Patent ID: 8368897

Claim:
An instrument for detecting changes in the optical properties of a sensor surface defining an image plane, said instrument including an optical system comprising: a source of collimated, polarized light of a predetermined wavelength; a selectively rotatable reflecting surface defining an object plane, said reflecting surface situated to redirect said polarized light; an optical telescope optically connecting said object plane to said image plane to transfer said polarized light from said object plane to said image plane, including the angle at which said polarized light is reflected from said object plane; and an imaging detector arranged to form an image of said sensor surface from light reflected from said sensor surface, wherein rotation of said reflecting surface alters the angle at which said polarized light is incident upon said sensor surface, while said polarized light remains at a substantially fixed position on said sensor surface and changes in said image at different angles of incidence correspond to changes in said optical properties, said rotatable reflecting surface and optical telescope defining an optical path, each image of said sensor surface contains a fiduciary mark that changes position within the image as a linear function of the angle at which said light is incident upon said sensor surface, the fiduciary mark created by a relatively fine opaque object positioned within the optical path.