Инфракрасные камеры и термография: история и развитие

Imaging device using infrared radiation

A thermographic camera (also called an infrared camera or thermal imaging camera, thermal camera or thermal imager) is a device that creates an image using infrared (IR) radiation, similar to a normal camera that forms an image using visible light. Instead of the 400–700 nanometre (nm) range of the visible light camera, infrared cameras are sensitive to wavelengths from about 1,000 nm (1 micrometre or μm) to about 14,000 nm (14 μm). The practice of capturing and analyzing the data they provide is called thermography.

Infrared was discovered in 1800 by Sir William Herschel as a form of radiation beyond red light. These "infrared rays" (infra is the Latin prefix for "below") were used mainly for thermal measurement. There are four basic laws of IR radiation: Kirchhoff's law of thermal radiation, Stefan–Boltzmann law, Planck's law, and Wien's displacement law. The development of detectors was mainly focused on the use of thermometers and bolometers until World War I. A significant step in the development of detectors occurred in 1829, when Leopoldo Nobili, using the Seebeck effect, created the first known thermocouple, fabricating an improved thermometer, a crude thermopile. He described this instrument to Macedonio Melloni. Initially, they jointly developed a greatly improved instrument. Subsequently, Melloni worked alone, creating an instrument in 1833 (a multielement thermopile) that could detect a person 10 metres away. The next significant step in improving detectors was the bolometer, invented in 1880 by Samuel Pierpont Langley. Langley and his assistant Charles Greeley Abbot continued to make improvements in this instrument. By 1901, it could detect radiation from a cow from 400 metres away and was sensitive to differences in temperature of one hundred thousandths (0.00001 C) of a degree Celsius. The first commercial thermal imaging camera was sold in 1965 for high voltage power line inspections. The first advanced application of IR technology in the civil section may have been a device to detect the presence of icebergs and steamships using a mirror and thermopile, patented in 1913. This was soon outdone by the first accurate IR iceberg detector, which did not use thermopiles, patented in 1914 by R. D. Parker. This was followed by G. A. Barker's proposal to use the IR system to detect forest fires in 1934. The technique was not genuinely industrialized until it was used to analyze heating uniformity in hot steel strips in 1935.

In 1929, Hungarian physicist Kálmán Tihanyi invented the infrared sensitive (night vision) electronic television camera for anti aircraft defense in Britain. The first American thermographic camera developed was an infrared line scanner. This was created by the US military and Texas Instruments in 1947 and took one hour to produce a single image. While several approaches were investigated to improve the speed and accuracy of the technology, one of the most crucial factors dealt with scanning an image, which the AGA company was able to commercialize using a cooled photoconductor. The first infrared linescan system was the British Yellow Duckling of the mid 1950s. This used a continuously rotating mirror and detector, with Y axis scanning by the motion of the carrier aircraft. Although unsuccessful in its intended application of submarine tracking by wake detection, it was applied to land based surveillance and became the foundation of military IR linescan. This work was further developed at the Royal Signals and Radar Establishment in the UK when they discovered that mercury cadmium telluride was a photoconductor that required much less cooling. Honeywell in the United States also developed arrays of detectors that could cool at a lower temperature, but they scanned mechanically. This method had several disadvantages which could be overcome using an electronic scanning system. In 1969 Michael Francis Tompsett at English Electric Valve Company in the UK patented a camera that scanned pyro electronically and which reached a high level of performance after several other breakthroughs during the 1970s. Tompsett also proposed an idea for solid state thermal imaging arrays, which eventually led to modern hybridized single crystal slice imaging devices. is possible. This technology was a precursor to modern microbolometer technology, and mainly used in firefighting thermal cameras.

One of the essential areas of development for security systems was for the ability to intelligently evaluate a signal, as well as warning of a threat's presence. Under the encouragement of the US Strategic Defense Initiative, "smart sensors" began to appear. These are sensors that could integrate sensing, signal extraction, processing, and comprehension. There are two main types of smart sensors. One, similar to what is called a "vision chip" when used in the visible range, allow for preprocessing using smart sensing techniques due to the increase in growth of integrated microcircuitry. The other technology is more oriented to specific use and fulfills its preprocessing goal through its design and structure. Towards the end of the 1990s, the use of infrared was moving towards civilian use. There was a dramatic lowering of costs for uncooled arrays, which along with the significant increase in developments, led to a dual use market encompassing both civilian and military uses. These uses include environmental control, building/art analysis, functional medical diagnostics, and car guidance and collision avoidance systems.

Infrared energy is just one part of the electromagnetic spectrum, which encompasses radiation from gamma rays, x rays, ultraviolet, a thin region of visible light, infrared, terahertz waves, microwaves, and radio waves. These are all related and differentiated in the length of their wave (wavelength). All objects emit a certain amount of black body radiation as a function of their temperature. Generally speaking, the higher an object's temperature, the more infrared radiation is emitted as black body radiation. A special camera can detect this radiation in a way similar to the way an ordinary camera detects visible light. It even works in total darkness because ambient light level does not matter. This makes it useful for rescue operations in smoke filled buildings and underground. A major difference with optical cameras is that the focusing lenses cannot be made of glass, as glass blocks long wave infrared light. Typically the spectral range of thermal radiation is from 7 to 14 μm. Special materials such as Germanium, calcium fluoride, crystalline silicon or newly developed special type of chalcogenide glasses must be used. Except for calcium fluoride all these materials are quite hard and have high refractive index (for germanium n=4) which leads to very high Fresnel reflection from uncoated surfaces (up to more than 30%). For this reason most of the lenses for thermal cameras have antireflective coatings. The higher cost of these special lenses is one reason why thermographic cameras are more expensive.

Images from infrared cameras tend to be monochrome because the cameras generally use an image sensor that does not distinguish different wavelengths of infrared radiation. Color image sensors require a complex construction to differentiate wavelengths, and color has less meaning outside of the normal visible spectrum because the differing wavelengths do not map uniformly into the system of color vision used by humans. Sometimes these monochromatic images are displayed in pseudo color, where changes in color are used rather than changes in intensity to display changes in the signal. This technique, called density slicing, is useful because although humans have much greater dynamic range in intensity detection than color overall, the ability to see fine intensity differences in bright areas is fairly limited. For use in temperature measurement the brightest (warmest) parts of the image are customarily colored white, intermediate temperatures reds and yellows, and the dimmest (coolest) parts black. A scale should be shown next to a false color image to relate colors to temperatures. Their resolution is considerably lower than that of optical cameras, mostly only 160 x 120 or 320 x 240 pixels, although more expensive cameras can achieve a resolution of 1280 x 1024 pixels. Thermographic cameras are much more expensive than their visible spectrum counterparts, though low performance add on thermal cameras for smartphones became available for hundreds of dollars in 2014. Higher end models are often deemed dual use military grade equipment, and are export restricted, particularly if the resolution is 640 x 480 or greater, unless the refresh rate is 9 Hz or less. The export of thermal cameras is regulated by International Traffic in Arms Regulations. A thermal camera was first built into a smartphone in 2016, into the Cat S60. In uncooled detectors the temperature differences at the sensor pixels are minute; a 1 °C difference at the scene induces just a 0.03 °C difference at the sensor. The pixel response time is also fairly slow, at the range of tens of milliseconds. Thermography finds many other uses. For example, firefighters use it to see through smoke, find people, and localize hotspots of fires. With thermal imaging, power line maintenance technicians locate overheating joints and parts, a telltale sign of their failure, to eliminate potential hazards. Where thermal insulation becomes faulty, building construction technicians can see heat leaks to improve the efficiencies of cooling or heating air conditioning. Thermal imaging cameras are also installed in some luxury cars to aid the driver (automotive night vision), the first being the 2000 Cadillac DeVille. Some physiological activities, particularly responses such as fever, in human beings and other warm blooded animals can also be monitored with thermographic imaging. Cooled infrared cameras can be found at major astronomy research telescopes, even those that are not infrared telescopes.

Thermographic cameras can be broadly divided into two types: those with cooled infrared image detectors and those with uncooled detectors.

Uncooled thermal cameras use a sensor operating at ambient temperature, or a sensor stabilized at a temperature close to ambient using small temperature control elements. Modern uncooled detectors all use sensors that work by the change of resistance, voltage or current when heated by infrared radiation. These changes are then measured and compared to the values at the operating temperature of the sensor. Uncooled infrared sensors can be stabilized to an operating temperature to reduce image noise, but they are not cooled to low temperatures and do not require bulky, expensive, energy consuming cryogenic coolers. This makes infrared cameras smaller and less costly. However, their resolution and image quality tend to be lower than cooled detectors. This is due to differences in their fabrication processes, limited by currently available technology. An uncooled thermal camera also needs to deal with its own heat signature. Uncooled detectors are mostly based on pyroelectric and ferroelectric materials or microbolometer technology. The material are used to form pixels with highly temperature dependent properties, which are thermally insulated from the environment and read electronically. Ferroelectric detectors operate close to phase transition temperature of the sensor material; the pixel temperature is read as the highly temperature dependent polarization charge. The achieved NETD of ferroelectric detectors with f/1 optics and 320x240 sensors is 70 80 mK. A possible sensor assembly consists of barium strontium titanate bump bonded by polyimide thermally insulated connection. Silicon microbolometers can reach NETD down to 20 mK. They consist of a layer of amorphous silicon, or a thin film vanadium(V) oxide sensing element suspended on silicon nitride bridge above the silicon based scanning electronics. The electric resistance of the sensing element is measured once per frame. Current improvements of uncooled focal plane arrays (UFPA) are focused primarily on higher sensitivity and pixel density. In 2013 DARPA announced a five micron LWIR camera that uses a 1280 x 720 focal plane array (FPA). Some of the materials used for the sensor arrays are amorphous silicon (a Si), vanadium(V) oxide (VOx), lanthanum barium manganite (LBMO), lead zirconate titanate (PZT), lanthanum doped lead zirconate titanate (PLZT), lead scandium tantalate (PST), lead lanthanum titanate (PLT), lead titanate (PT), lead zinc niobate (PZN), lead strontium titanate (PSrT), barium strontium titanate (BST), barium titanate (BT), antimony sulfoiodide (SbSI), and polyvinylidene difluoride (PVDF).

Some specification parameters of an infrared camera system are number of pixels, frame rate, responsivity, noise equivalent power, noise equivalent temperature difference (NETD), spectral band, distance to spot ratio (D:S), minimum focus distance, sensor lifetime, minimum resolvable temperature difference (MRTD), field of view, dynamic range, input power, and mass and volume.