Page 160 - Modern Optical Engineering The Design of Optical Systems

Prism and Mirror Systems 143

cementing two small right-angle prisms on the hypotenuse of a large
right-angle prism as indicated in Fig. 7.25b. The lateral displacement
of the axis is less than that for the Porro #1 system.

Abbe prism
The Abbe (or Koenig, or Brashear-Hastings) prism (Fig. 7.26) is an
erecting prism which can be used when it is desired to erect the image
without displacing the axis as the Porro prisms do. The roof is necessary
to provide the left-to-right reversal of the image; the roof angle must
be made accurately to avoid image doubling.
If this prism is made without the roof, it will invert the image in one
meridian only, just as the Dove prism. However, since its entrance and
exit faces are normal to the system axis, it may be used in a converging
beam without introducing astigmatism.

Other erecting prisms
Among the many prisms designed to erect an image are those sketched
in Fig. 7.27. The fact that the image is inverted and reversed left to right
after passing through these prisms may be verified by the methods
outlined in Sec. 7.7. Notice that each prism (except Fig. 7.27f) has been
arranged so that the axial ray enters and leaves the prism normal to
the prism faces and that all reflections are total internal reflections. In
the Leman and Goerz prisms, the axis is displaced but not deviated.
In the Schmidt and modified Amici prisms, the axis is deviated
through a definite angle, which can be selected by the designer (within
the limits allowed by total internal reflection). Note also that the roof
surface is used at the location where the angle of incidence is small
and where there would be light leakage through an ordinary surface.

Figure 7.26 Abbe prism. Used as
an in-line erecting system, it
does not displace the axis as the
Porro systems do, nor does it
materially displace the image
longitudinally.