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How to get great results with an infrared thermometer

Five ways to make better measurements

July 2011

Infrared thermometer distance-to-spot ratio

Measurement spot diameters of a Fluke 63 Infrared Thermometer at three distances.

Infrared (IR) thermometers enable you to measure temperature quickly, at a distance, and without touching the object you’re measuring. They are so useful, easy, and even fun to use that they have become as common in kitchens as they have on factory floors. Infrared thermometers are often used to find overheated equipment and electrical circuits, but they have hundreds of other uses.

However, there are a few “gotchas” when using an infrared thermometer that can generate readings that are misleading or just plain wrong. Fortunately, these sources of error are easy to avoid or work around.

Common uses for infrared thermometers in industry

  • Finding faulty terminations in high power electrical circuits
  • Locating overloaded circuit breakers
  • Identifying fuses at or near their current rated capacity
  • Identifying problems in electrical switch gear
  • Monitoring and measuring bearing temperatures in large motors or other rotating equipment
  • Identifying "hot spots" in electronic equipment
  • Identifying leaks in sealed vessels
  • Troubleshooting steam traps
  • Finding faulty insulation in process pipes or other insulated processes
  • Capturing process temperature readings

Measuring More Than You Thought?

Every infrared thermometer has a “distance-to-spot” (D:S) ratio that tells you the diameter of the area being measured compared to the distance from the target. For example, if your thermometer has a distance-to-spot ratio of 12:1, it measures an approximately one-inch-diameter spot when it’s 12 inches from the target (about 2.5 cm at 30 cm). If you try to use that thermometer to measure a two-inch (5-cm) area from even just a few feet (1 m) away, you’re not going to get an accurate result because the thermometer will also be measuring the temperature outside the area you want to measure.

Distance-to-spot ratios vary a lot (from about 1:1 on the least expensive thermometers to about 60:1 on top-of-the-line models) and vary slightly with distance, so be sure to check the label on your thermometer or in the manual.

Lead Astray by the Laser?

Most handheld infrared thermometers have laser pointers that show the approximate center of the measurement area. It’s important to know that the laser is only a pointer and not used for the actual temperature measurement. Another common misconception is that the thermometer is measuring the area illuminated by the laser beam. The measurement spot is always wider.

Confused by Bright Shiny Objects?

Infrared thermometers have good accuracy when measuring most objects, but shiny, reflective surfaces can be a challenge. You should be especially wary when measuring the temperature of shiny metal objects, but even reflections off of glossy paint can affect accuracy. Putting a piece of non-reflective tape (such as electrical tape) over the shiny surface or applying some flat paint gives you a target from which you can get a better measurement.

Measuring temperature on a low emissivity surface with an infrared thermometer

Taking measurements on non-reflective tape or flat paint helps avoid errors caused by shiny surfaces.

The reason for this is that not all materials emit the same amount of infrared energy when they are at the same temperature. In general, most materials emit more infrared energy than shiny metals do - they have higher “emissivity.” (Emissivity is expressed as a number between 0 and 1, with 0 being non-emissive and 1 being perfectly emissive). Reflective surfaces are less emissive than dull surfaces. Weathered or oxidized metals are more emissive than polished, shiny metals.

If you need to take temperature readings on low emissivity objects regularly, consider an IR thermometer that enables you to compensate for variations in emissivity. For example, the Fluke 561 Infrared Thermometer enables you to set emissivity to “High” (for measuring most surfaces, such as wood, paint, rubber, plaster, or concrete), “Medium” (for oxidized metals or granite, for example), or “Low” (for shiny metals).

Obscured Optics?

Where you use your infrared thermometer can also affect its accuracy. For example, if there is steam or dust in the between the target and the thermometer, some of the IR energy may be deflected before reaching the thermometer. Similarly, a dirty or scratched lens on your IR thermometer may impair its ability to “see” the IR energy that it needs to make a measurement. A lens that has fogged when the thermometer is brought into a warm room from a colder environment can also affect accuracy.

Temperature Shocked?

Finally, for highest accuracy, it’s best to allow some time (about 20 minutes is usually enough) for your IR thermometer to come to the temperature of its surroundings when bringing the thermometer into surroundings that are significantly warmer or colder than where it has been stored.

Noncontact infrared thermometers offer a great combination of speed, convenience, and accuracy, but only when they’re used correctly.

To get the best results possible, remember to:

  • Know your IR thermometer’s distance-to-spot ratio, and get close enough to the target so your thermometer reads only the area that you want it to measure.
  • Watch out for (and compensate for) shiny, “low emissivity” objects.
  • Remember that steam or dust can affect the accuracy of IR thermometers.
  • Keep the lens of your thermometer clean and free of scratches.
  • To get the most accurate results, allow some time for the thermometer to come to the temperature of its surroundings.

Learn more

Why Use IR Thermometers in My Application? »
Chip Wade demonstrates how to use an IR thermometer in a short video »