Infrared Sensor is most used sensor in wireless technology where remote controlling functions and detection of surrounding objects/ obstacles are involved. This post will discuss about what is Infrared Sensor, its working principle, how it works, types, applications, advantages and disadvantages.

What is Infrared Sensor

IR sensor is a simple electronic device which emits and detects IR radiation in order to find out certain objects/obstacles in its range. Some of its features are heat and motion sensing.

IR sensors use infrared radiation of wavelength between 0.75 to 1000µm which falls between visible and microwave regions of electromagnetic spectrum. IR region is not visible to human eyes. Infrared spectrum is categorized into three regions based on its wavelength i.e. Near Infrared, Mid Infrared, Far Infrared.

Wavelength Regions of Infrared Spectrum

  • Near IR – 0.75µm to 3 µm
  • Mid IR – 3 µm to 6 µm
  • Far IR –   > 6 µm

1 Introduction to Infrared Sensor

Fig. 1 – Introduction to Infrared Sensor

Working Principle of Infrared Sensor

Infrared Sensors works on three fundamental Physics laws:

  • Planck’s Radiation Law: Any object whose temperature is not equal to absolute Zero (0 Kelvin) 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 that is inversely proportional to Temperature.

Key Elements of Infrared Detection System

The key elements of Infrared Detection System are:

IR Transmitter

IR Transmitter acts as source for IR radiation. According to Plank’s Radiation Law, every object is a source of IR radiation at temp T above 0 Kelvin. In most cases black body radiators,  tungsten lamps, silicon carbide, infrared lasers, LEDs of infrared wavelength are used as sources.

Transmission Medium

As the name suggests, Transmission Medium provides passage for the radiation to reach from IR Transmitter to IR Receiver. Vacuum, atmosphere and optical fibers are used as medium.

IR receiver

Generally IR receivers are photo diode and photo transistors. They are capable of detecting infrared radiation. Hence IR receiver is also called as IR detector. Variety of receivers are available based on wavelength, voltage and package.

IR Transmitter and Receivers are selected with matching parameters. Some of deciding specifications of receivers are photosensitivity or responsivity, noise equivalent power and detectivity.

How Infrared Sensor Works

An Infrared Sensor works in the following sequence:

  • IR source (transmitter) is used to emit radiation of required wavelength.
  • This radiation reaches the object and is reflected back.
  • The reflected radiation is detected by the IR receiver.
  • The IR Receiver detected radiation is then further processed based on its intensity. Generally, IR Receiver output is small and amplifiers are used to amplify the detected signal.

Typical working of IR sensor detection system can be understood by Figure 2 below.

How Infrared Sensor Works

Fig. 2 – How Infrared Sensor Works

Incidence in an IR Detection System may be direct or indirect. In case of Direct Incidence, there is no hindrance in between transmitter and receiver. Whereas, in Indirect Incidence IR Transmitter and Receiver are kept side by side and the object is in front of them.

Types of Infrared Sensor

IR sensors can be classified in two types based on presence of IR source:

  • Active Infrared Sensor
  • Passive Infrared Sensor

Active Infrared Sensor

Active Infrared Sensor contains both transmitter and receiver. Most of the cases LED or laser diode is used as source. LED for non-imaging IR sensor and laser diode for imaging IR sensor are used.

Active Infrared Sensor

Fig. 3 – Active Infrared Sensor

Active IR Sensor works by radiating energy, received and detected by detector and further processed by signal processor in order to fetch information required.

Examples of Active IR Sensor: Break Beam Sensor, Reflectance Sensor.

Passive Infrared Sensor

Passive Infrared Sensor contains detectors alone. There won’t be a transmitter component.

These type of sensors use object as IR source/ transmitter. Object radiates energy and it is detected by IR receivers. A Signal processor is then used to interpret the signal to fetch information required.

Example of Passive IR Sensor: Thermocouple-Thermopile, Bolometer, Pyro-Electric Detector, etc.

Passive Infrared Sensor

Fig. 4 – Passive Infrared Sensor

There are two types of Passive Infrared Sensor:

  • Thermal Infrared Sensor
  • Quantum Infrared Sensor

Thermal Infrared Sensor

Thermal Infrared sensors are independent of wavelength. They use heat as energy source.

Thermal detectors are slow with their detection time and response time.

Infrared Sensor as Heat Sensing Device

Fig. 5 – Infrared Sensor as Heat Sensing Device

Quantum Infrared Sensor

Quantum Infrared Sensor are dependent on wavelengths. They have high detection time and response time. These type of IR sensors require frequent cooling for precise measurement.

Applications of Infrared Sensor

IR sensors have found their applications in most of today’s equipment. Following are the list of sensors which are named after its usage.

Proximity Sensor

These are used in smart phones to find distance of object. They use principle called Reflective Indirect Incidence. Radiation transmitted by transmitter is received by receiver after being reflected from object. Distance is calculated based on the intensity of radiation received.

Item Counter

This use direct incidence method to count the items. Constant radiation is maintained in between transmitter and receiver. As soon as object cuts the radiation, item is detected and count is increased. The same count is shown on display system.

Burglar Alarm

This is one of widely and commonly used sensor application. It is another example for direct incidence method.

It works similar to item counter, where transmitter and receiver are kept on both the sides of door frame. Constant radiation is maintained between transmitter and receiver, whenever object crosses path alarm starts off.

Applications of Infrared Sensor

Fig. 6 – Applications of Infrared Sensor

Radiation Thermometers

It is one of key application of Infrared sensors. Working of radiation thermometer depends on temperature and type of object.

These have faster response and easy pattern measurements. They can do measurement without direct contact of object.

Human Body Detection

This method is used in intrusion detection, auto light switches, etc. Intrusion alarm system sense temperature of human body.

If the temperature is more than threshold value, it sets on the alarms. It uses electromagnetic system which is suitable for human body in order to protect it from unwanted harmful radiations.

Gas Analyzers

Gas Analyzers are used to measure gas density by using absorption properties of gas in IR region. Dispersive and Non Dispersive types of gas analyzers are available.

Other Applications

IR sensors are also used in IR imaging devices, optical power meters, sorting devices, missile guidance, remote sensing, flame monitors, moisture analyzers, night vision devices, infrared astronomy, rail safety, etc.

Advantages of Infrared Sensor

The advantages of Infrared Sensor are:

  • Their low power requirements make them suitable for most electronic devices such as laptops, telephones, PDAs.
  • They are capable of detecting motion in presence/ absence of light almost with same reliability.
  • They do not require contact with object to for detection.
  • There is no leakage of data due to beam directionality IR radiation.
  • They are not affected by corrosion or oxidation.
  • They have very strong noise immunity.

Disadvantages of Infrared Sensor

The disadvantages of Infrared Sensor are:

  • Required Line of sight.
  • Get blocked by common objects.
  • Limited range.
  • Can be affected by Environmental conditions such as rain, fog, dust, pollution.
  • Transmission Data rate is slow.
Author: Anitha Shetty, BE(Electronics & Communication)


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