Limitations of Infrared Thermometers

Infrared thermometers are extremely useful when used in the right way and for the right applications. However, before you can develop confidence in their ability to give fast temperatures, you must understand their limitations.


Infrared thermometers:

  • Only measure surface temperatures and NOT the internal temperature of food or other materials
  • Require adjustments depending on the surface being measured
  • Can be temporarily affected by frost, moisture, dust, fog, smoke or other particles in the air
  • Can be temporarily affected by rapid changes in ambient temperature
  • Can be temporarily affected by proximity to a radio frequency with an electromagnetic field strength of three volts per meter or greater
  • Do not see through glass, liquids or other transparent surfaces - even though visible light like a laser passes through them (i.e. if you point an IR gun at a window, you'll be measuring the temperature of the window pane, not the outside temperature).


* In some cases, Infrared Thermometers can be MORE accurate than a surface probe because surface probes have their own temperature and can affect the surface being measured by coming into contact with it.

Infra red thermometer FAQ's

What Lasers are in in ETI Products?

All Infrared thermometers sold by ETI Ltd, that have Laser Pointers, comply with BS EN 60825-1: 2014: - Safety of laser products: Part 1. Equipment classification and requirements:- Class 2. The basis of this EU Harmonised Standard is that the laser should be strong enough to make someone blink but not strong enough to damage tissue.This therefore prevents damage to the eye unless someone deliberately stares at the laser light for a long time, causing self-inflicted injury. i.e. For consumer laser products in general there is a requirement that, by employing technical means (i.e. engineering controls), lasers shall not cause damage to the eyes or skin under normal and reasonably foreseeable conditions of use, including momentary accidental or unintended exposure.Also any laser should not be shone in anyone’s eyes; it can be very distracting and could result in a serious incident.We strongly recommend that the laser is only used to help target the area being measured, indicating the optical ratio of the instrument.

Can I check grilling temperatures with an infrared thermometer?

Aiming an infrared thermometer at a porous surface like a grill or grate, will factor in the surface temperature of whatever surfaces are visible through the holes of the grill or grate when calculating a final temperature for your reading. To accurately measure the temperature of a porous grate or grill, place a solid surface like an iron plate or skillet on the grill, let it come to temperature and measure the plate or skillet. Then spray a little cooking oil on the plate or skillet to ensure proper emissivity.

What is emissivity?

"Emissivity" is a measure of a material's relative ability to emit radiated energy. It is measured on a scale from just above 0.00 to just below 1.00.

Emissivity depends on factors such as temperature, emission angle, and wavelength. Generally, the closer a material's emissivity rating is to 1.00, the more that material tends to absorb reflected or ambient infrared energy and emit only its own infrared radiation. Most organic materials, including the byproducts of plants and animals, have an emissivity rating of 0.95.

Check your infrared thermometer to see if it has adjustable emissivity settings as a feature. Then check your target material against this Emissivity Table.

Can I check if food is properly cooked with an infrared thermometer?

Infrared thermometers only measure surface temperatures, so they are not very effective at testing if food is properly cooked. Use traditional probe thermometers for this.

If using an infrared thermometer with liquids like soups and sauces, ensure you stir vigorously before taking a measurement to more closely approximate the internal temperature of the liquid. Be aware that steam, even when a liquid is not boiling, can condense on your thermometer and affect the accuracy of your measurements.

Can infrared thermometers work through glass or clear plastic?

An Infrared thermometer will NOT measure temperature accurately through glass, liquids or other transparent surfaces even though visible light (like a laser) passes through them. If you point an infrared thermometer at a subject through a closed window, you will be measuring the surface temperature of the window itself, not the object you are aiming at.

Can an infrared thermometer measure temperature through water?

No. As in the answer above and for the same reasons, an infrared thermometer will only measure the surface temperature of water, not the object's temperature.

What do spot size, spot ratio and distance to target ratio mean?

The "spot size" of any measurement is controlled by two variables:

  • the "distance to target ratio" or "spot ratio" of your infrared thermometer
  • the distance between your infrared thermometer and the target

Usually listed on the thermometer itself, the "distance to target ratio" (DTR) or "spot ratio" tells you the diameter of the "circle" of surface area an infrared thermometer will measure at a given distance.

For example, an infrared thermometer with a 12:1 ratio will measure the temperature of a 1" diameter circle of surface area from 12" away, a 2" diameter circle of surface area from 24" away, and so on.

Do I need to clean my infrared thermometer?

For accuracy, an infrared thermometer should be kept free of dirt, dust, moisture, fog, smoke and debris. Always take the time to clean your infrared thermometer after exposure to a dirty, dusty, smokey or humid environment. You should also plan a regular cleaning every six months or so. Particular care should be taken to keep the infrared lens or opening clean and free of debris.

To clean your infrared thermometer:

  • use a soft cloth or cotton swab with water or medical alcohol
  • carefully wipe first the lens and then the body of the thermometer
  • allow the lens to dry fully before using the thermometer

Never submerge any part of the thermometer in water.

How do I turn the laser on?

That will depend on the model or type of your infrared thermometer. Read the user manual that came with your infrared thermometer to lear about the full range of features it offers and how to use them.

I am getting strange readings on shiny metals, why?

Highly-polished metals typically have very low emissivity ratings, as they tend to be very reflective of ambient infrared energy and less effective at emitting their own electromagnetic waves. If you point an infrared thermometer with fixed emissivity at a stainless steel pot filled with boiling water, you might get a reading closer to 100°F (38°C) than 212°F (100°C). That's because the shiny metal is better at reflecting the ambient radiation of the room than it is at emitting its own infrared radiation.

Some infrared thermometers have fixed emissivity settings of (usually of 0.95 or 0.97) to simplify their operation while leaving them suitable for most material surfaces, including almost all foods.

Other infrared thermometers come with adjustable emissivity settings, so you can more accurately prepare your thermometer for the type of surface being measured, particularly when measuring non-organic surfaces.

Can I calibrate my infrared thermometer?

Infrared thermometers can be calibrated for accuracy just like other thermometers. In calibration labs like the in-house UKAS certificated ETI Calibration Laboratory at our factory in Worthing, our technicians use industrial black bodies to calibrate infrared thermometers. You can contact them by emailing or ask for the calibration lab on 01903 202151

If neither an industrial black body or a comparator cup are available, however, you can do a quick calibration using a properly made ice bath.


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