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Infrared Thermometers

Quick Tips #370

Handheld Infrared (IR) Thermometers are becoming a widely used technology throughout many industries and environments to determine surface temperature. The IR thermometer was originally developed to measure the temperature of an object where normal contact thermometers could not be used such as: a moving object, an object in a vacuum where time-sensitive readings are required or to measure the temperature of an object with limited access. They are now commonplace and used everywhere.

But before we discuss handheld infrared thermometers in detail, let's first define infrared radiation and its role in determining temperature.

Infrared Radiation

Infrared radiation (IR) is just one type of radiation that exists within the Electromagnetic Spectrum. Additional types of electromagnetic radiation include microwaves, X-rays and visible light. The illustration below shows wavelength and frequency of the electromagnetic spectrum.

Electromagnetic Spectrum

The wavelength of infrared rays is longer than that of red rays. If we could see infrared rays on the color spectrum, they would appear just after or below red. The Latin infra translates to "below."

Infrared radiation is, quite literally, heat. Although our eyes cannot visibly detect IR, we can surely feel it. Wrap your hand around a cup of coffee, take a walk in balmy weather or enjoy sizzling fried chicken; in all of these experiences, you are interacting directly with IR. It is this IR (heat) that can be measured and used.

How is IR used to determine temperature?

All matter emits energy in the form of IR (heat). If there is a temperature difference between objects, including the surrounding environment, then this gradient can be measured and used. If the object in question is at the same temperature as its surroundings, the net radiation energy exchange will be zero. In either case, the characteristic spectrum of the radiation depends on the object and the surrounding absolute temperature, which is relative to absolute zero (0 K, –273.16°C, –459.69°F). Handheld infrared thermometers take advantage of this "radiation dependence" on temperature to produce a value for the targeted object and to display the results for the operator to read.

Infrared light works like visible light—it can be focused, reflected or absorbed. Handheld infrared thermometers typically use a lens to focus infrared light from one object onto a detector, called a thermopile. The thermopile absorbs the infrared radiation and turns it into heat. The more infrared energy, the hotter the thermopile gets. This heat is turned into electricity. The electricity is sent to a detector, which uses it to determine the temperature of whatever the thermometer is pointed at. The more electricity, the hotter the object is. The higher the temperature, the more electricity sent to the detector, the higher the reading.

Handheld Infrared Thermometer Advantages

The advantages of handheld infrared thermometers are fast, accurate response times and convenience—ideal for remote monitoring. The noncontact feature of IR thermometers allows for temperature measurements to be taken without touching the product being tested. This allows for the safety of the worker as well as limiting contamination to the product being measured.

The response time (detection to display) of an IR thermometer is typically about 0.5 seconds. Maximum measuring distance is determined by the quality of the internal optics and atmospheric conditions. A handheld IR thermometer can only measure the surface temperature of an object and not the internal temperature. In such a case, you may want to look into an IR Camera. Because the maximum measuring range and accuracy can be affected by atmospheric conditions (water vapor or CO>2), the maximum range (generally) is limited to approximately 100 feet.


The accuracy of the handheld infrared thermometer is primarily determined by the distance-to-spot ratio (D/S Ratio). This ratio is the size of the area being evaluated by the infrared thermometer as it relates to distance. In other words, the area being measured becomes larger as the distance increases. This ratio will have a significant impact on the accuracy or precision of the reading. If the target you are measuring is 6 inches in size, and your handheld infrared thermometer has a D/S ratio of 8:1, then the maximum distance at which you can reliably measure the temperature of the target is 48 inches. Beyond this distance, not only is the target being measured, but whatever else falls within the "spot" is being measured as well.

This means that if a very hot object is the target, and it is in cooler surroundings, then measurements taken beyond the maximum distance will include cooler elements, lowering the "average" of what is in the "spot."

Ratios & Targets

D/S Ratio x Target Size or 8:1 x 6 = maximum measuring distance of 48 inches.

As the target size decreases, or the distance to the target increases, a larger D/S ratio becomes necessary. Using the same example above, and changing first the target size and then the D/S ratio, you see that this formula helps you decide the correct D/S ratio and, subsequently, the handheld infrared thermometer for your needs.

D/S Ratio x Target Size or 8:1 x 2 = maximum measure distance of 16 inches.

D/S Ratio x Target Size or 12:1 x 2 = maximum measure distance of 24 inches.

D/S ratios vary greatly, so carefully compare this feature of handheld infrared thermometers when comparison shopping. This ratio and temperature range are the two biggest factors to consider when shopping for an infrared thermometer. The example below explains the D/S ratio.

Field-of-view—Make sure that the target is larger than the spot size the unit is measuring. The smaller the target, the closer you should be to it. When accuracy is critical make sure that the target is at least twice as large as the spot size.

Distance-to-spot ratio—The optical system of an infrared thermometer collects the infrared energy from a circular measurement spot and focuses it on the detector. Optical resolution is defined by the ratio of the distance from instrument to the object compared to the size of the spot being measured (D:S ratio). The larger the ratio number, the better the instrument's resolution, and the smaller the spot size that can be measured. The laser sighting included in some instruments only helps to aim at the measured spot.

A recent innovation in infrared optics is the addition of a Close Focus feature, which provides accurate measurement of small target areas without including unwanted background temperatures.

Handheld Infrared Thermometer Disadvantages

Handheld infrared thermometers have a disadvantage when measuring an object that reflects light well. This characteristic is called emissivity, or the ability of an object to absorb or emit energy. Infrared thermometers that can adjust for emissivity will be able to more accurately measure shiny, metal objects.

With the characteristics given above, you can see that the primary disadvantage to the use of a handheld infrared thermometer is in how the tool is used. It is up to the user to ensure the tool is being used properly, taking into account its specific application and has a handheld infrared thermometer that is appropriate for that application. It should be understood that the tool is a screening device and not an actual temperature of the item being measured.

Levels of Infrared Thermometers

There are many different handheld infrared thermometers for all levels of speed, accuracy and applications. Your basic and most economical infrared thermometer has a measurement range from 0 to approximately 600°F, with an accuracy of +/– 3.5° F (Fluke® 61 IR Thermometer and Extech® IR Thermometer). These simple yet effective tools are used by HVAC technicians to measure air temperature and motor surface temperature. They can also be used in the food industry to measure the temperature of foods in storage or of foods being served. The more expensive handheld infrared thermometers have larger measurement ranges and higher distant-to-spot ratios for measuring smaller objects at greater distances. These units also have the capability to data log for data evaluation at a later date and may be used in hazardous locations. An example of this is Fluke 568 IR and Contact Thermometer. These more expensive units might be used in production environments where a process temperature needs to be monitored and evaluated or if the object that needs to be monitored is at a greater distance, and your distance-to-spot ratio needs to be considered.

Infrared Thermometer Questions / Resources

Questions and Answers

1. What is emissivity?

Emissivity is the ability of an object to emit or absorb energy. Perfect emitters have an emissivity of 1, emitting 100% of incident energy. An object with an emissivity of 0.8 will absorb 80% and reflect 20% of the incident energy. Emissivity may vary with temperature and spectral response (wavelength). Handheld infrared thermometers will have difficulty taking accurate temperature measurements of shiny, metal surfaces unless they can adjust for emissivity.

2. How can the emissivity of an object be determined?

  1. First, measure the surface temperature of the object to be measured with a surface-type thermocouple probe. Measure the same surface with a handheld infrared thermometer, adjusting emissivity on the thermometer until the temperature readings on both the thermocouple and IR meters agree.
  2. According to Cole Parmer, manufacturer of Oakton IR thermometers, for temperatures up to approximately 500°F (260°C), place a piece of regular masking tape on the object to be measured. Allow the tape to reach thermal equilibrium with the object. Using a handheld infrared thermometer with the emissivity set at 0.95, measure and note the temperature of the masking tape. Then, measure the surface temperature of the object. Adjust the emissivity until the temperature of the object is the same as that of the tape.


(Rev. 1/2012)

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