Avoiding Reflections During IR Testing

Jason Tranter | Founder & CEO, Mobius Institute

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  Avoiding Reflections During IR Testing

  Thermal imaging cameras and spot radiometers detect the thermal radiation coming from a surface. That’s how they work. Therefore, what you are measuring is the reflection, the transmission, and the emission. So they’re all displayed there, but imagine your camera is in a particular place. You’re going to see whatever reflects off that surface, whatever is emitted from that surface, and the transmission of heat that comes through what you’re testing to where you are testing. It’s unlikely for most of the things that we test with condition monitoring that you would be measuring the transmission of heat through the surface that you’re looking at. It’s going to be a combination, therefore, of the emission from that surface—because of its temperature—and the reflection of another heat source. That reflection could be from the sun if you’re outdoors, it could be reflection from another heat source, or it could be reflection from you. If you look at the situation here (shown in video), there’s a person standing against a wall, and if we look at that through an infrared camera, there is the person standing there (mostly orange), but there is the reflection (mostly violet). We could be doing a test, and there could be heat sources in the area, including the sun and including you, that heat reflects off what you’re testing and then comes into the camera, and you can think that the object you’re testing looks hot, but it’s actually just a reflection off something else. In this image, which came from the Fluke.com website, it looks like there’s a lot of heat there, but it’s actually just the reflection from the operator of the infrared camera.

  So what’s the tip then?

  1. Look at the shape of the hot spot. If you see that something looks hot, you have to think—and this is a big question—about what makes something hot to be tested with infrared energy, and a reflection might be more of a point source, and something that isn’t actually hot wouldn’t look like that.
  2. Look at the surface. If it’s shiny, it could be a reflection. What is the angle of you to the surface that you’re testing?
  3. The other thing you can do—and this is really the point of the tip—is move. If you just move from side to side and up and down, and if the hot spot appears to move on that surface, then you know it’s reflection. If it stays in the same place, then you know it is coming from that surface.

  If you look at this reflection here, imagine if the operator just moved a little bit to the left or to the right, that hot spot would move as well; it would reflect off a different part of the surface, and you’d see that there is nothing hot there, it’s just the reflection.

Tags: Testing, emissivity, infrared, Jason Tranter, Mobius Institute, radiation, reflection, reflectivity, transmission

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