Infrared thermography is used to visualise hot & cold areas or dampness in a subject. However, getting the right info is not achieved with a quick snapshot.
Thermal cameras (infrared cameras) are easy to handle and thermal images quick to take. Images are often included in reports after inspecting electrical installations in buildings, and provide evidence of any faults detected.
But choosing the correct camera settings for different tasks is actually quite difficult. Many thermographers, especially those without training, produce reports with thermal images devoid of meaning or, even worse, images that support the wrong conclusion.
This is often the case in companies where thermography is more of a sideline and staff haven’t been trained to understand what a good thermal image is and how to take one.
The aim of infrared thermography
The aim of infrared thermography is to show an object’s condition. Thermal patterns must be illustrated clearly and temperatures measured accurately. The image must contain a clear level of detail and display the object at an appropriate size and position.
Whereas a normal photograph captures reflected light, infrared thermography captures both emitted and reflected radiation. The relationship and intensity of the infrared radiation emitted by both the object and by the surrounding environment are therefore important.
Brightness and contrast in the thermal image are optimised by adjusting the displayed temperature interval that is displayed.
Image adjustment
By default, most infrared cameras continuously adjust the image in the display automatically. They do this by analysing the radiation distribution across the image and then setting the upper and lower limits of the displayed temperatures in a way that optimises the contrast.
However, the danger of always letting the camera revert to its default settings is that the contrast will be optimised for the whole image and not just for the target object.
Check out these side-by-side images to see how thermal detail on the target object is lost…
Most infrared thermal imaging cameras allow the operator to manually adjust the brightness and contrast in the image to get the type of result pictured in the right-hand image above. The image is controlled by selecting a temperature range and setting the level and span…
Span and level
Span is the portion of the total temperature range being used to generate the image. We can make the span wider or narrower.
If, for example, our span is set to 50°C, the colours will be spread out over a 50°C temperature width. But will it be 0-50°C, or 25-75°C, or 50-100°C?
If it is 50-100°C and we are looking at a room-temperature scene, no image will be shown. The whole image will instead have the colour that corresponds to below 50°C in our scale, which is usually black.
The solution to this problem is to force the colours to cover the room-temperature scene we are looking at by adjusting the ‘level’ downwards.
Level is the mid-point of the span. Another way to think of level is “thermal brightness.” Once the level is optimised, we may find that we are still not using all of the colours in the scale. We can fine tune it further to improve the contrast on the target object.
Thermal tuning
The process of adjusting the level and span has a name: thermal tuning.
Definition: Thermal tuning is the process of putting the colours of the image on the object of analysis, in order to maximise contrast.
A comparison of the methods for adjusting the image is shown below…
Level is the mid-point of the span. Another way to think of level is “thermal brightness.” Once the level is optimised, we may find that we are still not using all of the colours in the scale. We can fine tune it further to improve the contrast on the target object.
Thermal tuning
The process of adjusting the level and span has a name: thermal tuning.
Definition: Thermal tuning is the process of putting the colours of the image on the object of analysis, in order to maximise contrast.
A comparison of the methods for adjusting the image is shown below…
The auto-adjust feature may be the easiest option to use, but as described earlier it is a blunt instrument and often not the optimal adjustment for an image.
If the thermal detail displayed on an object is poor, there is a risk that defects may be missed.
A professional thermographer must be confident in their ability to thermally tune an image in the field rather than defaulting to the camera’s auto mode. In order to see more detail and perform better fault analysis, thermal tuning is a key skill.
Click here for details on our infrared thermography service… |
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