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35 as electro-optical (EO) cameras, but with a few notable differences. For one, lenses on thermal imaging sensors must be stable over a range of temperatures in order to give consistent and reliable transmission of IR radiation. Confirming this stability forms a critical part of performance testing, as it ensures that the lenses will transmit known thermal readings accurately over a range of environmental temperatures. Also notable is the relative rarity of optical zoom functionality in IR and NIR (near-infrared) sensors, because zoom lenses are prohibitively heavy and bulky for integration on many UAVs. Selecting the lens material must take several factors into account. Perhaps most important is the transmission of IR radiation at the desired waveband, known as the transmittance of a material. For example, germanium transmits light from 3 to 14 µm, making it versatile across the MWIR (3-8 µm) and LWIR (8-14 µm) ranges. The transmittance of sapphire, on the other hand, runs from 0.3 to 5 µm, making it useable for visible light cameras, as well as thermal cameras in the NIR band and parts of the MWIR band. Cost considerations often follow closely. Germanium is typically the most expensive material, chalcogenide glasses are second, and silicon is often the least costly. Also, a high index of refraction – how much the IR light slows down (and is therefore bent or focused) as it enters the lens material at an angle – means similar performance can be achieved with less lens material, reducing overall system weight and cost. Among thermal optics materials, germanium has the highest refractive index, at 4.003, with silicon typically coming second at about 3.4, giving these the biggest weight and cost reduction advantages. And for the aforementioned stability over temperature, silicon has the advantage, with a thermal expansion coefficient of 2.55. That is lower than most other materials, and makes it potentially the most versatile for use in hot environments or those with explosive hazards. There are various lens manufacturing processes to consider as well. Germanium (and to a lesser extent chalcogenides) is often processed via diamond-turning to determine lens curvature. It is an automated process but can only produce single lenses at a time. Automated batch processes such as stamping or moulding of chalcogenide lenses – and to a greater extent, those made from silicon – are being adopted more widely though, and the cost-effectiveness of this approach is contributing to lower thermal imager prices. Going even further, silicon lenses can now be produced through a wafer- level optic process. Here, multiple lens surfaces can be shaped in parallel across a broad wafer of silicon, similar to how silicon semiconductors are produced but with curvature and smoothness in mind rather than MEMS- like geometric details. That leads to a highly cost-effective LWIR lens output. Detectors The detector forms the sensing element of a thermal imager. As with a visible- light camera’s imager, it consists of a 2D array of electrical resistors made from a specially chosen material. During imaging, current passes through the resistors, row by row, which causes them to change their resistance according to changes in the heat energy being focused on them from the lens. This gives an electrical measurement of the thermal ‘contrasts’ of every resistor, hence each resistor corresponds to a pixel in the thermal image that is reconstructed to produce the scene in front of the lens. Detector materials are thus most often piezoelectric transducers, to translate the temperature change into something more measurable, such as current or resistance. Possibly the most commonly used LWIR detector material is vanadium oxide (VOx), as it is highly sensitive to very minor changes in observed radiation (VOx detectors can measure temperature changes as small as 50 mK (millikelvin). Amorphous silicon is used slightly less widely as VOx, but it has advantages such as thermal stability and long lifetime. Different materials are used to build resistors that are sensitive to changes in the MWIR band, for cameras often used in surveillance or border monitoring. InAsSb for example has found wide use in UASs, as they are HOT (high operating temperature) detectors that operate at 150 K (-123 C). This reduces their cooling requirement and therefore overall power consumption (and enhances their lifespan) relative to other materials, improving the return that UAV manufacturers and operators get from using them. Many other MWIRs currently use indium antimonide (InSb) for their detector arrays, for its high quantum efficiency (a key measurement of an array’s electrical sensitivity to light) and low susceptibility to external noise. However, InSb emits photons in the IR bandwidth, so it must be kept Thermal imaging sensors | Focus Unmanned Systems Technology | February/March 2020 A range of materials, coatings and manufacturing methods must be considered in order to optimise each thermal camera’s lens for its preferred infrared band (Courtesy of Controp)