1.  Technical Field
The present invention relates to the field of interferometric measuring methods, for applications such as profile measurements, roughness measurements, plainness measurements, and measurements of the radius of curvature as well as to an arrangement for implementing this method. 
2.  Description of the Related Art
Interferometers are being increasingly used in a number of production processes to characterize and measure surfaces.  Optical measuring devices of this kind are distinguished by a high degree of measuring accuracy.  For manufacturing, it is desirable to make measurements without contacting the measured part to avoid damage.  It is also desirable that measurements be fully automated. 
A measuring device of this kind is the Laser Spot Scanning Interferometer (LASSI) described in U.S. Pat.  No.  4,298,283 assigned to the assignee of the present invention, incorporated herein by reference.  The underlying measuring principle is based on scanning the surface to be tested with two laser light beams which are simultaneously focused adjacent to each other onto the surface.  During this process, the optical phase difference between the two light waves, which are reflected from the surface, changes linearly as a function of the height difference between the two laser spots on the surface.  The phase difference is determined by phase shifting.  For this purpose, an electro-optical light modulator is used which periodically shifts the phase difference between the two light waves by a fixed amount.  At the same time, the intensity of the two interfering light beams is measured by a photodiode. 
U.S. Pat.  No.  5,392,116 which is assigned to the assignee of the present invention and is incorporated herein by reference, shows an interferometric phase measurement method, which permits simultaneous signal evaluation. 
The orthogonally polarized light beams with the phase difference are initially split by a beam splitter into several partial beam pairs, which, by means of a lens, are focused as parallel beams into a phase shifter, a polarizer, and an array of light sensors.  Phase differences of the light beams create intensity differences between the beams received by the different light sensors.  High measuring speed and accuracy are thus provided.  When combined with means for directing two spatially separated orthogonally polarized beams on a surface, the method and apparatus can be used to determine height differences along the surface. 
The present invention aims to provide an improved interferometric method and apparatus for enabling an increased precision of the interferometric measurement.