How far from an object can an infrared pyrometer be used?

Optris CSlaser infrared temperature sensor with dual laser aiming

A common question we are asked by our customers is “is there a limit to the distance an infrared pyrometer can be used from an object?”. In this short article, we explain what is possible.

For all infrared temperature sensors, optical resolution is a key part of the technical specifications and always needs to be carefully considered based on the size of the object you are measuring and the distances from the object the sensor will be mounted. For Optris sensors optical resolutions range from 2:1 to 300:1 depending on the model selected.

An explanation of distance to spot size ratio is provided elsewhere and is outside the scope of this article. It is however worth considering what is meant when optical resolutions are provided for each sensor. For example, in the optical chart shown below for our CTLaser LT CF3 it can be seen that the smallest spot size is at a distance of 200mm (the spot size is just 2.75mm).


Optical chart for Optris CTLaser LT CF3


Is the sensor limited to use at this distance?

The simple answer is no. Spot size data is provided for all distances up to 800mm on the chart but even this distance is not a limit. In theory, the sensor can measure temperature over much larger distances of many metres. The spot size at any distance can be calculated by using the D:S, in this case, 9:1. For example, at a distance of 5 metres, the spot size would be approximately 555mm (5000/9). Of course, your object would need to be very big and in this specific case there might be a better solution but this helps to illustrate what is possible.


Is there a theoretical maximum distance?

For general purpose long-wavelength sensors, the transmissivity of air is consistently very high and therefore you could use a pyrometer over a distance of 100 metres or more. The limiting factor becomes the optical resolution and the size of the object being measured.

For short wavelengths, the transmissivity of air varies considerably and “windows” of high transmission are selected for our sensors wavelength sensitivity to avoid any environmental effects. In general applications, distances of many metres will not cause any issue.


What can limit the measurement distance?

Aside from the optical resolutions, the primary limiting factor is any suspended matter between the object and the sensor. For example, dust, steam or smoke will limit the transmission of infrared energy from the object and lead to a false measurement. In these instances, it may not be possible to mount the sensor remotely and control the content of the air. Positioning the sensor closer to the object and fitting a suitable air purge system may help to solve this issue.


This article is intended as a guide and does not replace our own free technical assistance which is always readily available.

If you would like additional assistance, we can help you. Please contact us and we would be glad to discuss your application and help with your product selection.

T: 01628 778688