Remember the hullabaloo a few years ago about camcorders capable of infrared photography — folks modifying their camcorders to see through clothing? One of the more annoying elements of the press coverage was the label “x-ray” for that kind of image. Sure, it’s a quick way of saying the modified cameras can see through clothing that appears opaque to the eye, but the infrared wavelengths are about a thousand times less energetic than x-rays.

And speaking of wavelength, there’s a lot of confusion about infrared cameras in general. The confusion stems from the fact that the infrared region of the spectrum is about 200 times as wide as the visible light spectrum. That is, if you reflect a beam of deep blue light off a mirror, it will act (just about) exactly like a beam of red light — so the two ends of the visible light spectrum act almost exactly the same way.

The Infrared Spectrum

Not so for the infrared. The infrared is roughly and loosely divided into three (or more) regions: the near-infrared (NIR), the midwave-infrared (MWIR), and the longwave- (or far-) infrared (LWIR). Those regions act completely different from each other. A material that absorbs energy in the NIR can be transparent in the MWIR and reflective in the LWIR.

Those regions of the infrared spectrum are generated in different ways as well. Every object in the universe emits radiation, at wavelengths that correspond to the object’s temperature. The human body, for example, emits light in the LWIR region. An army tank or an airplane emits in the MWIR. A hot stove emits in the NIR — and when it gets even hotter it emits in the visible…leading to our familiar experience of something being “red hot.” There are dozens of different types of infrared cameras, imaging different parts of the infrared spectrum.

The modified camcorders that do the “x-ray” infrared photography work in the near-infrared spectrum, so they aren’t creating pictures from the infrared energy originating in the human body. They create images from reflected NIR. At night, when there is not much visible light around, these cameras would shine a NIR “flashlight” and capture the infrared wavelengths reflected off the object. Even though the scene would be dark, the camera would capture a perceptible scene.

Near-infrared photography creates subtly different — almost eerie — effects.

The sun and artificial light sources emit near-infrared radiation, but they also emit visible light. The detector in the camcorder could sense the NIR, but it’s usually not the image you want during the daytime, so an infrared blocking filter is put in front of the detector. To take the night-time photos, the infrared blocking filter is flipped up, out of the optical path. If the filter is flipped up during the day, the detector senses the NIR, but there’s so much visible light around that the infrared image would be swamped by the visible light. To get around that, some users put a visible light blocking filter in front of the camera. Then, during the daytime, the camera senses the NIR without all the extra visible light. That NIR image captures NIR reflected from the scene, in the same way that visible light images capture light reflected from the scene.

Seeing Through Clothes

So how did that “see through the clothes” thing work? Well, there are some materials that are transparent in the NIR and opaque in the visible. Some (usually thinner cotton) fabrics do not reflect near-infrared. Meanwhile, the undergarments, fabricated from different materials, do reflect NIR. The effect is almost as if the outer garment isn’t there.

It’s really not too common a situation, where the external clothes are transparent in the NIR, but the uproar over the situation was enough to cause camcorder manufacturers to make it much more difficult to use the cameras to image NIR under daylight conditions. It’s a shame, because there are some really interesting effects possible with daytime infrared photography. Still, since there are some scumballs who do things like take “naked” photos of the Chinese diving team with their modified cameras, one can see why the manufacturers have tried to eliminate the infrared imaging capabilities of their cameras.

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Related article at Salon.com.