Compact, linear measurement interferometer with zero abbe error

Abstract

A linear measurement interferometer 10 with a measurement axis directed toward a surface of a work piece 17 has a light source 11, a detection system 12, and a beam splitter 13 arranged to divide light from the source into test beams 21, 22, and 34 and reference beams 23, 24, and 33 that travel test and reference paths and recombine for detection by the detection system. Beam splitter 13 is arranged on the measurement axis with the work piece surface on one side of the beam splitter and the test beam path straddling the measurement axis on the opposite side of the beam splitter. A test beam retroreflector 25 mounted in the test path on the measurement axis reflects back the test beam from beam splitter 13 and is movable along the measurement axis without causing abbe error. A probe or focusing lens 15 movable along the measurement axis for measuring the work piece surface is interconnected with the test beam retroreflector 25 for movement together to determine a distance moved in measuring the work piece surface. A preferred way of arranging beam splitter 13 on the measurement axis while accommodating a probe is to bore a central aperture 20 in the beam splitter allowing the probe to pass through the beam splitter.