Tips on achieving meaningful images

Software tools allow many factors to be adjusted after the thermal image is stored but there are three important parameters that must be considered in the field – optical focus, dynamic temperature range and image composition. If ignored, resultant images are either useless or, perhaps even worse, display inaccurate temperature measurements that could cause a significant fault to be overlooked.

Focus and temperature range

It may seem obvious to many but the most fundamental requirement when taking a thermal image is getting the target in focus. It’s not just about making sure the thermal image is clear and crisp so you can identify what you’re looking at after you’ve stored the image.  If it is out of focus, temperature measurement will be inaccurate too. 

A typical, professional thermal camera can measure temperatures from around -40 to +1500°C but needs several dynamical ranges to cover this span. These ranges must be adjusted, either manually or automatically, at the point of image capture to suit the application.  If an image is saved outside the appropriate range you won’t be able to see the thermal detail and measurements won’t be accurate.  

The width or narrowness of the thermal span directly affects thermal contrast and the distribution of colour in whichever colour palette you choose.  If the span is too narrow, thermal data will be saturated, too wide and thermal anomalies won’t be visible.  While the choice of colour palette is largely a matter of personal preference, as a rule of thumb it’s best to use low contrast palettes on high contrast targets and vice versa. 

Image composition

All optical imaging systems reach a limit in their ability to resolve very small targets effectively and an infrared camera is no different, except that as well as ‘seeing’ small objects it is also required to measure their temperature.  

A thermal image is built from an array of pixels, tiny sensors in the camera that detect heat radiated from the object. An 80 x 60 pixels array camera provides 4800 heat measurement points and a 640 x 480, a total of 307,200. So, there is huge difference in capability between a basic point-and-shoot model and one designed for the professional thermographer.  

Quite simply, the more pixels your camera has at its disposal, the further you can stand away from the target and still be able to see important detail. So, making a judgement on distance according to the specification of your camera is essential to capturing a great thermal image.  For longer distance targets, maybe use a lens with a smaller field of view, move closer, or do both. And remember digital zoom only magnifies the pixels, it doesn’t compensate for distance or improve measurement.

As well as distance, angle of view and image perspective are important considerations in the field. Both need to be optimal to prevent your body heat reflecting on the target or reflections from other surfaces requiring correction for emissivity. Also, be aware of the conditions around you, especially if you are inspecting a building as sun and shade will seriously skew your findings.

And finally, do remember that thermal imaging can’t see through walls or solid objects such as doors or panels, it can only measure surface temperature.  To get the full picture of the problem, a direct line of sight is essential.