Two-grating interferometer for measurement of a step-surface altitude difference

A method of measuring step-surface altitude difference using a two-grating interferometer is described. The advantage of this method is that the measuring range of the interferometer is large and can be changed freely by turning the grating. The interference fringes are produced by the (+1,0)th- and (0,+1)th-order beams diffracted from two identical binary step gratings with the same line spacing. Sinusoidal phase modulation is easily applied to detect the phase distribution of interference pattern by sinusoidal vibration of the grating. A plane reflector of ∼3 mm thickness is measured with this two-grating interferometer.