Page 159 - Modern Optical Engineering The Design of Optical Systems

142 Chapter Seven

Figure 7.24 Porro prism system
(first type) (a) indicating the
way the Porro system erects an
inverted image. (b) Porro prisms
are usually fabricated with
rounded ends to save space and
weight. Note that the spacing
between the prisms has been
shown increased for clarity.

Porro prism of the second type
The Porro prism of the second type is shown in Fig. 7.25, and serves
the same purpose as the Porro #1 system. Both Porro systems function
by total internal reflection so that no silvering is required. It is common
to round off the ends of the prisms to conserve space and weight.
The second Porro is somewhat more difficult to fabricate than the
first type, but in some applications its compactness, and the fact that
the prisms can be readily cemented together, offer compensating
advantages. The Porro #2 may also be made in three pieces, by

Figure 7.25 Porro prism system (second type) (a) indicating the erection
of an inverted image. This system is shown made from two prisms in
(a) and from three prisms in (b).