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44 released from the substrate and coated with a vanadium oxide temperature- sensitive material. The design of these elements is complex. The bridges have thin legs to get the signal out of the micro- bridge quickly, but at the same time they have to have a high thermal resistance to avoid introducing errors into the sensor array. At the same time, the pixels in the array have to have a low thermal mass, to give the sensor a short time constant of 20 ms or less and allow a fast response to changes in the IR image. The optics are made out of etched silicon. These are special lenses that are a couple of millimetres across, and they can be designed with different fields of view. The industry is pushing towards more pixels in these micro-cameras, for example with IR sensors with 160 x 120 pixels, but this then raises the issue of processing the data. Taking the data and processing it into an image has a computing cost to it – the more pixels you have, the more computing steps per frame, and that drives up the camera’s power consumption. That’s where the technical challenges lie, in dealing with more data while meeting the power requirements that influence battery life for portable systems. One way around this is to use a custom controller with proprietary image processing algorithms for taking the raw sensor data, as it is not easy to get a good image out of these arrays. You have to calibrate the image sensor in the camera housing to produce a good image, regardless of the ambient temperature, whether it be -10 C to 65 C, which is a serious technical challenge. There are also thermal gradients across the camera, where one side of it can be hotter than the other because of where it sits in the overall system, so you have to make it immune to these effects or compensate for them. The problem with IR cameras is that they have all this thermal energy hitting the sensor from all directions. You want the camera to ignore all IR radiation except for that coming from the scene being imaged through the lens. However, it’s never really dark for an IR camera – even with the lens cap on, the sensor sees it – so the camera has to calibrated to compensate for the out-of- field radiation, and if that changes then it has to be dealt with. One way of doing so is to fit the camera with a shutter so that the sensor momentarily sees a uniform field of view and compensates each pixel based on that. This process, called tuning, can be done with an external and manually controlled shutter. Sensor combinations One application where a combination of IR sensors is being used is for studying the Arctic Ocean, where researchers are using UAVs with two hyper-spectral sensors to investigate climate change. These sensors look at more than just the red, green and blue components of visible light: digital signal processing captures the light intensity (the radiance) for a large number (typically a few tens to several hundred) of spectral bands. Every pixel in the image thus contains a continuous spectrum (in radiance or reflectance) and can be used to look at the objects in a scene with greater precision and detail. One sensor covers the visible near- infrared spectrum (400-1000 nm) via a standard silicon CCD (charge-coupled device) with a focal plane array of 7.4 x 7.4 µm pixels. This gives a resolution down to 1.9 nm with a dynamic range of 60 dB, and a frame rate of greater than 90 fps to capture more detail. The second sensor covers the near-infrared spectrum, from 900-1700 nm. This is a cooled sensor with an InGaAs IR array and a 25 x 25 µm pixel size. Similarly, other developers are combining SWIR and long-wave IR (LWIR) sensors for the 0.9-1.7 µm and 8-14 µm bands into a single camera with its own interface for unmanned intelligent Spring 2015 | Unmanned Systems Technology The technical challenges lie in dealing with more data while meeting the power needs that influence battery life for portable systems This shortwave infrared module is used in airborne cameras (Courtesy of 3D-One) An unboxed camera developed for UAV systems to minimise weight (Courtesy of 3D-One)