Science: Thermography: Coloring with Heat

THE British astronomer Sir William Herschel performed a curious little experiment some 170 years ago. After bending a beam of sunlight through a prism, he found that a thermometer heated up most if it was placed just beyond the red end of the spectrum. Herschel concluded that the mysterious heat source was invisible rays from the sun, but he could hardly have known that infra-red radiation—as it was called —would eventually let man see the world in an entirely new light.

Today, infra-red detectors are providing stunning images that were once totally invisible to the naked eye (see color pages). The new medium is called color thermography—the technique of translating heat rays into color. Unlike ordinary color photographs, which depend on reflected visible light, thermograms, or heat pictures, respond only to the temperature of the subject. Thus the thermographic camera can work with equal facility in the dark or light.

The camera’s extraordinary capability is built around a characteristic of all objects, living or inanimate. Because their atoms are constantly in motion, they give off some degree of heat, or infra-red radiation. If the temperature rises high enough, the radiation may become visible to the human eye, as in the red glow of a blast furnace. Ordinarily, the heat emissions remain locked in the invisible range of infra-red light.

Since World War II, there has been an intensive effort to produce better infra-red detectors for the military. In Viet Nam, for instance, such devices have often been used to detect Communist troops in even the most densely foliated jungle. Other applications include heat-seeking missiles and spy-in-the-sky satellites. One of the leaders in the field, the Barnes Engineering Co. of Stamford, Conn., has developed detectors that can “see” the dark part of a crescent moon from a quarter-million miles away.

Though their composition may vary, all these devices are based on the same technology: they are capable of transforming tiny amounts of heat into electrical currents. Once amplified, those currents are fed into a display unit that shows the rise and fall of the infra-red radiations as visible light. The display may be as uncomplicated as an ordinary light bulb whose fluctuations are recorded on photographic film. In some cases it is a more sophisticated cathode-ray tube system that produces a TV-type image.

In fact, the optics of a typical thermograph somewhat resemble early television. Using tilting and moving mirrors, the Barnes cameras scan the target horizontally and vertically. With each movement of the mirrors, the infra-red detectors take what are, in effect, quick temperature readings of a tiny portion of the subject. Before a picture is completed, as many as 40,000 “bits” of such information may be needed. The picture may be shown simply in black and white with shades of gray representing different temperature ranges. But color can be added with the use of appropriately positioned filters. Whenever there is a sufficient change in heat intensity, a different color filter pops in front of the thermograph’s internal light bulb. The resulting flickerings are then recorded on color film, with each hue representing a different temperature range. Colors are arbitrarily selected. Warmest areas are represented by shades of red and orange. Medium temperatures come out in yellow and green, while the coldest spots are violet, blue and black. The advantage of a color thermogram over black and white is that most people can distinguish vivid colors more easily than shades of gray.

At least half a dozen companies are now producing thermographic equipment. Two of the pioneers in the field are Sweden’s AGA and Bofors. The newest system in AGA’s line, which is called Thermovision, can show color pictures on a TV screen at the fast rate of 16 frames per second. Therefore it can provide cinematic-style color thermograms that actually show changes in temperature as they occur. The Barnes and Bofors cameras, on the other hand, are slower, but their manufacturers claim better resolution. In any case, the heat of the competition is a measure of thermography’s potential in the marketplace.

Hot Cargo. Unlike thermometers and other ordinary heat-measuring devices, thermographs do not touch or disturb the objects they photograph. They are useful tools in the growing field of nondestructive testing—analyzing a product without damaging it. Utility companies, for example, are able to uncover dangerous overheating in equipment without interrupting service.

One promising use of thermography is in medicine. By spotting unusual temperature changes on the skin, doctors have been able to locate tumors, detect symptoms foreshadowing strokes, explore the extent of arthritic inflammation, gauge the severity of burns. If human skin is too warm, it may well mean increased metabolic activity and blood temperature underneath it, one of the signs of a malignancy. Cold skin may indicate dead tissue, as in severe burns, or reduced blood circulation, a clue to circulatory blockages.

Thermography has also proved helpful in finding flaws in aircraft assemblies, checking electronic circuitry and discovering diseased crops. It has even trapped smugglers. Equipped with a thermograph, border police in one Middle Eastern country found unusual heat coming from one area of a water-tank truck. An immediate inspection revealed that part of the cargo was indeed hot; it was a huge haul of hashish.