1. An imaging system for detecting the presence of a substance that has a detectable signature in a known spectral band, the system comprising: a thermal imaging sensor comprising: a detector sensitive to a given spectral band corresponding to a detectable signature of the substance and optics; and two interchangeable band-pass uncooled filters located between the optics and the detector, a first filter that transmits only electromagnetic radiation in a first spectral band that includes the known spectral band, and a second filter that transmits only electromagnetic radiation in a second spectral band in which the substance has no detectable signature, a processor for processing 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.