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Reflecting on Electromagnetic Radiation

Reflections, no matter what part of the electromagnetic spectrum we are operating in, are part of life. All materials are partially reflective of some wave lengths and none are perfectly reflective.

We are all familiar with mirrors, but watch a baby or a puppy when they are first looking into a mirror and learning how to distinguish a reflection from reality. We are so accustomed to seeing the world around us, we forget that nearly all of what we see is based on reflected visible light. Don’t believe me? Turn the lights out and tell me what you see!

Colors are simply selective reflectors—energy of different wavelengths being reflected by changes at the atomic level of the surfaces around us. That energy is then received by our eyes and processed in our brain as “red” or “blue” or any of thousands of subtle variations in shade and tone.

Glass is not highly reflective of longwave thermal energy, only about 20%, but the reflections are quite clear due to the smooth surface of the glass. Because of this, it is easy to see the reflections of surrounding buildings (yellow) in this glass-faced skyscraper (blue). The blue is, in part, a reflection of the clear—and colder—sky.

The world of infrared is similar. Each surface has a unique ability to reflect the wavelengths we are detecting with our imagers, typically a longwave band from 8 to 12 microns. Of course they also emit at the same time and a few transmit. But all surfaces reflect to some extent. Most non-metal and non-polished surfaces don’t reflect very highly, but they still reflect some radiation. Even human skin, regardless of visual color, is approximately 2% reflective.

Nearly all of us are very familiar with the obvious reflections we see in a piece of glass or a window. While such reflections are not strong, they are quite clear or, as it is termed, specular. Most of us are also familiar with reflections off bright metals. These too are often specular, at least when the metal if flat and polished, but they are often much stronger than glass because metals are more reflective for longwave radiation.

Aluminum can make a great thermal mirror. You can get surfaces that are more reflective, but the price goes up quickly as you move on to a front-surfaced mirror or a gold-plated surface.  If you have not yet experienced the beauty of a thermal mirror, get yourself a flat piece of 1/8” aluminum plate. Even if it is not highly polished, you’ll be amazed by what you can see. Have fun exploring what you can see and how the mirror behaves. Try measuring the temperature of the reflected object and compare that to the reflection of the object itself.

Reflections off a piece of aluminum plate are not only very clear but also very strong. Typically 95% of the energy is reflected. In this thermal image the author appears on the left and the reflection of the author on the right.

How fast do reflections happen? At any wavelength electromagnetic radiation moves at the speed of light. How far can radiation be reflected? As I’ve said in previous blogs, you can easily see the sun’s infrared radiation reflected by the surface of the moon to your thermal imager.

I once had a wonderful experience  that demonstrated both how fast and how far light can be reflected when I saw an “iridium flare.” Perhaps you’ve experienced this amazing flash of light in the early evening sky as sunlight reflects off the solar panel of one of a number of communications satellites that periodically circle Earth. See for more details and, if you like, download an app that will allow you to predict when you can see the flash!

Have fun reflecting on infrared radiation this week.

Thinking Thermally,
John Snell—The Snell Group, a
Fluke Thermal Imaging Blog content partner

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