This paper describes a method using a multiple-beam interferometer for establishing oscillatory displacements in the range 0.25λ to 12.5λ or greater, where λ = 546.078 nm, the nominal wavelength of mercury green light. The interferometer is used to calibrate accelerometers at frequencies up to 1000 Hz and accelerations up to 2g, where g = 9.80665 m/s². The smallest displacement in the range can be established with a precision better than 1% and the largest better than 0.05%.

The oscillatory displacement of frequency f is imparted to one surface of the interferometer relative to a fixed surface, and the variation of the light flux through a fixed aperture is photo-electrically detected. The detector is tuned to the first even harmonic 2f and its variation in amplitude is observed as the displacement is increased. Provided that the fundamental in the photo-current is suppressed by continuously correcting any drift of the mid position of the oscillating mirror, a pair of characteristic curves is obtained each of which has a series of turning points corresponding to changes of about λ/2 in the displacement. The turning points of one curve are offset by about λ/4 from those of the other. The precise values of the displacement at all these turning points have been calculated.

Errors in the displacement may arise from failure to take account of non-sinusoidal and non-rectilinear motion of the oscillating mirror, from the departure of the intensity distribution in the fringe image from the symmetrical Airy type, and from induced vibration of the fixed mirror. All of these possible sources of error have been investigated and their significance evaluated.