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individual soldier, whereas previously they were far too expensive to be distributed on this scale.’ The detectors in modern, uncooled thermal


imagers are focal-plane arrays of uncooled microbolometers (simply referred to as FPAs), and it is these FPAs that account for much of the expense of a device such as Armstrong’s IR513. ‘Anyone can buy a visible sensor off-the-shelf for a few pounds,’ says Johnstone, ‘and the same applies to digital compasses and GPS modules. Even laser rangefinders tend to be for sale here and there. It’s the integration of the whole that adds value, as well as the usability of the final product.’


But integrators can only work with what they’re


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able to purchase, and when it comes to certain advanced military technologies such as FPAs, some limitations are in place as to which countries are permitted to make use of which technologies. ‘The problem for the Chinese manufacturers is that the FPA in the IR513 product is a 384 x 288px uncooled microbolometer array. They would love dearly to get their hands on a 640 x 480px or even 1,280 x 960px arrays, but the Americans are guarding 1,280 x 960px array, and I think even the UK has trouble getting hold of them.’ Johnstone notes that although 640 x 480px FPAs are commercially available, from French specialist Sofradir for example, a licence to export the devices from Europe to China would not be granted. This may not be an issue in coming years, as the capabilities of Chinese foundries mature to match those in the West.


Breaking the mould Detectors are expensive, as are the lenses for thermal imaging. When choosing lenses for handheld applications, care must also be given as to the weight of the system. The increasing affordability of military thermal imaging has as much to do with cheaper lenses as it does with cheaper detectors but, where FPAs are cheaper due to greater production volumes, thermal lenses are becoming cheaper due to the use of new materials. Currently, Johnstone says, lenses for thermal


imaging are usually made from germanium – an expensive material that is also difficult to process. ‘The rear surface of the main germanium lens is not spherical, which allows the designers to remove at least one lens from the system, reducing the cost and weight of the device,’ he says. Aspheric shapes are, however, a manufacturing challenge, which is currently met by single-point diamond turning, one component at a time, greatly adding to the cost of the system. A promising innovation has come in the form chalcogenide glasses, which offer the same IR-focusing properties as germanium with the


12 electro optics l APRIL 2011


advantage that they can be moulded to any shape. ‘Once we can start moulding these lenses we can really start bringing the unit cost down,’ he says. Rangefinders generally incorporate pulsed lasers and work on a time-of-flight principal to give an accuracy of approximately one metre. Neukum of Dilas outlines some of the requirements made on lasers for rangefinders and target designators: ‘In these applications you need to have a defined pulse in order to measure distance to a certain target in a certain direction. Therefore you need a well- defined beam, and units capable of supplying this beam are all DPSS. In the old days it was always Nd:YAG, but now there are many different types of crystal in use – Yt-doped crystals, or even Yb-glass lasers. Rangefinders have a low duty cycle and low power, and so a glass laser is an option too. Target designators, on the other hand, have much higher power and can be used at distances of up to 70 miles in some naval applications. Where rangefinders used in commercial sporting applications are invariably near-IR at


C perhaps the sci-fi world of the 1950s is nearer than we thought


around 905nm, military customers have more exacting demands. Modulight’s Uusimma explains the requirement for unusual lasing media such as Nd:YLF (1,047nm) and Ho:YAG (2.1µm): ‘At 905nm wavelengths, a simple silicon CCD will detect the rangefinder beam; that basically means that anybody with a mobile phone can detect it. At 1.5µm, detection would typically require the use of an avalanche photodiode (APD), and the expense and scarce availability of these detectors makes that unlikely.’ For covert military operations, unusual wavelengths are preferable. Finally, as well as being able to spot and


provide coordinates of a target several miles away, the soldier of tomorrow may well be able to see through dust, fog, or even camouflage nets. Dilas’ Neukum highlights the use of the company’s diodes in a novel application of 1.5µm laser radiation – so- called gated imaging, which is similar in principle to lidar: ‘The idea is to synchronise a pulse of laser light and a camera precisely, and to only record pictures that correspond to a certain time-of- flight for the light. If there is fog, smoke, or even camouflage between the camera and the subject, the technique can take pictures behind it,’ he says. Futuristic as this seems, Neukum says it is already being trialled by several militaries. Perhaps the sci-fi world of the 1950s is nearer than we thought. l


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