The present invention contributes to the resolution of the problem of detecting, for example, plastic explosives, which are not easily detected by conventional inspection techniques, as those based on X-ray apparatuses; or those using more sophisticated means from traces of explosive material which may remain, “contaminating” the external surface of luggage. As regards the first technique, the invention has an additional advantage in that it is fully automatic, that is, it is independent from the operator's ability to interpret low contrast images. As regards the second technology, the main advantage of the present invention consists of its speed and safety when inspecting luggage.