Infrared Sensors and PIR Sensors Breakdown

Infrared sensors are a vital component of almost every alarm system. In this guide we see how they work.

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What are Infrared Sensors?

Infrared sensors are a specific type of motion sensors that use infrared radiations. The main use-case of this device is within physical security and, in particular, for intrusion detection purposes. The two types of infrared sensors are active infrared sensors and passive infrared sensors with these last ones being the preferred ones in the context of physical security.

In the next paragraphs, we will dive into how these sensors work, what are the ideal use-cases, and how to choose the best infrared sensor for your business or home.

How do Infrared Sensors Work?

Infrared sensors, as mentioned above, work with infrared radiations. The functioning depends on if the sensor is active or passive (PIR).

Active infrared sensors work with radar technology and they both emit and receive infrared radiation. This radiation hits the objects nearby and bounces back to the receiver of the device. Through this technology, the sensor can not only detect movement in an environment but also how far the object is from the device. This is especially useful in robotics to detect proximity.

For the physical security use case, however, passive infrared sensors (PIR sensors) are definitely more widespread. The PIR sensors do not emit radiation but simply receive the one that the objects nearby are naturally emitting. The basic functioning is that the passive infrared sensors trigger the alarm when there is an anomaly in the infrared waves measured in the room. This happens if, for example, a warm object (like an intruder) crosses ways to the signal of the device. More about the functioning of PIR in the later paragraphs.

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A Digression on Infrared Radiations

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 into 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.

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 use infrared lasers and LEDs with infrared wavelengths.

In order for the thermal energy to reach 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 afterward 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.

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PIR Sensors Overview

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 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.

PIR Sensors Components

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 a 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.

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

PIR Sensors and Temperature

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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Infrared Sensors and Access Control

When mounted on the wall or on the ceiling, PIR sensors can be used in an 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 on 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.

Kisi has its own native Intrusion Alarm features and uses passive infrared sensors. If you want to know more about this feature, do not hesitate to check our product page or contact our sales team.

If you want to learn more about intrusion alarms and how to choose one, feel free to check out our overview page.

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