User's Manual

The secret to a good thermal image33
sofas, indoor plants or curtains. Figure 33.6 illustrates this situation—which occurs all
too regularly.
Figure 33.6 “Thermographic inspection” of an inaccessible object.
It is also important that the object under investigation, or its areas of interest, take up the
whole thermal image. This is particularly true when measuring the temperature of small
objects. The spot tool must be completely filled by the object to enable correct tempera-
ture measurements. Since the field of view and therefore the spot size are determined by
both the distance to the object and the camera’s optics, in such situations the distance to
the object must either be reduced (get closer!) or a telephoto lens must be used (see Fig-
ure 33.7).
Figure 33.7 Supply and return lines from radiators in an open-plan office. The left image was taken from a
distance of 1 m: the measurement spot is filled and the temperature measurement is correct. The right im-
age was taken from a distance of 3 m: the measurement spot is not completely filled and the measured
temperature values are incorrect (31.4 and 24.4°C (88.5 and 75.9°F) instead of 33.2 and 25.9°C (91.8 and
78.6°F)).
33.5 The changeables—image optimization and temperature measurement
33.5.1 Level and span
After choosing the appropriate temperature range, you can adjust the contrast and
brightness of the thermal image by changing the temperature intervals displayed. In
manual mode, the false colors available in the palette can be assigned to the tempera-
tures of the object of interest. This process is often referred to as “thermal tuning.” In au-
tomatic mode, the camera selects the coldest and warmest apparent temperatures in the
image as the upper and lower limits of the temperature interval currently displayed.
A good or problem-specific scaling of the thermal image is an important step in the inter-
pretation of the image, and is, unfortunately, often underestimated (see Figure 33.8).
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