Optical surface roughness measuring apparatus with polarization detection

An optical surface roughness measuring apparatus for measuring roughness of a surface of a subject in a non-contacting manner, by irradiating said surface with two linearly polarized beams of light which have mutually perpendicular polarization planes and different frequencies, and detecting a change in a beat frequency of said two beams reflected by said surface, comprising:

• a laser device for producing two linearly polarized laser beams as said two linearly polarized beams;
• a converging lens device for converging said two laser beams at a first and a second point of convergence on said surface of said subject;
• a polarizing beam splitter disposed between an optical path between said laser device and said converging lens device, and having a reflector surface which permits transmission therethrough of a linearly polarized beam having a polarization plane parallel to a plane of incidence thereof, and which reflects a linearly polarized beam having a polarization plane perpendicular to said plane of incidence, said polarizing beam splitter causing one of said two laser beams to be incident upon said converging lens device and converged by the converging lens device at said first point of convergence, said polarizing beam splitter causing the other of said two laser beams to deviate from said optical path;
• and
• optical means for causing said other laser beam which has deviated from said optical path, to be again incident upon said polarizing beam splitter and emitted from the beam splitter such that said other laser beam is incident upon said converging lens device and thereby converged at said second point of convergence, such that a direction of incidence of said other laser beam upon said converging lens device is inclined by a predetermined angle with respect to that of said one laser beam, so that said second point of convergence is spaced by a small distance away from said first point of convergence.