Heat-Sensitive Eva

Electronic devices for seeing in very dim light have become commonplace, but all of them are blind in total darkness. Last week Baird Associates. Inc. of Cambridge, Mass, showed a recently declassified "camera" that needs no light at all, only infra-red (heat) radiation from faintly warm objects.

Baird's Evaporograph (Eva for short) is based on a prewar German idea which until recently was not followed up diligently. It has a concave mirror which concentrates heat rays as the mirror of an astronomical telescope concentrates light.

Just before they come to a focus, the rays enter a vacuum chamber through a sheet of salt (transparent to infrared) and form their image on the blackened surface of a thin sheet of plastic. The other side of the plastic is covered with a film of silicone oil.

When the heat-ray image forms on the plastic, the "bright" parts of it are warmer than the dim parts. Their heat passes through the plastic and evaporates part of the oil film, making it locally thinner. When light is turned on the oil film, it glows in the bright "interference" colors of an oil slick floating on water. The colors have nothing to do with the real colors in visible light of the object that Eva is viewing. They show thin or thick parts of the oil film--and therefore outline the object by its temperature. Hot parts show in one color, cool parts in another. Eva can distinguish 1DEG differences in temperature.

When Eva looks at an airplane in total darkness, the hot engine parts may show up yellow while the cold wings look blue. A heated house is visible against its cooler background, and factory chimneys stand out conspicuously with trails of hot gas. The heat-pictures on the film are bright enough to be photographed in black and white or color with an ordinary camera. A picture can be erased by heating the film momentarily and evaporating all the oil. In about two seconds the oil film forms again, ready for another picture.

Baird Associates workers had lots of fun looking at distant islands in Boston harbor on pitch-black nights and taking darkroom pictures of the office staff. One of the girls, photographed by the heat-rays flowing out of her skin, proved to have a cold nose.

But Eva (cost: $9,500) was not built for such frivolity. The military uses are obvious. Blacked-out cities, whose warmth cannot be eliminated, will stand out conspicuously on Eva's screen. An underground factory will be betrayed by heat rising from it.

Many nonmilitary uses are also showing up. Since Eva was declassified, Baird Associates has been getting inquiries from industries that want to chart hot spots in electronic apparatus, find flaws in hot metal parts. Another obvious use is to check the insulation of a building by taking a snapshot of the heat escaping through its walls.

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