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 &# 39 ; 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 .