Page 166 - Modern Optical Engineering The Design of Optical Systems

Prism and Mirror Systems 149

retains its original orientation, because the reflecting surfaces are in
the form of a rhomboid prism.
Often two Porro systems are used in a similar rotatable configura-
tion which allows a change in the eye separation.

7.15 Plane Mirrors

In the preceding discussions we have indicated several times that
reflecting prisms may be replaced by mirrors. For most applications, it is
necessary that the mirrors be first-surface mirrors, as opposed to ordi-
nary second-surface mirrors. The two types are sketched in Fig. 7.34.
The first-surface mirror is usually preferable because it does not pro-
duce a ghost image as does the second-surface mirror. In addition, the
second-surface mirror requires the processing of an extra surface in its
fabrication. It also requires the light to pass through a thickness of
glass which may introduce aberrations and which will absorb energy
in ultraviolet and infrared applications. The second-surface mirror can
be made more durable, however, since its reflecting coating can be pro-
tected from the elements by electrodeposited copper and painted cove-
rings. First-surface mirrors are usually made with vacuum-deposited
aluminum films protected by a thin transparent overcoating of silicon
monoxide or magnesium fluoride.

7.16 The Design of Prism
and Reflector Systems
Ordinarily it is required of a prism (or reflector) system that it produce
an image with a certain orientation and with the emergent beam of
light redirected in a given manner. The design effort is usually best
begun by establishing the minimum number of reflectors which will
produce the desired result. This is most simply (and perhaps best)

Figure 7.34 (a) Second-surface
mirror. (b) First-surface mirror.