Page 155 - Modern Optical Engineering The Design of Optical Systems

138 Chapter Seven

Figure 7.18 (a) The right-angle prism used in the
manner shown is a constant-deviation prism, in
that each ray is reflected through exactly 180°.
The entering and emergent paths are parallel,
regardless of the initial angle the ray makes with
the prism. (b) A pair of constant-deviation mirrors.
In this case, the deviation produced by the two
reflections is always exactly 90°.

(shown in dashed lines) indicates that this prism is the equivalent of
a plane parallel plate which is tilted with respect to the axis of the
bundle, whereas in the preceding examples the prism faces have been
normal to the axis. If this prism is used in a convergent light beam, it
will introduce a substantial amount of astigmatism (roughly equal to
one-quarter of its thickness). For this reason, this prism, which is
known as a Dove prism, is used almost exclusively in parallel light.
Since the apex of the prism is not used by the light beam, the prism
is usually truncated at AA′.
The Dove prism has a very interesting effect on the orientation of
the image. In Fig. 7.20a, the arrow and crossbar pattern is shown to
be inverted from top to bottom but not left to right. If the prism is
rotated 45°, as in Fig. 7.20b, the image is rotated through 90°; if the
prism is rotated 90° as in Fig. 7.20c, the pattern is rotated 180°. Thus,
the image is rotated twice as fast as the prism. (The analysis of the
image orientation in Fig. 7.20b is an example of the use of an auxiliary
pattern as described in Sec. 7.7. The auxiliary pattern is shown in dotted
lines in Fig. 7.20b.)

Figure 7.19 The Dove prism.
The dashed lines show that the
Dove prism is equivalent to a
tilted plate and will introduce
astigmatism when used in con-
vergent or divergent beams.