This article was updated on the 24th July 2018.
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An infrared sensor is an electronic instrument that is used to sense certain characteristics of its surroundings. It does this by either emitting or detecting infrared radiation. Infrared sensors are also capable of measuring the heat being emitted by an object and detecting motion.
Infrared technology is found not just in industry, but also in every-day life. Televisions, for example, use an infrared detector to interpret the signals sent from a remote control. Passive Infrared sensors are used for motion detection systems, and LDR sensors are used for outdoor lighting systems. The key benefits of infrared sensors include their low power requirements, their simple circuitry and their portable features.
Infrared Radiation Theory
Infrared waves are not visible to the human eye. In the electromagnetic spectrum, infrared radiation can be found between the visible and microwave regions. The infrared waves typically have wavelengths between 0.75 and 1000µm.
The infrared spectrum can be split into near IR, mid IR and far IR. The wavelength region from 0.75 to 3µm is known as the near infrared region. The region between 3 and 6µm is known as the mid-infrared region, and infrared radiation which has a wavelength greater higher than 6µm is known as far infrared.
The Foundations of Infrared Science
The theory of infrared spectroscopy had been around since F.W. Herschel discovered infrared light in 1800. Herschel conducted an experiment using a prism to refract light from the sun and was able to detect the presence of infrared radiation beyond the red part of the visible spectrum using a thermometer to measure an increase in temperature.
The Types of Infrared Sensors
Infrared sensors can be active or passive and they can be split into two main types:
- Thermal infrared sensors – use infrared energy as heat. Their photo sensitivity is independent of the wavelength being detected. Thermal detectors do not require cooling but do have slow response times and low detection capabilities. Read more about Thermal Infrared Sensors here.
- Quantum infrared sensors – provide higher detection performance and faster response speed. Their photo sensitivity is dependent on wavelength. Quantum detectors have to be cooled in order to obtain accurate measurements.
The Working Principle of Infrared Sensors
The physics behind infrared sensors is governed by three laws:
- Planck’s radiation law: Every object at a temperature T not equal to 0 K emits radiation
- Stephan Boltzmann Law: The total energy emitted at all wavelengths by a black body is related to the absolute temperature
- Wein’s Displacement Law: Objects of different temperature emit spectra that peak at different wavelengths
All objects which have a temperature greater than absolute zero (0 Kelvin) posses thermal energy and are sources of infrared radiation as a result.
Sources of infrared radiation include blackbody radiators, tungsten lamps and silicon carbide. Infrared sensors typically use infrared lasers and LEDs with specific infrared wavelengths as sources.
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A transmission medium is required for infrared transmission, which can be comprised of either a vacuum, the atmosphere or an optical fiber.
Optical components such as optical lenses made from quartz, CaF2, Ge and Si, polyethylene Fresnel lenses and Al or Au mirrors are used to converge or focus the infrared radiation. In order to limit spectral response, band-pass filters can be used.
Next, infrared detectors are used to detect the radiation which has been focused. The output from the detector is usually very small and hence pre-amplifiers coupled with circuitry are required to further process the received signals.
The Key Applications of Infrared Technology
Night Vision Devices
Infrared technology is implemented in night vision equipment if there is not enough visible light available to see unaided. Night vision devices convert ambient photons of light into electrons and then amplify them using a chemical and electrical process before finally converting them back into visible light. Read more about infrared technology in night vision devices here.
Infrared astronomy is a field of astronomy which studies astronomical objects that are visible in infrared radiation. By using telescopes and solid-state detectors, astronomers are able to observe objects in the universe which are impossible to detect using light in the visible range of the electromagnetic spectrum.
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Infrared observatories have been set up in space such as the Spitzer Space Telescope and the Herschel Space Observatory have been set up in space. The observatories are not affected by the absorption of infrared light by water vapour in the Earth's atmosphere.
Infrared tracking, also known as infrared homing, is a missile guidance system which operates using the infrared electromagnetic radiation emitted from a target to track it. These missile systems are often known as 'heat-seekers' as infrared is radiated strongly by hot bodies such as people, vehicles and aircraft.
Art History and Restoration
Infrared reflectography is used by art historians in order to reveal hidden layers in paintings. This reflectography technique is useful in helping to decide whether a painting is an original version or a copy, and whether it has been altered by restoration work. Read more about infrared in art history and restoration here.
Hyperspectral imaging accumulates and processes information from across the electromagnetic spectrum and can be used to track nanoparticles inside large living organisms.
Other Key Application Areas
Other key application areas that use infrared sensors include:
- Gas detectors
- Water analysis
- Anesthesiology testing
- Petroleum exploration
- Rail safety
References and Further Reading