What is a Bolometer : Circuit & Its Working

The American scientist namely “Samuel P. Langley” was invented the first bolometer in the year 1880. Both the galvanometer as well as Wheatstone bridge is used to generate a deflection. Here generated deflection can be proportional to the radiation intensity used for tiny deflections. The next bolometer mainly includes 4-platinum gatings where each gate is designed with a sequence of strips. The arrangement of these strips can be done within the resistance bridge arms. These gratings are located in opposite to the bridge arms. So the bolometer device is used to measure radiation once the temperature of a black end metal strip raises in the resistance bridge. This article discusses an overview of a bolometer, working, circuit, advantages, and applications.

What is Bolometer?

Definition: An instrument that is used to detect as well as measure the microwave energy radiation & heat is known as a bolometer. This device works by using a temperature-sensitive resistive element where the resistance of this element will change through the temperature. The most frequently used resistive elements are Barretter and Thermistor. The speed, as well as the sensitivity of this device, can be changed by changing the thermal resistance among the bolometer as well as its environment. But, both the sensitivity and speed are inversely proportional in the direction of the thermal resistance. Consequently, sensitive bolometer is frequently slow.

Bolometer Working

A bolometer includes an absorptive part that is made up of a slight metal layer. The connection of this part can be done through a thermal reservoir with the help of a thermal link. Once radiation hits the absorptive part, then its temperature will be a change within the temperature. So compared with reservoir temperature, this temperature is high because of the radiation absorption using the absorptive part.

The thermal time constant of intrinsic can be equivalent to the heat capacity ratio among the absorptive element as well as the reservoir. Therefore, the temperature change is measured directly through a resistive thermometer that is connected to the absorptive part. Sometimes, the absorptive parts resistance is used for calculating the change in temperature.

Bolometer Circuit

The bolometer circuit diagram is shown below. The arrangement of this can be done in the bridge form, where one arm of this includes the temperature-sensitive resistor. The arrangement of this resistor can be done in a microwave energy field where the power can be measured.

Bolometer Circuit
Bolometer Circuit

This resistor absorbs the measurand power because the heat generates within it. This generated heat can change the resistance of an element. The change in resistance can be measured by the bridge circuit.


The construction of a bolometer can be done using the combination of a differential amplifier and oscillators. One the circuit is unbalanced then it will oscillate. The resistive element in the meter will absorb the power to get the circuit balanced. So the bridge circuit can be balanced by adjusting the dc bias.

The bolometer circuit can be arranged within the microwave field. So the radiation can be absorbed through the element to increase their temperature and causes a change in their resistance.

The inequality will occur in the reverse direction due to the cold resistance. So the oscillator output will be decreased by the imbalance to make the bridge circuit balance. The reduced power in the circuit can be measured by the electronic voltmeter so that it displays the increased power through the oscillator. This power can be absorbed in the microwave field through the resistive element.

The bolometer bridge mainly uses two elements which include the following.


Barretter is one kind of wire made with metal. This wire has a property that is a positive temperature coefficient. Once the temperature increases then the metallic wire temperature also increases.


A thermistor is one kind of thermal resistor that can be made with a semiconductor material. The main property of this is a negative temperature coefficient which means once the temperature increases then their resistance will be decreased.

So, barretter is a very sensitive metal wire as compared with a thermistor. This is frequently used to measure the power which ranges from 0.01 – 10mW. To measure the power which is above the 10mW, then the bolometer & the attenuator combination is used.

New Bolometer

The new bolometer devices are simple, faster, and also cover more wavelengths. These are designed under the conditions of laboratory and used for measuring the whole energy that is carried through the received electromagnetic radiation photons. This radiation comes from the distant galaxies and is in the form of radio waves, visible light, microwaves otherwise spectrum parts.

New bolometers are totally different as compared with traditional bolometers because they use metal for absorbing the radiation as well as to measure the increased temperature. There are some other bolometers which rely on the vibrations of atoms within a material to reduce its response


The main advantages of the bolometer include the following.

  • These instruments are very efficient in terms of resolution of energy and sensitivity as compared with other conservative particle detectors.
  • These instruments don’t need cooling because they function at room temperature.
  • They can also calculate non-ionizing elements, photons, and ionizing particles and photons.


The major applications of the bolometer include the following.

  • A bolometer is an extremely sensitive device used for detecting electromagnetic radiation or heat.
  • Emerging applications of this device are thermal imaging, scientific, monitoring of the remote environment, solar probes, and THz communication.
  • It is used in particle detectors, thermal cameras, scanners of fingerprint, forest fire detection, concealed weapons detection, air surveillance, and astronomical applications.

At present, modern bolometers are frequently used, because the platinum of the device can be replaced by a semiconductor strip. This device has a much high-temperature coefficient of resistance so that it makes the device more responsive.

Thus, this is all about an overview of a bolometer and an alternate name of this device is the calorimeter. This is one kind of detector that is mainly used for particles or radiation and also used for detecting light in mm waves and the far-infrared. Here is a question for you, what are the disadvantages of a bolometer?

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