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Thermal imaging & vision systems


The challenges of infrared M


Measuring radiance is the central aim of researchers using IR cameras on the military test range. However, a number of factors need to be considered to make the most accurate measurements, as Koen Jacobs, FLIR Systems, explains


ost users of infrared (IR) cameras in the commercial world rely on the instruments to measure the


temperature of objects. They might want to know, for example, how hot a certain part of their product gets. On the military test range, however, the main characteristic researchers want to measure is the radiance of an object. With that as their goal, they need to be sure that the IR camera they are using is calibrated for radiance. Radiance is the amount of light — that is,


the number of photons — coming off an object in a particular part of the spectrum at a specific temperature. Radiance is important on the military test range because knowing what an object looks like at particular wavelengths lets detection equipment in the field identify those objects. While temperature readings of the tip of a bullet or the engine block of a vehicle, for instance, may be useful for some work, radiance is the real key to most of what the researchers do.


By measuring radiance in specific wavebands,


researchers understand IR signatures that allow them to identify enemy equipment and personnel, as well as other objects. Based on the radiance they have measured, they can predict what a different detector will see. They can then translate that signature for use by other detectors, such as the IR sensor in the tip of a heat-seeking missile or the scope on a sniper rifle. Thermal cameras, in fact, are often deployed in tanks and carried by many personnel, so providing a reliable signature to


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those detectors is valuable. Knowing what an object looks like in the IR also allows the military to camouflage its own equipment.


CaLIbRatIng FoR RadIanCe Unfortunately, many IR camera manufacturers only calibrate their cameras for temperature. They put a series of different reference sources in front of the camera, one emitting at, say, 10°C, one at 50°C, one at 100°C, and so on until they have covered the whole range of temperatures the camera can detect. Often, they will include software that allows users to convert those temperature measurements to radiance, but those conversions use a standard formula. They do not take into account the actual spectral properties of the camera, such as how much light a particular lens transmits or the individual spectral response of the detector. Overlooking those can add error to the measurement and throw off the accuracy of the IR signature. A better way is to calibrate the cameras


for both temperature and radiance. The system works the same as before, checking the camera’s performance against various reference sources. But at the same time, it is possible to measure how much radiance is emitted by the light source and how much is received by the detector, then report that in terms of energy rather than temperature. Then the user can know how much radiance their camera is detecting, rather than measuring just the temperature and using a


standardised, often inaccurate, estimate to make the conversion. The manufacturer of the camera will


calibrate the device as built, taking into consideration the specific lenses and filters the customer has chosen. Sometimes, though, users will decide they need to image in a waveband they had not anticipated, so they will buy a new filter for that waveband. That filter will not be calibrated to the camera, but it is possible for users to do this sort of calibration themselves. Many advanced users have blackbody sources, and software is available that allows them to perform calibrations.


tHe eFFeCtS oF atMoSpHeRICS The calibrations, of course, take place with a very short distance between the camera and the object, usually less than 20 m, and so do some measurements in the field. But it’s common for the objects being imaged to be much farther away; an aircraft in flight might be, say, two kilometres away, and that distance can change rapidly. The distance affects the measurement of radiance. The issue is that light travelling from the object to the detector has to pass through the intervening atmosphere, a mixture of gases and water vapour that can absorb the photons coming off the object or emit photons at a new wavelength. Each component of that mixture has its own effect on different wavelengths of light. At short distances, changes in atmospheric transmission or atmospheric absorption make little difference. But if the


April 2019 Instrumentation Monthly


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