Optical Shop Testing

Chapter 8 - Foucault, Wire, and Phase Modulation Tests

8.1.   INTRODUCTION

The knife-edge method, introduced by Foucault (1858, 1859), and, in general, all the
Schlieren techniques—Topler (1864, 1866, 1868), Ritchey (1904), Hartmann
(1908), Platzeck and Gaviola (1939), Wolter (1949 and 1956)—have proved to be
extremely useful for testing optical surfaces. Rays may be moved from their expected
trajectories (or wavefronts may be deformed) by optical aberrations, by diffraction,
or by a deformed, unfinished surface. The basic idea behind the Schlieren techniques
is to detect lateral displacements of rays by blocking out or modifying these
displaced rays. The blocking or modification can be accomplished by placing screens
in any of the planes of convergence of the light passing through, or being reflected
from, the optical surface under test.

The main advantages of the Schlieren techniques are their high sensitivity and
their simplicity, both in apparatus and in qualitative interpretation (at least from the
geometrical point of view). Of course, to appreciate the relative merits of one method
over others in this class, it is necessary to study its characteristics when it is used to
detect the presence of (a) aberrations greater than the wavelength of the illuminating
radiation (geometrical theory of image formation) and (b) aberrations smaller than
the wavelength (physical theory of image formation).

The choice of test depends on the circumstances.

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