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In
the " 90
¡ã Deflection"
orientation, the prism acts like a single mirror. Light enters
the prism through one of its legs, reflects off its hypotenuse
by total internal reflection, and then exits through its second
leg. The centerline of the incoming light must be perpendicular
to the entrance face. As is the case with a flat mirror at
45¡ã to the incoming light, the prism in this orientation inverts
the image while deflecting its direction of propagation by
90¡ã.
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In
the " 180
¡ã Deflection"
orientation, the prism acts like two mirrors. Light enters
the prism through its hypotenuse, reflects at its first and
second legs by total internal reflection, and then exits back
through its hypotenuse.
In
the " 180
¡ã Deflection"
orientation, as
long as the incoming light remains parallel to the plane that
contains the vertex angle, the alignment of the prism within
that plane is not critical; exact retroreflection will still
occur.
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The
dimension that controls the accuracy of the retroreflection
is the right angle at the vertex of the prism. The outgoing
beam will be inclined to the incoming beam by an amount equal
to twice the deviation of the vertex angle from 90¡ã. For example,
if a right-angle prism were manufactured with a tolerance
of ¡À1 minute of arc, then the incoming and outgoing beams
could cross each other with an inclination of no more than
¡À2 minutes of arc.
The
retroreflective capability of a right-angle prism is limited
to action in the plane that includes its right-angle vertex.
If retroreflective action is required for randomly oriented
light, then the designer must use a corner cube retroreflector.
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