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Infrared Radiation and Its Sensor

Introduction – What is Infrared Radiation?

Infrared light was discovered in the year 1800 by a British astronomer named William Herschel. It can be defined as a type of energy radiated that's invisible to human eyes but that we can feel as heat. Every object emits some amount of electromagnetic radiation when it is above absolute zero (-273°C). Infrared is a kind of electromagnetic radiation, a continuous sequence of frequencies which are produced when atoms absorb and then release energy. In order of highest to lowest frequency range, electromagnetic radiation consists of various rays which include gamma-rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.

The peak wavelength of radiation and the body’s temperature is related by the following equation:

λmax=2898/T, where T is temperature is Kelvins, and λ is wavelength in µm.

The emission from typical bodies spans over a wide spectrum. Multispectral detection of infrared from different infrared bands helps in providing enhanced target discrimination and identification. The shortwave infrared (SWIR) waveband helps in detecting reflected light which produces more intuitive and visible images. On the other hand, longer wavelength provides thermographic images. The mid-wave infrared (MWIR) detection is capable of detecting hot plumes while long-wave infrared (LWIR) detection, as the peak of the radiation spectrum for room temperature objects, can detect cooler bodies. Read here for a further introduction to Infrared.

Infrared Detectors

There are basically 2 main types of infrared detectors: photon detectors and thermal energy detectors. Photon detectors work by directly converting the absorbed photons into electrons or electron/hole pairs, and the carrier density is dependent on the intensity of the radiation. Thermal energy detectors work by absorbing the heat energy and increasing the temperature of the sensor element, which in turn modifies some electrical property of the sensor, dependent on the choice of sensor used. This article focuses on infrared detection in the MWIR and LWIR detection.

Common Semiconductor Infrared Detectors

1) Detectors for MWIR and LWIR: Detectors that are functional and used in the thermal infrared region (including mid-wave infrared (MWIR) and long-wave infrared (LWIR), are able to capture object temperature by analyzing the heat they emit. There are various kind of systems used to detect temperature. Some of the commonly used systems are discussed below:

a. Thermopile: Thermopiles work by absorbing the heat energy and changing the electromotive force. This is detected as a potential difference. Simply put, a thermopile detector can be described as a passive radiation sensing voltage-generating device. It does not emit any radiation and does not require cooling or bias. As per the target size, radiance and temperature, the output of thermopiles is typically in the range of microvolts to millivolts.

b. Pyroelectric Sensors: Pyroelectric sensors work by producing a change in the charge of the surface of the pyroelectric crystal material as a result of a change in the surface temperature. Pyroelectric sensors are essentially AC devices and would require the use of chopper to detect static temperatures. Unlike thermopiles, these are active devices and require electrical bias. Pyroelectric sensors can be operated in voltage mode or current mode. While such detectors cover the whole ambit, they are mainly used for mid-wave and long-wave infrared detection (MWIR and LWIR).

c. Microbolometers (used mainly for IR imaging): A microbolometer is one of a kind bolometer that is used as a detector in a thermal camera. Its working principle is based on changing electrical resistance. Infrared radiation having wavelengths between 7.5–14 μm strikes the detector material; this heats it, and changes its electrical resistance. This resistance change is detected and processed into temperatures which can be used to create an image.

Advantages of IR Detectors as Temperature Sensors:

  • IR sensors can read moving objects. Since contact-based temperature sensors do not work well on moving objects, Infrared temperature sensors are ideally suited for measuring the temperatures of tires, brakes and similar devices.

  • IR sensors do not suffer from wear and tear. No contact means no friction. Infrared sensors experience no wear and tear, and consequently giving them longer operating lives.

  • IR sensors can provide more detailed information. An IR sensor can provide greater details during a measurement than contact devices, simply by pointing it at different spots on the object being read.

  • IR sensors can also be used to detect motion by measuring fluctuations in temperature in the field of view.

Usage of Infrared Radiation in day to day life

Infrared radiation is used extensively in various industries today. It finds its usage in industrial, scientific, military, law enforcement, and medical applications.

Infrared science and technology have been predominantly dedicated to security and surveillance especially in military field. Specialized techniques have also evolved in thermal imaging for medical diagnostic and building structures and recently in energy savings and aerospace context.

In the aftermath of the COVID-19 pandemic, infrared detection has also found its way into the first line defense against the spread of the virus. IR cameras can be seen in public buildings, as a means of detection of person with elevated temperatures. Non-contact thermometers that are based on infrared detection are also routinely used to measure the temperatures of individuals before allowing entry into establishments.

Infrared temperature sensor cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus. Extensive uses for military and civilian applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm (micrometers). Non-military applications include thermal efficiency analysis, environmental monitoring, industrial facility inspections, detection of grow-ops, remote temperature sensing, short-range wireless communication, spectroscopy, and weather forecasting.

In more recent developments, T-SMART is introducing state of the art Thermopile 2.0 with a responsivity of 2 orders of magnitude higher than the current generation of thermopiles. The company is focused on taking thermal detection technology to an elevated level, using sensing integration algorithms for a cognitive understanding through thoughts, experience and senses. Click here to learn more about Thermopile 2.0.

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