handbook of modern sensors

4.4 Mirrors 135
Fig. 4.10. Spectral reflectances of some mirror coatings.
surface where the substrate is fabricated of ZnSe or other long-wavelength transparent
materials. Materials such as Si or Ge have too strong a surface reflectivity to be useful
for the fabrication of the second surface mirrors.
Reflecting coatings applied to a surface for operation in the visible and nearinfrared
ranges can be silver, aluminum, chromium, and rhodium. Gold is preferable
for the far-infrared spectral range devices. By selecting an appropriate coating, the
reflectance may be achieved of any desired value from 0 to 1 (Fig. 4.10).
The best mirrors for broadband use have pure metallic layers, vacuum-deposited
or electrolytically deposited on glass, fused silica, or metal substrates. Before the
reflective layer deposition, to achieve a leveling effect a mirror may be given an
undercoat of copper, zirconium–copper, or molybdenum.
Another useful reflector which may serve as a second surface mirror without the
need for reflective coatings is a prism, where the effect of total internal reflection (TIR)
is used. The angle of a total internal reflection is a function of the refractive index:
( 1
0 = arcsin . (4.23)
n)
The total internal reflectors are the most efficient in the visible and near-infrared
spectral ranges, as the reflectivity coefficient is close to unity. The TIR principle is
fundamental for the operation of the optical fibers.
A reflective surface may be formed practically in any shape to divert the direction
of light travel. In the optical systems, curved mirrors produce effects equivalent to
that of lenses. The advantages they offer include (1) higher transmission, especially
in the longer-wavelength spectral range, where lenses become less efficient due to