How does a bolometer work


Concept drawing of a bolometer.

A Bolometer, also bolometric detector (Greekbole "Throw", "beam") is a radiation sensor for measuring over 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. The heat must be dissipated for continuous measurement of the radiant power. Defined thermal bridges are used for this, which are made of solid metal for the measurement of high outputs and are water-cooled at the other end. The measured variable 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. NTCs are used in the imaging microbolometer arrays because they have a steeper characteristic curve, but only in a relatively narrow temperature range and in a small design combined with considerable shot noise.

The alternative are thermocouples that directly measure temperaturedifferences measure up. 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 - unlike the pyrgeometer - the radiation to be measured can be focused on one contact of the thermocouple, a separate absorber surface is not required. B. a gold foil.

The inclusion in a vacuum reduces external disturbances through heat transfer to air or convection. The temperature of surrounding parts of the detector must be precisely controlled, gold-plated surfaces emit less heat radiation. The highest sensitivity is achieved with frozen bolometers.

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