1. A method for obtaining a reflectance image of a concealed object at a scene from a plurality of thermal images, the method comprising: obtaining a plurality of images, the plurality of images containing both temperature and reflectance data; synchronously changing illumination conditions at the scene between at least two different states for pairs of first and second images of the plurality of images; and deriving a third image from a pair of the images, the third image being a reflectance image, wherein a concealed object, with one of lesser or greater reflectance than reflectance generated by a human surface, generates a visible contrast on the third image.