Time Constant of Bolometers

The thermal time constant of bolometers measures how quickly the bolometer responds to changes in incident infrared radiation. It indicates the speed at which the bolometer detects and responds to temperature changes in the observed scene. Specifically, the thermal time constant is the time it takes for the bolometer to reach approximately 63% (1 – 1/e) of its final temperature when exposed to a step change in incident radiation. This time constant is related to the exposure time.

A shorter thermal time constant means the bolometer responds faster to temperature changes, making it more suitable for applications requiring rapid thermal measurements. On the other hand, a longer thermal time constant implies a slower response, which might be beneficial in applications where signal stability and noise reduction are more critical than response speed. In practical terms, the thermal time constant is influenced by the materials and design of the bolometer. For instance, microbolometers used in uncooled infrared cameras are designed with materials and structures that optimize heat capacity and thermal conductance to achieve a balance between sensitivity and response time.

Amorphous silicon (a:Si) as a bolometer material usually has a time constant of around 7ms, while vanadium oxide (VOx) has a time constant of around 15ms. According to the Nyquist-Shannon sampling theorem, a signal must be sampled at a rate greater than twice its maximum frequency. In practice, oversampling should be done to avoid aliasing effects.

The maximum permissible scene frequency is half of the infrared camera’s frame rate. For instance, if the infrared camera runs at 32 Hz, the scene frequency should be slower than 16 Hz. In this case, the time constant of the bolometer material is much shorter than the illumination time of 62ms.