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1945 Theatre Catalog, 4th Edition, Page 321 (297)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 321
Page 321

1945 Theatre Catalog, 4th Edition, Page 321

To protect the silver, the redector receives a heavily electroplated coating of copper, after which the back is sprayed with several coats of the familiar backing which is slow baked at a low temperature to extreme hardness.

The reflector is carefully cleaned, and wrapped.

The results of the optical tests for;

each individual reflector are then recorded according to the serial number etched thereon and these records are kept in the engineering department for future reference. The test records on the thousands of reflectors now in service are on file. These files contain the specifications of over 200 types and sizes of reflectors, as required in the various types of projection lamps.

The Status Quo

The modern technique, by which hard, durable coatings of transparent materials can be produced has given impetus to the idea of new procedures in mirror manufacture, as well as to resurrecting the old idea of using thin films to protect front-surface reflectors from corrosion and abrasion.

It is apparent, then, from the discussion of lens coating and transmission films, that films, having a lower refractive index than the glass upon which they are deposited, cause a decrease in reflection. If the film has a refractive index greater than the glass, reflection is increased. Zinc sulphide is colorless and stable and possesses an index of refraction of 2.35. When a film of this material is formed upon glass having a refractive index of 1.52, it is possible to increase reflection from the normal 4.2 percent to as high as 53 percent. Here, then, is a method for making highly efficient and non-absorbing mirrors. The time is not far distant when these mirrors and semi-reflectors will completely displace the ineflicient ones new made of metal.

What is urgently needed is a frontsurface reflector having the corrosion resistance, durability, and hardness of rhodium, coupled with the high reflectance of silver. The thought has been that a thin, impervious layer of transparent material could be deposited upon the silver to protect it from tarnish. The same idea has been applied to aluminum to increase its resistance to the corrosive action of salt-water atmospheres and to permit easier cleaning without scratching. The results to date, however, have not been satisfactory.

Undoubtedly, the most perfect reflecting surface is that which makes use of internal reflection. No film is required and the reflection is 100 per cent perfect. The designers have demonstrated great ingenuity in devising intricately-shaped pieces of glass to utilize this most perfect type of reflection for conserving light while changing direction and inverting images.

Many cases arise, however, where it is impossible to use internal reflection and recourse must be found in the next best source of reHectors, the metals. Except for sodium and potassium--excluded because of their chemical activity -silver, aluminum, and rhodium are the only ones which are being used to any great extent as reflectors.



Of these, silver is by far the best, its reflectance being about 96 percent when used as a back-surface reflector. It can also be deposited from chemical solution upon polished surfaces to give flawless mirrors. Silvering by chemical means has been for years the standard method of making mirrors and redectors, irrespective of whether they are to be used in precision optical instruments or are to be hung on the walls at home.

Unfortunately, an exposed silver surface tarnishes rapidly, so that its reflecting power is destroyed. Silver, therefore, is always used as a back-surface redector and the unused surface covered with other metallic coatings and protective paints. It is indeed unfortunate that silver tarnishes so readily, because the design of optical instruments is unduly complicated, and materials having inferior redective power must be used as front-surface reflectors.

Aluminum, second to silver in reflecting power of all the known useful metals, can be used as a front-surface mirror. It has been used as the reflecting suface of astronomical telescopes for several years because of its non-tarnishability and retention of reflecting power. Its re flectance, however, is only 89 percent, and this low figure excludes it from use in instruments which must have the highest possible transmission. Its softness, which introduces difficulties in cleaning with scratching, and its susceptibility to rapid deterioration in saltwater or salt-water atmosphere further restricts its use to enclosed locations where it is protected from mechanical abuse and atmospheric conditions.

The need for a front-surface reflector which is very hard and possesses excellent resistance to abrasion and chemical corrosion has been found in nlms of metallic rhodium. The result has been obtained at considerable sacrifice of reflectance, however, for 75 per cent is about the maximum. Rhodium plating of metallic objects came into commercial use ten to fifteen years ago, but its application to optical elements was largely a development of the wartime years. However, for some time Heyer-Schultz, Inc., had been making an all-metal, rhodium-plated reflector for motionpicture projectors.

Rhodium is also used as the reflecting surface in large searchlights, because of its hardness and resistance to corrosion.

THE SILVER IS DEPOSITED by a continuous flow of the silvering solution (frequently with ommoniacal silver nitrate as the base) over the backs of the reflectors so as to insure a uniform and brilliant reflecting surface. Following the silvering, the reflector receives a heavily electroplated coating of copper to protect the silver from scratches and other forms of physical damage and to prevent the tarnishing of an exposed silver surface.
1945 Theatre Catalog, 4th Edition, Page 321