Page 16 - Chalcogenide Glasses for Infrared Optics

Introduction

he purpose of this book is to describe the technology devel-
oped over 50 years to utilize chalcogenide glasses as infrared
Toptical materials. Chalcogenide glasses are based on the chal-
cogen elements sulfur, selenium, and tellurium excluding oxygen,
the first member of the family. The name is a misnomer since chalco-
gen is from the Greek meaning chalk former and oxygen is the only
member of the family that forms chalk. All its compounds are called
oxides. Methods used to identify qualitatively chalcogenide glass
compositions with promise to become useful infrared optical materi-
als are discussed. Once identified, the optical and related physical
properties must be measured quantitatively. The method best suited
for the production of homogeneous glass in high purity and quantity
must then be developed. Thus, a great deal of effort is required before
a glass composition is considered by optical designers ready for use
in an infrared system. For this reason, only a few glass compositions
have been fully developed and used in quantity over the years.
Infrared light by definition is light with a wavelength greater
than the sensitivity region of the human eye, 4000 to 8000 Å. For
infrared discussions, the more commonly used terms are 0.4 to 0.8 µm
withµm being the abbreviation for micrometers. Of special importance
are materials useful for infrared imaging systems designed to respond
to infrared energy transmitted through the atmosphere. Figure I.1
illustrates infrared light absorption in the air at sea level due to water
vapor, carbon dioxide gas, and ozone. The bottom illustration is the
resultant total for the three gas molecules. Notice there are two
windows indicated where energy is transmitted well, from 3 to 5 µm
(hot window) and about 7 to 14 µm (thermal window). The window
is called thermal since the peak of emitted radiation from a body at
room temperature, about 300 K, occurs in this window. Thermal
imaging of a living subject is based on emitted radiation, which is
transmitted in this atmospheric window. The hot window refers to
the fact that heated objects emit at the shorter wavelengths in this
range. Examples might be the tailpipe of a jetplane or a missile
exhaust.

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