■ Avoid forced convection
■ Look for a thermally stable surrounding that will not generate spot reflections
■ Use high quality tape that you know is not transparent, and has a high emissivity
you are certain of
■ This method assumes that the temperature of your tape and the sample surface
are the same. If they are not, your emissivity measurement will be wrong.
20.3 Reflected apparent temperature
This parameter is used to compensate for the radiation reflected in the object. If the
emissivity is low and the object temperature relatively far from that of the reflected it
will be important to set and compensate for the reflected apparent temperature cor-
rectly.
20.4 Distance
The distance is the distance between the object and the front lens of the camera. This
parameter is used to compensate for the following two facts:
■ That radiation from the target is absorbed by the athmosphere between the object
and the camera.
■ That radiation from the atmosphere itself is detected by the camera.
20.5 Relative humidity
The camera can also compensate for the fact that the transmittance is also dependent
on the relative humidity of the atmosphere. To do this set the relative humidity to the
correct value. For short distances and normal humidity the relative humidity can nor-
mally be left at a default value of 50%.
20.6 Other parameters
In addition, some cameras and analysis programs from FLIR Systems allow you to
compensate for the following parameters:
■ Atmospheric temperature – i.e. the temperature of the atmosphere between the
camera and the target
■ External optics temperature – i.e. the temperature of any external lenses or windows
used in front of the camera
■ External optics transmittance – i.e. the transmission of any external lenses or win-
dows used in front of the camera
128 Publ. No. T559580 Rev. a486 – ENGLISH (EN) – November 17, 2010
20 – Thermographic measurement techniques
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