) that involves phenomena transmitted through the earth and manmade structures including emitted or reflected sounds, pressure waves, vibrations, and magnetic field or ionosphere disturbances.

A flame detector is a sensor designed to detect and respond to the presence of a flame or fire, allowing flame detection. Responses to a detected flame depend on the installation, but can include sounding an alarm, deactivating a fuel line, and activating a fire suppression system. When used in applications such as industrial furnaces, their role is to provide confirmation that the furnace is working properly; it can be used to turn off the ignition system though in many cases they take no direct action beyond notifying the operator or control system. A flame detector can often respond faster and more accurately than a smoke or heat detector due to the mechanisms it uses to detect the flame.

Thermographic inspection refers to the nondestructive testing (NDT) of parts, materials or systems through the imaging of the temperature fields, gradients and/or patterns ("thermograms") at the object's surface. It is distinguished from medical thermography by the subjects being examined: thermographic inspection generally examines inanimate objects, while medical thermography generally examines living organisms. Generally, thermographic inspection is performed using an infrared sensor.

This is a list of infrared topics.

<span class="mw-page-title-main">Adaptiv</span>

Adaptiv is an active camouflage technology developed by BAE Systems AB to protect military vehicles from detection by far infrared night vision devices, providing infrared stealth. It consists of an array of hexagonal Peltier plates which can be rapidly heated and cooled to form any desired image, such as of the natural background or of a non-target object. Its goal is to develop stealth ground vehicles.

References

  1. ↑ 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
  2. ↑ 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.
  3. ↑ 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.
  4. ↑ 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.
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ 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.
  8. ↑ 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.
  9. ↑ 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.
  10. ↑ 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.
  11. ↑ Optical Warfare - The New Frontier
  12. ↑ GAO-10-669R Warfighter Support
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