Appendix
12.2
Typical
Emissivity
Values
The
following
table
provides
a
brief
reference
guide
for
determining
emissivity
and
can
be
used
when
one
of
the
above
methods
is
not
practical.
Emissivity
values
shown
in
the
table
are
only
approximate,
since
several
parameters
may
affect
the
emissivity
of
a
material.
These
include
the
following:
1.
Temperature
2.
Angle
of
measurement
3.
Geometry
(plane,
concave,
convex)
4.
Thickness
5.
Surface
quality
(polished,
rough,
oxidized,
sandblasted)
6.
Spectral
range
of
measurement
7.
Transmission
(e.g.
thin
films
plastics)
To
optimize
surface
temperature
measurements,
consider
the
following
guidelines:
•
Determine
the
object
emissivity
using
the
instrument
which
is
also
to
be
used
for
the
measurements.
•
Avoid
reflections
by
shielding
the
object
from
surrounding
temperature
sources.
•
For
higher
temperature
objects
use
instruments
with
the
shortest
wavelength
possible.
•
For
translucent
materials
such
as
plastic
foils
or
glass,
assure
that
the
background
is
uniform
and
lower
in
temperature
than
the
object.
•
Mount
instrument
perpendicular
to
surface
if
possible.
In
all
cases,
do
not
exceed
angles
more
than
30°
from
incidence.
MID
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