Thermography

• 1. Introduction

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  Thermography is the practice of turning invisible heat into a visible picture, and it has been reshaping how humans see the world since Sir William Herschel discovered infrared radiation in 1800. Stand in a field at night, and your eyes see nothing. Point a thermal imaging camera at that same field, and every warm-blooded creature in it glows like a lantern. That gap between what we can see and what the camera can reveal is the whole story of this technology.

  The camera does not detect light in the way our eyes do. It reads infrared radiation, the heat energy that every object above absolute zero constantly emits. A thermal image, called a thermogram, is not a photograph of a surface. It is a map of radiant energy. And reading that map accurately turns out to be far more complicated than it first appears. What drives a fire into the walls of a building, how a power line signals its own failure before it breaks, why a doctor scanning for fever can see what a standard thermometer misses: these are the questions that decades of thermographic development have tried to answer.

• 2. The Physics of Invisible Light

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  Black-body radiation is the foundation on which all of thermography rests. Every object above absolute zero emits thermal radiation, and the amount it emits climbs with temperature. A theoretical perfect emitter, called a black body, has an emissivity of 1. Real materials fall somewhere below that ceiling. Silver, one of the lowest, sits at an emissivity coefficient of 0.02. Asphalt, at the other end, reaches 0.98.

  Emissivity is not the only complicating factor. The signal a thermal camera receives is a mixture of three components: radiation the object emits, radiation from a remote source that passes through the object, and radiation from surrounding sources that bounces off the surface. The formula that governs this relationship states that incident radiant power equals emitted plus transmitted plus reflected radiant power. In practice, this means the camera is always integrating multiple sources, and what it shows is an approximation.

  Materials behave differently across the infrared spectrum. Clean metal surfaces have emissivity that falls at longer wavelengths. Many dielectric materials, including quartz, sapphire, and calcium fluoride, show the opposite pattern, with emissivity rising at longer wavelengths. Iron oxide displays a relatively flat emissivity profile across the infrared range. A thermographer who ignores those differences risks reading the wrong temperature entirely.

  Wind adds another layer of uncertainty outdoors. Convective cooling from moving air can alter the surface temperature of the measured object, pulling the reading away from the true value. Thermographers must account for atmospheric absorption, ambient temperature, and humidity before they can claim an accurate result.

• 3. From Thermocouple to Camera

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  In 1829, Leopoldo Nobili built the first known thermocouple by exploiting the Seebeck effect, creating what was then an improved thermometer and a crude thermopile. He described the instrument to Macedonio Melloni, and the two initially developed it together before Melloni continued alone. By 1833, Melloni had built a multielement thermopile sensitive enough to detect a person standing ten metres away.

  The next major leap came in 1880, when Samuel Pierpont Langley invented the bolometer. Langley worked with his assistant Charles Greeley Abbot to refine it over decades. By 1901, their instrument could detect the thermal signature of a cow from 400 metres and distinguish temperature differences as small as 0.00001 degrees Celsius.

  The path toward practical cameras moved through two landmark inventions in the twentieth century. In 1929, Hungarian physicist Kalman Tihanyi built an infrared-sensitive electronic television camera, designed for anti-aircraft defense in Britain. Then, in 1947, the US military and Texas Instruments jointly developed the first American thermographic camera, an infrared line scanner that required a full hour to produce a single image. The AGA company later commercialized a system using a cooled photoconductor that could scan images far more quickly.

  The British Yellow Duckling linescan system of the mid-1950s used a continuously rotating mirror and detector, with scanning along one axis handled by the motion of the carrier aircraft. Although it failed in its intended role of tracking submarines by their wake, it found a second life in land-based surveillance. Work at the Royal Signals and Radar Establishment then showed that mercury cadmium telluride could serve as a photoconductor requiring much less cooling than previous materials. In 1969, Michael Francis Tompsett at English Electric Valve Company patented a camera that scanned pyro-electronically. Tompsett also proposed the concept of solid-state thermal-imaging arrays, which eventually led to the hybridized single-crystal-slice devices used today. The first commercial thermal imaging camera reached the market in 1965, initially sold for high voltage power line inspections.

• 4. How Cameras See Heat

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  Modern thermographic cameras divide into two broad families: those with cooled detectors and those without. Cooled detectors are typically sealed in a vacuum case and brought down to cryogenic temperatures. Most modern cooled detectors operate between 60 Kelvin and 100 Kelvin, which translates to -213 to -173 degrees Celsius. Without that cooling, the sensor would be swamped by its own thermal radiation.

  Cooling can be achieved several ways. Peltier coolers are relatively simple and compact but limited in how cold they can reach. Stirling cryocoolers deliver better image quality, particularly for objects near or below room temperature. A third option uses gases bottled at high pressure: nitrogen expanded through a micro-sized orifice passes over a miniature heat exchanger and cools via the Joule-Thomson effect. For field deployments, keeping a supply of pressurized gas is a significant logistical challenge.

  Uncooled cameras rely on sensors that operate at or near ambient temperature. In these devices, a 1 degree Celsius difference at the scene produces only a 0.03 degree Celsius change at the sensor pixel. Silicon microbolometers can achieve a noise-equivalent temperature difference, known as NETD, as low as 20 mK. They consist of a layer of amorphous silicon or a thin film of vanadium oxide suspended on a silicon nitride bridge above the scanning electronics. Ferroelectric detectors, which operate near the phase transition temperature of the sensor material, can reach NETD of 70 to 80 mK using f/1 optics and a 320x240 sensor.

  In 2013, DARPA announced a five-micron long-wave infrared camera using a 1280x720 focal plane array. For smartphones, a thermal camera first appeared as an add-on accessory in 2014 for a few hundred dollars, and the first integrated thermal camera in a smartphone shipped in the Cat S60 in 2016. The first automotive night vision system using thermal imaging appeared in the 2000 Cadillac DeVille. Resolution for most thermographic cameras remains considerably lower than optical cameras, with many models producing 160x120 or 320x240 pixel images and the highest-end models reaching 1280x1024.

• 5. Reading the Image Correctly

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  Standard infrared cameras produce monochrome images, because their sensors do not distinguish between different wavelengths of infrared radiation. Color image sensors require complex construction to differentiate wavelengths, and color carries less meaning outside the visible spectrum because infrared wavelengths do not map uniformly into human color perception.

  To make temperature differences easier to read, these monochrome images are often displayed in pseudo-color using a technique called density slicing. Under this convention, the warmest parts of a thermogram are displayed in white, intermediate temperatures in reds and yellows, and the coolest zones in black. A calibrated scale shown alongside the image translates those colors into actual temperatures.

  Getting accurate temperature readings from a thermogram requires careful calibration. For a quick estimate, a thermographer can consult an emissivity table and enter the appropriate value into the imager, which then calculates contact temperature from the detected infrared radiation. For greater accuracy, a thermographer may apply a material of known high emissivity directly to the surface being measured. Standard black insulation tape, with an emissivity of about 0.97, is one practical choice. Most cameras carry an accuracy of plus or minus 2 percent in temperature measurement, and all are limited to detecting surface temperatures. They cannot see what is happening inside a solid object.

  Higher-end thermographic cameras are classified as dual-use military equipment and are export-restricted. Under International Traffic in Arms Regulations, cameras with a resolution of 640x480 pixels or greater are subject to export controls unless their refresh rate is 9 Hz or lower. Quality thermal cameras often cost US$3,000 or more, though basic models with pixel arrays as small as 40x40 are available at lower price points.

• 6. Where Thermography Is Used

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  Firefighters rely on thermography to see through smoke, locate trapped people, and identify the base of a fire. Power line maintenance technicians use it to find overheating joints before those joints fail, turning what would be a reactive repair into a scheduled maintenance task. Building inspectors use thermograms to find gaps in thermal insulation, trace moisture in walls and roofs, locate pest infestations, and detect refrigerant leaks. For structures and bridges, the ASTM standard D4788 covers the use of infrared thermography to detect delamination in bridge decks.

  In medicine, thermography monitors physiological responses including fever detection without contact. During the 2009 swine flu pandemic, government and airport personnel used it specifically to screen travelers for suspected cases. Healthcare applications extend to peripheral vascular disease screening, carotid artery stenosis screening through skin thermal maps, thyroid gland abnormalities, and facial emotion recognition. Some practitioners promote thermal imaging for breast cancer screening, but the FDA has warned that choosing this approach instead of mammography may cause patients to miss cancer at its earliest detectable stage.

  The military and law enforcement value thermography for its ability to make warm-blooded targets visible in complete darkness and in fog, smoke, or other visibility-obscuring conditions. In an unexpected application within computer security, a thermal attack uses heat traces left on touchscreens or keyboards after a user interacts with them to reconstruct what input was entered.

  For astronomy, cooled infrared cameras are installed at major research telescopes including UKIRT, the Spitzer Space Telescope, WISE, and the James Webb Space Telescope. In cricket, the Umpire Decision Review System uses thermography to detect faint contact between ball and bat by identifying the heat patch a collision leaves on the bat's surface.

• 7. Natural Thermographers

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  Some living creatures do not need cameras. Infrared sensing in snakes is one of the most well-known examples of biological thermoception, the natural analog to what a bolometer does. These organs function as crude thermal imagers, allowing certain snakes to detect warm-blooded prey in darkness through sensed radiation rather than visible light.

  The field of thermography by strict definition is an instrumented measurement technique. But the existence of natural thermoception shows that detecting thermal radiation as a form of environmental perception has deep roots in the biology of sensing. The work begun with Nobili's thermopile in 1829 and carried through Langley's bolometer of 1880 was, in a sense, an effort to match a capability that evolution had already demonstrated was possible. What the 1969 pyroelectric camera patent by Tompsett and the uncooled microbolometer arrays that followed eventually built was a technology accessible enough to move from military labs and astronomy observatories into smartphones.