Three decades later the lure of x-ray vision is still strong. A mini furor erupted a few years ago when photographers discovered that placing lens filters on certain Sony camcorders enabled them to see through thin skirts and swimsuits. The secret wasn't x-rays but infrared radiation, which we perceive as heat. The CCD (charge-coupled device) chips in the video cameras can detect some infrared rays as well as visible light; if you filter out the visible wavelengths, the images show the radiant heat from the sun reflected off objects.
But the alarmist stories that appeared on television ("A Camera That Can See Through Clothes!") greatly exaggerated the threat of thermal voyeurism. Only sheer fabrics are transparent to infrared, and the camcorders can observe just the near-infrared part of the spectrum--wavelengths shorter than one micron--so they cannot detect body heat, which peaks in intensity at about nine microns.
Until recently, true thermal cameras--those that can sense the long-wavelength infrared rays--were bulky and very expensive. The standard instrument was a photon detector that had to be cooled to cryogenic temperatures, usually by liquid nitrogen. (At room temperature, the signal would be swamped by thermal "noise.") Heat-seeking missiles use these systems for targeting, and astronomers employ them to peer into the dusty hearts of galaxies. Over the past 10 years, however, manufacturers have developed cheaper and more portable infrared cameras that do not have to be cooled. Instead of detecting photons, these devices focus the incoming infrared radiation on heat-sensitive materials; the resulting changes in the physical properties of the material are then used to generate an image.
When I heard about this new technology, I felt like I was nine years old again. Now I could finally get a glimpse of superhuman vision! One of the leading manufacturers of uncooled thermal cameras, L-3 Communications Infrared Products in Dallas, Tex., let me evaluate the Thermal-Eye 250D, a model designed to meet the needs of law-enforcement agencies. Because the unit can observe infrared rays with wavelengths between seven and 14 microns, it is perfect for detecting body heat. Police officers can use the 250D as a night-vision scope, scanning dark alleys for fugitives or searching the woods for lost hikers. Thermal cameras have an advantage over conventional night-vision scopes, which show greenish images and are widely used by the military: whereas the conventional scopes simply intensify the available visible light, the 250D can operate in total darkness.
You can just imagine how delighted I was when the unit arrived at my office. (I didn't even have to pay for postage.) At first glance, the 250D resembles a run-of-the-mill video camera. It has a rechargeable battery, an adjustable eyepiece and a pair of focus buttons. In fact, you won't find any major differences from an ordinary camcorder until you remove the lens cover and behold the oversize circle of polished germanium. Unlike glass, germanium is transparent to infrared.
The 250D is a pyroelectric detector--it focuses infrared rays on barium strontium titanate (BST), a ceramiclike material that acts like a capacitor. When connected to a circuit, the material undergoes electric polarization: one end becomes positive, the other negative. Changes in the temperature of BST alter the degree of polarization, inducing a displacement current in the circuit. The BST is divided into a rectangular array; by measuring the displacement current for each section of the array, the detector creates a two-dimensional image showing the intensity of the incoming radiation.
I turned on the camera, squinted into the eyepiece and surveyed my office. I discovered that the lens is optimized for long-range use--I couldn't focus on anything closer than 10 feet away. But by backing up a bit and adjusting the controls, I was able to get a nice infrared picture of my work space. The image as a whole was bluish and grainy, but the hot spots stood out well. The rear of my computer monitor put out a warm glow, and the vent of the radiator flared like a blast furnace. The window, though, was solid black (it was quite cold outside at the time).
Next, I wandered the hallways of Scientific American, noting the brilliant heat signatures of all the printers and copiers that were turned on. I was a bit leery of training the camera on my colleagues; although the officials at L-3 Infrared Products had assured me that the 250D couldn't see through clothes, I didn't want to take any chances. In the end, I focused the camera on our editor in chief, John Rennie. Luckily for both of us, I saw nothing too revealing. The infrared image delineated the folds of his shirt, but that was about it. His face, though, showed interesting variations--his nose was black, much colder than his cheeks and forehead. Also, his fingers were darker than his palms. Maybe it wasn't as exciting as seeing the bones in your hand, but it was still pretty neat.
John and I played a few tricks with the camera. John placed his hand on the door to his office for about 10 seconds; after he removed it, I could see a handprint of heat on the metal. Then we decided to see if the camera really worked in complete darkness. We went into an office without windows, shut the door and turned off the lights. It was pitch-black, but when I looked in the 250D and scanned the room, I found John hiding in the corner, as bright as a neon sign.
Finally, I took the camera to the famous skating rink at Manhattan's Rockefeller Center. Because the 250D looks so much like an ordinary camcorder, I figured I could pose as a tourist and unobtrusively observe the holiday crowds. In infrared, the scene was like something out of Dante's Inferno: a swarm of ghastly figures with glow-paint faces. In the background, the cars streamed down 49th Street, their front ends lit up by engine heat.
Whereas the 250D was designed for police work, other thermal cameras have been developed for firefighters, who use the devices to locate people trapped in dark, smoky buildings. L-3 Infrared Products has introduced a newer generation of detectors based on microbolometer technology; the devices focus infrared rays on an amorphous silicon material whose electrical resistance drops as its temperature rises. These units require less power than the 250D--they can run on a pair of AA batteries--and can fit in the palm of your hand. The Thermal-Eye X100xp, for example, weighs just 13 ounces. The U.S. Army has bought thousands of these devices, which are now in use in Iraq and Afghanistan.
The costs of the cameras are dropping as the technology advances, but they still run at least several thousand dollars apiece if you are buying them retail. The steep prices make me nostalgic for the $1.98 x-ray goggles. I'm still hoping to find out if those glasses really work. My kids will be old enough to read comic books soon, and if they're anything like me, I'm sure they'll be clamoring for a pair.