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A technique for real-time measurement of interferograms, which circumvents the common sources of error in traditional methods of analysis, is described. By nulling the interferometer and simultaneously measuring the phase over a rectilinear grid, one can eliminate errors due to geometric distortion in the interferometer (which produces apparent coma terms in the analysis of straight-line interferograms), uneven pupil illumination (which shifts the apparent location of the fringe peaks), and the difficulties in the fitting and interpolation of polynomials to unevenly sampled pupil functions. Data are not interpolated or artificially smoothed, so localized irregularities in the wavefront are visible in the results.
Because on-line computer processing is used, contour and isometric plots are displayed less than two minutes after the taking of data is completed. A unique interface design permits the utilization of virtually all of the information present in the input video signal. By taking thousands of measurements per minute at each point in the wavefront and extending the measurements over several minutes, the effects of vibration and turbulence are averaged out of the data. With a reasonably stable interferometer, the effective instrument bandwidth can be reduced to 0.01 Hz, providing worst-point peak-to-peak repeatabilities of successive measurements of better than λ/100. For repeatabilities of λ/20, the data-taking time is normally one second.
interferograms, interferometers, interference fringes, optical tests
Director of research and development, Tropel, Inc., Fairport, N.Y.