Pyrometer- Handbook - Contika

14. Optics, Lenses and Window Material
One of the most important components of a pyrometer is
the optics. If lenses are used the material must be adapted
to the spectral properties of the detector. The material must
be permeable to the pyrometer's spectral range, which is
determined by the measuring range and the object to be
measured.
• Crown glass (BK7) is used in pyrometers which
measure in the short wavelength band (up to 2.7 µm).
Crown glass is very stable, resistant to chemicals and
easy to clean.
• Water-free quartz glass (Infrasil) is also used in
pyrometers which measure in the short wavelength band
(up to 3 µm).
• Calcium Fluoride (CaF 2 , Fluorspar) is used especially
when glass is measured. It can be used up to 10 µm and
has a high transmission coefficient.
• Germanium lenses are useful for pyrometers which
measure in the long wavelength band (up to 18 µm).
They have a non-reflective surface and are nontransparent
for visible light.
• Plastic lenses are used in simple pyrometers. They are,
however, attacked by cleaning agents and scratch easily.
They also do not tolerate high ambient temperatures.
The various colours of light have different focal lengths for
normal lenses. This divergence of colour is called
dispersion, and the resulting effect is called chromatic
error. To eliminate this error an achromat is used. This is a
combination of a convex lens and a diverging lens each
with a different refractive index. They are precisely
designed so that in the observed wavelength range, the
chromatic error is fully compensated for.
Pyrometer Handbook
Crown glass
Quartz glass
Calcium fluoride
Germanium
Plastic
dispersion
chromatic
errorachromat
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