Patent ID: 6885449

Claim:
Apparatus for comparing the surface structure of a test object with that of a reference object, the apparatus comprising: a source of laser light; means for focusing said laser light at least on said test object, for diffraction of said laser light by said test object, so as to produce a diffracted beam having an optical field conveying a diffraction pattern of the test object; a photorefractive crystal arranged to receive the diffracted beam, wherein the photorefractive crystal is arranged such that a portion of the diffracted laser light is interfered with itself by counterpropagation of laser light within the crystal so as to produce a refractive index grating, said grating having a modulation depth which is a function of position in two mutually orthogonal directions, said function corresponding to the diffraction pattern of the reference object within the crystal; and a detector of laser light passing through said crystal.