Choosing an Infrared Thermometer

Infrared thermometers are ideal for taking temperature measurements at a distance with relative accuracy. They provide a way to measure the temperature of a surface/object without having to touch or penetrate it. This is particularly useful when the surface to be measured is inaccessible or if it is impractical to insert a probe. Such conditions include:

Infrared thermometers are typically very fast and comparatively easy to use. They can provide a reading within a fraction of a second or in the time it takes for its processor to perform the calculations. They have become invaluable to public safety tools in the food service industry, manufacturing, HVAC, asphalt & concrete, labs and countless other industrial applications.

As with any situation, choosing the right thermometer depends on its application and desired outcomes. Infrared thermometers are extremely useful but it is important to be aware of their limitations. There are times when an Infrared is the better choice of thermometer type. For example, a surface probe has its own temperature and can affect the surface being measured by coming into contact with it. In this circumstance, an Infrared Thermometer would provide a more accurate reading.

There are three basic types of infrared thermometer optics, each with its own advantages and disadvantages:

Infrared Lens Cost Durability Temp Range Optics Ambient Effect
No Lens $ ‡‡‡ Hot Good ‡‡‡
Fresnel Lens $$ ‡‡ Hotter Better
Mica Lens $$$ Hottest Best ‡‡

An infrared reading is determined by the calculated average temperature within the full field of view. When choosing an Infrared Thermometer it is important to consider the anticipated temperature range you require as well as the environment and surface type of the selected target. It’s ideal to use a thermometer with a specified range that most narrowly covers what you are planning to measure.

It is a valuable approach to regularly clean and calibrate your Infrared Thermometer in order to sustain accurate readings from your device.

Infrared Thermometer Limitations

  • • Only measures surface temperatures, NOT the internal temperature of food or other materials
  • • Requires 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
  • • Does not see through glass, liquids or other transparent surfaces regardless if visible light passes through them (ie, if you point an IR gun at a window, you’ll be measuring the temperature of the window pane, not the outside temperature)

No Lens Thermometers

No-lens Thermometers, like the IR Pocket Thermometer, use a reflective funnel design to focus infrared energy against the thermopile rather than a lens.


  • • Typically cheaper
  • • Higher durability
  • • Generally smaller and easier to handle than other types of Infrared Thermometers
  • • Most accurate in cold spaces

The lack of lens between the electromagnetic waves being emitted by a surface and the thermometer’s thermopile circumvents significant contraction or expansion effects. In most units, an internal sensor compensates for the ambient temperature effect on the electronics themselves. This allows you to literally walk from a warm room directly into a sub-zero freezer and acquire accurate measurements immediately.

The primary drawback to no-lens thermometers is the limited distance to target ratio element. The DTR is always 1:1 or lower resulting in the need to hold no-lens thermometers as close to the target surface as possible. An alternative type of Infrared Thermometer would be more appropriate when taking measurements at a distance.

Mica Lens Thermometers

Mica lens thermometers have more rigid mineral-based ground lenses and are the most common type used in industrial settings.


  • • Take accurate measurements at much higher temperatures (above 1,000°C)
  • • Roughly half as susceptible than Fresnel lens thermometers to the thermal shock effects caused by sudden swings in ambient temperature
  • • Most accurate at greater distances (DTR above 20:1)

Mica lens thermometers generally come with one or two laser guides to assist in both the aiming of the thermometer and the estimation of the field of view being measured. This type of thermometer is the most fragile and likely to crack or break when dropped so are often supplied with a carrying case. Typically the most expensive, these units also require approximately 10 minutes to acclimate when providing accurate readings in extreme ambient temperatures.

Mica Lens Products: RayTemp 28, RayTemp 38

Fresnel Lens Thermometers

Fresnel lens thermometers are generally plastic and are the most common type used in the food industry.


  • • Less expensive than mica lens thermometers
  • • More durable than mica lens thermometers
  • • Can offer tight spot diameters at a greater distance than no lens thermometers
  • • Typically more accurate at a 6” to 12” distance than other technologies

Fresnel lens thermometers generally come with laser guides to assist when aiming the thermometer. These thermometers have a narrower temperature range than the more versatile mica lens and are more susceptible to thermal shock. This is when sudden swings in ambient temperature such as moving from room temperature into a walk-in freezer will cause inaccurate temperature readings. Sudden temperature changes may also change the shape of the Fresnel lens.

Most Fresnel lens thermometers will display an error alert when this happens and will give faulty readings until the lens has acclimated to the new environment. Allowing your device to rest in the new ambient temperature for approximately 20 minutes beforehand can dramatically reduce the distortions and inaccurate readings.

Fresnel Lens Products: RayTemp 2, Thermapen IR, Mini RayTemp, RayTemp 3, RayTemp 8

Cleaning and Caring for Your Thermometer

It is vital to ensure your Infrared Thermometer remains free of dirt, dust, moisture, fog, smoke and debris. If your device is exposed to any of these conditions it can affect its accuracy and will need to be suitably cleaned. Regular cleaning every six months or so will facilitate the ongoing efficiency of the unit with particular care to keep the infrared lens or opening clean and free of debris.

To clean your infrared thermometer:

  1. 1. Use a soft cloth or cotton swab with water or medical alcohol (never use soap or chemicals)
  2. 2. Carefully wipe the lens and then the body of the thermometer
  3. 3. Allow the lens to dry fully before using the thermometer
Never submerge any part of your infrared thermometer!

Store your thermometer between 4-65°C and protect it from extreme temperatures while in storage.

Calibrating an Infrared Thermometer

Infrared Thermometers can be calibrated for accuracy just like any other thermometer. In Ross Brown Sales’ Laboratory, using industrial black bodies, our technicians calibrate infrared thermometers within Australian Standards and include a 12 month traceable certificate.

Black bodies have approximate zero reflected ambient radiation and therefore unimpeded emission of infrared energy for a given emissivity value, typically 0.95. Without having access to a black body, a simple, inexpensive Infrared Comparator Cup is the next best thing. If neither option is available, a quick calibration using an ice bath is achievable.

Using a boiling point for calibration is problematic (more so with an infrared), factoring in variables in air pressure and elevation, as well as the steam generated by boiling. That steam and the evaporative cooling and condensation makes it difficult to obtain an accurate infrared measurement of a boiling liquid. The surface of a properly made ice bath is a reliable 0°C and is therefore the preferable method.