Bolometer
A bolometer , also known as a bolometric detector ( Greek bole , “throw”, “beam”), is a radiation sensor that measures the entire wavelength spectrum of electromagnetic waves . The measuring principle is absorption and heating.
The inventor was the American astronomer Samuel Pierpont Langley in 1878.
Blackened surfaces are used for absorption in the optical range. The temperature rise is evaluated for the measurement of power pulses. Must for the continuous measurement of the radiation power of heat to be dissipated. Defined thermal bridges are used for this, which are made of solid metal for the measurement of high powers and are water-cooled at the other end. The measurand is the heat flow . This results from the temperature difference across the thermal bridge divided by its thermal resistance .
Because of their simplicity, thermistors can be used as temperature sensors . The imaging microbolometer arrays are NTCs used as these a steeper curve have, but connected only in a relatively narrow temperature range and small construction with significant shot noise .
The alternative are thermocouples that measure temperature differences directly . For a higher thermal voltage, several thermocouples are connected electrically in series , but thermally in parallel. Such a thermopile itself often represents the entire thermal bridge.
This also applies to the most sensitive detectors consisting of just a single thermocouple made of very thin wires or thin-film structures . If - in contrast to the pyrgeometer - the radiation to be measured can be focused on one contact of the thermocouple, there is no need for a separate absorber surface that would have to be very thin and very good heat-conducting for short reaction times , e.g. B. a gold foil.
The inclusion in a vacuum reduces external disturbances through heat transfer to air or convection . The temperature of the surrounding parts of the detector must be precisely controlled, gold-plated surfaces emit less heat radiation . The highest sensitivity is achieved with frozen bolometers.
After Bolometerprinzip also often work motion detectors used pyroelectric infrared detectors . The radiation is converted into heat by means of an absorption layer applied to a thin pyroelectric crystal. This creates electrical charges in the crystal, which after amplification deliver the detector signal. The crystal, for example made of lithium tantalate , is mounted in such a way that a defined heat flow is ensured. In contrast to thermopiles, these detectors only provide a signal when there is a change in radiation. Another advantage is the higher level of detectivity. There are more demanding applications for the detectors in medical technology and technical safety: gas measurement technology based on the NDIR principle and flame detection .
See also
Web links
- Bolometer. Entry in the spectrum lexicon of optics.
- Brief description in a historical lexicon
Individual evidence
- ↑ FAQ on pyroelectric detectors. InfraTec GmbH infrared sensors and measurement technology, accessed on June 4, 2018 .