How Do Infrared Thermometers Work?

Infrared thermometer

There are many situations where knowing a surface’s temperature might be useful without having to touch it. Perhaps you might want to ensure your grill is hot enough to cook your food correctly or take the temperature of hard to reach parts when diagnosing car problems. Or maybe you’d like to be able to take temperature readings of cold-stored food without risking cross-contamination. These are all common applications for infrared thermometers.

If you’re looking to learn more about the technology behind infrared thermometers or considering purchasing one, keep reading.

What Is an Infrared Thermometer?

An infrared thermometer is a measuring device that allows you to take accurate “spot” measurements of surface temperatures without you (or the device) touching the object. They measure the amount of infrared radiation emitted by the object at a specific point. The readings can be as accurate as any other method of temperature measurement.

To use them, you point the device at the object, with the aid of a laser guide to help you identify the spot for the measurement. They are also called laser thermometers for this exact reason. More commonly used in research, commercial, and industrial applications, infrared thermometers are an ideal temperature measurement instrument for hard-to-reach objects, moving objects, or situations where extreme temperatures may make it potentially hazardous to get near.

Unlike standard thermometers where the instrument is touched against or inserted into the object, an IR thermometer is a non-invasive instrument that only measures surface temperature.

How Does an Infrared Thermometer Work?

Infrared thermometers work by focusing the infrared rays emitted by an object through a lens in the device itself, and then onto a detector, called a thermopile. As the thermopile absorbs infrared radiation, it heats up. This heat is converted into electricity, which is measured to determine the surface temperature.

If you’re more of a visual learner, the video below shows how infrared thermometers work in greater detail.

How Accurate Are Infrared Thermometers?

Just like every measuring device, you will pay more for better accuracy. A decent model can be as accurate as any other method to measure temperature, as long as you’re using it correctly. Cheaper IR thermometers will typically be accurate within ± 2°C or ± 2% of the actual temperature, while more expensive models will have lower tolerances, usually ± 1°C or within ± 1% of the actual temperature.

Accuracy may vary based on a variety of factors, including temperature range, distance to spot ratio, and emissivity, all of which we will discuss in detail below.

What Temperatures Do Infrared Thermometers Measure?

IR thermometer measuring temperature of asphalt

Non contact Infrared thermometers can measure a wide range of temperatures from below freezing to smoldering hot. However, the accuracy of the measurement can vary as the temperature changes. We’re going to use two popular models to give you an idea of how capable they are.

The budget Etekcity 1022D thermometer can measure temperatures between -58°F to 1022°F (-50°C to 550°C). Under 212°F (100°C), the thermometer should be accurate within 2°C. However, at temperatures higher than 212°F, readings are guaranteed to be accurate within 2%. While at (relatively) cooler temperatures, this shouldn’t be an issue, at the upper end of its range readings might be off as much as 20 degrees.

Contrast this with the more expensive Fluke 62 Max Plus, which has both a broader temperature range, -22°F to 1202°F (-30°C to 650°C), and much better accuracy, ± 1.0°C or ± 1.0% (whichever is greater) at non-freezing temperatures. Of course, it’s several times the price of the 1022D, so we’d expect higher performance.

Distance to Spot Ratio

Distance to spot ratio

Distance to spot (D:S) ratio is another critical factor to consider when using an IR thermometer. The ratio refers to the area that the thermometer will measure when a given distance away. A model with a 12:1 D:S ratio will measure a 1-inch diameter area from 12 inches away. Higher ratios are better in extreme temperature environments because you’re able to stay further apart without sacrificing accuracy.

In our example, both the 1022D and 62 Max Plus both have 12:1 D:S ratios, but there are IR thermometers with better D:S ratios, such as 16:1 and higher. Keep this in mind if you’re looking for a device to use in more extreme environments or hard to reach areas.


Emissivity is a measurement of how efficient a surface is at emitting infrared radiation. Emissivity can have a value anywhere from 0 (shiny mirror) to 1 (blackbody). A surface with an emissivity of 0 emits no thermal radiation, while a surface with an emissivity of 1 is a perfect emitter and will efficiently radiate the temperature of the object. Knowing the emissivity of a surface before measuring it will allow you to calibrate the thermometer to gauge the temperature accurately.

Emissivity settings are either fixed or adjustable on an infrared thermometer. The 1022D has a fixed emissivity of 0.95, which allows it to take accurate temperature readings of high emissivity surfaces such as bricks, soil, and asphalt. But polished metal surfaces will provide highly inaccurate readings.

For that, you need the Fluke 62 Max Plus, which has adjustable emissivity settings. Since we know the emissivity of objects, all that is necessary on the device is a small adjustment to ensure correct readings. See this table for the emissivity of many everyday objects.

How Do I Know If My Infrared Thermometer Is Accurate?

You’ll want to regularly check your infrared thermometer for accuracy, just like you would with any other measuring instrument.

The cheapest method to calibrate a laser thermometer requires a crushed ice and water mix in a bath. The mixture needs to be a slurry consistency, you’ll need to crush the ice thoroughly to get it to that point. The object here is to get a uniform temperature as close to freezing as possible. When measuring this mixture, the ice slurry mix should be 32°F (0°C). If it is off by a significant amount, your IR thermometer might require adjustment.

However, this is not a perfect solution as you may not be able to get the mix to exactly freezing, and you’re only making a reference check at one point.

We recommend using an infrared comparator cup instead. The cup provides a solid matte black base with a high emissivity rating for accurate readings. You can use the cup to compare the temperature measurements of an infrared thermometer with a calibrated reference thermometer.

Using the comparator cup method will result in far better calibration than the ice water bath method as you can compare a range of temperatures with greater accuracy.

When to Use Your Infrared Thermometer

Infrared thermometer measuring fuse box

Since an infrared thermometer measures quickly and without contact, they are useful in a wide variety of applications where the object may either be too hot to get close to, out of reach, hazardous, or at risk of cross-contamination.

In industrial applications, IR thermometers are commonplace at job sites where electrical and mechanical equipment, moving parts, or hard-to-reach components need to be routinely monitored to prevent expensive machinery breakdowns.

In foodservice applications, employees will use a non contact infrared thermometer to perform quick spot checks of hot and cold holding or buffet lines. They can also be useful when cooking to take a surface temperature reading of a grill or a cast-iron frypan.

There are disadvantages to the technology as well. IR thermometers can only take surface temperatures—they do not measure the internal temperature of objects. You also cannot measure the temperature of objects through glass, as you’ll be measuring the temperature of the glass instead. Visual obstructions like smoke and dust can obstruct readings, and they are sensitive to rapid changes in ambient temperature.

If you’re interested in learning more, check out our article on infrared thermometer uses.

How to Use an Infrared Thermometer?

You’ll first want to consult your IR thermometer instruction manual to find the D:S ratio of your device if you don’t know it already. Knowing the distance to spot ratio is essential so that you position the gun as close as possible for the most accurate temperature reading.

Next, turn on the infrared thermometer gun, and if necessary, chose your preferred measurement scale, either Fahrenheit or Celsius. If you have an adjustable emissivity gun, you’ll want to change the emissivity to that of the object you’re trying to measure. Fixed emissivity guns will be inaccurate for any surface not close to the gun’s fixed setting.

Point the thermometer at the object you’re measuring, using the laser as a guide to ensure you measure the correct point. You’ll want to select an area of the object that isn’t close to another area where the temperature might differ significantly. Pull the trigger, and the temperature should appear on the IR thermometer’s display.


Infrared thermometers are a valuable tool for various applications, even if they’re limited to measuring just the surface temperature. You’ll want to pay close attention to the distance to spot ratio and temperature range of the IR thermometer you choose, while also keeping in mind the emissivity of the surface you’re measuring.

Of course, read the instruction manual of your device thoroughly. Many of your questions on the operation of your specific model will be answered there.

We hope that this answers any questions you might have on infrared thermometers. If you’re in the market for one, be sure to check out our buying guide to find the best infrared thermometer for your needs. Armed with the information above, you’ll make smarter decisions on which one’s right for you.

Published: August 5, 2020

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