Page 331 - Modern Optical Engineering The Design of Optical Systems

310 Chapter Thirteen

surfaces in a convergent beam. The counterrotating wedges of Fig.
13.14e can be located in parallel light (region Y in Fig. 13.14a) and
thus avoid this difficulty. Note that as one wedge turns clockwise, the
other must rotate counterclockwise through exactly the same angle; in
this way the vertical deviation is maintained at zero while the hori-
zontal deviation can be varied plus or minus twice the deviation of an
individual wedge. These are sometimes called Risley prisms.
Another device to produce a variable angle of deviation consists of
a fixed plano concave lens and a movable plano convex lens of the
same radius with their curved surfaces nested together. When the
convex lens is located so that its plane surface is parallel to that of
the concave lens, the pair produces no angular deviation. However, if
the convex lens is rotated (about its center of curvature), the pair
effectively becomes a prism and will produce an angular deviation.
This device can be executed with spherical surfaces or with cylindrical
surfaces.
Single-lens reflex (SLR) cameras often incorporate a split-image
rangefinder which is based on an entirely different principle than the
coincidence rangefinder described above. The viewfinder of an SLR camera
consists of the camera objective lens, a field lens, and an eyelens. The
field lens is divided into three zones as indicated in Fig. 13.15b. The outer
zone functions as a straightforward field lens, redirecting the light at the
edge of the field so that it passes through the eyelens. It is made in the
form of a plastic Fresnel lens, in which the curved surface of a lens is col-
lapsed in annular zones to a thin plate, as shown in Fig. 13.15a. This has
the refracting effect of the lens without its thickness or weight. Such
Fresnel lenses are also used as condensers in overhead projectors, as well
as in spotlights and signal lamps. The center zone of the SLR field lens
is split into two halves. Each half is a wedge prism; the two prisms are
oriented in opposite directions. If the image formed by the objective lens
is in focus, it is located in the plane of the wedges and the two halves
of the image line up with each other. If the image is out of focus, the
image through one-half of the split wedge is deviated in one direction;
through the other half the deviation is in the other direction and the
image is split. The intermediate zone of the field lens has a surface
comprised of tiny pyramidal prisms which deviate and break up an
out-of-focus image so as to exaggerate the out-of-focus blurring.
For many applications the optical rangefinder has been superceded
by the laser rangefinder. This is essentially optical radar, where the
distance to the target is obtained by measuring the travel time for a
pulse of light to reflect from the target and return. In military appli-
cations a high-power laser is used; in surveying applications a cooper-
ative target such as a retrodirector (corner-cube prism) is used and a
much lower power source is adequate.