On the plus side, it wouldn’t require any physical contact, and you get an almost instantaneous reading. Actually, you might have seen pictures of handheld IR devices called temperature guns (which work somewhat differently) used like this in China. IR sensors are also used in factories to monitor the temperature of equipment without having to stop it.
But there are some issues with using this technology to screen people for illness. To do it well, you really need to understand how infrared sensors work. So I’m going to explain all that. Besides, the physics are just super interesting. I’m a huge fan of these cameras, because they let you see the world in a different light—literally.
With science, you don’t always get what you want. But if you try sometimes, you get something even better. That’s what happened to William Herschel in 1800. While testing some light filters, Herschel used a prism to split sunlight into its component hues. Then he set up some thermometers. He knew that light falling on an object would warm it up, but he wanted to measure the effects of each color separately.
But wait! There’s more. You can actually use the wavelength of light emitted by an object to determine its temperature. You’ve seen this when you use an electric oven. Once the element gets good and hot, say around 2,000 degrees Fahrenheit (that’s the temperature of the element, not the air in the oven that bakes your muffins), it glows a reddish-orange color:
Now you see the light. Of course, this is a false-color image. Since our eyes can’t detect infrared light, the camera basically translates, using visible colors to represent different wavelengths in the infrared range. In this palette (which you can change), yellow is hotter than orange, which is hotter than purple. (The thing you see in orange is a reflection off the top of the oven.)
All objects emit electromagnetic radiation—yes, that’s what light is—over a whole range of wavelengths. If you plot the intensity of the radiation (technically the spectral power density) vs. wavelength for a given object, you get a curve like this.
If you want to play with an interactive version of this plot, check out this cool PhET simulator.
It turns out that the highest-intensity wavelength produced—the peak in the curve above—depends on the temperature of the object. As it gets hotter, the wavelength of peak emission decreases—it moves to the left, back toward the visible spectrum.
So for something at room temperature (like 300 Kelvin), this peak wavelength is about 9.7 μm (micrometers). That puts most of the radiation in the infrared part of the spectrum. That’s why you usually can’t tell just by looking at things how warm they are.
You can get more information about infrared cameras for body temperature screening here.