A spiderweb bolometer for measurements of the cosmic microwave background radiation. Image credit: NASA/JPL-Caltech.
A bolometer is an instrument used to measure radiant energy, usually in the infrared and microwave regions of the electromagnetic spectrum. It comprises a lens or stop system which focuses the test radiation onto a thermoconductive device (usually a thermistor), which is set in a Wheatstone bridge circuit with another non-illuminated reference thermistor. Sensitive bolometers are used in conjunction with spectroscopes to measure the intensities of spectral lines.