Focus on a better thermal image

 

Before taking an image, the first thing to understand is that if there is little or no temperature difference between the target and its surroundings, the infrared camera is unlikely to reveal any useful information. A temperature difference between the target and its surroundings is vital, so either create one or only monitor equipment when it’s running.

 

Once ready to take an image, consideration should be given to factors such as optical focus, image composition and dynamic temperature ranges – none of which can be changed once an image has been saved. 

 

Firstly, optical focus has a huge impact not only on image appearance, but what it is possible to measure accurately. Attempting to analyse an out-of-focus image will only lead to incorrect conclusions.  

 

In terms of image composition, always place the target centrally in the viewfinder and ensure a distance is maintained where the target fills the screen. If this proves tricky, the advice is to either use a smaller field of view, or get closer. 

 

With regard to dynamic temperature ranges, advanced cameras will offer three or four ranges to cover extremes from -40°C to +2000°C. However, dynamic ranges need to be changed manually, with the camera indicating via numeric and visual warnings when the calibrated range is being exceeded.

 

Within the temperature ranges, cameras tend to adopt a scale, where a level or span can be exploited to achieve better thermal images. If a thermographer starts scanning around the room or site in automatic mode, he or she will notice that the scale constantly changes. This is because the camera is continuously looking at the upper and lower temperatures, or the amount of incoming radiation, and adjusting the scale accordingly. With this in mind, a manual approach must be adopted in order to start identifying anomalies. 

 

Thermal tuning is the manual control of the level of span. In essence, this entails placing colours from the scale on the part of the image that the thermographer wants to analyse, thus highlighting patterns such as thermal gradients. It allows users to better identify loose connections in electrical equipment or missing insulation in buildings, for example.

 

Another factor to consider is distance. Most cameras will measure a 20cm² target at a distance of 200m, for example, but it might be necessary to move closer if greater measurement accuracy is required. A good indication is the ability to fill the crosshair circle with the target. 

 

Where getting close to the target is not possible, it may be necessary to use a lens with a smaller field of view. Those thinking that the digital zoom serves the same purpose should think again. The zoom simply magnifies the pixels and offers little in the way of help when it comes to accurate temperature measurement.

 

Last but not least is the angle of view. Thermographers standing perpendicular to a target with low emissivity will see a reflection of themselves (a target with a shiny or reflective surface is said to have low emissivity). Conversely, getting too close to 90° will see natural emissivity drop. Indeed, this situation can also manifest itself on pipes, tubes and cylindrical objects. The advice from FLIR is to adopt a position of 45-50° from perpendicular, which is a known ‘safe zone’.

 

Ultimately, armed with a few handy hints and tips, professionals tasked with detecting hot spots or anomalies in machinery, equipment or buildings can achieve far better thermal images. 

 

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