History of Thermal Imaging
It All Began Back in 1800
Current thermal imagers are based on technology that was originally developed for the military. However, thermography is not as new as most wo
On Feb. 11, 1800, astronomer Sir William Herschel was searching for optical filter material to reduce the brightness of the sun’s image in telescopes during solar observations. While testing different samples of colored glass that gave similar reductions in brightness, he became aware that some of the filter samples passed very little of the sun’s heat. Others passed so much heat that the risk of eye damage was possible after only a few seconds of observation.
When using a red filter, Herschel found there was a lot of heat produced. He discovered infrared radiation in sunlight by passing it through a prism and holding a thermometer just beyond the red end of the visible spectrum. This thermometer was meant to be a control to measure the ambient air temperature in the room. He was shocked when it showed a higher temperature than the visible spectrum. Further experimentation led to Herschel’s conclusion that there must be an invisible form of light beyond the visible spectrum. When Herschel revealed his discovery, he referred to this new portion of the electromagnetic spectrum as Calorific Rays.
The first so-called heat-picture became possible in 1840, the result of work by Sir John Herschel. Based upon the differential evaporation of a thin film of oil when exposed to a heat pattern focused upon it, the thermal image could be seen by reflected light where the interference effects of the oil film made the image visible to the eye. Sir John also managed to obtain a primitive record of the thermal image on paper, which he called a thermograph.
The improvement of infrared detector sensitivity progressed slowly. However, in 1880 a major breakthrough, made by American astronomer Samuel Pierpont Langley, was the invention of the bolometer. This instrument is used to measure IR, or heat, radiation. The bolometer is essentially a very sensitive thermometer. The first unit consisted of a thin blackened strip of platinum connected in one arm of a Wheatstone bridge circuit upon which the infrared radiation was focused and a type of ammeter, called a galvanometer, measured the electric current. This instrument is said to have been able to detect the heat from a cow at a distance of 400 meters.
In 1929, Hungarian physicist Kálmán Tihanyi invented the first infrared-sensitive electronic television camera for anti-aircraft defense in Britain. The first conventional IR camera, the Evaporograph, was declassified around 1956.
The Evaporograph was a thermal imaging device that converted an IR image into a visible image by differential evaporation, or condensation of oil on a thin membrane. In this thermal imaging process, the first element is a germanium lens that collects the infrared radiation and brings it into focus to form an image. The second element of this process was the detector, consisting of a very thin nitrocellulose membrane placed at the image plane. The front surface was coated with a thin layer of gold-black that absorbed radiation. The membrane was then placed in an enclosure to evacuate the air. The irradiance at the membrane due to the focused image by the germanium lens was absorbed to produce a heating effect that formed a thermal reproduction of the IR image.
It was not until about 20 to 30 years later that real progress was made in the thermal imaging world. In the late 1950s and 1960s, Texas Instruments, Hughes Aircraft and Honeywell developed single element detectors that scanned scenes and produced line images. The military had a lock on the technology because it was expensive and had sensitive military applications.
These basic detectors led to the development of modern thermal imaging. The pyroelectric vidicon tube (a pyroelectric detector is an IR-sensitive optoelectronic component used for detecting electromagnetic radiation in a wavelength range from 2µm to 14µm) was developed by Philips and EEV in the 1970s. It became the core of a new product for firefighting, first used by the Royal Navy for shipboard firefighting.
In 1978, Raytheon R&D group, then part of Texas Instruments, patented ferroelectric infrared detectors, using barium strontium titanate (BST). Raytheon first demonstrated the technology to the military in 1979.
Also in 1978, FLIR was founded as a provider of IR imaging systems installed on vehicles for use in conducting energy audits. The company would later expand to other applications and markets for thermal imaging technology, including stabilized thermal imaging cameras for law enforcement aircraft, radiometry devices for use in monitoring industrial systems, and thermal imaging systems for use in ground-based security and search and rescue.
In the late 1980s, the federal government awarded HIDAD (HIgh-Density Array Development) contracts to both Raytheon and Honeywell for the development of thermal imaging technology for practical military applications. Raytheon went on to commercialize BST technology, while Honeywell developed vanadium oxide (VOx) microbolometer technology.
After the 1991 Gulf War, production of thermal units increased and costs decreased, which has led the way for the introduction of thermal imaging into many different industries.
In 1994, Honeywell was awarded a patent for a microbolometer detector array. Thermal imagers based on both BST and microbolometer technologies are available now for nonmilitary applications. In fact, thermal imaging has now expanded to be used in firefighting, law enforcement, industrial applications, security, transportation and many other industries.
Use Becoming More Widespread
One major question arises when discussing thermal cameras: Does a thermal imager work like a video camera? The answer is yes.
Video cameras use a detector that is sensitive to the visible light spectrum. Electronics process the information gathered on the video camera detector into a “video” signal that we can view on a standard video monitor or display. Thermal imaging detectors are sensitive to IR wavelengths. Again, electronics process the information gathered on the thermal imaging camera detector into a normal video signal that we can view on a standard video monitor or display. However, the overall image is not one would expect.
The surveillance industry has been slow to incorporate thermal technology.
Thermal images can either be black and white or in color; however, identification using thermal is next to impossible. Many of us are accustomed to viewing a scene and having the ability to identify persons or objects. Thermal applications are used to make viewers aware of the presence of persons or objects that do not fit into the surrounding environment. As for identification, well that is another matter. In most system applications, a thermal camera is linked with a standard composite video output camera for use during daylight hours of operation.
In defense of thermal, imagine the difficulty of locating a suspect in total darkness, or trying to find small evidence in a large, poorly lit a
rea. Previously, night vision technology needed IR illumination to work. This makes it more difficult if light is not available or if one wishes to be unseen. Thermal imaging cameras need no light and can be used to track down fugitives, suspects or evidence. Working on a crash site, law enforcement can determine potential causes of collisions and even locate passengers who may have been thrown from the vehicle during a violent collision.
Thermography has helped advance many other scientific areas such as medical, fire inspections, electrical wiring inspections and law enforcement. The ability to see objects in the dark allows us to go beyond natural capabilities and explore a new and interesting world. Thermal cameras have influenced many fields of today’s technologies.
Thermal technology is not just for security. Many of these changing technical fields that are now incorporating thermal technology are quite diverse.
In the automotive world, thermal is now being incorporated in cars, buses, trucks and trains for driver vision enhancement. Thermal cameras see up to five times further than headlights. Thanks to thermal, the driver can see pedestrians and obstacles on the road from a greater distance. This way, thermal cameras can help to avoid deadly accidents.
The automotive industry is also using thermal technology to improve the reliability of its products by testing all parts — from simple items such as transmission belts to complex ones like turbochargers or catalytic converters before they are cleared for production. A thermal imaging camera is also used on the engine test-bed. New engines are put on trial to determine heat build-up and distribution, as well to determine the failure point.
In the medical field, doctors and physicians have found ways to use thermal imaging cameras to help them provide more effective treatments for their patients. More precise medical imaging leads to more specific diagnoses, and more acute treatments. It can help detect disease, and could prove vital to maintaining and safeguarding one’s health.
With the recent major fires in Arizona, firefighters are using thermal technology to locate people in smoke-filled areas. Any application that needs to see in total darkness, through light fog and smoke, can benefit from the power of thermal imagery.
A great example of the increasing acceptance and utilization of thermal in the security field was well documented at this year’s ISC West exposition held in Las Vegas. There was a noticeable increase of manufacturers offering the surveillance industry a complete line of thermal cameras. There was a great emphasis placed on the advantages of thermal imagery in regard to low light surveillance and thermal use for video analytics.
The growth of thermal in the security industry is only in its infancy stage. With the need and desire to improve video surveillance and the ever-increasing application for system analytics, thermal is here to stay. With the introduction of lower costs and advancement in both detector design and germanium lenses required for thermal cameras, the future looks bright … even red hot.
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