Lateral shear interferometer for testing aspheric surfaces

Claim 1

A lateral shear interferometer for testing an aspheric surface, comprising: (a) means for generating a coherent light beam; (b) means for guiding the coherent light beam to the aspheric surface, the coherent light beam incident upon the aspheric surface being reflected back to form a reflected light beam; (c) means for guiding the reflected light beam into a beam path; (d) a lateral shear producing element including at least a pair of cubic prisms disposed with a same orientation relative to the beam path, each of said cubic prisms having at least one semi-reflective plane arranged therewithin such that a ray in the reflected light beam that propagates within a predefined range and incident thereupon with an incident angle θ i will exit therefrom with a pair of rays having a refraction angle θ r equal to the incident angle θ i , said pair cubic prisms being capable of converting rays propagating within the predefined range into a first beam of parallel rays and a second beam of parallel propagating divergently with respect to each other rays, the first beam of parallel rays and the second beam of parallel rays having lateral shear interferences; (e) an aperture stop allowing only one of the first beam of rays and the second beam of rays to pass therethrough while rejecting the other; and (f) means for analyzing the lateral shear interference pattern formed by the beam of rays passing through said aperture stop.