Emerson’s procedure (the one in use at the National Bureau of Standards) for determining the absolute contours of optical flats was refined to increase the precision of the method and the speed of taking readings. The fringes were scanned over a photoelectric detector and the intensity profile presented on an oscilloscope. Setting accuracy of a fraction of a hundredth of a fringe was achieved. The data were gathered on a data logger. A complete set of data was obtained in about 5 min. The precision of determining a set of differences of two plates was fringe and the precision achieved in determining the absolute plate contours is estimated as 0.005 fringe. The problem of extending the method to 0.002 fringe is discussed.
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