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.
The performance of a bolometer is measured in terms of its responsivity, or the electric signal generated per unit small change in incident radiation; its response time; its noise equivalent power, essentially the minimum detectable signal; and the spectral range over which the sensitive element produces a signal. Characteristics important for determining these quantities are the temperature coefficient of the sensitive element (how strongly its electrical properties change with temperature); the background electrical noise produced by the system; the thermal capacity of the element; and the thermal conductance between the element and a heat sink. These parameters can be controlled or improved by such means as encapsulating the element in a vacuum space, restricting its size, and cooling it to low temperature, such as to the boiling point of liquid helium, 4.2 K (−452.1°F), and below.
The bolometer was invented in 1880 by S. P. Langley, who used a thin, blackened platinum strip as the sensitive element. Similar strips or fine wires are still used, but demands for improved performance have led to the deployment of materials with increased temperature coefficients, mainly semiconductors, such as carbon; mixed oxides of nickel, cobalt, and manganese (thermistor materials); germanium doped with gallium or indium; and indium antimonide. A class of bolometers incorporating a capacitive element uses thin films of materials with temperature-dependent electrical polarization properties. See also: Semiconductor; Thermistor
Langley used his bolometer to measure the spectral distribution of solar radiation in the infrared region. Although bolometers are useful in studying a variety of systems where detection of small amounts of heat is important, their main application is in 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 also: Infrared astronomy; Infrared radiation
Bolometers are also used to detect and measure microwave energy or power. A bolometer constructed for this purpose is contained in a mount that protects the usually fragile element, guides the microwave energy to the bolometer, and has connection terminals for measuring the bolometer resistance. The bolometer resistance can be measured by using direct current or low-frequency instruments. By varying the direct-current or low-frequency power, the resistance of the bolometer can be adjusted so that most of the microwave power incident upon it will be absorbed. The direct current used to measure the resistance also dissipates power in the bolometer. Therefore, when the microwave power is applied, the direct-current or low-frequency power must be reduced to keep the bolometer resistance constant. The reduction of low-frequency power is a measure of the microwave power. The ratio of the low-frequency power change to the microwave power absorbed is the effective efficiency of the bolometer mount. The ratio of the low-frequency power change to the microwave power incident upon the bolometer is called the calibration factor of the bolometer mount.
A self-adjusting current loop (see illustration) can be designed to adjust the current that flows through a four-terminal resistor and a four-terminal bolometer so that the bolometer resistance is the same as that of the resistor. The power dissipated in the bolometer can be calculated from the voltage measured across the resistor.
Bolometers for measurement of microwave power are usually constructed by using one of four thermal-sensitive elements, as described below.
Fine metal wires
These bolometers are called barretters. They have positive temperature coefficients of resistance, are linear and stable, and are used for the most accurate measurements. Barretters are easily burned out and therefore are not used for routine measurements.
These bolometers have negative temperature coefficients of resistance and are called thermistors. Thermistors are nonlinear and not as stable as barretters, but with proper techniques they can be used for accurate measurements. They are not as easily burned out as barretters and are therefore used extensively for microwave power measurements.
Thin metal films
Resistive-film bolometers are made by depositing a very thin metal film on a nonconducting substratum, and have characteristics similar to those of barretters. The microwave power dissipated in the resistance film also heats the substratum, and so these bolometers are not as sensitive or as subject to burnout as barretters. Resistive-film bolometers can be fabricated in special configurations to better fit the bolometer mount.
Thin sheets of ferroelectric material can be metallized on both sides to form a capacitor. These are called pyroelectric bolometers. The dielectric constant and the polarization of ferroelectric materials change with temperature, resulting in a change in capacity and a small current flow or voltage change. The metallized surfaces serve as electrodes and also as resistors to dissipate microwave power. Pyroelectric bolometers also may be fabricated in special shapes and have been used for pulse-power and millimeter measurements, but they are not common. See also: Ferroelectrics; Microwave power measurement; Pyroelectricity; Radiometry