Page 267 - Modern Optical Engineering The Design of Optical Systems

Optical Coatings 247

Coatings consisting of a few layers are for the most part reasonably
durable and can withstand careful cleaning. However, coatings con-
sisting of a great number of layers (and coatings consisting of 50 or
more layers are occasionally used) tend toward delicacy, and must be
handled with due respect.
Some multilayer coatings are quite effective polarizers when used
obliquely (and as such, are occasionally responsible for “mysterious”
happenings). This is notably true in systems using linearly polarized
laser beams. One must also exercise care in photometric or radiomet-
ric applications (e.g., spectrophotometers), since polarization effects
can introduce significant errors.

11.2 Reflectors

Although polished bulk metals are occasionally used for mirror sur-
faces, most optical reflectors are fabricated by evaporating one or more
thin films on a polished surface, which is usually glass. Obviously the
interference filters described in the preceding section can be used as
special-purpose reflectors in instances where their spectral character-
istics are suitable. However, the workhorse reflector material for the
great majority of applications is an aluminum film deposited on a sub-
strate by evaporation in vacuum. Aluminum has a broad spectral band
of quite high reflectivity and is reasonably durable when properly
applied. Almost all aluminum mirrors are “overcoated” with a thin
protective layer of either silicon monoxide or magnesium fluoride. This
combination produces a first-surface mirror which is rugged enough to
withstand ordinary handling and cleaning without undue scratching
or other signs of wear.
The spectral reflectance characteristics of several evaporated metal
films are shown in Fig. 11.6. With the exception of the curve for
rhodium, the reflectivities given here can seldom be attained for prac-
tical purposes; the silver coating will tarnish and the aluminum film
will oxidize, so that the reflectances tend to decrease with age, espe-
cially at shorter wavelengths. The high reflectivity of silver is only use-
ful when the coating is properly protected.
Figure 11.7 indicates the variety of characteristics which are available
in commercial aluminum mirrors. A run-of-the-mill protected aluminum
mirror can be expected to have an average visual reflectance of about
88 percent. Two, four, or more interference films may be added to
improve the reflectance where the additional cost can be accepted.
This enhanced reflectivity within the bandpass of the mirror is
obtained at the expense of a lowered reflectivity on either side, as can
be seen from the dashed curve in Fig. 11.7.