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  • Sets of blackbodies for two point NUC of thermal imagers / BNUC


Article: 00249792

Sets of blackbodies for two point NUC of thermal imagers / BNUC

Raw image generated by IR FPA sensors used in thermal imagers is typically very noisy, mostly due to high spatial noise generated by significant variation of gain and offset of pixels of this image sensor. Therefore this spatial noise ( it varies from pixel to pixel but almost does not depends on time) must be corrected by some image processing. Practically it means that all thermal imagers must be factory-calibrated to generate non-uniformity compensation (NUC) coefficients which are applied automatically by the camera in real time to maintain good image quality. These coefficients are determined during so called two point NUC operation. NUC coefficients are typically calculated on basis of images of a large area, uniform blackbody that fully fills FOV of tested imager taken at typically two different blackbody temperatures. Because blackbody is uniform then manufacturer knows that any difference in signal from different pixels is spatial noise to be corrected. Manufacturers use myriad of different mathematical algorithms to correct spatial noise. However, from point of view of test equipment there are three two-point NUC methods: A) Tests at laboratory ambient temperature (about 20ºC) by capturing images of a hot blackbody (temperature more than about 30ºC over ambient) and of a neutral blackbody (temperature equal to ambient temperature). B) Tests at laboratory ambient temperature by capturing images of a hot blackbody (temperature more than about 30ºC over ambient temperature) and of a cold blackbody (temperature more than about 10ºC below ambient temperature). C) Tests at temperature chambers (ambient temperature varies from about -30ºC to about +60ºC) by capturing images of a hot blackbody (temperature more than about 20ºC over ambient temperature) and of a cold blackbody (temperature more than 20ºC below ambient temperature). Method A is the simplest and most typical. However, it enables to compensate spatial noise only if thermal imager work at temperatures close to typical laboratory temperature and of imagers having symmetric response function. The latter limitation is eliminated in case of method B, especially when blackbody of temperature below about -20ºC ambient temperature is used. However, there are possible problems with vapor condensation on such blackbody when emitter is at sub-zero temperatures. Method C is the most advanced as it enables to determine a set NUC coefficients for different ambient temperature and to design thermal imagers capable to deliver image almost free from spatial noise at any ambient temperature. Perfect blackbody for two point NUC tests should simulate a large thermal target of thermal non-uniformity and temporal non-stability that are not detectable by tested imager. Practically it means that the blackbody should be characterized by: 1. blackbody emitter must be large enough to fill totally FOV of tested thermal imager (blackbody is typically located at very short distance to optics of the imager), 2. very low and diffuse reflectivity of the blackbody emitter (no detectable reflections on blackbody surface from radiation of hot sources), 3. very good thermal uniformity (uniform emission of thermal radiation), 4. very good temporal uniformity (no detectable temporal variations of emitted thermal radiation), 5. ability to work at extreme ambient conditions from about -40ºC to about +60ºC. Two point NUC are many low cost blackbodies offered on international market that can be used potentially for two point NUC applications. However, due to significant and specular reflectivity, noticeable thermal uniformity and temporal stability, and not ability to work in temperature chambers these blackbodies are not suitable for professional applications. Inframet BNUC sets are optimized for the latter applications. BNUC sets for two point NUC are built as sets of two blackbodies with optional YLP linear platform and YLP Control program. The latter two blocks are needed to enable to enable fast movement of tested imager from one blackbody to another blackbody in order to increase NUC accuracy.


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