Abstract: Metrology for a microlithographic objective reducing lens 10 is integrated into a metrology block 20 secured to a lower region of the barrel 15 for lens 10. Block 20 has a central opening 27 around a lowermost element 11 of lens 10; and block 20 mounts distance detectors 28 around central opening 27, a pair of microscope objectives 30 and 31, and a pair of X and Y mirrors 40 and 41.

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.