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Dealing With That Big Heater in the Sky—the Sun!

Whether we work outside or inside, day or night, inspecting buildings or substations, we must learn to deal with the Sun because it is very powerful.

The sun radiates both long and mid-way infrared electromagnetic energy, but it also radiates considerably more energy in other parts of the spectrum—ultraviolet, short-wave IR, visual, and radio—that, when absorbed, cause materials to heat up. Full spectrum sunshine, as it is called, has significant heating power when absorbed by any material.

The classic example is that dark surfaces absorb more than light ones, as is the case of the Think Thermally™ t-shirt hanging on the clothesline—but it can be much more involved than that. The rate at which sunshine is absorbed (or transmitted or reflected) is often different than the way long-wave infrared alone behaves. As thermographers, we sometimes fail to appreciate that fact.

A thermographer’s life in an outdoor substation on a sunny afternoon—summer or winter—can be challenging. With long-wave imagers, reflections are not a problem, but solar loading can be. A brown insulator is hotter than a grey one—is it a problem or just the effect of the sun?

A weatherization specialist looks at a home on a bright, but overcast, winter day and is amazed that the light trim is so much cooler than the dark body of the house. She could even see where parts of the wall had probably been in the shadows before she arrived. Inside, she is stunned to find the walls appear to be uninsulated when she knows otherwise. Armed with patience and her Level I skills, she reasons out that the sun has caused a reversal of the normal winter direction of heat transfer!

Even with the winter sun behind the clouds, this house shows the cooler shaded areas and the affect of the sun on the dark paint (and light trim). Inside, the sun has caused the direction of heat flow to reverse so an insulated wall shows up as cooler. It is not unusual to see these effects even 4-6 hours after the sun has been on a wall.

We need to always pay attention to the sun—where it is and where it has been, what has been in the shadow and how it is absorbed. The sun is not a thermographer’s “enemy,” but it can be a significant “pain” at times.  However, it mostly just makes life more interesting and can even be an “ally” in some instances. We simply need to understand how it affects us and work with it.

Next week, we’ll begin a series on radiometric measurements—emissivity, background temperature, resolution and transmission. Imaging alone is great, but adding in the temperature values makes this technology much more powerful.

Thinking Thermally,

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

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