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Heat Transfer by Radiation Part 2

For young children and pets, the first look in a mirror can be very confusing! They may be asking themselves. “Is that real or…?” Most thermographers share a similar exasperation the first time they see a thermal reflection, most commonly from a bright piece of metal. Many go on to understand that reflections are not reality, but many also continue to be confused about exactly how to deal with them.

All surfaces are reflective to infrared radiation to some extent. Bright metals are the most reflective. You can feel this by moving a piece of common aluminum foil very close to your face. Suddenly your face will feel warm! Why? The foil reflects your body heat back on itself rather than having your body radiate to cooler surroundings.

Most surfaces are not particularly reflective. Human skin is among the least so, but even we—regardless of skin color—are about 2% reflective. Most painted or heavily oxidized surfaces are between 10-20% reflective.

Window glass is somewhat reflective to infrared radiation and, more importantly, highly specular or mirror-like.

Glass and still water are two common surfaces that are very thermally curious. While they are only 20% and 5% reflective, respectively, because they are very smooth they can look mirror-like or specular. When polished smooth, any materials  like stone, tile, wood or even glossy paint, will appear specular even though they many not be highly reflective.

There are two easy ways to understand how reflective a surface is. First, while looking through your imager, simply move back and forth. If what you see changes appreciably, the surface is probably fairly reflective or specular. Next, heat the surface 10-20F above ambient temperature and firmly apply a piece of electrician’s tape to it. If the tape shows up clearly, the material is probably fairly reflective. Try this!

For all materials not transparent to infrared radiation—and fortunately that means most materials—reflection and absorption have an inverse relationship. Human skin  is both 2% reflective and 98% absorbing. Bright aluminum is approximately 95% reflective and only 5% absorbing.

Over the course of the next week, have some fun exploring which materials are reflective and which are not so reflective. See what you can learn. We’ll come back next week and discuss the issue of emissivity, in particular how understanding it is essential to making an accurate radiometric temperature measurement.

Thinking Thermally,

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

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