What Are Bolometers?

Bolometers are thermal radiation detectors that measure incident power by sensing the temperature rise a radiation-absorbing element undergoes when it absorbs electromagnetic energy. The temperature rise is read out through a thermistor or other temperature-sensitive element whose electrical resistance changes as a function of heat input, producing a measurable signal proportional to the absorbed radiation. Unlike photonic detectors, which require photon energies above a bandgap threshold, bolometers respond to radiation across a broad spectral range, from millimeter waves through the far-infrared to X-rays, making them versatile instruments in physics, astronomy, and engineering.

The device takes its name from the Greek word for beam, bole, and was first demonstrated by Samuel Pierpont Langley in 1880. Modern versions bear little resemblance to Langley's platinum strips but share the same principle: a thermally isolated absorber coupled to a resistance thermometer. The two critical performance parameters are responsivity, the output signal per unit of incident power, and noise equivalent power (NEP), the minimum detectable signal set by thermal and electrical noise floors.

Operating Principle and Detector Materials

A bolometer consists of three functional parts: a radiation absorber, a thermal link to a heat sink that sets the detector's thermal time constant, and a thermometer that reads the absorber temperature. The absorber must have high emissivity across the spectral band of interest, while the thermometer material must have a large temperature coefficient of resistance (TCR) to convert small temperature changes into large resistance changes. Vanadium oxide (VOx) and amorphous silicon (a-Si) are the two dominant materials in commercial uncooled devices because both offer TCR values near -2 to -3 percent per kelvin at room temperature. Cooled bolometers, operating at cryogenic temperatures approaching 0.1 K, use superconducting transition-edge sensors (TES) that exploit the sharp resistance transition at the superconducting critical temperature to achieve NEP values orders of magnitude below those of room-temperature devices. The NIST high-accuracy infrared spectrophotometer bolometer system uses a silicon composite bolometer with a 3 mm diameter absorber thermally coupled to a doped-silicon thermistor, illustrating the precision achievable in metrological applications.

Microbolometer Focal Plane Arrays

The most commercially significant bolometer format is the uncooled microbolometer infrared focal plane array (IRFPA), in which tens of thousands of individual bolometer pixels are fabricated on a single silicon substrate using MEMS processes. Each pixel is a membrane suspended by thin support legs that thermally isolate it from the substrate, maximizing temperature rise per unit of absorbed power. The membrane carries a thin VOx or a-Si thermistor film connected to readout circuitry through the support legs. A review of low-cost microbolometer type infrared detectors published in PMC describes pixel pitches that have shrunk from 50 micrometers in early arrays to 12 micrometers in current devices, enabling higher-resolution cameras in smaller packages. Because they require no cryogenic cooling, microbolometer arrays are compact, low-power, and manufacturable at volumes that support consumer and military markets alike.

Readout and Signal Processing

The readout integrated circuit (ROIC) bonded beneath the detector array performs bias control, integration of the resistance signal, analog-to-digital conversion, and non-uniformity correction for each pixel. Fixed-pattern noise from pixel-to-pixel resistance variations is the primary image artifact in microbolometer cameras and must be suppressed through two-point correction algorithms applied to stored calibration data. Readout IC architectures for uncooled microbolometer focal plane arrays surveys resistance-based, current-based, and bridge-circuit readout schemes and their tradeoffs in power, dynamic range, and noise performance at the pixel level.

Applications

Bolometers have applications in a range of fields, including:

• Thermal imaging cameras for security surveillance, firefighting, and industrial inspection
• Astronomical observatories detecting cosmic microwave background radiation and far-infrared sources
• Spectroscopy and radiometry for measuring broadband infrared power with traceable accuracy
• Medical thermography for non-contact body temperature screening
• Automotive night-vision systems that extend driver visibility beyond headlight range