Bolometer

Bolmeter in "spider tea." to measure cosmic microwave radiation (Image: NASA/JPL-Caltech)

A bolometer (from the Greek βολόμετρον or bolometron; βολο- means thrown object; and -μετρον, meter) is an instrument that measures the total amount of electromagnetic radiation that comes from an object at all wavelengths. It works by measuring the temperature of a detector illuminated by the source to be studied. The bolometer was invented by the American astronomer Samuel Pierpont Langley around the year 1878. With it he studied the infrared radiation of the Sun. The bolometric magnitude of a star can be defined as its luminosity in the entire electromagnetic spectrum.

Langley Bolometer

The first bolometer devised by American aviation pioneer and inventor Samuel Pierpont Langley consisted of two strips of platinum coated with carbon black. One of the strips was shielded from radiation and the other exposed to it. The strips formed the two legs of a Wheatstone bridge, equipped with a highly sensitive galvanometer connected to a battery. Electromagnetic radiation heats the exposed strip on which it impinges and changes its resistance. In 1880, Langley's bolometer was sufficiently refined to "detect thermal radiation from a cow at a distance of a quarter of a mile". This instrument enabled him to thermally detect a wide spectrum of radiations, taking into account all the patterns of the Fraunhofer lines. He also discovered new atomic and molecular absorption lines in the invisible infrared part of the electromagnetic spectrum. Nikola Tesla personally applied to Langley for permission to use his bolometer in his energy transmission experiments in 1892. Thanks to this first application, a demonstration of the device was arranged between West Point and his laboratory on Houston Street.

Operation

A bolometer consists of a heat absorbing body connected to a heat sink (an object kept at a constant temperature) through an insulating material. The result is that any radiation absorbed by the detector increases its temperature above that of the reference heat sink. The absorbed radiation is therefore measured from the temperature contrast between the detector and the reference. In some bolometers the thermometer also acts as an absorber, while in others the thermometer and detector are different devices. These types of bolometers are called composite design. In bolometers of the first type, the temperature is measured by means of the variation of the resistance of the absorbent (metal) as a function of its temperature.

Although they can be used to measure the intensity of any type of electromagnetic radiation, there are currently more sensitive devices in most of the light spectrum. However, at sub-millimetre wavelengths (wavelengths around 200 µm - 1 mm), bolometers remain the most sensitive detection devices. At these wavelengths, bolometers are used that must be cooled to temperatures fractions of a degree above absolute zero, typically between 50 and 300 millikelvin. For this reason its use is technically very complex.