Thermal Imaging

Thermal imaging is a method of using infrared(IR) radiation and thermal energy to gather information about objects, in order to formulate images of them, even in low visibility environments.

Infrared radiation (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore generally invisible to the human eye.

When reflected light from book falls on our eyes, we can see the book. Hence, we cannot see the book in dark.

When the reflected light from the objects falls on our eyes, our eyes can see the objects. This is the reason we cannot see any objects in dark.

IR is emitted by all objects above 0 K. It is thus possible to see an object by detecting its IR emission, rather than detecting reflected radiations from illuminating source(as seen in above figure). The source of radiation is the object itself and the detection is entirely passive.

The below figure shows the image of a house when seen using a thermal camera.

Temperature map of house

Application of thermal imaging in thermal cameras: A thermal camera can record the infra red light that humans cannot see. Human eyes can see only in the visible range of the spectrum viz 400nm — 700nm. Every material in the universe emits infrared radiation. The warmer the material is the more infrared it radiates.

Hand is warmer than water in the glass

In the above figure, the fingers are looking bright when compared to the glass of water. Hence, we can conclude that the water is at a lesser temperature than the fingers.

Thermal Infrared Imaging requires the use of special cameras. The imaging sensors in these cameras are sensitive to wavelengths in the infrared region of the electro-magnetic spectrum. Thermal Infrared Imaging is also referred to as “non-visible” imaging, since the infrared spectrum is not visible to the human eye.

Infrared spectrum

The above illustration, from left to right, shows the visible spectrum (400nm to 700nm). That is the spectrum that we can perceive as humans with our eyes. As the wavelengths become longer, we enter the near infrared (NIR) and shortwave infrared (SWIR) region.

In electronics thermal cameras are used to know the circuit performance. We can know which components in the circuits are causing more heat.

Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nanometers or 9–14 µm) 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.

Thermal images or thermograms are visual displays of the amount of infrared energy emitted, transmitted, and reflected by an object. Because there are multiple sources of the infrared energy, it is difficult to get an accurate temperature of an object using this method. A thermal imaging camera can perform algorithms to interpret that data and build an image. Although the image shows the viewer an approximation of the temperature at which the object is operating, the camera is using multiple sources of data based on the areas surrounding the object to determine that value rather than detecting the actual temperature.

In general, objects emit infrared radiation across a spectrum of wavelengths, but sometimes only a limited region of the spectrum is of interest because sensors usually collect radiation only within a specific bandwidth. Thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wien’s displacement law. Therefore, the infrared band is often subdivided into smaller sections.

NIR and SWIR is sometimes called “reflected infrared”, whereas MWIR and LWIR is sometimes referred to as “thermal infrared”. Due to the nature of the blackbody radiation curves, typical “hot” objects, such as exhaust pipes, often appear brighter in the MW compared to the same object viewed in the LW.

wavelength windows in the atmosphere that are transparent to the IR radiation

Short wavelength IR (SWIR) 1–2 μm

Medium wavelength IR (MWIR) 3–5 μm

Long wavelength IR (LWIR) 8–14 μm

For 3rd generation ATGM with IIR (Imaging InfraRed) seeker, LWIR is used.

mercury cadmium telluride Hg Cd Te (MCT) material is used for IR sensor The detectors made out of this material can be operated only at cryogenic temperature (77 K) and therefore have to be encapsulated in a dewar and cooled to 77 K.

Thermography: Infrared radiation can be used to remotely determine the temperature of objects (if the emissivity is known). This is termed thermography, or in the case of very hot objects in the NIR or visible it is termed pyrometry. Thermography (thermal imaging) is mainly used in military and industrial applications but the technology is reaching the public market in the form of infrared cameras on cars due to greatly reduced production costs. Thermographic cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 900–14,000 nanometers or 0.9–14 μm) and produce images of that radiation. Since infrared radiation is emitted by all objects based on their temperatures, 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 (hence the name).

Applications of Infrared Thermal Imaging

Surveillance — see hiding burglars or track someone fleeing a crime scene.

Counter-surveillance — covert surveillance equipment such as listening devices or hidden cameras all consume some energy which gives off waste heat that is clearly visible on a thermal camera (even if hidden inside or behind an object).

Circulation problems — may help to detect the presence of deep vein thromboses and other circulatory disorders

PCB defects — check for electrical defects on printed circuit boards (PCB’s).

Power usage — quickly see which circuits on a switchboard are consuming the most power.

Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it. Missiles which use infrared seeking are often referred to as “heat-seekers”, since infrared is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and emit heat, and as such, are especially visible in the infrared wavelengths of light compared to objects in the background.

Infrared homing is a passive system that homes in on the heat generated by the target. Typically used in the ATGMs(Anti tank guided Missiles) to track the heat of MBT(Main Battle Tank). This means of guidance is also called to as “heat seeking”. Infrared seekers are passive devices, which, unlike radar, provide no indication that they are tracking a target.

Tracking:

Thermal image of the target is locked before launch, Image registration algorithms are used to compare the position of the target frame by frame during the flight of missile. LOS(Line Of Sight) rate error generated by the Integrated Electronics part in the IIR seeker is given to the servo control system. Servo system generates appropriate motor commands to actuate the fins position so that missile manoeuvres in such a way that target is always in the centre of Field Of View (FOV) of seeker optics.

Other useful links:

https://www.pyrosales.com.au/blog/thermal-imaging/thermal-imaging-where-is-it-used

https://www.quora.com/How-do-seeker-missiles-work-How-are-they-used