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Heat Transfer by Radiation

For the past several weeks, we’ve been reviewing heat transfer. Thermographers must understand the basics if they are to successfully interpret their images. Over the next two weeks, we’ll wrap up the review with a discussion of radiation.

Electromagnetic radiation is not only a powerful mode of heat transfer, it is also the way energy moves from a surface to the detector of our imaging systems. We already know a great deal about many forms of radiation. Electromagnetic radiation travels at the speed of light, in a straight line and can most easily be defined—even if not completely accurately—as a “wave.”

Radiation is defined by its wavelength. We define various “bands” of wavelengths, such as ultraviolet, visible light, and long-wave infrared, based mainly on how they interact with materials. While each band or form differs in significant ways, some also share commonalities. Some materials are transparent to certain forms, like light passing through a glass window. Look in a mirror and you are seeing electromagnetic energy being reflected. Sit outside on a sunny day or next to a fire and your skin absorbs radiant energy making you feel warm.

My wife, sitting by the fireplace, enjoys the fact that infrared radiation is strongly absorbed by human skin.

When infrared radiation interacts with materials, interesting things also happen. Thin-film plastic, regardless of color, is quite transparent. Most bare metals, even if somewhat oxidized, are fairly reflective. And nearly all non-metallic surfaces absorb infrared radiation efficiently—no matter what their color.

For thermographers, the sun is typically the most critical source of radiation affecting our work. Sunlight is composed of a full spectrum of radiant energy rather than just light or infrared so it can behave uniquely. Outside, its energy is quickly absorbed causing surfaces to heat various degrees (no pun intended). We need to pay attention to the color of the surface because absorption of full-spectrum sunshine, unlike infrared radiation, does vary with color. A brown ceramic insulator in a group of gray polymer insulators will often be warmer in the sun. The dark trim on a light-colored house will be warmer. You can have fun on a sunny day seeing dark-colored letters heat up enough to be able to read a sign in a thermal image. On a cloudy day the words are typically invisible!

A “Think Thermally” t-shirt differentially absorbs the sun and the lettering heats up enough to be seen clearly in a thermal image. What would this look like in the shade?

Next week, we’ll review how reflection of radiation affects our work. It is one of the most important aspects of thermography we need to pay attention to, especially if we are making quantitative assessments and measuring temperatures.

Thinking Thermally,

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

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