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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 trans

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.

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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; andtwo interchangeable band-pass uncooled filters located between the op

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; andtwo 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, anda 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, andan 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. 2. The system as claimed in claim 1, wherein the detector is cooled. 3. The system as claimed in claim 1, wherein the imaging system further comprises a switching mechanism arranged to switch between the two filters. 4. The system as claimed in claim 3, wherein the switching mechanism is selected from a group of switching mechanisms that includes a pivoting device and a sliding device. 5. The system as claimed in claim 1, comprising a control unit for synchronizing the acquiring of images with a position of the filters. 6. The system as claimed in claim 1, wherein the thermal imaging sensor comprises an infrared stills camera. 7. The system as claimed in claim 1, wherein the thermal imaging sensor comprises an infrared video camera. 8. A method of detecting the presence of a substance that has a detectable signature in a known spectral band, the method comprising: acquiring, by a thermal imaging sensor, at least a first and a second image of a scene in which the substance is supposed to be, the first image in only a first spectral band that includes the known spectral band corresponding to the detectable signature of the substance, and the second image in a second spectral band in which the substance has no detectable signature;using at least one mathematical transform defined by finding a minimal norm between the first and second images to transform one of the two images to an image data that represents an image with greatest similarity to the other of the two images; andreconstructing a fused image by subtraction between the transformed image data and the other of the two images,wherein at least one of the using and the reconstructing is carried out by a computer processor. 9. The method as claimed in claim 8, wherein the step of using at least one mathematical transform comprises using two mathematical transforms, a first transform defined by finding the minimal norm between the two images to transform the second image to an image data that represents an image with greatest similarity to the first image and a second transform defined by finding the minimal norm between the two images to transform the first image to an image data that represents an image with greatest similarity to the second image, andwherein the step of reconstructing the fused image comprises generating two subtractions between each of the two images and the transformed image data relating to it and adding the two subtractions. 10. The method as claimed in claim 8, wherein the step of acquiring at least first and second images of a scene in which the substance is supposed to be comprises: providing an imaging system for detecting the presence of the substance that has a detectable signature in a known spectral band, the system comprising: a detector sensitive to a given spectral band corresponding to a detectable signature of the substance and optics; andtwo 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, andswitching between the interchangeable uncooled filters so that the first image is acquired using one of the two filters and the second image is acquired using the other of the two filters. 11. The method as claimed in claim 8, wherein the step of acquiring at least first and second images of the scene comprises acquiring the images in a form of still images. 12. The method as claimed in claim 8, wherein the step of acquiring at least first and second images of the scene comprises acquiring the images in a form of video streaming images. 13. The method as claimed in claim 8, the wherein the known spectral band is within the infrared spectrum. 14. The method as claimed in claim 8, wherein the step of acquiring at least first and second images of the scene comprises acquiring the images using a single thermal imaging sensor. 15. The method as claimed in claim 8, wherein the step of acquiring at least first and second images of the scene comprises acquiring the images using more than one thermal imaging sensor directed at the scene.