How Do IR Thermometers Work?

by Andy on December 13, 2011

General Tools IR Thermometer While going through some of our products the other day I came across some IR thermometers, and it struck me that I did not fully understand how they work. It seems strange that a machine can take the temperature of an object that it is not touching, but after doing a bit of research it started to make sense. I figured there may be others who do not understand this technology, so I decided to share my findings on our blog!

The first thing to understand with IR thermometers is a bit of how heat works. As an object heats up, its molecules start to vibrate faster, generating heat. IR radiation is emitted whenever an object has a temperature above absolute zero. This basic principle is what IR thermometers use to measure the temperature of an object. However, there are some interfering factors that must be taken into account to get an accurate temperature reading.

When an object gives off heat it normally does so in three ways. It reflects heat from other sources (most notably lights), transmits heat from its internal temperature, and emits heat from its surface. Because IR thermometers are meant to measure surface temperatures they need to measure the emitted energy and disregard the reflected and transmitted energy.

One way this is done is through using variable settings for emissivity. According to the Merriam-Webster dictionary emissivity is “the ratio of the radiant energy emitted by a surface to that emitted by a blackbody at the same temperature”. A blackbody is any object that is completely black and therefore does not reflect any light. Blackbodies are used as a baseline to judge the emissivity of other objects. By changing the emissivity of the IR thermometer you let it know how much of the temperature can be accounted for by reflection, allowing the device to correct for that.

Some IR thermometers are set to always have an emissivity of 0.95. This is a common setting because it covers most non-metallic objects and materials. When using an IR thermometer with set emissivity care must be taken not to use it on shiny surfaces. Polished and shiny surfaces have emissivities closer to 0.1, so using a fixed emissivity IR thermometer will give you a bad measurement. To compensate for this you can put non-shiny tape or paint over the surface and measure that instead. Just wait for the paint or tape to become the same temperature as the object you are measuring.

Extech IR Thermometer To compensate for transmitted energy, it is best to store your IR thermometer in the same temperature environment as the objects you will be measuring. If you have to take the thermometer to another environment you need to allow it time to adjust, or else the measurements will be off.

Once the IR thermometer is set to only measure emitted energy it is a simple process to read the temperature of an object. The device receives the IR radiation that is naturally given off from the object and uses a system of lenses to focus the energy on a receiver, which turns the energy into an electrical signal. After this signal is interpreted by the device and all settings are taken into account it gives you the temperature reading on its display.

Have you ever wondered how a particular instrument works? Let us know in the comments and we may write a blog post on it!

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