Non-contact infrared radiation thermometers
All objects emit thermal radiation.
Radiation thermometers work like cameras, with an optical system (lenses or mirrors) and a filter which selects the wavelength range (waveband) over which the thermometer is sensitive and focuses radiation onto a detector whose output indicates the intensity of the radiation and hence the temperature.
Detector could be:
- 'Photo-detector' - incident photons give rise to an electric current
- 'Thermal' - senses the temperature rise produced by the energy absorbed
A detector is temperature-controlled to make its response more repeatable. For low temperature applications, it must be cooled to improve the signal-to-noise ratio and to reduce the intensity of its own radiation.
Temperatures can be measured remotely using this technique, with the sensor situated some distance away from the object. Hence it is useful for objects that are very hot, moving or in hazardous environments.
The amount of radiation emitted can be measured and related to temperature using the Planck law of radiation.
Planck law of radiation
Planck law
The intensity, Lλ(T), of a perfect radiator (a blackbody) is given by the Planck law and depends only on the temperature T (in kelvin) and the wavelength of the radiation λ (in micrometres). The figure shows that the intensity rises to a peak and then falls off as the wavelength increases.
At low temperatures the intensity is low and entirely in the infrared (λ > 0.7 μm), but as the temperature increases, the intensity of the radiation rapidly increases and the peak moves toward shorter wavelengths. The sensitivity is then very high, roughly exponential, but at longer wavelengths the lines are bunched closer together and so the sensitivity is lower.
Planck curves
The intensity of thermal radiation versus wavelength at various temperatures. The dashed line indicates the wavelength range of visible radiation. Note that the intensity (radiance) is plotted as a logarithm, so each division on the y-axis marks an increase by a factor of ten.
5 of 11