9 – Infrared Primer
A6700sc/A6750sc User’s Manual
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When the plate becomes opaque this formula is reduced to the single formula:
This last relation is a particularly convenient one, because it is often easier to measure reflectance
than to measure emissivity directly.
9.3 The Measurement Formula
As already mentioned, when viewing an object, the camera receives radiation not only from the object
itself. It also collects radiation from the surroundings reflected via the object surface. Both these
radiation contributions become attenuated to some extent by the atmosphere in the measurement
path. To this comes a third radiation contribution from the atmosphere itself.
This description of the measurement situation, as illustrated in the figure below, is so far a fairly true
description of the real conditions. What has been neglected could for instance be sun light scattering
in the atmosphere or stray radiation from intense radiation sources outside the field of view. Such
disturbances are difficult to quantify, however, in most cases they are fortunately small enough to be
neglected. In case they are not negligible, the measurement configuration is likely to be such that the
risk for disturbance is obvious, at least to a trained operator. It is then his responsibility to modify the
measurement situation to avoid the disturbance e.g. by changing the viewing direction, shielding off
intense radiation sources etc.
Accepting the description above, we can use the figure below to derive a formula for the calculation of
the object temperature from the calibrated camera output.
Figure 8-14: A schematic representation of the general thermographic measurement
situation.1: Surroundings; 2: Object; 3: Atmosphere; 4: Camera
