Figure 7-2. Bolometer circuit.

TM 9-258

g. Bolometers may be of three types: metal,

associated with low temperature operation and precise

temperature control are severe.

semiconductor, and superconductor.

Metal and

h. The radiation thermocouple was one of the

semiconductor bolometers are operated at ambient

earliest infrared detectors. It consists of a junction of two

temperatures, whereas the superconducting bolometer

dissimilar metals.

Absorbed radiation causes the

must be cooled to temperatures near absolute zero. A

junction temperature to rise, and the heating of the

form of metal bolometer used in microwave work

junction generates a small flow of electric current. This

consisting of an encapsulated platinum wire is known as

action is responsible for generating a voltage which is

a barretter; semiconductor bolometers are known as

proportional to the temperature rise and, therefore, is

thermistor bolometers, thermistor denoting thermally

proportional to the intensity of the radiation. A widely

sensitive resistors. Unlike a barretter the resistance of a

used form of the thermocouple is the radiation

thermistor drops as its temperature rises.

This

thermopile; it consists of a parrellel array of

characteristic has caused semiconductors to be much

thermocouples. The thermopile has a higher response

more widely used than metals from bolometers. A

than the thermocouple because of the use of multiple

superconducting bolometer has the advantage over

junctions. However, its response time is long and it is not

thermistor and metal bolometers of reduced thermal

suitable for ac amplification techniques. The fragile

noise, reduced heat capacity, and a step resistance-

construction makes it of little use in applications where it

temperature curve. On the other hand, the problems

would be subject to vibration and shock.

7-3