LumaSense Technologies BoilerSpection-SD - Manual (Page 49)

BoilerSpection HD/SD Manual Principles of Thermal Imaging 49
2. Have no high temperature object behind the measured object, such as the sun shining on
the back of the measured object.
3. Do not allow direct sunlight to strike thermal imager.
4. Do not allow obstacles such as dust or vapor (which attenuates the infrared signal) between
the measured object and the thermal imager.
6.7 Practical Measurement
There are a number of methods for correcting emissivity in order to obtain the true temperature.
The correction procedure with each method will be explained next.
1. Method of comparison or direct measurement with emissivity equal to approximately
1.0
1. Stabilize the temperature of the measured object or similar material.
2. Open a very small hole (hereafter called blackbody part) in the object which the thermal
imager must measure as to satisfy blackbody conditions.
3. Then set the emissivity correcting function of thermal imager so that the temperature of
the blackbody part and the measured surface will be the same. The obtained emissivity
will be the emissivity of the measured surface.
4. Thereafter when measuring the same type object, it is unnecessary to change the
emissivity setting.
2. Method of direct measurement of emissivity
If a hole cannot be made as in method 1, then apply black high emissivity paint and carry
out the same procedures to obtain the emissivity. Since the black paint will not provide a
perfect blackbody, first set the emissivity of the black paint and then measure the
temperature.
3. Indirect measurement
Measure a sample similar to the measured object, and place it in a condition able to be
heated by a heater, etc. Then measure the object and the sample alternately with the
camera and when the indicated values are identical, measure the sample with a contact-
type thermometer. Adjust the emissivity of the thermal imager to cause the temperature
readout to match that of the contact measurement. The resulting emissivity is that of the
sample.
4. Measuring by Wedge effect
With this method, the emissivity of the measured surface itself is enhanced through use of
the wedge or semi-wedge effect. But one must be careful about the number of reflections
and/or the measuring angle.
A small change in angle will reduce the emissivity enhancement.
Figure 31: Measuring by Wedge effect