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Reflected Background and its Potential Impact on a Good Thermal Image

For today’s subject I would like to first draw an analogy between infrared energy and visible light.  We all know that in our world, light interacts with us in different ways.  It can pass through certain objects such as the windows of your house, it can be reflected like the glare you see when looking at the water on a bright sunny day, or it can be absorbed by an object.  Ever touch a dark-colored car that has been sitting in the direct sun on a hot day?  Infrared energy, or ‘heat’ energy, can be transmitted (pass through certain objects), reflected, or absorbed.

Infrared energy is all around us and every object in our industrial plants, offices and homes emit it to various degrees.  The intensity of the infrared energy from background objects depends on a number of factors, including how hot it is.  The ‘reflected’ background is simply the infrared energy all around us that is being reflected off of objects we are inspecting and into the lens of our camera. The bad news is that our camera doesn’t know the difference between the actual heat being emitted from our target and that of the ‘reflected energy’ from somewhere else—it just tells us that heat energy is there.

The funny thing about all of this is that WE often can’t tell the difference right away either!  So you may ask, “Who cares?” Well, if we want to avoid confusion and the possibility of making an incorrect analysis, we need to learn to adjust our position, adjust the position of the camera, or control the background.  Understanding the difference between reflected energy and emitted energy is important in helping to identifying where the real problem is and may also be important when trying to make more accurate temperature measurements.

Below are two images portraying an actual fuse interconnect showing spots of reflected energy from a background heat source.  As highlighted by the red arrows, note that from Figure 1 and Figure 2 the reflection has moved.

First Step – Being aware that background energy exists! Here are a few easy steps that will help uncover potential reflections and separate them from actual problems, if a problem even exists.

  • Be aware of ‘emissivity’!  Emissivity – in the simplest terms, it is the reflective properties of surfaces.  All materials or objects we look at with infrared have reflective properties.  Some things are very reflective—like polished aluminum, a bus bar, copper wire or other newer metal objects.  Other surfaces are less reflective—like rubber, wall paint or even our skin.  The latter group is considered to have a high emissivity and the first group (newer metal objects) tends to be very reflective to infrared energy, therefore having a low emissivity.  Referring back to Figures 1 and 2, the background reflections are a result of the enclosure and interconnect materials being reflective and having a low emissivity.  (Emissivity will be covered in more detail in a future blog!)
  • Be aware of ‘background energy’! We may naturally think of ‘background energy’ as energy coming from behind us and reflecting off of a shinier object in our field of view, but this isn’t always the case.  Background energy can come from anywhere depending on what you are viewing and your position relative to it.  Note the example below where the reflected energy is coming from straight in front of us.

In Figure 3 we are looking at a truck in a garage and can see the exhaust system being reflected off the floor in the foreground.

In Figure 4 you may see a legitimate problem with the elevated interconnect on the A Phase when examining an electrical cabinet.  Also, notice the apparent warm spot at the arrow—this could be a reflection of energy from the background; probably a reflection of the thermographer in the shiny metal of the fuse cap and clip.

How do you take control of the target area and your camera to get a good image?  An effective practice is when you see something you suspect may be a reflection, move to the left or right a step or so. By changing position, those thermal spots that are just reflections from another source will move with you.  This is similar to the changes you would see when looking at what’s behind you in a bathroom mirror.

Some reflected spots can be eliminated very easily by positioning your body to block the heat energy so the reflected spot moves off of the targeted area.  In other situations, you may want to place an object, such as cardboard, between the background and the target.  In some cases you may be able to switch off the hotter or colder objects in the background.

Simple case study: Figure 5 is an example of a hot, polished aluminum drum showing reflections of other drums above and to the side (indicated by arrows).  Trying to do a thermal study in this application is difficult because of the nature of the material (very reflective) and its size.  In this case we used wax to change the emissivity.

There are times when you may be interested in knowing how much thermal energy is in your background. Here are a few things that you can do to get an idea of the potential impact there might be on your target:

Ways of estimating the background temperature

  1. Simply turn around and scan the area around the targeted asset and estimate the average temperature (eye ball it!) in the field of view as you pan the area behind you.
  2. Use room temperature, and/or take images of the background.
  3. More specific way is to use an aluminum foil curtain.  Crumple up a piece of aluminum foil and then flatten it back out. Hold it in between your target and camera and note the average temperature observed in your field of view as shown in the illustration below—it acts like a diffuse reflector!

For general thermography work it’s important to understand your background and take reasonable steps to control it and/or compensate for it.  This often gives you less confusing thermal images, and can sometimes allow you to obtain more accurate temperature measurement results as well.

Reflected infrared energy is only one part, albeit a very important part, of understanding what our thermal imagers are really showing us.  We must make sure that we know how to recognize it!

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