In thermography, “Span” refers to the difference between the high and low temperature settings on an infrared image.
“Level” is the mid-point of that span.
Let me use an example to help further explain this.
All Fluke thermal imagers can be set to either auto or manual span control. In the auto mode, the camera automatically sets the span to the highest and lowest temperatures within the (FOV) Field of View (Figure 1). In manual mode, the user can define the highest and lowest temperatures of the span (Figure 2).
In auto mode, Figure 1 shows the camera with a span of 31°F from a range of 64°F to 95°F. The area of interest is the potential insulation issue seen just above the window curtain (lighter colored area on the ceiling). While we can partially see this potential problem, the issue is somewhat masked due to the higher temperature light fixture on the left wall in the field of view (FOV).
Taking advantage of the “manual mode” we can remove or “saturate out” the higher and lower temperatures not associated with the area of interest, in this case the light on the left. Doing so improves the clarity of the thermal image allowing us to better highlight the insulation issue. This new “thermal window” or defined span within the image offers more detail and can then be adjusted up or down to accommodate the changing scene within the field of view.
Again, a wide span gives less detail and a narrow span offers more detail in the image. According to “best practice” in thermography, keep your span as narrow as possible and adjust the level as needed.
Check back tomorrow for the answer to yesterday’s infrared image teaser! Not too late to still input your thoughts