Page 33 - Chalcogenide Glasses for Infrared Optics

12 Cha pte r O n e

There is usually a slight difference in the refractive index of the crys-

tallite from the glass from which it formed. Because of this difference,
light is reflected (scattered) at each surface interface and is a transmis-
sion loss. The scattering particles may range from the microscopic to
the macroscopic. The intensity of light scattered at right angles to the
incident light depends upon the particle size relative to the wave-
length of light and upon the difference between the particle and the
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surrounding medium. Such would be the case for large-grain poly-
crystalline materials. As particles become very small relative to the
wavelength, the intensity drops off dramatically, as the inverse of the
wavelength to the fourth power.
Another source of loss of transmitted light and optical distortion
is striae in glass. During the glass process, local variations in compo-
sition or density produce regions where the refractive index is differ-
ent from that of the whole. The beam can be diverted in direction in
these local regions, harming the integrity of the transmitted image or
reducing the intensity of light reaching the detector plane. The sup-
plier must develop a reliable process to supply high-purity, homoge-
neous glass, free of bubbles, particles, striae, and crystallites.

1.5 Optical Constants and Dispersion due to Strong
Absorption
If a beam of light in air incident on a flat surface at an angle of O from
1
the normal and the refracted beam is O from the normal, the refrac-
2
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tive index may be calculated from N sin O = N sin O . Since N for
1 1 2 2 1
air is ~1.000,
N = sin O 1
2
sin O 2
The refractive index of an optical material is not a constant, and how
it varies with wavelength is perhaps the most important parameter
for its use by an optical designer. Already we have seen that Fresnel
reflection losses decrease transmission for a material. The Fresnel
reflection coefficient R can be calculated from the expression
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R = ( N − )1 2
( N + )1 2

Again, the value of R is not a constant since N changes with wave-
length. The greater the value of N, the higher the value of R. Thus far,
we have accounted for the Fresnel reflection losses only from trans-
mission. Losses due to absorption must be measured and calculated
from the expression

Re
T = (1 − ) 2 −α x
1 − Re −2α x
2