Question from a customer: “Scanning through the steel grating I read a temperature 5 to 15 degrees Celsius lower than scanning without the grating. I know it is to do with the steel grating but I was wondering why.”
Great question. Let’s take a look at a typical situation with and without a grating, and then placing the grating at different distances to the camera.
The distance between the fuse and the camera remains constant, and the camera is always focused on the fuse. The only changes are the insertion of the grating, and the distance of the grating from the infrared camera.
Figure 1. Looking at a fuse directly with no grating. Max temperature is 51.4 C.
Figure 2. We have inserted the grating close to the fuse. Max temperature dropped to 51 C, a small error.
Figure 3. Now we have moved the grating away from the fuse and closer to the camera. Note that the grating has become blurred and the maximum temperature has dropped to 44.6 C.
Figure 4. We have moved the grating closer still to the camera. Note that the blurring effect of the grating has virtually disappeared, leaving a fairly good IR image, but also note that the maximum temperature has now dropped to 41.5 C.
Figure 5. The grating has been positioned up against the camera lens. The image has regained clarity, but the temperature measurement remains low due to the defocussed grating adding its lower temperature radiation while blocking some of the target higher radiation at the same time.
So the bottom line is, that temperatures read through a grating, will not be accurate unless the grating is close to the target and appears well focused in the image (and your high temperature spots are large enough to be resolved by your camera, but that’s another post!).
In order to make measurement through a grating, the effective transmission must be measured and entered into your camera in a manner similar to that used for characterizing infrared windows.
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