Overview of the Infrared Sensor

Infrared sensors detect and measure radiation at wavelengths “beneath” that of visible red light (“infra” is the Latin term for “below”). They not only detect motion but can measure the extent of heat being emitted by an object: infrared radiation is thermal radiation.

Sensors that simply receive infrared rays without generating any thermal radiation themselves are referred to as passive infrared (PIR) detectors, and are commonly used at the external entrances of buildings to switch on lights to illuminate pathways and doorways as someone approaches in the dark. When linked to an alarm system, they function as intruder alerts for building security.

The sensor typically receives infrared radiation through an aperture at the front of the device but the sensor itself, which interprets and measures it, is located in the centre of the gadget. There may in fact be several sensors in one individual device, often manufactured from “pyroelectrical” materials that generate changing voltages in response to variations in heat or the absence of heat.

Often, when the pyroelectrical surface identifies a heat source in an area of its viewing field, it will send a signal through wiring on the circuit board on which it is housed to a motion detector, whereupon an alarm will be activated. Sensors of this sort are widely used in security systems.

 

History

The German-born astronomer Sir William Herschel discovered the existence of radiation beyond visible red light in 1800 while he was studying the composition of sunlight. Early IR detectors were used initially as components of thermometers but in 1831 the Italian physicist, Macedonio Melloni, devised a multi-element thermopile capable of detecting warmth emitted from a person standing ten metres away from the instrument. In 1913, while the trauma of the 1912 sinking of the Titanic was still fresh in the public mind, a device fashioned from a mirror and a thermopile was developed for ocean-going vessels to detect the presence of other ships and icebergs. A thermopile-free IR iceberg detector was patented a year later, and in 1934, the same technology was used to detect the presence of forest fires.

IR sensors began to be developed for infrared cameras by the US military in conjunction with the technology firm Texas Instruments in 1947, a process that accelerated during the Korean War of 1950-53. By the 1970s, the technology had advanced considerably and cameras capable of pyro-electric scanning with impressively high performance became widely available.

Today, “Smart Sensors” represent the cutting edge of IR sensor technology. They are capable not simply of sensing IR radiation, but by integrating the sensor with high-speed digital computer technology, they can also process and interpret it. They have paved the way for new developments in functional diagnostics in medicine, collision avoidance and guidance systems in vehicles.

 

Technical aspects

While infrared radiation extends from 0.7 to 1000 microns, the range for temperature measurement is confined to 0.7 to 14 microns. Typical infrared sensors incorporate five essential components. Firstly, in active sensors which generate their own heat radiation, the infrared source, defined as any object at a temperature above 0°Kelvin (all such objects emit infrared rays). Usually, specific IR wavelengths, LEDs or infrared lasers are used as infrared source in these sensors.

Secondly, the infrared sensor, whether active or passive, requires a transmission medium.  Infrared transmission occurs mainly through three media: the atmosphere, the vacuum and optical fibres. In the atmosphere, water vapour, CO2 and atmospheric trace elements all influence infrared transmission, because molecules of each will absorb the radiation. The infrared wavelengths that are not absorbed are known as infrared windows, and these are the parts of the IR spectrum principally utilised by remote thermal sensor devices and thermal imaging applications.

For sensors integrated into thermal imaging devices, IR rays must be focussed via optical lenses, the third component. These are usually manufactured from calcium fluoride, quartz, polyethylene Fresnel, germanium or silicon and are incorporated into the sensor in conjunction with mirrors made using gold or aluminium.

The fourth essential component of the infrared sensor is the IR detecting material itself, which must conform to infrared photosensitivity specifications. This is known as the IR “responsivity” and is calculated by dividing the output voltage by the current per watt of incident energy. The higher the value, the better the material is for IR detection. “Detectivity” or “D*” (D-star) is another important specification for the detector material, and is a measure of the degree of photosensitivity per unit area of the detector. Infrared detectors are also designed to capture different segments of the IR spectrum, and must be stable in different thermal environments as well as demonstrating sound linearity in their measurement of IR (i.e., measures must accurately reflect and be in proportion to the amount and type of IR radiation being received). The devices response time and cooling mechanisms are also important parameters.

Finally, because sensor outputs are usually exceptionally small, there is a need for signal processing or signal enhancement: this is usually achieved by means of preamplifiers, which enhance the signals they receive from the detector.

 

Where the infrared sensor is used in manufacturing

In addition to military/surveillance/security usage and medical and veterinary thermography, infrared sensors are used in early failure warning systems for industrial machinery and electrical equipment, the manufacture of automobiles and quality control during many production processes.

How the infrared sensor differs from other sensors

Many sensors measure electromagnetic radiation but thermal infrared sensors confine their detection and measurement to a specific span of radiation beyond the visible red wavelength of light: radiation of wavelengths between 0.7 and 14 microns.