Everything you need to know about an infrared thermometer

Everything you need to know about an infrared thermometer

Knowing the temperature value is one of the essential needs in industrial, household, agricultural, food, laboratory, and medical units. Two common temperature units are Celsius and Fahrenheit.

The most common tool for measuring temperature is a thermometer. The simplest thermometer is a mercury and alcohol thermometer for household and medical use, with the change of the mercury heat and cold in the graduated mercury thermometer tube, It goes up and down and thus shows the temperature. In addition to environmental applications, mercury thermometers are also used to measure fluid temperatures.

Thermometer has various types. Although, from a point of view thermometers can be classified into two types: laser thermometer and contact thermometer. A contact thermometer is a contact sensor that can be a thermocouple or thermoresistance (PT100), and a non-contact method is usually called a laser thermometer or a gun thermometer.

The importance of knowing the temperature of objects and the environment in various industries has led to the production of various thermometers as follows:

  • Liquid mercury thermometer
  • Resistance thermometer
  • Laboratory thermometer with liquid alcohol
  • Digital laser thermometer
  • Infrared thermometer

The essential role of heat in the industry has encouraged manufacturers to make digital thermometers equipped with data loggers to record data and send it to the computer to analyze and check the data at a predetermined time.

What is infrared radiation?

Infrared radiation is one of the invisible non-magnetic lights and energies that can be reflected, absorbed, and focused. By radiating and returning from any material, this ray vibrates concerning that material and changes in temperature, and this vibration is different in each material and temperature. The hotter the object, the more vibration and infrared it emits, and in the case of a lower-temperature object, this process is the opposite. Since the infrared wavelength can be measured; So this feature can be used in measuring devices.

What is an infrared thermometer?

Non-contact thermometers are better known as infrared thermometers, and laser thermometers can be used at a greater distance and range. Due to the lack of direct contact with the surface of the body and skin, these products are ideal for different tasks, especially health and safety operations during public diseases such as the Corona pandemic. The non-contact thermometer can measure temperatures between -30 and 1300 degrees Celsius. It is better to know that the laser is a guide for the user to set the desired range. The whole mechanism of heat measurement is done by the infrared lens and parts inside the laser thermometer device.

All objects emit infrared radiation, the amount and intensity of which is proportional to the object's temperature. The infrared thermometer concentrates the energy received from the object at one point through the lenses and mirrors built into it. After converting it into an electrical signal, it indicates the temperature.

In short, it can be said that these thermometers, with the help of a lens, focus the infrared energy reflected from an object and deliver it to a piece called a thermopile or detector. Inside the device, the operation of converting this light into electrical energy is done, and after oscillating and calculating the ambient temperature, the object's temperature is displayed on the screen.

Infrared thermometers allow users to measure temperature in applications where conventional contact sensors cannot. In particular, where you are dealing with moving objects (for example; rollers, moving machinery, or conveyor belts), or in cases where non-contact measurements are required(such as high voltage), these temperature sensors can be used where the distance is too high or where the measured temperature is too high for thermocouples or contact sensors. This type of thermometer is also used to measure human body temperature. With the help of the non-contact feature, they are prone to avoid the risk of contagion when used to measure body temperature.

What parameters should we keep in mind when choosing an infrared thermometer?

Important considerations for any infrared thermometer selection include the field of view (target size and distance), type of surface being measured (emissivity considerations), spectral response (due to atmospheric effects or transmission through the surface), temperature range, and installation (portable or fixed). Other considerations include response time, environment, installation limitations, and desired signal processing.

What are the advantages of using infrared thermometers?

Are infrared thermometers worth using? Why can they be used to measure human body temperature? Here are some benefits of infrared thermometers:

Portable and easy to use

Infrared thermometers are always beneficial and portable due to their lightweight. They are shaped like guns and are comfortable to hold. Also, they are very simple to use within seconds. A memory function is also available on most infrared thermometer models, making it easy and efficient to record multiple temperature logs of multiple objects at once.

The instant and accurate temperature reading

The infrared thermometer can measure surface temperature instantly within seconds. The rapid temperature reading of infrared thermometers makes it possible to measure large groups of people in crowded scenarios such as airports and train stations.

In addition, the infrared thermometer is capable of moderately accurate readings with only a measurement error of ±2.0%. Many infrared thermometers today also use emissivity adjustment, which allows you to calibrate the thermometer to match the emissivity of different types of objects, resulting in more accurate temperature measurements.

No contact and no contamination

Since they are non-contact, infrared thermometers allow temperature measurement from a distance. It is very useful for measuring high-temperature levels where it is impossible to touch or detect objects targeted for access directly.

Also, the non-contact feature allows infrared thermometers to measure a person's temperature without actually touching the mouth, armpit, or colon for extended periods. Lack of contact makes contamination less or even non-existent.

What is meant by field of view? Why is it important?

The field of view is the viewing angle in which the laser or infrared thermometer works and is determined by the optical unit. To obtain an accurate temperature reading, the target to be measured must fill the field of view of the device. Since the infrared device determines the average temperature of all surfaces within the field of view, a measurement error can occur if the background temperature is different from the object temperature. Most advanced thermometers from famous companies provide a solution for this problem and display the target in the form of a circle with several lasers or points in the center with laser targeting.

For example, in the case of a circle, a laser sight showing the diameter or circumference of a circle clearly shows the measurement of the desired area. In spot mode, a single laser spot marks the center of the measurement area.

Does the infrared thermometer harm the human body?

While infrared thermometers with many powerful features are now used to quickly and accurately measure human body temperature, there are concerns about their being dangerous to the human body. The main reason for these concerns is that infrared thermometers work by emitting infrared radiation, which is harmful to the body.

But the fact is that they do not send any infrared light or wavelength to the body. This device is designed to detect and absorb heat in the form of infrared rays emitted from the human body and convert it into electricity, so it is safe to use these types of thermometers. Infrared radiation can damage many tissues, but this does not apply to thermometers. According to the first sentence, the thermometer sensor only measures the electromagnetic radiation emitted by the user and does not produce it.

Infrared thermometer parameters definitions


Emissivity is a term used to describe the energy-emitting characteristics of materials. Most (90% of typical applications) organic materials and painted or oxidized surfaces have an emissivity of 0.95 (preset in the unit). Inaccurate readings will result from measuring shiny or polished metal surfaces. To compensate, cover the surface to be measured with black tape or flat black paint. Allow time for the tape to reach the same temperature as the material underneath it. Measure the temperature of the tape or painted surface.

Distance to Spot Ratio or D:S

Field of view is often described by a distance-to-spot ratio (D:S). 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 distance from the instrument to the object that is compared to the size of the spot being measured. The larger the ratio number, the smaller the spot size that can be measured. The laser sighting included in some devices only helps to aim at the measured spot.

The Distance to Spot ratio [D:S] is the size of the area (“spot”) being measured as it relates to the distance of the thermometer from the spot. For example, a D:S ratio of 8 to 1, would measure a 1-inch spot at a distance of 8 inches. In terms of field of view, if the distance to the object divided by the diameter of the object is exactly 8, then the object exactly fills the instrument’s field of view.

As the distance increases, the size of the spot measured would increase. For example at 16 inches, the spot being measured would be 2 inches wide; at 24”, it would be 3” wide.

The D:S ratio impacts the accuracy of the reading. If the target you are measuring is 6 inches in size, and your infrared thermometer has a D:S ratio of 8:1, the maximum distance you can reliably measure the temperature of the target is 48 inches. Beyond this distance, the size of the spot being measured increases, and not only is the target being measured but whatever else falls within the "spot" (or field of view) 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 and lower the average temperature of what is in the "spot". 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.

How do you know what D:S ratio you need?

Using this relationship between the D:S ratio, size of the target spot, and measure distance, you can determine if an IR thermometer meets your needs or, conversely, which IR thermometer you need:

1. If you know your target size and the D:S ratio of the IR thermometer, you can tell how far away you can be from the target and still get an accurate measurement by using the following formula:

D:S Ratio x Target Size = maximum Measure Distance

8:1 x 6 inches = 48 inches

2. If you know how far away from the target you will be, and the size of the target spot, you can determine the minimum D:S ratio required as follows:

Measure Distance/Target Size = minimum D:S ratio required

48 inches / 6 inches = 8 [or 8:1]

3. If you know your distance from the target and the D:S ratio of the IR thermometer, you can tell the smallest target size that can be measured accurately by using the formula:

Measure Distance/D:S ratio = minimum target size for accurate measurement

48 inches / 8 = 6 inches

As the target size decreases or the distance to the target increases, a larger D:S Ratio becomes necessary. We can see this using example 2 above let’s first decrease the target size to 2 inches:

48 inches / 2 inches = 24:1 required D:S ratio

Now let’s go back to the original target size of 6 inches, but increase the measure distance to 60:

60 inches / 6 inches = 10:1 required D:S ratio

D:S Ratios vary greatly, so carefully compare this feature of IR Thermometers when selecting the best IR thermometer to meet your needs.

Accuracy of infrared thermometer

Accuracy depends on the temperature of whatever you're measuring - typically the accuracy gets worse at very high or very low temperatures. Quoted accuracy figures often refer to the accuracy within a specific temperature range (not across the entire temperature range covered), so take care when comparing the specs of different models. You have to notice just because the screen displays the temperature to the nearest 0.1 degrees, it doesn't mean the reading is accurate to within 0.1 degrees.

The accuracy of an infrared thermometer can be checked using a comparator at any stable temperature. However, to reduce the possibility of a difference in temperature between the inside surface and the base test hole, it is more accurate at 22°C, ambient room temperature.

In the end, it should be mentioned that:

  • The unit is not recommended for measuring shiny or polished metal surfaces (stainless steel, aluminum, etc.).
  • The unit cannot measure through transparent surfaces such as glass. It will measure the surface temperature of the glass instead.
  • Steam, dust, smoke, etc. can prevent accurate measurement by obstructing the unit’s optics.
28th Jan 2023

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