Theory of Thermography

 
1.1 The emission, reflection, transmission
 - Emissivity (e)
 - The reflection coefficient (p)
 - Transmittance (T)
 - The law of thermal radiation Kirgofa
1.2 point measurement and the distance to the object to be measured
 

All objects with a temperature above absolute zero (0 K = -273.15 ° C), emit infrared radiation. The human eye can not see infrared light.

Back in the 1900s, physicist Max Planck showed an association between body temperature and the intensity coming from a stream of infrared radiation.

Imager measures the infrared radiation in the longwave spectrum within the field of view. On this basis, the calculation is the temperature of the measuring object. Factors for calculating the emissivity (ε) the surface of the object to be measured and reflected temperature compensation (CAT = reflected temperature compensation) - the values of these variables can be manually set in camera.

Each pixel of the detector is an infrared point displayed on the display, using the video effect "false color".

Thermography (measurement of temperature by the thermal imager) is a passive, contactless method of measurement. IR image shows the temperature distribution on the surface of the object. Therefore, using the thermal imager you can not "see" inside the object or see it through.

1.1 The emission, reflection, transmission.

The radiation, recorded by the thermal imager, consists of emitted, reflected and transmitted long-wave infrared radiation emanating from objects located within the field of view thermal imager.

Thermal imager

Figure 1.1: The radiation reflection and transmission

Emissivity (ε)

Emissivity (ε) is a measure of the ability of the material to emit (highlight) the infrared radiation.

  • ε varies depending on the properties of the surface material, and in the case of some materials - the temperature of the measured object.
  • The maximum emissivity: ε = 1 (t.100%).

ε = 1 in fact does not occur.

  • The living body: ε <1, since the living body also reflect and transmit light as possible.
  • Many non-metallic materials (eg, PVC, concrete, organic matter) have a high emissivity in the longwave infrared, which is independent of temperature (ε ≈ 0.8 to 0.95).
  • Metals, especially material with a shiny surface, have a low emissivity, which can vary depending on the temperature.
  • Emissivity ε can be manually set in camera.

The reflection coefficient (ρ)

The reflection coefficient (ρ) is a measure of the ability of a material to reflect infrared radiation.

  • ρ depends on the properties of the surface temperature and the type of material.
  • As a rule, smooth, polished surfaces are more reflective than a rough, matte surface, made of the same material.
  • Reflected temperature compensation can be manually configured in camera (CAT).
  • In many areas of applications of the reflected temperature corresponds to the ambient temperature. You can measure it, for example, with an air thermometer testo 810.
  • CAT can be determined by the emitter Lambert.
  • The angle of reflection of the reflected infrared light is always equal to the angle of incidence.

Transmittance (τ)

Transmittance (τ) is a measure of the ability of the material to pass (to pass through itself) infrared radiation.

  • τ depends on the type and thickness of the material.
  • Most materials are impervious material type, ie resistant to long-wave infrared radiation.

The law of thermal radiation Kirgofa.

Infrared radiation, recorded by the thermal imager, consisting of:

  • radiation emitted by the object of measurement;
  • external radiation and the reflected
  • Missed the object of measurement of radiation.

The sum of these components is always taken as 1 (or 100%):

ε + ρ + τ = 1

Since the transmittance rarely plays a significant role in practice, τ is omitted and the formula

ε + ρ + τ = 1

simplifies to

ε + ρ = 1

For thermography, this means that:

The lower the emissivity,

  • the higher the level of the reflected infrared radiation,
  • more difficult to carry out an accurate measurement of temperature and
  • the more important becomes the correct setting reflected temperature compensation (CTO).

The relationship between the radiation and reflection.

A. Measurement objects with a high emissivity (ε ≥ 0.8):

  • have a low reflection coefficient (ρ): = ρ = 1-ε.
  • The temperature of these objects can be easily measured using a thermal imager.

Two. Measurement of objects with an average emission factor (0.8 <ε <0.6):

  • have an average reflection coefficient (ρ): ρ = 1-ε.
  • The temperature of these objects can be easily measured using the thermal imager.

Three. Measurement objects with low emissivity (ε ≤ 0.6)

  • have a high reflection coefficient (r): r = 1-ε.
  • Measurement of temperature by the thermal imager is possible, but you need to carefully examine the results.
  • It is important to follow the correct setting reflected temperature compensation (CTO), since this is a major factor in the calculation of the temperature.

Proper adjustment of the coefficient of radiation is critical if a significant difference between the temperature measurement object and the operating temperature of the environment.

  • When the temperature of the object to be measured above ambient temperature:
  • Extremely high coefficient of radiation will lead to excessive temperature readings.
  • Extremely low coefficient of radiation will lead to too low temperature readings.
  • When the temperature of the object to be measured below the ambient temperature:
  • Extremely high coefficient of radiation will lead to too low temperature readings.
  • Extremely low coefficient of radiation will lead to inflated values of the temperature.

1.2 The point of measurement and the distance to the object to be measured

There are three variables that must be considered when determining the optimal distance to the object to be measured and the maximum of the visible and the object to be measured:

  • field of view (FOV);
  • the smallest visible object (IFOVgeo) and
  • the smallest measured object / point measured (IFOVmeas).

Emmisivity measurement

In Fig. 1.2: The influence of incorrect settings on the emissivity measurements of temperature

Note: the larger the temperature difference between the measured object and the ambient temperature and the lower the emissivity, the more likely the occurrence of errors. The number of errors increases if the emissivity is incorrect.

  • With the help of the thermal imager, you can only measure the temperature of surfaces using this instrument, it is impossible to look inside the facility or to see through it.
  • Despite the fact that many materials such as glass, transparent, seem to us, they behave as materials are transmissive, ie resistant to long-wavelength infrared materials
  • If necessary, remove the object to be measured pouch / packaging, as in the presence of the latter measure the surface temperature of the thermal cover / packaging.

Please note:

Always follow the operating instructions with respect to the measured object!

  • Certain transmissive materials include, for example, thin plastic or germanium - the material of construction and a protective lens Lens Imager Testo.
  • If the components are located under the surface affect the temperature distribution over the surface of the object to be measured through the conductivity structure of the internal design of the measurement object can often be considered for IK-izobrazhenii/termogramme received. However, the imager can measure only the surface temperature. The exact definition of the temperature inside of the object using the thermal imager can not be implemented.

Field of view

In Fig. 1.3: The field of view thermal imager

The field of view (FOV) is an area imager, a visible imager. The dimensions of the area defined by the lens used with a thermal imager.Moreover, you need to know specifications of the smallest visible object (IFOVgeo) your thermal imager. With this pixel size is determined depending on the spatial resolution of the lens rasstoyaniya.S 3.5 mrad and the distance to the object to be measured 1 m, the smallest visible object (IFOVgeo) side of the pixel is equal to 3.5 mm and displayed as a 1 - th pixel. To obtain accurate measurement results measured object should be 2-3 times greater than the smallest visible object (IFOVgeo). Consequently, the following rule of thumb applies to the lowest measured object (IFOVmeas):

IFOVmeas ≈ 3x IFOVgeo

To increase the field of view to use a wide angle lens.

 
 
 

 

The thermal imager testo 870 was specially developed for your applications, in cooperation with heating constructors, building contractors, service engineers and facility management specialists – for example for detecting leakages, localizing cold bridges or visualizing overheated connections.