What is the thermal time constant?

The thermal time constant τth of a bolometer is determined by the thermal mass C and by the thermal conductance G between the pixel and its environment. It expresses the physical time a bolometer needs to heat up and give an electrical output that equals or represents the input. Typical values for A-Si are between 7 and 10ms.

The rate of heating up or cooling down is expressed by (1-e-t/τth). This means that after 5 times the duration of the thermal time constant the output signal represents 99.9% of the input signal. After 1 thermal time constant this is 63.2%. When the camera is used to measure absolute temperatures typically 3 times the duration of the thermal time constant is needed to represent the input signal. This explains the physical limitation of the radiometric calibration speed (3x7ms = 21ms -> 50FPS).

Note that some applications will use bolometers with a radiometric high speed calibration (+50FPS). The reliability of the measurement will depend on the amount of time the pixel is exposed to the object. If the pixel is exposed for a smaller duration than 21ms then the accuracy of the measurement will drop. The only alternative for high speed thermal measurements is jumping to a MWIR/LWIR cooled photodetector, prices are typically 7 times higher. In terms of market perspective the higher speed thermal calibrations for bolometers exist to fill in a gap between the huge price differences of the two technologies.

Thermal time constant

Thermal Imagers in mobile systems for Security & Safety Market

It was a challenge to produce a mobile system for border control in just 6 months, but with Xenics as a partner we could apply for this project.

Ivica Mijić, Audio-Video Trend