Page 249 - Modern Optical Engineering The Design of Optical Systems

Optical Materials 229

Figure 10.13 Polar intensity plots
of diffusing materials. (Left) For a
“perfect diffuser”, the intensity of
a unit area of the surface varies
with cos . (Right) The relative
intensities of single- and double-
ground glass and flashed opal
glass.

scattering from these particles. The transmitted light is slightly yel-
lowish since the shorter wavelengths are scattered more than the
longer. Opal glass is ordinarily used as flashed opal, which is a thin
layer of opal glass fused to a supporting sheet of clear glass. The diffusion
of flashed opal is quite good. When illuminated normally, the bright-
ness at 45° from the normal is about 90 percent of what one would
expect from a perfect diffuser. Its total transmission is quite low, about
35 or 40 percent. It should be noted that, since good diffusion means
that the incident light is scattered into 2 steradians, the axial bright-
ness of a rear-illuminated screen of good diffusion is very low when
compared with a poor diffuser.
Ground glass is produced by fine grinding (or etching) the surface of a
glass plate to produce a large number of very small facets which refract
the incident light more or less randomly. The total transmission of
ground glass is about 75 percent. This transmission is quite strongly
directional, and ground glass is far from a perfect diffuser. Its characteristics
vary somewhat, depending on the coarseness of the surface. Typically, for
a normally illuminated surface, the brightness at 10° from the normal is
about 50 percent of the normal brightness; at 30°, the brightness is about
2.5 percent of the brightness at the normal. This characteristic is of
course quite useful when partial diffusion is desired. By combining two
sheets of ground glass (with the ground faces in contact), the transmis-
sion is lowered about 10 percent but the diffusion is improved; at 20° to