An electronic fringe counter for use with optical interferometers is described. Provision is made for automatic counting of integral fringes, interpolation to within 1/200 wavelength or approximately 10−7 inch, and indication of the direction of motion of the interferometer plates. Automatic interpolation is achieved by vibrating one plate of the interferometer and analyzing the resulting ac signals from a photomultiplier exposed to a portion of the fringes in the image plane of the optical system. The use of an ac analyzer eliminates troublesome dc drifts associated with photoelectronic circuits and provides an oscilloscope display of the phase progression from fringe to fringe.
© 1954 Optical Society of America
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