The infrared temperature sensors determine the surface temperature of an object based on the infrared radiation. Infrared radiation is emitted by all objects at any temperature above the absolute zero. Some types of infrared temperature sensors include thermal imaging camera, infrared thermometer sensor and handheld pyrometer and camera. These modern non-contact thermometers utilise infrared measurement principle which offers an advantage over contact-type measurement in terms of reliability, accuracy, maintenance and response rate. They also do not physically interfere with the process during the measurement, allowing the users to gain the accurate temperature that occurs during the heating/ cooling process. However, there are several factors that users need to understand as conducting measurement using an IR camera may sometimes produce erroneous results if correction is not applied. This resource is aimed to clarify these issues to avoid such events from happening and assist the users in obtaining correct and reliable data.
Materials Emissivity
Emissivity is defined as thermal radiation energy emitted by a real object in comparison to the black body at a given temperature, T which value lies between zero and one. As IR sensors calculate the object’s temperature based on the emitted radiation energy, failure to set the correct value of materials emissivity will produce errors in the measured value.
However, 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 object temperature. Understanding all parameters in the measuring instrument is crucial to make the correct temperature measurement. There are several infrared cameras and laser temperature sensors specifically designed to measure at a particular wavelength to suit a specific industry, such as 7.9µm for glass and 1.1µm for hot glowing metals.
Measurement Spot Size
The accuracy of infrared pyrometers is also greatly influenced by the measurement spot size, especially when measuring temperature of small objects. This is often referred to as “distance-to-spot ratio” which specifies the diameter of the area being measured in response to the distance of the sensor from the target. For example, for a sensor with distance-to-spot ratio of 5:1, it will measure approximately 1cm of diameter when the sensor is located 5cm away from the target.
It should be noted that the measurement spot size should be adjusted to the size of the measurement object. In some applications, the sensors must be installed at a certain distance from the measuring objects. Therefore, a sensor with a low distance to spot ratio might not be suitable. On the other hand, you do not want the spot size to be unfocused and cover the whole object as it will return inaccurate measurement if there are temperature variation within the object. Some models for laser temperature measurement 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 and store unreliable data into a temperature data logger when measuring shiny and reflective objects. In general, shiny and reflective materials emit less infrared energy than normal objects, meaning the sensors have to be adjusted to lower emissivity when used to measure on these objects. Although using a non-reflective tape to bypass this issue may work occasionally, this approach is not feasible for measuring temperature of hot objects. Some IR sensors that are available to measure at very short wavelength have been developed to reduce measurement errors due to the emissivity change.
