Microbolometer

A microbolometer is a miniaturized bolometer in which absorbed electromagnetic radiation heats a thermally isolated sensing element and the readout detects the resulting change in electrical resistance. Arrays of microbolometers can form an infrared image without cryogenic detector cooling. A pixel commonly has an absorbing membrane suspended above its readout circuit on narrow supports.^[ref]

An elementary small-signal heat transfer model writesHere $C$ is the sensing element's heat capacity, $G$ its thermal conductance to a fixed-temperature substrate $T_b$, $P$ incident radiant power and $\eta$ its absorbed fraction. The model treats the element as having a single temperature and neglects changes in readout self-heating. A step to constant incident power $P_0$ givesThe thermal time constant therefore connects sensitivity to response speed. Lowering $G$ increases the steady temperature rise, but at fixed $C$ also lengthens the response.^[ref]

For an original example, choose $C=10^{-9}\mathrm{J}{\mathrm{K}}^{-1}$, $G=10^{-7}\mathrm{W}{\mathrm{K}}^{-1}$, $\eta =0.8$ and $P_0=1\mathrm{nW}$. Then $\tau =10\mathrm{ms}$ and the final sensing-element rise is $8\mathrm{mK}$; after $30\mathrm{ms}$ it has reached about $95\%$ of that rise. If the initial resistance is $100\mathrm{k\Omega }$ and the temperature coefficient of resistance is $\alpha =-0.02{\mathrm{K}}^{-1}$, the linear approximation $\Delta R\approx \alpha R\theta$ gives $\Delta R=-16\Omega$ at steady state. These stipulated values illustrate the model, not a particular camera's specification.

The sensing element's temperature rise is not the observed object's temperature rise. Inferring object temperature from the measured infrared signal additionally depends on emissivity, reflected radiation, the optical path and measurement calibration. “Uncooled” also permits temperature stabilization of the array; it describes the absence of required cryogenic operation, not the absence of thermal design.