Bolometer

A bolometer is a thermal detector: an absorber of heat capacity $C$ is connected to a heat sink through a weak thermal link of conductance $G$, and a thermometer on the absorber, usually a resistor with a large temperature coefficient, reads its temperature. Absorbed power $P$ raises the temperature by $\Delta T = P/G$, and the resistance changes by $\Delta R/R = \alpha\,\Delta T$, where $\alpha$ is the temperature coefficient of resistance (TCR). Because the response depends only on absorbed heat, a bolometer covers any wavelength its absorber can take in, from the visible to the millimetre band. Uncooled vanadium-oxide or amorphous-silicon microbolometers, with TCRs of roughly 2–3 %/K in magnitude and pixel pitches of 10–17 µm, form the arrays in most thermal cameras for the 8–14 µm long-wave infrared. Samuel Langley built the first one around 1880 to measure the solar spectrum.

Response and time constant

The absorber reaches equilibrium with a thermal time constant

$$\tau = \frac{C}{G},$$

and for a modulated input at angular frequency $\omega$ the voltage responsivity of a current-biased bolometer is

$$\mathcal{R}_V = \frac{\eta\,\alpha\,I_b R}{G\sqrt{1+\omega^2\tau^2}},$$

where $\eta$ is the absorptance and $I_b$ the bias current (electrothermal feedback is neglected here). A small $G$ gives high responsivity but a slow detector, since $\tau$ grows as $G$ falls; the design trade is between these two.

A worked example with microbolometer-scale numbers: $C = 10^{-9}$ J/K and $G = 10^{-7}$ W/K give $\tau = 10$ ms and a 3 dB frequency $1/(2\pi\tau)$ of 15.9 Hz. One nanowatt absorbed raises the temperature by 10 mK. With $\alpha = -0.02$ K⁻¹ that is a fractional resistance change of 2 × 10⁻⁴; a 100 kΩ element biased at 10 µA (1 V across it) then produces a 0.2 mV signal. The small $G$ is achieved by suspending the absorber on thin legs above the readout circuit and packaging the array in vacuum, because air conduction would otherwise dominate the heat loss.

Noise and sensitivity

The fundamental limit for a thermal detector is the fluctuation of energy across the thermal link, which sets a noise-equivalent power of

$$\mathrm{NEP}_{\mathrm{th}} = \sqrt{4 k_B T^2 G}.$$

At 300 K with $G = 10^{-7}$ W/K this is 7.1 × 10⁻¹³ W/√Hz. Johnson noise of the resistor, described under thermal noise, and 1/f noise of the thermistor film add to it, and in practice the readout and 1/f noise usually dominate in uncooled arrays. The same formula explains why astronomical bolometers are cooled: a superconducting transition-edge sensor at 0.1 K with $G = 10^{-10}$ W/K has a thermal-fluctuation NEP of 7.4 × 10⁻¹⁸ W/√Hz, five orders of magnitude lower. Thermal cameras quote sensitivity instead as noise-equivalent temperature difference (NETD), the scene temperature change that gives a signal equal to the noise; values below 50 mK at f/1 are typical of current uncooled cameras. Comparison between detectors of different area uses the specific detectivity.

Where bolometers are used

Uncooled microbolometer arrays dominate thermal imaging for building inspection, industrial monitoring, firefighting and automotive night vision, because they need no cryocooler and cost far less than cooled HgCdTe cameras. Cooled semiconductor and superconducting bolometers are the standard detectors for far-infrared and millimetre-wave astronomy, including measurements of the cosmic microwave background, whose spectrum is a blackbody at 2.725 K. Single-element bolometers also serve in broadband radiometers and in terahertz spectroscopy.

Pitfalls

Bias current heats the element, and because $R$ depends on $T$ this creates electrothermal feedback: for a negative-TCR material under voltage bias it can run away at high bias, while a transition-edge sensor, with positive TCR under voltage bias, is stabilized by it and uses it deliberately. The response is slow, so microbolometer images smear moving scenes at frame rates of 30–60 Hz. Absorptance varies with wavelength unless a dedicated absorber layer or resonant cavity is used, and a window on the vacuum package limits the band further. Uncooled arrays drift with ambient temperature and need periodic shutter-based non-uniformity correction.

Common questions

What is the difference between a bolometer and a thermopile?

Both are thermal detectors. A thermopile reads the temperature rise with thermocouples and generates its own voltage with no bias, which makes it simple and stable for power measurement; a bolometer reads it as a resistance change under bias, which gives higher responsivity and suits dense arrays. A pyroelectric detector, the third common type, responds only to changes in temperature and so needs a chopped or pulsed input.

Does a microbolometer need cooling?

No cryogenic cooling is needed; the array operates near room temperature, often with a thermoelectric stabilizer or with software correction for ambient drift. Cooling improves the fundamental noise limit, which is why research bolometers run at liquid-helium or sub-kelvin temperatures.

What wavelengths can a bolometer detect?

Any wavelength its absorber absorbs. Microbolometer cameras are built for 8–14 µm because a room-temperature scene emits most strongly there and the atmosphere is transparent; bolometers for astronomy extend to millimetre wavelengths with antenna-coupled or mesh absorbers.

References: P. L. Richards, "Bolometers for infrared and millimeter waves," J. Appl. Phys. 76, 1 (1994); A. Rogalski, Infrared and Terahertz Detectors, 3rd ed. (CRC Press, 2019); E. L. Dereniak, G. D. Boreman, Infrared Detectors and Systems (Wiley, 1996).