Page 259 - Modern Optical Engineering The Design of Optical Systems

Optical Coatings 239

With the exception of the reflecting films, such films have an optical
thickness (the optical thickness is the physical thickness times the
index) which is measured in wavelengths, typically one-quarter or one-
half wavelength. The deposition of thin films is carried out in a vac-
uum and is done by heating the material to be deposited to its
evaporation temperature and allowing it to condense on the surface to
be coated. The thickness of the film is determined by the rate of evap-
oration (or more accuratelly condensation) and the length of time the
process is allowed to continue. Since interference effects produce col-
ors in the light reflected from thin films, just as in oil films on wet
pavements, it is possible to judge the thickness of a film by the apparent
color of light reflected from it. Simple coatings can be controlled visu-
ally by utilizing this effect, but coatings consisting of several layers are
often monitored photoelectrically, using monochromatic light, so that
the sinusoidal rise and fall of the reflectivity can be accurately
assessed and the thickness of each layer controlled. By using two dif-
ferent wavelengths (often from lasers), this technique can achieve high
precision. Another popular monitoring technique utilizes a quartz
crystal of the type used to control radio broadcast frequencies. The
oscillation frequency of such a crystal varies with its mass or thick-
ness. By depositing the coating directly on the crystal and measuring
its oscillation frequency, the coating thickness can be accurately
monitored.
Let us first consider a single-layer film the optical thickness of which
(nt) is exactly one-quarter of a wavelength. For light entering the film
at normal incidence, the wave reflected from the second surface of the
film will be exactly one-half wavelength out of phase with the light
reflected from the first surface when they recombine at the first surface,
resulting in destructive interference (assuming that there is no phase
change by reflection). If the amount of light reflected from each surface is
the same, a complete cancellation will occur and no light will be
reflected. Thus, if the materials involved are nonabsorbing, all the
energy incident on the surface will be transmitted. This is the basis of
the “quarter-wave” low-reflection coating which is almost universally
used to increase the transmission of optical systems. Since low-reflection
coatings reduce reflections, they tend to eliminate ghost images as
well as the stray reflected light which reduces contrast in the final
image. Before the invention of low-reflection coatings, optical systems
which consisted of many separate elements were impractical because
of the transmission losses incurred in surface reflections and the fre-
quent ghost images. Even complex lenses were usually limited to only
four air-glass surfaces. A magnesium fluoride coating has an addi-
tional benefit in that it is actually (when properly applied) a protective
coating; the chemical stability of many glasses is enhanced by coating.