Page 163 - Modern Optical Engineering The Design of Optical Systems

146 Chapter Seven

Figure 7.29 The Penta prism
(a) and its equivalent mirror
system (b).

weight and/or economy. However, a prism, as a monolithic glass block, is
a very stable system and is not as subject to environmental variation of
angles as is an assemblage of mirrors on a metal support block.
The Penta prism is used where it is desirable to produce an exact 90°
deviation without having to orient the prism precisely. The end reflec-
tors of rangefinders are often of this type, and in optical tooling and
precise alignment work, the Penta prism is useful to establish an exact
90° angle. In large rangefinders, however, the prism is replaced by two
mirrors (Fig. 7.29b), securely cemented to a block in order to avoid the
weight, absorption, and cost of a large block of solid glass. The devi-
ation of two mirrors as shown in Fig. 7.29 is equal to twice the angle
between the mirrors.
Occasionally a roof is substituted for one of the reflecting faces of the
Penta prism to invert the image in one meridian.

7.14 Rhomboids and Beamsplitters
The rhomboid prism is a simple means of displacing the line of sight
without affecting the orientation of the image or deviating the line of
sight. The rhomboid prism and its mirror system equivalent are shown
in Fig. 7.30.

Figure 7.30 (a) Rhomboid prism.
(b) An equivalent mirror system.
Both systems displace the optical
axis without deviation or reori-
entation of the image.