Using an infrared camera to understand radiative transfer

Submitted by Shawn Reeves
2008-10-31 21:23:12

Many people have misconceptions about the different subsets of electromagnetic radiation, misconceptions that make it difficult for them to learn about solar cooking or global warming. Many people think that only infrared light can carry energy the energy necessary to raise the temperature of everyday objects.
In my physics classes, usually in the context of astronomy, I ask my students to learn about all the known wavelengths of light, how astronomers use different wavelengths to study different phenomena. For example, microwaves are used to study the subtle anisotropy of the earliest known period in the history of the cosmos, milliseconds after the Big Bang. X-rays are used to study high energy activity like matter falling into black holes. But what do all electromagnetic wavelengths have in common? They all represent an interaction that carries energy (and equivalent mass, by the way) from one place and time to another. The relationship between the differences in those places and times is represented by the speed of light. Thus, not only can infrared light transfer "heat," so can visible light, x-rays, radio waves, etc. Most of the sunlight that reaches the earth's surface is visible light. We feel warmed by the sun's light when we step into it. Many people forget this experience when they state that it is only infrared that carries "heat."
There is a spark of truth to that common misconception. It is that objects at room temperature do exchange energy mostly with infrared light, since, unlike the sun, they are not hot enough to emit much visible light. So, ignoring sunlight, most radiative energy transfers here on earth occur through infrared light. But even solar experts, like the kind you may have heard on radio shows like NPR's Science Friday, talk about solar radiation as if there are two distinct types of radiation, light and heat, that act on materials in completely different ways.
An infrared camera allows users to see that light which is otherwise unseen. There are two types of infrared photography: in the older type, a film with an emulsion sensitive to infrared light is exposed in a camera with a filter that blocks visible light. This type is used by portrait photographers and landscape photographers. It offers high contrast yet it smoothes skin by seeing a little bit deeper into it (the higher the wavelength, the less scattering and more "skin depth" there is). It sees through fog better. This type of camera is sensitive to "near-infrared," wavelengths not far from red, the wavelengths that are emitted very strongly by incandescent lamps and comprise a significant portion of the solar spectrum.
The second, newer kind of infrared camera is made up of thousands of "microbolometers," miniature thermometers made of semiconductors that amplify the current flowing through them proportional to the light falling on them. These microbolometers are tuned to be sensitive to infrared light, and, like the other type of camera, a filter is attached to the lens to prevent unwanted wavelengths from adding noise to the image.
Infrared light is emitted by all terrestrial objects; seeing that light with a special camera helps users to understand that radiation is carrying energy from warmer objects to colder ones. The usage of infrared cameras to study temperature is called "infrared thermography."
The accompanying photos show some of the interesting conditions and interactions seen in infrared. An album of dozens of photos is available at:
http://energyteachers.org/photos/main.php?cmd=album&var1=IRImages/

This photo shows how it is difficult to study the inside of a solar cooker because cookers are surrounded by IR-reflective glass or plastic to help trap energy inside. But the photo is good at showing how well the aluminum reflector works.

This photo shows cold air infiltrating the house at the top of a window-frame.

Here you see the reflection of the photographer in this glass door.

Although glass reflects infrared, it also emits infrared. When the sky is very cold, there is little radiation from the sky to be reflected, so what we see in this photo is the emission from the windows, which, being poorly insulated, are warmer than the walls of this building. If the sky were very warm, it would affect the way the windows look to the camera. Since any earth-bound material is neither 100% emissive (a so-called black body) nor 100% reflective (would be some dark form of matter, not interacting with light), we must keep in mind that we are always seeing in our camera some combination of emitted and reflected light, and maybe some transmitted light.

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