Applications Guide
Modline 5
133
8.4
Emissivity Settings
An ideal infrared radiator, called a
blackbody
, emits the maximum amount of infrared energy possible at
a given temperature. It has an Emissivity (
E)
that
equals 1.0. However, targets measured in processes
are
non-blackbodies
. Their emissivity values are less than 1.0, which means they emit a fraction of the
infrared energy a
blackbody
would emit at a given temperature.
All Modline 5 Sensors are factory calibrated using
blackbody
standards. For accurate measurements, you
must compensate for the difference between the emissivity of your target and that of a
blackbody
radiator.
Modline 5 Series Sensors, except the Series 5R, are single color brightness thermometers. Brightness
thermometers depend upon the intensity of the infrared radiation. Since most process targets have an
emissivity of less than 1.0, setting the Emissivity value to match the emissivity of the target material is
necessary.
Series 5R Sensor use the two color ratio measurement described in Section 8.5
Methods of determining emissivity are described below.
8.4.1
Using Emissivity Tables
One way to determine an approximate Emissivity setting is to refer to a set of emissivity tables.
and
are provided for this purpose. Emissivity values in the tables
are based on actual tests on samples of the materials. Because the emissivity of most materials changes
with wavelength, a separate column of emissivity values is provided for each Sensor series. To use the
tables, locate your target material and obtain a value from the column for the Sensor series you are
using. Adjust the Emissivity value setting to this value.
Most table values are in the form of ranges. This is because the details of an object’s form and
characteristics affect its reflectance (R) and transmittance (T). These factors in turn reduce the emissivity
(E) value, as shown in the following general equation relating the three characteristics:
E
= 1 – R
– T
shows how a target’s surface characteristics affect its reflective properties, hence its emissivity.
It is assumed in the figure that each sample of Material A is thick enough to be completely opaque (T =
0). Notice that a cavity in an opaque object comes very close to having blackbody characteristics; E = 1
when both R = 0 and T = 0.
, you can see why the lower value of a given range represents a flat, highly polished
sample of the material. The upper value represents a sample of the material that has a flat surface that
is as rough as might be expected when in its “crude” or “unfinished” form.
Some entries in
and
are marked with asterisks (*) to signify that the range of values
results from more than just the target's reflective characteristics. These materials, in their specified
forms, are partially transparent to infrared radiation. In spectral regions where the materials transmit
energy, the emissivity typically increases with target thickness.
Follow the guidelines in the tables to estimate the emissivity of your target. Even a rough estimation
can significantly improve the accuracy of your measurements over only using “averaged” values. Note
that table entries having a single value represent targets in forms that are clearly specified.
