Infrared signature

Infrared signature

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The term infrared signature is used by defense scientists
Military science
Military science is the process of translating national defence policy to produce military capability by employing military scientists, including theorists, researchers, experimental scientists, applied scientists, designers, engineers, test technicians, and military personnel responsible for...

 and the military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 to describe the appearance of objects to infrared sensors. An infrared signature depends on many factors, including the shape and size of the object, temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and emissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...

, reflection of external sources (earthshine, sunshine, skyshine) from the object's surface, the background against which it is viewed 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.

Infrared stealth is an area of stealth technology
Stealth technology
Stealth technology also termed LO technology is a sub-discipline of military tactics and passive electronic countermeasures, which cover a range of techniques used with personnel, aircraft, ships, submarines, and missiles, to make them less visible to radar, infrared, sonar and other detection...

 aimed at reducing infrared signatures. This reduces a platform's susceptibility to infrared guided weapons and infrared surveillance sensors, and thus increases the platform's overall survivability. Infrared stealth is particularly applicable to military jets because of the detectable engines and plumes from non-stealth aircraft, but it also applies to military helicopters, 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
Anti-ship missile
Anti-ship missiles are guided missiles that are designed for use against ships and large boats. Most anti-ship missiles are of the sea-skimming type, many use a combination of inertial guidance and radar homing...

s while also having an infrared signature below the detection threshold of the infrared sensor guiding
Infrared homing
Infrared homing refers to a passive missile guidance system which uses the emission from a target of electromagnetic radiation in the infrared part of the spectrum to track and follow it. Missiles which use infrared seeking are often referred to as "heat-seekers", since infrared is just below the...

 the missile.

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
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

) 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.

Free software

  • osmosis is free software currently being built to allow basic infrared signatures simulations.

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 objects physical existence in the infrared wavebands.

An example of a commercial infrared signature prediction tool is the Ship EDF software offered by the Italian company IDS.

See also

  • Infrared
    Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

  • Electromagnetic modeling
  • Radar cross section
    Radar cross section
    Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

  • Infrared detector
    Infrared detector
    An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....

  • Infrared countermeasures
  • Survivability
    Survivability is the ability to remain alive or continue to exist. The term has more specific meaning in certain contexts.-Engineering:In engineering, survivability is the quantified ability of a system, subsystem, equipment, process, or procedure to continue to function during and after a natural...