Common errors in infrared temperature measurement

While modern non-contact sensor technologies offer advantages over contact-type measurements, there are occasions when an IR sensor may mislead the user.

Infrared temperature sensors determine the surface temperature of an object based on the infrared radiation emitted by the object at any temperature above absolute zero (-273°C). Some types of infrared temperature sensors include thermal imaging cameras, infrared sensors and handheld pyrometers. These modern non-contact sensor technologies offer advantages over contact-type measurement in terms of reliability, accuracy, maintenance and response rate. However there are several occasions when the IR sensor may mislead the user leading to erroneous results. This article aims to clarify these issues to avoid such events from happening and assist the user in obtaining correct and reliable data.

Material emissivity

Emissivity is defined as the ability of a real object to emit thermal radiation in comparison to a theoretical black body at a given temperature, T. Its value lies between zero and one. As IR sensors calculate the object’s temperature based on the emitted energy, failure to specify the correct value of material emissivity will definitely produce errors in the measured value.

Specifying the true material emissivity is a challenge. While the emissivity of the material is known at a certain temperature range, the emissivity is also a dynamically changing property that largely depends on the surrounding ambient temperature and the wavelength of the measuring instrument. Understanding all parameters applied in the measuring instrument is crucial to make the correct temperature measurement. Currently, there are several infrared sensors specifically designed to measure at a particular wavelength to suit a specific industry, such as 7.9 µm for glass and 1.1 µm for hot glowing metals.

Measurement spot size

The accuracy of infrared sensors is also greatly influenced by the measurement spot size, especially when measuring the temperature of small objects. This is often referred to as ‘distance-to-spot ratio’, which specifies the diameter of the area being measured relative to the distance of the sensor from the target. For example, for a sensor with a distance-to-spot ratio of 5:1, it will measure an area of approximately 1 cm diameter when the sensor is located 5 cm away from the target.

It should be noted that the measurement spot size should be adjusted to the size of the measured object. Using the above-mentioned sensor to measure a 10 cm area from 1 m away will produce erroneous results as it will measure the temperature of the area outside of the object. Some models are designed with a laser sighting to point at the target object, however, the size of the laser beam does not always specify the true measurement spot size and users are always encouraged to check the correct specifications of their purchased model before making the measurement.

Reflective and shiny objects

Most infrared temperature sensors produce unreliable data when measuring shiny and reflective objects. In general, shiny and reflective materials emit less infrared energy than normal objects, meaning they have lower emissivity. Although using a non-reflective tape to bypass this issue may work occasionally, this is not feasible for measuring the temperature of hot objects commonly found in most of the manufacturing industry. Some IR sensors designed for measuring at very short wavelength have been developed to reduce measurement errors due to the emissivity change.