Extended-source low coherence interferometer for flatness testing

An interferometer system including in combination:

• an extended spatially non-coherent light source producing a beam of light;
• a first beam splitter angularly oriented to receive light from said light source to split said beam of light from said light source into first and second divergent wavefronts;
• a reference surface located to reflect said first wavefront;
• a surface under test located to reflect said second wavefront;
• a second beam splitter located to receive and combine said first and second related wavefronts;
• an imaging lens and interference detector located to receive said first and second reflected wavefronts after combination thereof by said second beam splitter;
• an align projector including an alignment light source and distance and tilt reticles, with said alignment light source projecting light through said distance reticle and said tilt reticle and through a projecting lens onto said second beam splitter to produce first and second divergent alignment beams directed to said reference surface and said surface under test, respectively;
• an align detector located to receive said first and second alignment beams reflected from said reference surface and said surface under test from said first beam splitter;
• and
• apparatus for adjusting the distance between said reference surface and said surface under test and adjusting the tip and tilt of said surface under test relative to said reference surface.

A method for measuring the flatness of an article under test relative to the flatness of a reference surface comprising the steps of:

• producing an extended spatially non-coherent, non-collimated beam of light;
• splitting said beam of light into first and second spatially non-coherent divergent wavefronts;
• locating a reference surface to reflect said first spatially non-coherent wavefront;
• locating a surface under test to reflect said second spatially non-coherent wavefront;
• recombining said first and second spatially non-coherent reflected wavefronts;
• and
• imaging and detecting said recombined first and second spatially non-coherent wavefronts whereby the distance traveled by said first and second wavefronts is an identical distance.