This article will cover the basic functions of infrared radiation (IR) temperature sensors as well as some frequently asked questions about their operation and technical characteristics.
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What Is IR?
Infrared radiation is part of the electromagnetic spectrum. It’s higher frequency than microwave radiation and lower frequency than visible light ( Fig. 1) .
Fig. 1: Electromagnetic Spectrum
All solid bodies emit various amounts of energy in the form of heat (IR) when they are above absolute zero (-273C). Scientist Wilhelm Wien stated the relationship between a solid body’s temperature and its peak wave length can be determined by the following equation:
lmax =2898 / T
T = Temperature in K ( Kelvin )
l = Wavelength in µm
How do Thermal Detectors Work?
There are two major classes or IR detectors: photon detectors and thermal detectors. This article is focused on thermal detectors – thermal detectors receive radiation and use it to raise the temperature of a sensing material such as a thermopile. If there’s a difference in temperature between objects, then it can be measured. IR temperature sensors produce a value for the targeted object and sent a signal to represent the temperature value. IR temperature sensors typically use a lens to focus light from one object onto a thermopile. The thermopile absorbs the IR radiation and turns it into heat - more IR energy absorbed means a hotter temperature reading. The heat from the thermopile is transduced into a constant voltage output, which is proportional to the net radiation.
The Thermoelectric Effect
The thermoelectric effect (Fig. 2) works by applying a temperature difference to two dissimilar materials (A and B). By getting a reading from the junctions of the materials (at voltage V), a voltage proportional to the temperature difference can be observed.
Fig. 2: The Thermoelectric Effect
Advantages/Disadvantages of Contactless Temperature Sensors
IR temperature sensors are quick, accurate, and ideal for remote monitoring. The non-contact measuring is useful for the safety of the operator and it limits potential contamination. Some quick examples for contactless usage of IR temperature sensors include:
- Check surface temperature
- Check for hot and cold spots
- Check electric heating coil operation
Typically IR thermometers are limited to surface temperature readings only and don’t measure through transparent surfaces or liquids. IR sensor readings can also be negatively affected by environmental conditions/air particles such as dust, fog, excess moisture, or smoke.
What Does Resolution Mean for Temperature Sensors?
This specification is the smallest detectable incremental change of input parameter that can be detected in the output signal. For example, a sensor that can detect and output temperature readings to a hundredth of a degree (99.68°) has a higher resolution than one that would only show the tenths of a degree (99.7°). Resolution can be expressed either as a proportion of the reading (or the full-scale reading) or in absolute terms. When resolution is expressed in bits, such as 14 b, 15 b, et cetera, higher values mean higher amounts of data will be expressed in every sample.
How to Clean and Maintain IR Sensors
Since IR sensors function by focusing IR light, keeping them clean is ideal for proper performance. To clean, use a soft cloth or cotton swab with water or medical grade rubbing alcohol and carefully wipe the lens of the part. Allow the lens to dry fully before using the part. Never use soap or chemicals and never submerge the sensor or thermometer in water.
If you have any questions or comments, please feel free to leave a reply below.
Information courtesy of:
Excelitas Thermal Infrared Sensors
For tips and application notes for handheld IR thermometers, check out these links from Fluke: