US Patent 7561097 Method and system for detection of objects

A method for detection of an object, the method including irradiating a target with two electromagnetic wave energy beams, a first beam at a first frequency and a second beam at a second frequency, the first frequency being lower than the second frequency, both beams being polarized in a first direction, and determining a presence of an object by analyzing reflections of the first and second beams, wherein if there is a dominant reflection polarization in the first direction for both the first and second frequencies, then the target is considered not to have the object, and if there is a dominant reflection polarization in the first direction for only one of the first and second frequencies and a depolarized reflection at the other of the first and second frequencies, then the target is considered to have the object.