Some infrared cameras do not have the ability to manually adjust the Level and Span, which is the equivalent of brightness and contrast. This can often be accomplished with image processing software after the image has been captured and saved. However, anomalies can go undetected in the field if the level and span are not adjusted well. If no problem is detected in the field, the image will probably not be saved for further analysis.
The FLIR i3, i5, and i7 cameras can be set to “Auto” or “Lock.” In the Auto mode the camera will continuously adjust the level and span. The adjustments depend upon the apparent temperatures in the scene. If there is something very cold in the image, such as the clear sky, the camera will increase the span by decreasing the low end of the temperature span. If there is something very hot, the camera will increase the span by increasing the high end of the temperature span. Whenever the span is increased the contrast is decreased. The span adjustment should be based on your target of interest, or a specific part of your target of interest, not necessarily the entire scene.
Many thermographers using the i series cameras are trying to detect relatively small temperature differences due to lack of insulation, air infiltration, or moisture. If there is a hot or cold object in the scene, the auto mode often sets the span too high and the contrast is too low for easy detection of the anomalies they are seeking. There is a way to use the features on these cameras to get the best possible results.
Consider this image, captured with a FLIR i7:
This is an exterior wall taken inside a building when the outside temperature was about 19°F. A hot cup of water was placed in the scene to simulate a hot transformer or baseboard heating system. Notice that the “Auto” adjust set the temperature span from 52°F to 132°F. This is a span of 92°F. If the “Lock” mode was selected at this time, it would simply lock the temperature scale at this setting. Since we are interested in thermal patterns on the wall, we certainly do not need the high end of the scale at 132°F.
Here is what was done to obtain a more useful temperature span:
- The camera was set to the “Auto” mode and placed very close to the wall, where the wall was uniform in temperature. In this case it was placed between the studs, above the cup, where it appeared quite uniform. This had changed the span. It was now 56°F to 67°F. We had essentially forced a minimum span of 11°F. Here is what the image looked like with the camera close to the wall:
- Next, with the camera still close to the wall, it was set to the “Lock” mode. The wall was then observed from the original position. Here is that image:
In this case, the contrast was maximized because the span was minimized, but the image is still not adjusted optimally because we need to change the “Level.” The “Level” in this image is 61.5°F. Level is also the “Center Temperature.”
- To lower the level, we repeated steps 1and 2, but we observed a slightly cooler part of the wall before changing to the “Lock” mode. Here is the image with our new setting:
In the field, it is not essential to adjust the thermal tuning perfectly, only good enough to determine if there is an anomaly. If there is a problem, the image should be saved. Once saved, the image can be finely tuned using the FLIR QuickReport analysis software. Consider the images below, which were thermally tuned using QuickReport. On the left image, the span has been maximized, producing very low contrast. The image on the right has been optimized to best illustrate anomalies in the wall.