Infrared Sensors

‍Infrared radiation is at the lower, invisible end of the electromagnetic spectrum

Infrared radiation works on the lower end of the electromagnetic spectrum and is therefore invisible to the human eye. The infrared section of the electromagnetic spectrum is found between the visible waves and the microwaves. The infrared wavelength is between 0.75 and 1000µm and is separated in three regions:

  • Near infrared - from 0.75 to 3 3µm
  • Mid-infrared - from 3 to 6µm
  • Far-infrared - higher than 6µm

Astronomer Herschel discovered the infrared section of the electromagnetic spectrum with the famous prism refraction experiment.

Prism refraction
‍Prism refraction

Infrared radiation is a characteristic of all objects that have a temperature higher than the absolute zero (0 Kelvin or -273 Celsius). Such objects have thermal energy and can emit infrared waves. IR sensors usually uses infrared lasers and LEDs with infrared wavelengths.

In order for the thermal energy to reach to the IR sensor, it must use a transmission medium. Compatible mediums are the atmosphere, vacuum or optical fibers. Optical lenses made from combinations of metals and minerals, such as quartz, calcium fluoride, polyethylene, germanium, aluminum and silicon are used as radiation convergents. The converged or focused radiation is afterwards detected by infrared detectors. Infrared detectors must additionally use pre-amplifiers to strengthen the signal.

Infrared technology is widely used for commercial purposes for:

  • Night vision devices.
  • In astronomy, to detect objects in the universe by telescopes and solid state detectors.
  • In military activities for missile tracking.
  • In art restoration to analyze paintings and discover hidden painting layers.
  • For tracking nanoparticles in living organisms.

IR sensors also find use in studying the weather, gas detection, petroleum examination and water analysis, as well as in medicine for anesthesiological purposes. IR sensors are used for security as part of access control systems.

PIR motion sensors

PIR motion sensors are specific IR sensors, also called passive infrared sensors or pyroelectric sensors. The acronym PIR stands for “passive infrared”. PIR motion detectors are intended for a specific use of the infrared radiation - the part which detects the infrared wavelength coming from the environment. They are inexpensive but very sensitive to detect and indicate whether a person is in or has left a detection field.

The PIR motion sensor is made from a pyroelectric component (a combination of metal and crystal) and additional electrical elements, such as circuitry, resistors and capacitors. The passive IR sensor is typically protected by a metal shell and has a silicone window to let the radiation pass through it. Most PIR sensors are rectangular in shape, have a sensitivity range of up to 20 feet. In terms of power supply, they belong in the range of 3.3V - 5V input voltage.

The sensor in the PIR motion detector consists of two halves because its purpose is not only to detect infrared waves but also to indicate a change as a signal for motion. The sensor is idle when there is no motion in the detection field. However, when a person or another living thing that radiates heat, such as an animal passes in the field range, it produces positive differential change in the first half on its way in and a negative differential change in the second half of the sensor on its way out. To be able to detect and pass subtle signals, the PIR motion sensor includes a lens or concretely, more lenses integrated as smaller sections in a larger component called fresnel lenses.

‍Infrared sensors are placed in TV remotes and computer mouses

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PIR sensors can be used to detect movement of people, animals and objects.

When mounted on the wall or on the ceiling, PIR sensors can be used in access control system to communicate to the central board that a movement in the area has been noticed. Infrared sensors then trigger alarms and report the event to the owner, the law enforcement or the security contractor, depending who is in charge of taking the action. The sensors can also instigate video recording or activate physical barriers to prevent the intruder from escaping.

PIR motion detectors can also be placed in thermometers to measure the temperature of a distant moving object. Emergency teams use PIR sensors for rescue purposes, for example, if a person has been lost on a mountain or for medicinal purposes to detect the skin temperature. They are placed in TV remotes and computer mouses to manage specific functions, such as on and off or scrolling.

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