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Optical Shop Testing

Chapter 2.6.1 - Some Special Designs

2.6.1.   Some Special Designs

With the advent of the laser, it became practical to use Twyman–Green interferometers
with large optical path differences. Probably the first one to suggest this was
Hopkins (1962). An instrument of this type following a Williams arrangement was
made by Grigull and Rottenkolder (1967) for wind-tunnel observations and the
testing of spherical mirrors.

Avery versatile unequal-path interferometer for optical shop testing was designed
by Houston et al. (1967). A schematic diagram of this interferometer is shown in
Figure 2.30. The beam-splitter plate, which is at the Brewster angle, has a wedge
angle of 2–3 min of arc between the surfaces. The reflecting surface of this plate is
located to receive the rays returning from the test specimen in order to preclude
astigmatism and other undesirable effects. A two-lens beam diverger can be placed in
one arm of the interferometer. It is made of high index glass, all the surfaces being
spherical, and has the capability for testing a surface as fast as f /1.7. A null lens can
be used to test an aspheric element, with the combination beam diverger and null lens
spaced and aligned as depicted in Figure 2.31 (see Chapter 12).

Another unequal-path interferometer was designed by Kocher (1972). This instrument,
shown in Figure 2.32, is quite similar to the Twyman–Green interferometer in

FIGURE 2.30. Houston’s unequal path interferometer.

FIGURE 2.31. Null lens and lens diverger for unequal path interferometer.

Figure 2.2. A significant feature is the use of an optically thick beam-splitter substrate
in the diverging beam. Such a plate introduces aberrations, but they are intentionally
made equal on both arms. To a first approximation there is no effect on the fringe
pattern, mainly if the total thickness is kept small. Buin et al. (1969) reported a
successful industrial use of unequal-path interferometers.


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