INFRA-RED HEAT RAYS differ from light rays, such as are reflected by a mirror. Light rays can penetrate only glass, or other transparent or translucent substances. The invisible • infra-red rays pass through dust, moisture, and are absorbed by plaster, wood, brick, iron, etc. But alumi­num surfaces TURN BACK these heat rays where there is a proper kind of air cell. This kind of "reflection" has no relationship to the brightness of the foil, which is JUST AS EFFECTIVE WHEN DULL as when bright.

Quite by coincidence, aluminum foil ALSO has a re­flectivity for visible light of about 85%, and therefore is extensively used EXPOSED in factories, auditoriums, etc.

The National Bureau of Standards, U. S. Dept. of Com­merce, in its Letter-Circular-535, states:

"Installations are reported where no appreciable dete­rioration of the aluminum has occurred over a considerable period of years. Thin layers of dust readily visible to the eye do not cause very serious lowering in the reflecting power." "The appearance of the surface is not a reliable guide as to its reflectivity for radiant heat, and foil which appears dark or discolored may have lost little in insulating value if the surface film is thin."

Prof. Wilkes of M.I.T. and Profs. Queer and Hechler of  Penn. State College Experiment Station state in "Thermal Test Coefficients of Aluminum Insulation for Buildings,"*

"A mirror, consisting of glass with a silvered surface on the back of the glass, is an excellent reflector of light but it is a very poor reflector of infra-red radiation correspond­ing to room temperature. In fact, such a mirror would have about the same reflectivity for infra-red as a heavy coating of black paint.

"With this in view, it is obviously impossible to judge the infra-red reflectivity or emissivity of a surface by its appear­ance to the eye. Consequently, in a discussion of reflective surfaces for building insulation, the term brightness has no specific meaning. The terms emissivity or reflectivity defi­nitely define the radiating or reflecting power of a surface and values may be determined for the long wave-length radiation corresponding to room temperature."

"Aluminum foil suspended vertically in laboratory for three years and measured with accumulated dust and fume. The emissivity was 0.05."

In another paper,* Prof. Wilkes says:

"Several samples of Aluminum Foil Insulation, that have been exposed to the air and dust of the laboratory from five to seven years show no appreciable deterioration in insulating value and there are also on record numerous cases where aluminum foil has been removed for examina­tion after from five to nine years service and found prac­tically unchanged in insulating value."

In Close's book, "BUILDING INSULATION," page 109, we find:

"The visible brightness of a surface is not a gauge of its emissivity, for a surface may appear to have lost its reflec­tive value and yet have a comparatively low emissivity as tested by a radiometer or emissivity-testing instrument."

The American Society of Heating and Ventilating Engineers on page 188 of the transactions at its 45th Annual Meeting, published by C. S. Taylor and J. D. Edwards, Physical Chemist and Asst. Director of Research of the Research Laboratories of the Aluminum Co. of America,

“The penetrating characteristics of the infra-red radiation are illustrated by the aerial photographs which have been taken by Captain Albert Stevens.  With infra-red sensitized photographic plates, views of mountains as far as 331 miles distant from the photographer have been taken, though the mountain itself was invisible to the eye because of intervening haze and fog. 

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