bolometer

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bolometer

(bōlŏm`ətər, bə–), instrument for detecting and measuring radiationradiation
, term applied to the emission and transmission of energy through space or through a material medium and also to the radiated energy itself. In its widest sense the term includes electromagnetic, acoustic, and particle radiation, and all forms of ionizing radiation.
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, e.g., visible lightlight,
visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.
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, infrared radiationinfrared radiation,
electromagnetic radiation having a wavelength in the range from c.75 × 10−6 cm to c.100,000 × 10−6 cm (0.000075–0.1 cm).
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, and ultraviolet radiationultraviolet radiation,
invisible electromagnetic radiation between visible violet light and X rays; it ranges in wavelength from about 400 to 4 nanometers and in frequency from about 1015 to 1017 hertz.
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, in amounts as small as one millionth of an ergerg
, unit of work or energy in the cgs system of units, which is based on the metric system; it is the work done or energy expended by a force of 1 dyne acting through a distance of 1 centimeter.
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. The bolometer was invented in 1880 by Samuel P. Langley. Basically it consists of a radiation-sensitive resistance element in one branch of a Wheatstone bridge; changes in radiation cause changes in the electrical resistance of the element. The radiation-sensitive element may be a platinum strip, a semiconductor film, or any other substance whose resistance is altered by slight changes in the amount of radiant energy falling on it.

Bolometer

A device for detecting and measuring small amounts of thermal radiation. The bolometer is a simple electric circuit, the essential element of which is a slab of material with an electrical property, most often resistance, that changes with temperature. Typical operation involves absorption of radiant energy by the slab, producing a rise in the slab's temperature and thereby a change in its resistance. The electric circuit converts the resistance change to a voltage change, which then can be amplified and observed by various, usually conventional, instruments.

Although bolometers are useful in studying a variety of systems where detection of small amounts of heat is important, their primary application remains as the instrument of choice for measuring weak radiation signals in the infrared and far infrared, that is, at wavelengths from about 1 to 2000 micrometers, from stars and interstellar material. See Barretter, Infrared radiation, Radiometry, Thermistor

bolometer

(boh-lom -ĕ-ter) A radiation-sensing instrument that is used in astronomy to measure the total energy flux of radiation entering the Earth's atmosphere, and is especially useful in the infrared and microwave regions of the spectrum. It is essentially a small resistive element capable of absorbing electromagnetic radiation. The resulting temperature rise is a measure of the power absorbed. There is a large variety of bolometers, including semiconductor and supercooled devices; the semiconductor ones are normally of doped silicon or germanium. See also infrared detectors.

Bolometer

 

a device which measures radiant energy by measuring the change in electrical resistance of a heatsensitive element which has been heated up by the absorption of radiation. The bolometer is used for the measurement of the strength of integral (total) radiation and, along with the spectrometer, for the measurement of the spectral composition of radiation. The heatsensitive element is often a thin (0.1–1.0 micron) layer of metal (such as nickel, bismuth, or gold) whose surface is covered with a layer of black enamel, which has a high absorption coefficient in a broad range of wave lengths; the element may also be a semiconductor with a high temperature coefficient of resistance (0.04° C-1 to 0.06° C-1 and higher), or it may be a dielectric.

The size and shape of the bolometer’s sensitive element are determined by the nature of the radiation source. For spectral measurements this element is usually made in the form of two identical strips. The radiation is directed onto one element, and the other is used to compensate for the temperature variations of the surrounding medium and for radiation interference. In form the bolometer for spectral measurements is bellshaped. The bolometer is connected to an electrical bridge circuit, which is fed by a DC or AC source. Under the influence of a radiation flux, the temperature of the sensitive element changes by a certain value ΔT, which leads to a corresponding change of the bolometer’s resistance by ΔR. The change in resistance causes a change in current intensity in the electrical circuit of the bolometer, and across the load resistance there is a voltage drop which serves as a measure of the power of the radiation flux being measured.

The bolometer is characterized by a conversion ratio of radiant energy into electrical potential or sensitivity r, by a resistance R, by a threshold of bolometer sensitivity, and by a time constant τ which serves as a measure of the time to establish its stationary operating conditions during irradiation.

Metallic bolometers are usually used without cooling. They are made either of thin foils, or by the vacuum deposition of a metal on a thin film or solid base layer. The choice depends on the desired value of τ. The best bolometers are of nickelized foils having a resistance of 5–10 ohm, a sensitivity of 7–10 volts per watt (V/W); and a time constant of 0.02 sec.

Deposition bolometers are made of bismuth, antimony, and nickel by the vacuum vaporization of the metals onto a thin organic film. The most widely used of the metallic bolometers are those made of bismuth with the following parameters: resistance, 150–200 ohms; sensitivity, 13–15 V/W; time constant, 0.02 sec.

Semiconductor bolometers are also usually used without cooling. They are made by the pressing of oxides of nickel, manganese, and cobalt or by the vacuum vaporization of certain semiconductors onto a base layer. Their parameters are the following: resistance, 1–10 megohms; sensitivity, 50–1,000 V/W; time constant, 1–5 milliseconds (msec).

The most sensitive semiconductor bolometers, which are cooled to a very low temperature, are made of germanium alloyed with gallium. At a temperature of 2°–4° K, a collecting area of 10 mm2, and a resistance of 12,000 ohms, the sensitivity is 4.5 kilovolts per W, and the time constant is 0.4 msec.

Also very sensitive are superconducting bolometers with a sensitive element in the form of a thin wire which is at a transition temperature in a superconducting state and along which the measuring current flows. Under the influence of radiation, the resistance of the wire is partially restored, leading to an increase of the voltage drop. The best of these, tin bolometers, have a deposit of a thin mica base layer; they operate at a temperature of 3.7° K, which corresponds to the middle of the interval of tin’s transition from the normal to the superconducting state. The drawback of such bolometers is the necessity of maintaining the temperature with a high degree of accuracy and the difficulty of matching a bolometer with an amplifier.

Bolometers are widely used in technology as detectors of infrared radiation.

REFERENCES

Smith, R., F. Jones, and R. Chasmar. Obnaruzhenie i izmerenie infrakrasnogo izlucheniia. Moscow, 1959. (Translated from English.)
Teplovye priemniki izlucheniia. Kiev, 1967.

bolometer

[bə′läm·əd·ər]
(engineering)
An instrument that measures the energy of electromagnetic radiation in certain wavelength regions by utilizing the change in resistance of a thin conductor caused by the heating effect of the radiation. Also known as thermal detector.

Bolometer

A device for detecting and measuring small amounts of thermal radiation. The bolometer is a simple electric circuit, the essential element of which is a slab of material with an electrical property, most often resistance, that changes with temperature. Typical operation involves absorption of radiant energy by the slab, producing a rise in the slab's temperature and thereby a change in its resistance. The electric circuit converts the resistance change to a voltage change, which then can be amplified and observed by various, usually conventional, instruments.

Although bolometers are useful in studying a variety of systems where detection of small amounts of heat is important, their primary application remains as the instrument of choice for measuring weak radiation signals in the infrared and far infrared, that is, at wavelengths from about 1 to 2000 micrometers, from stars and interstellar material. See Thermistor