General thermography
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Transcript of General thermography
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General Thermography Testo Product Management
02/05/2016 Author
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Thermography is the graphical representation of the
temperature distribution on a “solid” body
=> We always only see the surface!
What is this thing called Thermography?
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Thermographers see the world in a
different perspective.....
What is this thing called Thermography?
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Visible light...
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... and infrared light:
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Each body, that is warmer than -273.15 ºC, emits electromagnetic radiation.
An IR - camera doesn‟t detect temperature but infrared - radiation.
The camera doesn‟t emit radiation.
The possibility of a thermal imager to “see through” materials is very limited.
Basics of Thermography
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•
•
•
The electromagnetic spectrum
visible light
Visible Short wave Long wave
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Transmission curves for the atmosphere
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Planck‟s curves
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Radiation and temperature
The Thermal Imager doesn‟t measure temperature but radiation!
For the calculation of the correct temperature a Thermal Imager needs
certain information.
Most of the information the camera gathers by itself, however, some
parameter must be provided by the user.
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The two most important parameters of the measured object are:
Reflected Temperature –
temperatures that are reflected by the measured object.
Emissivity –
describes the capability of the inspected surface to emit
radiation.
Parameter, that must be provided by the user....
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Blackbody (perfect emitter)
Absorbs and emits 100%
(Emissivity = 1)
Real body
A part of the radiation is
reflected as well as transmitted
(Emissivity < 1)
The inspected object
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Reflection, Transmission, Emission
=> Kirchhoff„s Law
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Reflection, Transmission, Emission
Most of the materials are not permeable for IR-radiation.
In this case τ = 0 and
ρ + τ + ε = 1 equals to ρ + ε = 1
=> Conclusion: The lower the emissivity, the higher is the share of reflected radiation and, therefore, the more difficult is an exact temperature measurement.
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The emissivity can vary with the surface condition, the material and the temperature.
With long-wave IR-cameras the emissivity is not depending on the colour of the inspected object.
Most of the construction materials have an emissivity between 0.85 und 0.96.
Emissivitiy tables can be used. However, these values should be used only as a guide.
It is also possible to determine the emissivity: Reference measurements with contact thermometers or surfaces where ε is known.
Reflection, Transmission, Emission
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IR
Optic
2 dim.
Detector-
Matrix
FPA-Camera
(Focal Plane Array)
I
vertical
scanning
Single
detector
horizontal
scanning
Scanner-
Camera II
IR – camera systems
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Thermal sensitivity
The most important criterion for building inspections
is the thermal sensitivity or NETD (Noise Equivalent
Temperature Difference) of a thermography system.
NETD describes the ability to resolve small
temperature differences.
The NETD in professional building thermography
should not be above 100mK (or 0,1°C).
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Field of View
The Field of View describes the
area seen by the detector
The Field of view is independent of:
the distance to the object
The Field of view is dependent on:
The size of the array
The lens used
The larger the field of view, the more fits on one image at the same
distance to the object.
160 Pixel
12
0 P
ixe
l
32°
24°
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Instantaneous Field of View
Instantaneous Field of View describes the field of view of one single pixel.
The unit of IFOV is mrad.
160 Pixel
12
0 P
ixe
l
1,3 mrad
1m
1,3
mm
The smaller the instantaneous field of view, the better is
the geometrical resolution of the system.
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Instantaneous Field of View
IFOVgeometric : smallest object that can be detected by
the system at a given distance.
IFOVmeasured : in real life the information of only one
pixel is not sufficient to get a precise
measuring value.
Therefore as a rule of thumb is used:
IFOVmeasured = 3 * IFOVgeometric
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Distance and smallest object
Distance D Smallest object
standard lens (32°)
Smallest object
telephoto lens (12°)
0,1 m 1,0 mm -
0,3 m 3,1 mm -
0,6 m 6,3 mm 2,4 mm
1 m 10 mm 4 mm
2 m 21 mm
8 mm
10 m 105 mm
39 mm
Attention: all values are based on edge lengths of 3x3 Pixel!
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