Temperature Measurement

Choosing the correct infrared thermometer

June 2009 Temperature Measurement

After time, temperature is the second most measured physical unit in production – as well as in quality control and maintenance.

Temperature represents an important indicator of product quality, safety and equipment conditions. Knowledge of the advantages and the limitations of this technology is needed if the correct infrared thermometer for the application is to be selected. It is also helpful to know the principal of operation of infrared thermometers.

During the last five years the IR sensor market has seen two major trends:


IR thermometers are significantly lower in cost. The most expensive part of an IR thermometer is the lens and detector. New lens materials and technologies (plastic, Fresnel lenses) and mass production of IR detectors have resulted in lower prices. Together, with the increased demand for industrial IR thermometers, high volume production of standard sensors has resulted in greater manufacturing efficiency for the IR thermometer manufacturers. Some types of IR sensors are up to 50% lower in cost than they were five years ago.


IR sensors are becoming smaller in size. In the past, the measurement of low temperatures made it necessary to use fast lenses with relatively large diameters in order to capture enough emitted energy. Progress in detector technology has helped reduce the dimensions of the IR sensing head dramatically. Because the newer sensors exhibit greater sensitivity they require less input energy to produce a usable output signal. As a result, lenses can have smaller diameters. The diameter of a lens may be only a few millimetres, but the sensor has a temperature resolution as fine as 0,1°C, and can measure down to -40°C.

The advantages of IR thermometers are:

* Fast response time (millisecond range), user gets more information per time period.

* Measure the temperature of objects that are moving, rotating or vibrating.

* Measure high temperatures up to 3000°C. (Contact probes will not work or have a short lifetime at high temperatures.)

* No mechanical damage or contamination of the surface (painted surfaces, food, and soft plastics).

* Measure actual product being manufactured instead of another part of the process or inferred measurement.

IR thermometers have special requirements:

* Must have direct line-of-sight.

* The optics of IR thermometers need to be protected against dust or condensation (a variety of accessories are available).

* In general, IR thermometers measure only surface temperatures and the ability to emit thermal radiation depends on the kind of material and especially on the surface finish.

Choice of IR unit

The operating wavelength of the instrument must coincide with that of the application in order for reliable, accurate measurement. It is best to use the shortest possible wavelength for the temperature range and Figure 1 indicates the common wavelengths used with respect to target temperature and error. From this it can be seen that to measure above 900°C one would not use the common 8-14 μm instrument, but rather one having a sensor rated for 1 or 2 μm wavelength.

Figure 1. Target temperature (°C) with emmisivity assumed to be 10% and the ambient temperature lower than the target. This graph clearly illustrates how the wavelength affects the accuracy of the measurement
Figure 1. Target temperature (°C) with emmisivity assumed to be 10% and the ambient temperature lower than the target. This graph clearly illustrates how the wavelength affects the accuracy of the measurement

These factors are important when choosing the correct IR thermometer. Note that there are many units on the market that quote low error on high temperatures and have the wrong wavelength detector. (For example, a range of -50 to 1500°C with a 8–14 μm wavelength.)

Advances in IR thermometer electronic design

Process industries continue to need sensors with higher accuracy and repeatability as they look for ways to cut costs, maximise their production lines, and monitor quality. Easy installation, use, and maintenance continue to be important, as does the ability to withstand harsh environmental conditions. Field checking and field calibration features are becoming more and more important as companies look to optimise their personnel and equipment resources. Manufacturers need IR sensors with a wide operating temperature range that covers more applications and eliminates the demand for water/air-cooled housing, which keeps the cost down due to less water/air consumption and hardware required.

Advances to detectors and optics of infrared thermometers are helping manufacturers and their customers expand the types of applications where IR can be used effectively, gain speed and accuracy, and get a better return on their investments. Two-colour or ratio measurement and units with built-in visual cameras are some of the recent advances.

Raytek and Ircon, suppliers in the infrared measurement industry, design, manufacture, and market a complete line of online and portable noncontact thermometers (IS units included) and line scanners for industrial, maintenance, and quality control applications.

R&C Instrumentation is offering a simple handbook which explains the principles and types of measurement. The handbook covers important factors like different frequencies for different temperatures and single and two colour (ratio) operations. (For a free copy of this handbook e-mail R&C Instrumentation at the address below.)


Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Thermocouples in gasification reactors
November 2019, WIKA Instruments , Temperature Measurement
Gasification reactors, due to their harsh process conditions, place high demands on the instruments used there. Many manufacturers have reacted to this and adapted their products accordingly, but what ...

Hotspot detection in pressboard manufacturing applications
October 2019, R&C Instrumentation , Temperature Measurement
In the building industry, there is a product used that is commonly referred to as pressboard, also known as chipboard. Sheets of pressboard are typically 1,2 m x 2,4 m in size and are used in the construction ...

Pair temperature probes with different cable lengths
October 2019, ASSTech Process Electronics + Instrumentation , Temperature Measurement
In heat meters for heating systems, two paired temperature probes are usually used for symmetrical installation. These must have identical cable lengths to comply with the legal regulations. Now, Jumo ...

Vibration measurement in machine maintenance
October 2019, R&C Instrumentation , Maintenance, Test & Measurement, Calibration
Vibration and temperature are the most common parameters measured on process plants for machine maintenance. Temperature measurement has been around for longer, probably because it was available first ...

IR thermometers in the steel industry
September 2019, R&C Instrumentation , Temperature Measurement
In many foundry applications, temperature readings show whether processes are operating within their proper ranges, whether a reheater is too cold or too hot, whether a stand needs adjusting, or how much ...

Ten facts about infrared windows
September 2019, R&C Instrumentation , Maintenance, Test & Measurement, Calibration
Infrared (IR) inspection is widely used and quickly becoming indispensable in electrical maintenance programmes. With the advent of IR windows, this procedure is now both safe and cost-effective. However, ...

Compact IR camera with industrial accessories
September 2019, Comtest , Temperature Measurement
Comtest has announced an addition to the Optris Compact line of IR cameras – Xi 80 and Xi 400 – with new industrial accessories for use in harsh conditions. The range has a modular design and as a result, ...

IR measurement optimises plastic stationary and rotary thermoforming
August 2019, R&C Instrumentation , Temperature Measurement
Background Thermoforming is the process by which a thermoplastic sheet is made soft and pliable by heating, and bi-axially deformed by being forced into a three-dimensional shape. This process may take ...

R&C Instrumentation receives accreditation from Fluke
July 2019, R&C Instrumentation , News
R&C Instrumentation is pleased to announce that the company has received accreditation from Fluke Process Instruments for its Infrared Temperature Calibration facility. This facility is housed in a climate-controlled ...

Temperature profiling in aluminium heat treatment
July 2019, R&C Instrumentation , Temperature Measurement
Many high volume production foundries use continuous furnaces for the heat treatment of castings. Cylinder heads, wheels, suspension components and many other aluminium alloy products are placed in wire ...