If, like me, you were an avid reader of comic books during the 60s and 70s, you'll probably remember a small advertisement that often ran on one of the back pages. Under the words "X-RAY GOGS," the ad showed a boy wearing a pair of glasses with lightning bolts radiating from the lenses to the boy's upraised hand, which looked like a skeleton's. The text below the picture asked, "Ever seen the bones in your hand?" For a nine-year-old, this come-on was irresistible, and I would've certainly mailed in $1.98 (plus postage) to purchase the goggles if my parents hadn't stopped me. "Don't waste your money," my father said.
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.