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; and two interchangeable band-pass uncooled filters located between the optics and the detector, comprising: a first uncooled filter arranged to transmit only electromagnetic radiation in a first spectral band that includes the known spectral band, and a second uncooled filter arranged to transmit only electromagnetic radiation in a second spectral band which does not include the known spectral band and in which the substance has no detectable signature, wherein the given spectral band of the detector is selected to include the first and the second spectral bands, and the detector is arranged to acquire, by interchanging the filters, at least one first image in the first spectral band and at least one second image in the second spectral band, the images being of a scene in which the substance is supposed to be, and an image processor arranged to enhance a similarity between the at least one first image and the at least one second image and reconstruct at least one fused image therefrom, wherein enhancing the similarity is carried out by applying at least one transform to at least one of the images.