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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