Thermal Imaging Chronicles (Part 1)
William Herschel (1738-1822) The English astronomer, the discoverer of infrared radiation. Source - en.wikipedia.org
The next significant person in stories thermal imaging became the discoverer of Uranus and his satellites English astronomer William Herschel. The scientist discovered in 1800 the existence of invisible rays, “possessing the greatest heating power,” located outside the human visible spectrum. He succeeded in this with the help of a glass prism, which decomposes light into its components, and a thermometer, which fixed the maximum temperature just to the right of the visible red light. As a follower of the corpuscular teachings of Newton, Herschel firmly believed in the identity of the light and radiant heat, however, after experiments with the refraction of invisible infrared rays, his faith was badly shaken. But in any history it does not do without the authoritative clever people from science, who spoil the picture with their false speculation. In this role was made by the physicist John Leslie from Edinburgh, who declared the existence of heated air, which, in fact, is the very "mythical heat rays." He was not too lazy to repeat the experiment by Herschel, he invented a special differential mercury thermometer for this, which recorded the maximum temperature just in the area of the visible red spectrum. Herschel was declared almost a charlatan, pointing out the insufficient preparation of experiments and the falsity of conclusions.
However, time judged differently - by 1830-th year, numerous experiments by leading world scientists proved the existence of "rays named after Herschel", which Becquerel called infrared. The study of various bodies on the ability to transmit (or not transmit) such radiation led scientists to understand that the fluid that fills the eyeball absorbs the infrared spectrum. In general, it was precisely such a mistake of nature that created the need for the invention of a thermal imager. But in the XIX century, scientists only cognized the nature of heat-transfer and invisible radiation, going into all the nuances. It turned out that different sources of heat - a hot kettle, red-hot steel, an alcohol lamp - have a different quality composition of the “infrared pie”. It was experimentally proved by the Italian Machedonio Melloni with the help of one of the first heat registering devices - bismuth-antimony thermo-column (thermomultiplicateur). Interference of infrared radiation allowed to deal with this phenomenon - in 1847, with its help, the spectrum with a wavelength up to 1,94 μm was used for the first time.
Spider bolometer - thermal radiation recorder. Source - en.wikipedia.org
And in 1881, a bolometer came to the aid of experimental physics - one of the first devices for fixing radiant energy. This miracle was invented by the Swedish mathematician and physicist Adolf-Ferdinand Swanberg, setting an extremely thin blackened plate in the path of infrared radiation that can change its conductivity under the influence of heat. Such a radiation receiver made it possible to reach the maximum possible wavelength at that time to 5,3 μm, and by the year 1923, 420 μm were detected in the radiation of a small electric oscillator. The beginning of the 20th century is marked by the emergence of a mass of ideas relating to the practical implementation of the theoretical searches of previous decades. Thus, a photoresistor of thallium sulphide treated with oxygen (thallium oxysulfide), capable of changing its conductivity under the action of infrared rays, appears. German engineers created on their basis talloid receivers that have become a reliable means of communication on the battlefield. Until the 1942, the Wehrmacht managed to keep its system secret, capable of operating for a distance of up to 8 km, until they pierced at El Alamein. Evaporographs are the first true thermal imaging systems, allowing to obtain more or less satisfactory thermograms.
The device is as follows: a thin membrane with supersaturated vapors of alcohol, camphor or naphthalene is located in the chamber, and the temperature inside is such that the rate of evaporation of substances is equal to the rate of condensation. Such thermal equilibrium is violated by the optical system focusing the thermal image on the membrane, which leads to the acceleration of evaporation in the hottest areas - as a result, a thermal image is formed. The endless seconds in the evapororograph went to the formation of a picture, the contrast of which left much to be desired, the noise sometimes overshadowed everything, but there was nothing to say about the high-quality transmission of moving objects. Despite the good resolution of 10 degrees Celsius, the combination of minuses did not leave the evapororograph a place in mass production. However, the small-series EV-84 device appeared in the USSR, EVA in Germany, and experimental searches were also conducted in Cambridge. Since the 30s, semiconductors and their special relationships with the infrared spectrum have attracted the attention of engineers. Here the reins passed to the military, under whose leadership the first cooled lead sulfide-based photoresistors appeared. The idea that the lower the temperature of the receiver, the higher its sensitivity, was confirmed and the crystals in the thermal imagers began to freeze with solid carbon dioxide and liquid air. And the technology for spraying the sensitive layer in a vacuum, developed at the University of Prague, was already a very high tech for those pre-war years. Since 1934, the zero-generation electron-optical converter, better known as the “Canvas Cup”, became the progenitor of the mass of useful equipment - from night driving devices tanks to individual sniper sights.
An important place night vision received in the naval navy - ships gained the ability to navigate in complete darkness in the coastal zone, while maintaining a regime of blackout. In 1942, the fleet’s achievements in the field of night navigation and communication were borrowed by the air forces. In general, the British were the first to detect an airplane in the night sky by its infrared signature in 1937. The distance, of course, was modest - about 500 meters, but for that time it was an undoubted success. The closest to the thermal imager in the classical sense came in 1942, when a superconducting bolometer based on tantalum and antimony with cooling with liquid helium was obtained. German heat detectors "Donau-60" based on it made it possible to recognize large sea vessels at a distance of up to 30 km. Forty years became a kind of crossroads for thermal imaging technology - one path led to systems similar to television with mechanical scanning, and the second to infrared video recorders without scanning.
The history of domestic military thermal imaging technology dates back to the end of the 1960s, when work began at the Novosibirsk Instrument-Making Plant in the framework of the Evening and Evening-2 research projects. The theoretical part was supervised by the leading scientific research institute of applied physics in Moscow. The serial thermal imager did not work out then, but the developments were used in the research work “Lena”, the result of which was the first thermal imager for reconnaissance 1PN59, equipped with a photodetector “Lena FN”. 50 photosensitive elements (each 100x100 μm in size) were arranged in one row with 130 μm pitch and ensured the device operation in the medium wave (MWIR - Middle Wave Infrared) 3-5 μm spectral range with a target detection range up to 2000 m. Nitrogen-based gas mixture under high pressure entered the photoheater microheat exchanger, cooled it to -194,5OS and returned to the compressor. This is a feature of the devices of the first generation - high sensitivity required low temperatures. And the low temperatures required, in turn, large dimensions and impressive power consumption in 600 watts.
Installed 1PN59 on the domestic reconnaissance machine PRP-4 "Nard" using the BMP-1 base.
By 1982, domestic engineers decided to shift the working spectral range of thermal imaging devices to 8-14 µm (Long Wave Infrared long-wave LWIR) due to the better “throughput” of the thermal radiation atmosphere in this segment. The product under the symbol 1PN71 was the result of a similar design work in the direction of "Benefit-2", having as a "all-seeing eye" a photo-receiver from cadmium-mercury telluride (CdHgTe or КРТ).
Product 1PN71. Source - army-guide.com
They called this sensing element "Zero Gravity-64" and it had ... correctly, 64 crystal CMT of sizes 50х50 in increments of 100 μm. Freeze "Weightlessness" had even stronger - to-196,50С, but the weight and dimensions of the product decreased markedly. All this allowed to achieve long-sighted 1PN71 in 3000 meters and significantly improve the picture in front of the user. The imager was installed at the Deuterium PRP-4М mobile artillery reconnaissance point, which, in addition to the 1PN71 device, is armed with a night vision device, a radar and a laser range finder. A rare species in the Russian army - BRM-3 "Lynx" is also equipped with a thermal imaging device intelligence Novosibirsk instrument-making plant. To change this technique in the troops, the 1PN126 “Argus-AT” thermal imager, developed in 2005 by the Tochpribor Central Design Bureau and equipped with microscopic sensing elements of 30x30 micron size from the proven CdHgTe, is intended to be changed. A real highlight of the one hundred and twenty sixth thermal imager was a rotating octagonal germanium prism, transparent to infrared radiation. It is this scanner that generates two frames on a photodetector device in the registration mode of the thermal signature of the observed object in one revolution. For comparison - in 1PN71 this role was performed by a flat mirror - in the Soviet Union there were no inexpensive technologies for the production of germanium glasses. Under the new domestic thermal imager a reconnaissance platform of the front edge of the PDP-4А or, as it is often called, the “all-seeing eye of the god of war” was prepared. Bristled with numerous lenses of optical reconnaissance devices, the car is quite similar to the ancient Greek multi-eyed giant, after whom it was named.
- Evgeny Fedorov
- zodiak.uu.ru army-guide.com cris9.armforc.ru ru.wikipedia.org
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