Optical Shop Testing

Chapter 14 - Phase Shifting Interferometry

14.1.   INTRODUCTION

The single biggest change in all types of instrumentation over the past 30 years or so
has been the integration of computers into the measurement system. Interferometry is
no exception, and the prime manifestation of this situation has been the development
of phase shifting interferometry (PSI). Unlike many of the techniques discussed
elsewhere in this book, PSI is not a specific optical hardware configuration, but rather
a data collection and analysis method that can be applied to a great variety of testing
situations.

While the analysis of static interferograms has benefited from computerization, it
suffers from the need to find fringe centers, and the resulting trade-off between
precision and number of data points. Data is collected only along the fringe centers,
and most analyses need the data on a regular grid, therefore requiring interpolation.
This low spatial sampling along the fringe centers also limits the lateral resolution of
the method. With a static interferogram, an additional piece of information is also
required to determine the polarity of the wavefront.

PSI electronically records a series of interferograms while the reference phase
of the interferometer is changed. The wavefront phase is encoded in the variations
in the intensity pattern of the recorded interferograms, and a simple point-by-point
calculation recovers the phase. The need to locate the fringe centers is eliminated,
as are the associated problems. While the earliest reference to this technique dates
back to 1966 (Carre, 1966), the development of PSI techniques really began in the
early 1970’s (Crane, 1969; Bruning et al. 1974; Wyant, 1975; Johnson and Moore,
1977; Hardy et al. 1977; Stump, 1979; Bruning, 1978; Moore, 1973; Massie, et al.
1979). The applications for this early work included optical testing, real-time
wavefront sensing for active optics, and microscopy.

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