Imaging system and method for detecting the presence of a substance that has a detectable signature in a known spectral band. The system comprises a thermal imaging sensor and optics, and two interchangeable band-pass uncooled filters located between the optics and the detector. A first filter transmits electromagnetic radiation in a first spectral band that includes the known spectral band and blocks electromagnetic radiation for other spectral bands. A second filter transmits only electromagnetic radiation in a second spectral band in which the substance has no detectable signature. The system also includes a processor for processing the images to obtain a reconstructed fused image involving using one or more transforms aimed at obtaining similarity between one or more images acquired with the first filter and one or more images acquired with the second filter before reconstructing the fused image.