Eary I.R. Imager (Aug, 1946)

AN EYE FOR HEAT

New bolometer that “sees” warmth miles away will help fight disease, warn of fire, catch burglars, and spot heat leaks.
By MARTIN MANN

SCIENCE has outdone the cat with a new device that can really see in the dark. The superconducting bolometer, developed at Johns Hopkins University’s Cryogeny (refrigeration) Laboratory by Dr. Donald H. Andrews and three student associates, will spot a truck moving in complete darkness five miles away—and instantly trace its outline on a screen.

Actually an ultrasensitive heat-measuring instrument, the bolometer detects heat radiating from men, vehicles, and buildings. Unlike the Army’s sniperscope (PSM, June ’46, p. 73), which reveals a night-hidden object by sending out a beam of infra-red rays and showing on a screen the reflections from the object, the bolometer does not emit rays.

Like early television cameras, the bolometer employs a mechanically oscillated mirror to scan the area under observation. Instead of a cell sensitive to visible light, however, it has a tiny strip of alloy that responds to the invisible light of the infra-red spectrum—heat rays. This alloy—the rare metal, columbium, alloyed with nitrogen-converts the varying heat radiation it receives from the mirror into electrical impulses, which are amplified and fed into a cathode-ray tube. Movement of the cathode beam is synchronized with the oscillating mirror, while the intensity of the ray is governed by the impulses from the alloy strip —thus the object being observed appears on the fluorescent screen of the tube just as in a television receiver.

Its high sensitivity and quick action should make the bolometer valuable in science and everyday life. An instrument that can sense the heat of a truck five miles away might remove the danger from night driving. In a bolometer-equipped car, the driver would see a pedestrian or oncoming vehicle on a screen on the dashboard long before he could see either of them naturally. The bolometer also might be used to detect heat losses caused by faulty insulation of buildings or equipment. Photographic film placed over the viewing screen would make a heat picture of a house, showing exactly where heat was leaking through walls and roof. And suggestions have already been made for employing the bolometer in fire and burglar alarms.

But most important, its inventors believe, will be the use of the superconducting bolometer as a new tool in scientific research —particularly in medicine and physics.

For the first time doctors will have an in-strument sensitive and fast enough to measure accurately the heat radiated from the human body. More precise analysis of body heat is expected to disclose additional information about the fundamental nature of disease and life itself. In physics, the bolometer will make more accurate investigation of the infra-red spectrum possible, and perhaps add importantly to existing knowledge of atomic structure.

Dr. Andrews will begin research this fall into the nature of heat radiations from sugar, fat, and other simple organic substances. Actual medical work, he estimates, can be started next spring.

The bolometer represents eight years’ hard work. When Dr. Andrews, while relaxing on the beach at Nassau, first thought of using a heat-measuring instrument as a detector, it was merely a good idea and little more. There was then no suitable heat-measuring instrument. The existing bolometers used a simple platinum strip set in a balanced electrical circuit; thus a change in the temperature of the platinum, altering its electrical resistance, changed the current in the circuit. A galvanometer would then, in effect, register small temperature changes. But this bolometer was not sensitive enough to pick out a truck at five miles.

To improve the bolometer to fit his purpose, Dr. Andrews drew on 25 years’ experience in low-temperature research. He knew the strange properties that matter acquires when cooled almost to absolute zero, the unattainable point 459 degrees below zero Fahrenheit where molecular motion would stop completely. One of these properties—superconductivity—was eventually to solve his problem.

Low-temperature researchers had discovered earlier that electrical resistance— the “friction” with which substances oppose passage of an electrical current—suddenly disappeared in some metals when they were brought to a temperature near absolute zero. If a current were started circling through a ring of metal at superconducting temperature, it would continue indefinitely, provided the temperature did not rise.

Aware of this phenomenon, Dr. Andrews reasoned that an unusually sensitive bolometer could be made by maintaining the bolometer element just barely above the superconducting temperature. Since the drop from normal resistance to superconductivity occurs very suddenly, an element kept at this transition temperature would show very large changes in resistance for minute differences in temperature.

Assisted by Drs. Robert M. Milton and Warren DeSorbo, Dr. Andrews made the first superconducting bolometer by cooling a tantalum element with liquid helium. It was remarkably sensitive, but the apparatus was bulky and expensive to operate. Through diligent research, Dr. F. Hubbard Horn discovered that an alloy of nitrogen and colum-bium became superconducting at 434 degrees below zero F —a temperature easily obtainable with liquid hydrogen, which costs only one-tenth as much as liquid helium.

In the present model, the columbium nitride element rests under a rock-salt window in the center of three concentric copper cans, called a cryostat. The inner can contains liquid hydrogen, the next liquid nitrogen, and the outer can a vacuum.

In operation, a small, steady electric current is passed through the columbium nitride element. Heat rays striking the element change its resistance and cause the electric current output to vary. These variations are amplified by standard, radio-type equipment and fed to a cathode-ray tube.

If the bolometer were mounted in an automobile, for example, the mirror would “look” from one side of the road to the other, shift down a bit and then “look” across again—in much the same way the human eye moves when reading. Whenever the mirror found an object radiating heat, such as a man walking in the road, its heat rays would be reflected to the bolometer element, which would convert them into electrical impulses.

The cathode beam, moving in synchronism with the mirror, shoots electrons at the screen when it receives an electrical im-pulse, causing the screen to glow at every point where the mirror found heat radiation. In that way, a zigzag outline of the man would appear on the screen. If the object in the road were another automobile, the engine, being hottest, would glow brightest.

As a detecting device, the bolometer is rather expensive to operate, since it requires about $8 worth of liquid hydrogen and nitrogen for every 24 hours’ use.

The initial investment would not be great, however, since even the first, custom-made bolometers cost only about $100. And if they were to be mass-produced, Dr. Andrews estimates this would drop to $25.

Eventual elimination of the need for any liquefied gases, with resulting economies, is Dr. Andrews’ present goal. He has already built the first model of the cryodyne, a refined mechanical refrigerating unit. The first cryodyne produced a temperature of 384 degrees below zero F„ and Dr. Andrews expects that improvements will enable it to reach the superconductivity zone.

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