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Infrared Thermometers

Quick Tips #370

Handheld Infrared (IR) Thermometers are widely used throughout many industries and work environments to determine surface temperature. Elevated temperatures are often the first sign of trouble for mechanical equipment, electrical circuits and building systems. A quick temperature check of key components and equipment can detect potential trouble areas and prevent catastrophic failures. The IR thermometer was originally developed to measure the temperature of an object where a normal contact thermometer could not be used such as on 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.

Explaining the use of an IR thermometer begins by defining what these devices measure to determine temperature—infrared radiation.

Infrared Radiation

Infrared radiation, or 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 IR rays is longer than that of the visible spectrums red rays. If we could see IR rays on the electromagnetic spectrum, they would appear just after red on the visible spectrum. The Latin infra translates to "below."

IR 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. Handheld IR 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.

IR light works like visible light—it can be focused, reflected or absorbed. Handheld IR thermometers typically use a lens to focus light from one object onto a detector, called a thermopile. The thermopile absorbs the IR radiation and turns it into heat. The more IR 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. The higher the temperature, the more electricity sent to the detector, the higher the reading.

Handheld Infrared Thermometer Advantages

Handheld IR thermometers are fast, accurate and convenient—ideal for remote monitoring. The noncontact feature allows temperature measurements to be taken without touching the product being tested. This is not only important for the safety of the worker but also limits potential product contamination.

The response time (detection to display) of an IR thermometer is typically about one-half second. 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. Because the maximum measuring range and accuracy can be affected by atmospheric conditions (water vapor or carbon dioxide) the maximum range is generally limited to approximately 100 feet.


The accuracy of the handheld IR 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 IR thermometer as it relates to distance. In other words, the area being measured becomes larger as the distance increases. The smaller the target, the closer you should be to it. This ratio will have a significant impact on the accuracy or precision of the reading. If the target you are measuring is six inches in size, and your handheld infrared thermometer has a D/S ratio of eight to one, then the maximum distance at which you can reliably measure the temperature of the target is 48 inches (8:1 x 6 = 48). 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."

As the target size decreases, or the distance to the target increases, a larger D/S ratio becomes necessary. Using the same example 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 IR 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 IR thermometers when shopping. The two biggest factors to consider when selecting an IR thermometer are the D/S ratio and temperature range.

A recent innovation in IR 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

IR thermometers only measure surface temperature and do not measure through glass, liquids or other transparent surfaces. They also may require adjustments depending upon the surface being measured especially if it is a highly reflective surface. IR thermometers can be temporarily affected by frost, moisture, dust, fog, smoke, other particles in the air, rapid changes in ambient temperature and proximity to a radio frequency with an electromagnetic field strength of three volts per meter or greater.

Levels of Infrared Thermometers

There are many different handheld IR thermometers for all applications and all levels of speed and accuracy. The basic and most economical IR thermometer has a measurement range from 0 to approximately 600° F, with an accuracy of ± 3.5° F (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 IR 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 and some 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 at great distances.

Commonly Asked Questions
Q: What is emissivity? A: Emissivity is the ability of an object to emit or absorb energy. Perfect emitters have an emissivity of one, emitting 100% of incident energy. An object with an emissivity of eight-tenths will absorb 80% and reflect 20% of the incident energy. Emissivity may vary with temperature and wavelength. Handheld IR thermometers will have difficulty taking accurate temperature measurements of shiny, metal surfaces unless they can adjust for emissivity.

Q: What is the best way to clean an IR thermometer? A: To be accurate, IR thermometers must be kept clean. To clean, use a soft cloth or cotton swab with water or medical grade rubbing alcohol and carefully wipe first the lens and then the body of the thermometer. Allow the lens to dry fully before using the thermometer. Never use soap or chemicals and never submerge any part of the thermometer in water.


(Rev. 8/2015)

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Please Note:
The information contained in this publication is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This publication is not a substitute for review of the current applicable government regulations and standards specific to your location and business activity, and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.

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