The radiation recorded by a thermal imager consists of the emission, transmission and reflection of infrared radiation which is emitted from objects in the surroundings of the thermal imager.
Emission, reflection and transmission
Transmission (t)
Transmission is the ability of a material to allow IR radiation to pass through it (transmit). A thin plastic sheet, for example, has a very high transmissivity – meaning that if one wants to use a thermal imager to record the temperature of a thin plastic sheet hanging in front of a house wall, one measures the temperature of the wall and not that of the sheet. Most materials do not allow IR radiation to pass through, so that the degree of transmissivity of a material is as a rule almost 0, and can thus be neglected.
Emission (ε)
Emission is the ability of a material to emit IR radiation. This ability is expressed in the level of emissivity. It depends on, among other things, the material itself and its surface structure. The sun, for example, has an emissivity of 100 %. However, this value never otherwise occurs. Concrete, on the other hand, is close, with an emissivity of 93 %. This means that 93 % of the IR radiation is emitted by the concrete itself.
Reflection (ρ)
The other 7 % are reflections from the surroundings of the material / the object which one wishes to measure, i.e. the temperature which is reflected from the object. One can enter the degree of emissivity and the reflected temperature into a thermal imager in order to obtain as precise a thermal image as possible.
Connection between emission and reflection
Measurement objects with high emissivity (ε ≥ 0.8):
- have a low reflectivity (ρ): ρ = 1 - ε
- their temperature can be very well measured with a thermal imager
Measurement objects with medium emissivity (0.6 < ε < 0.8):
- have a medium reflectivity (ρ): ρ = 1 - ε
- their temperature can be well measured with a thermal imager
Measurement objects with low emissivity (ε ≥ 0.6):
- have a high reflectivity (ρ): ρ = 1 - ε
- temperature measurement with a thermal imager is possible, however you should critically question the results
- a correct setting of the reflected temperature compensation is indispensable, as it makes a large contribution to the temperature calculation
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