FEATURE THERMAL IMAGING & VISION SYSTEMS
Figure 1: Spectral emissive power of a 30°C ideal blackbody
Table 3: LWIR MCT Camera Performance Metrics
Figure 3: MCT image at start-up
we look at Figure 2, we see that as objects heat up, the representative spectral radiant emittance curve’s peak shifts to the left and tails off to the right. The change in power in the LWIR band is less dramatic over a range of temperatures than the more dramatic change that happens in the MWIR band. This is how the LWIR SLS detector is able to avoid over or under exposure for a given integration time, compared to the MWIR InSb detector. Note that the change in power in the MWIR band is substantial; therefore, as an object heats up, the camera would soon saturate for a single integration time. In summary, SLS allows you to tackle
challenging applications where the target heats up across a wide temperature range quickly, such as a combustion research application. However, operating in the LWIR band
is not the only factor. If we look at LWIR mercury cadmium telluride (MCT) detectors, we see they also are limited in their ranges, similar to MWIR InSb detectors. You will notice the LWIR MCT cameras have both shorter individual ranges per integration time as well as limitations on how high they can measure before needing an ND filter to cut down the signal (see Table 3).
BETTER UNIFORMITY AND STABILITY AT A LOWER COST One of the best features of LWIR SLS cameras in comparison to other LWIR cooled camera options is the dramatically improved uniformity and stability through cool downs, especially when compared to LWIR MCT cameras. LWIR MCT detectors generally suffer from poor uniformity and stability. As a result, any time the user
14 NOVEMBER 2017 | INSTRUMENTATION
turns on a LWIR MCT camera, the last uniformity correction performed needs updating (See Figure 3). This presents problems for field based
applications, which are simply not conducive to equipment that requires you to update gain, offset and bad pixel maps due to environmental conditions. Those applications may include controlling the camera remotely as it sits in a test chamber, or controlling it from outside the blast zone for a government test range. In comparison, LWIR SLS operates much like MWIR InSb, in that you just need to turn it on and start testing (see Figure 4). The uniformity correction done in the lab works just as well in the field with no extra image uniformity updates beyond possibly a one-point offset update using the internal NUC flag inside the camera. The NUC also holds well through multiple cool downs over a long time duration. The camera tested for this article has not needed a new NUC since initial fielding of the camera more than a year ago. While SLS cameras cost more than their MWIR InSb counterparts, they are
Figure 2: Spectral radiant emittance of black bodies at various temperatures
Figure 4: SLS image at start-up
40 per cent lower in price than comparable LWIR MCT cameras. Therefore, if your application requires shorter exposure times, wider temperature ranges, or a spectral signature only offered with cooled LWIR detector cameras, SLS offers a clear cost and uniformity advantage over current cooled LWIR MCT detector options.
SUMMARY SLS LWIR detector materials are exciting because they fit a perfect niche in the performance/price spectrum by offering shorter integration times and wider temperature bands than MWIR InSb and LWIR MCT materials, along with better uniformity, stability, and price than current LWIR MCT cameras. An SLS LWIR detector is a great arrow to have in your quiver when the application calls for this special blend of performance and price.
FLIR Systems
www.flir.co.uk
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