Patent ID: 7218386

Claim:
A method of determining if a surface contains a specific OVM comprising the steps of: (1) illuminating the surface using light containing two substantially monochromatic components of wavelength λ 1 and λ 2 , (2) detecting the intensity of scattered light from the surface at two scattering angles Ø 1 and Ø 2 selected according to the OVM of interest, (3) computing the magnitude of the difference between the intensity values for λ 1 light at Ø 1 and Ø 2 less the difference between the intensity values for λ 2 light at Ø 1 and Ø 2 , and (4) generating an output signal indicating the presence or absence of the specific OVM depending on the computed difference magnitude relative to a predetermined value, wherein the surface is illuminated separately first with monochromatic light of one wavelength and then with monochromatic light of the other wavelength, and wherein detection is performed by a single detector which is moved between two positions so as to receive light reflected/scattered from the surface first at one and then the other of the two angles Ø 1 and Ø 2 .