1. How do infrared temperature sensors work?
Infrared temperature sensors (also know as infrared thermometers or Infrared pyrometers) are non-contact sensors that determine temperature through measurement of infrared radiation emission. Infrared radiation (abbreviated as IR radiation) is part of the electromagnetic radiation spectrum. All bodies with temperatures above absolute zero emit infrared radiation. The intensity of the emitted radiation is a function of both temperature and emissivity. Emissivity is the radiation emitted by a body as a proportion of that emitted by an ‘ideal black body’ of the same temperature. Emissivity is defined on a scale between 0 and 1. An emissivity of 0.8 indicates that the body radiates 80% as much radiation as an ideal black body. The graph below (from Omega engineering) shows the IR emission intensity of a black body at various temperatures. If the emissivity of the body is known, its temperature can be determined from the intensity of the emitted IR radiation at a given wavelength. Note, a low emissivity indicates that the body is highly reflective.
Infrared temperature sensors are typically packaged within small stainless steel cylindrical housings, which include external screw threads to aid in mounting. The main components of the sensor are its lens and photo detector. The sensor is pointed at a target object whose temperature we desire to measure. IR radiation emitted by the object is focused onto the photo detector by the lens. The photo detector converts the incoming IR radiation into an electrical voltage. The voltage is measured by the sensors built in electronics which then determines the objects temperature. For temperature to be accurately determined, the sensors emissivity rating must closely match that of the target object. However, many sensors include adjustable emissivity which can be programmed by the user (see application tips for determining emissivity).
2. General characteristics of infrared temperature sensors
IR temperature sensors are non-contact sensors that detect the infrared emission intensity from an object and use it to determine its temperature. They have fast response times and can measure temperatures as high as 3,000 °C. However, they are also expensive and suffer from low accuracy. IR temperature sensors are typical packaged within small stainless steel cylindrical housings, which include external screw threads to aid in mounting. They include built in electronics which convert the photo detector voltage to a standard output signal form (e.g. 0-10 V or 4-20 mA). This makes it simple to connect them to a process controller or data logger.
3. Input and output signals
IR temperature sensors require a fixed 12-24 Vdc supply voltage to power the photo detector and post processing electronics. The photo detector outputs a small voltage as a function of the incoming IR radiation intensity. The voltage is then converted into a standard output signal form (e.g. 0-10 V or 4-20 mA). Many IR temperature sensors include the option of a thermocouple style mV output, enabling them to be connected to the thermocouple input of a process controller. Furthermore, many include a programmable alarm which sounds an audible warning if a user programmed temperature has been crossed.
4. Applications of infrared temperature sensors
IR temperature sensors are expensive and achieve only low accuracy. As such, they are typically limited to applications where non-contact sensing is important. These Applications include extremely high temperatures, rotating parts (e.g. motor rotors), corrosive liquids, fragile objects, and for checking mechanical or electrical equipment for temperature hot spots.
5. Typical specification
|Measurement range||-50 to 500 °C (up to 3,000 °C available)|
|Max. operating temperature||70 °C (uncooled)|
200 °C (water cooled)
|Long term stability||High|
|Supply voltage||12-24 Vdc|
|Output voltage||0-10 V, 4-20 mA, k type|
|Ingress protection||IP65 typical|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- Field of view: The field of view determines the area on the target object, visible to the sensor at a given sensing distance. It is defined as the ratio between the linear sensing distance and the detection region diameter. A field of view is 5:1 indicates that at a distance of 500 mm the detection region diameter is 100 mm. Sensors are available with fields of view ranging from 2:1 to 30:1
- Maximum sensing distance: Maximum sensing distance is about 1,000 mm but varies slightly between sensor models.
- Emissivity: IR temperature sensors are available with either fixed or adjustable emissivity. Most fixed emissivity sensors have an emissivity of 0.9 or 0.95. Adjustable emissivity sensors may have emissivity ranges as large as 0.1 – 1.
- Spectral range: The spectral range is the range of IR wavelengths that can be detected by the sensor. Most sensors have a range of about 8 to 14 μm, though ranges as large as 1-20 μm are available.
- Operating temperature: IR temperature sensors cannot withstand exposure to high temperatures. The maximum operating temperature is typically about 70°C for uncooled sensors and 200 °C for water cooled sensors.
- Air purge jacket: Contamination of the sensor window with dirt or oil reduces accuracy. To prevent this, some sensors utilize a technique called air purging in which a protective air barrier if formed around the window to repel dirt and oil. Sensors that utilize air purging require a compressed air supply.
- Guide laser: Many sensors include a guide laser to provide visual feedback when aiming the sensor.
- Response time: IR temperature sensors are available with response times of about 0.1- 0.5 seconds.
7. Advantages of infrared temperature sensors
Infrared temperature sensors:
- Have fast response times because there is no thermal mass associated with their principle of operation.
- Enable non-contact temperature measurements.
- Can measure extremely high temperatures, beyond those possible with RTDs and thermocouples.
- Are not prone to self-heating or thermal shunting.
8. Disadvantages of infrared temperature sensors
Infrared temperature sensors:
- Are more expensive than other types of temperature sensors.
- Are of lower accuracy than most other types of temperature sensors.
- Are not capable of measuring temperatures below -50 °C because of the reduction in IR radiation intensity at low temperatures.
- Are sensitive to material emissivity; the sensor’s rated emissivity must match that of the object (though many sensors include user programmable emissivity).
- Are prone to error due to IR radiation from a nearby hot body being reflected off of the target object and into the sensor.
- Are prone to error due to the sensor window becoming contaminated with dirt or oil. The probability of contamination can be reduced by using an air purge jacket.
9. Application tips
- Calibration for improved accuracy: The long term accuracy of IR temperature sensors can be improved by performing calibration every 12 months to account for long term temperature drift.
- Reflective surfaces: Many lower cost sensors have fixed emissivities of 0.9 or 0.95, making them unsuitable for use with reflective objects. The emissivity of an object can be reduced by painting its surface with a non-reflective paint.
- Obtaining emissivity values: Adjustable emissivity sensors require the material emissivity to be selected by the user. The material emissivity can be obtained from the literature or by accurately measuring the temperature of the object by some other means (e.g. with an RTD) and adjusting the sensors emissivity until the correct temperature is obtained.
- Spatially non-uniform temperature within the detection region: Infrared intensity is proportional to the forth power of the temperature (see Stefan-Boltzmann law). As such, hot spots within the detection area emit disproportionately high intensity IR radiation. This results in the sensors output being strongly biased towards the maximum temperature within the detection region.
- Stable operating temperature: For accuracy to be maintained, the sensor should be kept at a stable operating temperature.