User's Manual

The secret to a good thermal image33
Figure 33.3 The same radiator from the same distance with the same settings, taken by three different
thermal cameras: FLIR C2 (left), FLIR T440 (middle), and FLIR T640 (right).
33.4.2 Temperature range
For hand-held uncooled microbolometer cameras, the “exposure” is essentially preset by
the image frame rate. This means that it is not possible to choose for how long—and
therefore how much—radiation hits the camera detector. For this reason, an appropriate
temperature range must be selected that matches the amount of incident radiation. If a
temperature range is selected that is too low, the image will be oversaturated, as objects
with higher temperatures emit more infrared radiation than colder objects. If you select a
temperature range that is too high, the thermal image will be “underexposed,” as can be
seen in Figure 33.4.
Figure 33.4 Images from a FLIR T440 with temperature ranges of –20 to +120°C, (left, –4 to +248°F), 0 to
+650°C (middle, +32 to +1202°F) and +250 to +1200°C (right, +482 to +2192°F). All other settings are
unchanged.
To take an image or temperature measurement, the lowest possible temperature range
available on the camera should be selected. However, it must also include the highest
temperature in the image (see Figure 33.5).
Figure 33.5 An image of the same object taken with different temperature ranges: –20 to 120°C (left, –4
to +248°F) and 0 to 650°C (right, +32 to +1202°F). The temperature in the left image is displayed with a
warning sign (a red circle with a white cross) because the measured values are outside the calibrated
range.
Depending on the camera model and configuration options, overdriven and underdriven
areas can be displayed in a contrasting color.
33.4.3 Image detail and distance from the object
Illumination in photography corresponds in thermography to the interplay of radiation
from the object and reflected radiation from the surrounding environment. The latter is
unwanted because interfering—or, at the very least, spot—reflections need to be
avoided. This is achieved by choosing a suitable position from where to take images. It is
also advisable to select a position from which the object of interest can be seen clearly
and is not hidden. This may seem obvious but in the building sector, for example, it is
common to find reports in which pipes or windows to be investigated are hidden behind
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