Measurement of length, surface form and thermal expansion coefficient of length bars up to 1.5 m using multiple-wavelength phase-stepping interferometry

A new interferometer has been developed for absolute length measurement of length bars between 100 and 1500 mm in length. The interferometer uses a combination of three-wavelength interferometry and phase-stepping interferometry to measure the length, flatness and mutual parallelism of the measuring faces, and thermal expansion of the bars, with direct traceability to realizations of the metre and the kelvin. The interferometer is operated inside a sealed chamber to minimize the effects of refractive index variations. The design and operation of the interferometer are presented, with representative results of length, flatness and parallelism and thermal expansivity measurements. The instrument has an estimated uncertainty of length measurement (at 95% confidence level) of +or-30 nm+or-6.2*10 -8 L, where L is the length of the bar.