Known for its accurate and sensitive readings regarding heat, light, and electromagnetic radiation, the bolometer has a number of applications in many industries, such as the military and space exploration. In this article, we will look at exactly what a bolometer measures, how it works, and what it is used for.
Did You Know?
Although bolometers are one of the most heat- and light-sensitive devices known to man, complete enclosures are cooled in liquid nitrogen or hydrogen to obtain the highest degree of sensitivity.
What is a Bolometer?
First invented by an American astronomer named Samuel Pierpont Langley in 1878, bolometers are extremely sensitive instruments which help measure heat or electromagnetic radiation. The device invented by Samuel Langley was used solely along with telescopes to measure infrared radiation from astronomical objects. It was made of two strips of lamp-black coated platinum, which connected to a galvanometer and a battery. One strip of platinum was shielded from radiation, while the other was not. Comparing the feedback of the radiation hitting the two strips led to accurate measurements.
Newer versions were developed in 1880, which were able to detect thermal radiation along with electromagnetic radiation. Modern devices use absorbent elements made out of superconductors or semiconductors, rather than metals, giving better responsivity, and can detect changes in temperature as little as 0.0001°C.
- Unlike other particle detectors, bolometers do not need to be cooled, and can function effectively even at room temperature.
- In addition to photons and ionizing particles, they can also measure non-ionizing particles.
- Compared to other particle detectors, these instruments are very sensitive, and give extremely accurate measurements of energy resolution.
- Because the sensors do not need to be heated or cooled to be operated, it gives a fast response, and can take sharp images of even moving objects.
- Microbolometers consume very less power during operation.
- They are also quite cheap to manufacture, making operations cost-effective.
As you can see in the image, the instrument consists of an absorbent metallic element shaped in a thin layer. This layer is then connected to a heat reservoir through a thermal conductance link.
Whenever radiation/heat hits the absorbent element, the power is absorbed, and the overall temperature of the element increases, heating up the thermal mass substrate. Through the connection between the substrate and the heat reservoir, the measure of the change in temperature is noted in the resistive thermometer. The calculation of the thermal input is equal to the ratio between the constant temperature of the heat reservoir and the variable temperature of the absorbent element.
These devices are specifically built as a detector inside thermal/IR cameras. They consist of grids of heat sensors, made with vanadium oxide. These grids are built over corresponding silicon grids. Whenever infrared radiation hits these sensors, the electric resistance of vanadium oxide changes. These changes can be measured and displayed graphically as temperatures. Depending upon the size of the grid, the camera’s field of view increases or decreases.
Hot Electron Bolometer
This is another type of bolometer, which is usually operated at extremely cold temperatures, i.e., approximately a few degrees near absolute zero. At these low temperatures, the electrons of the absorbent elements are very weak in their bonds to the molecules. Therefore, when heat/radiation power hits the element, the electrons get driven out of thermal equilibrium, causing the formation of hot electrons. Since the molecules of the metal element have strong bonds with the thermal substrate, it also acts as a heat reservoir. If the absorbing element’s resistance depends on the temperature of the electrons, this resistance can be used to calculate the change in temperature/radiation. However, in some cases, the resistance does not depend on the electron’s temperature. Therefore, current- or voltage-based resistive thermometers are used. They give very accurate readings due to the low temperature of operation.
- Thermal cameras use microbolometers to get accurate infrared images.
- Particle detectors also use these instruments to calculate radiation levels.
- Security services, such as the police and military, use varied equipment to detect concealed weapons. Most of these devices operate through the use of bolometers.
- Some types of fingerprint scanners use this device to detect the changes in reflected light, which is used in the identification of fingerprints.
- Air surveillance with manned aircraft or drones use thermal cameras with this equipment to survey concealed areas.
- Forest departments use bolometers to detect and study forest fires, which helps in planning extinguishing measures and evacuations.
- Agencies such as NASA and ESA, which are involved in astronomical studies, make use of these equipment in their observation devices to study outer space in the far-infrared spectrum.
- Bolometers can also measure microwaves that are emitted by a pulsing power source.
As we can see, bolometers are essential devices across numerous industries, and play an important role in our day-to-day lives. With further advances in technology, newer models will be able to provide readings, with reduced complexity, power consumption, and cost.