Page 250 - Modern Optical Engineering The Design of Optical Systems

230 Chapter Ten

the normal, the brightness is about 20 percent; at 30°, about 7 percent.
With two sheets, the diffusion can be increased by spacing them apart,
although this will destroy their utility as a projection screen.
A sheet of tracing paper has diffusion characteristics quite similar to
ground glass, and there are several commercially available plastic screen
materials which are somewhat better diffusers than ground glass. The
plastic surface also can be shaped to control the beam spread.
A rear projection screen, when used in a lighted room, is illuminated
from both sides. The room light reduces the contrast of the projected
image. This situation is sometimes alleviated by introducing a sheet
of gray glass (that is, a neutral filter) between the diffusing screen
and the observer. When this is done, the light from the projector is
reduced by a factor of T, the transmission of the gray glass, but the
2
room light is reduced by T , since the room light must pass through
the gray glass twice to go from the room to the diffuser and back to
the observer’s eye.

10.8 Polarizing Materials

Light behaves as a transverse wave in which the waves vibrate
perpendicular to the direction of propagation. If the wave motion is
considered as a vector sum of two such vibrations in perpendicular
planes, then plane polarized light results when one of the two components
is removed from a light beam. Plane polarized light can be produced by
passing the radiation from an ordinary source through a polarizing
prism, several types of which are available. These prisms depend on
the birefringent characteristic of calcite (CaCO 3 ), which has a different
index of refraction for the two planes of polarization. Since light of one
polarization is refracted more strongly than the other, it is possible to
separate them either by total internal reflection (as in the Nicol and
Glan-Thompson prisms) or by deviation in different directions (as in
the Rochon and Wollaston prisms).
Such prisms are large, heavy, and expensive. Sheet polarizers,
which are made by aligning microscopic crystals in a suitable base, are
thin, light, relatively inexpensive, useful over a wide field of view, and
simple to fabricate into an almost unlimited range of sizes and shapes.
Thus, despite the fact that they are not quite as efficient as a good
prism polarizer and are not effective over as large a wavelength range,
they have largely supplanted prisms for the great majority of applica-
tions where polarization is required. Several companies, produce a
number of types of sheet polarizers. For work in the visible region,
several types are available, depending on whether optimum transmission
or optimum extinction (through crossed polarizers) is desired. Special
types are available for use at high temperatures and also for use in the