Thermal imaging
terminology explained
Palette—
Color representation of the
temperatures (temperature scale) in a displayed
image. Certain color palettes meet personal
preferences or optimize the image for different
applications and/or problems. An example of the
different palettes appear to the right.
Sensor Size—
Similar to digital cameras the
sensor size describes the amount of displayed
points per image of a thermal imager. A sensor
size of 160 x 120 captures and displays more
than 19,000 measurement points with each
measurement. If the imager is fully radiometric
then it also truly measures and stores all
captured points with the image.
Field of view (FOV)—
Indicates what the
thermal imager sees or measures at a given
moment. The combination of the Field Of View
specification and the distance to the measured
object determines which surface or part of an
object will be measured as a total. A thermal
imager with a FOV of 23° x 17° (20 m), F=0.8
lens) can detect an object that is 6 m (20 ft)
wide by 4.5 m(15 ft) high. This same imager can
infer a temperature measurement of a smaller
section within that object of 3.8 cm X 3.8 cm
(1.5 in x 1.5 in). A FOV calculator on www.fluke.
com/FOV helps you calculate the measurement
surface at various distances to the object.
Thermal sensitivity —
Indicates what the
smallest temperature difference is which can
be measured/displayed in an image. It basically
is the maximum resolution of the image and
is referred to as NETD (noise equivalent
temperature difference).
Emissivity adjustment—
All surfaces emit
infrared energy or heat. The level of emission
varies much per surface and is described
with the term emissivity. Painted coatings
and materials usually have a high emissivity
while polished aluminum has a low emissivity.
Visit www.fluke.com/emissivity for a table
with emissivities for different materials. If you
perform qualitative inspections with the imager
(most applications) then emissivity does not
have to be adjusted. To measure the temperature
of a material accurately it will be necessary to
adjust for the material’s emissivity in specific
applications.
Span—
The set of temperature values that can be
measured within a preset range. Adjusting the
span allows you to see more subtle temperature
gradients (or contrast) in a captured image.
When the span is optimized the imager shows
256 different shades of color in an image.
Ironbow
Blue-red
High contrast
Amber
Hot metal
Grey
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