The present invention discloses a detection device for discriminating between different materials, and a method for doing so. The device comprises an optical system having at least one optical focussing element and a receiving element. The receiving element is sensitive to electromagnetic radiation, typically in the millimeter wave band, and the optical system being arranged to focus incident energy from a scene onto the receiving element. The optical system comprises a prism element having a first surface and a second surface, the first surface being opposite the second surface. At least a portion of the first surface is positioned at an angle θ to the second surface. The angle θ varies between a minimum at a first position on the first surface and a maximum at a second position on the first surface.