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1945 Theatre Catalog, 4th Edition, Page 324 (300)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 324
Page 324

1945 Theatre Catalog, 4th Edition, Page 324

Despite further extensive experimentation in 1918 by Dr. Fred E. Wright, then and now petrologist of the Carnegie Institutionls Geophysical Laboratory, the process fell into oblivion again following World War I. However, both Doctor Kollmorgen and Doctor Wright correctly foresaw the application to military instruments, applications which future research and World War II should bring into being.

The modern leaching method applicable to most glasses involves immersion in dilute mineral acids (such as nitric and hydrochloric acids) at elevated temperatures and for periods of time depending on the type of glass. This method, while producing very hard films, has never been used to any extent because it is inferior to method (3)eevaporation of fluoridesein reducing refiection.

The second method for adding films to optical surfacesethe building up of monomolecular layers of certain metallic soapsean outgrowth of the researches of General Electric Companyls great Dr. Irving Langmuir-is the particular contribution of Dr. Katherine B. Blodgett, research physicist since 1918 at that companyls Research Laboratory.

The fundamental procedure is essentially this: The glass to be coated is dipped into a solution on the surface of which the coating material is fioating as a monomolecular layer. When the glass is dipped into the solution and then withdrawn, the coating material is trans THE METHOD OF COATING LENSES is briefly pictured here. The optical parts are placed in a rack (left) and placed under a belliar. When the air is exhausted and the proper degree of vacuum has been obtained, magnesium fluoride is evaporated within the chamber. As the chemical, in a gaseous form, streams out in all directions, a portion is deposited on the optical parts in the rack. By watching the

ferred to the glass surface as a monoe molecular layer. Thus, it is necessary to dip the glass many times in order to build up a surface of a desired thickness. A subsequent treatment, analogous to chemical leaching, alters the structure of the film so that a film of low refractive index is formed. This method has never been used in practice because the films are too soft and fragile, but a study of the properties of such films has been of great academic interest.

The third method of applying transmission films to glass-the deposition of certain metallic salts on optical surfacesewas proposed about 1936, by Dr. John Strong, of the California Institute of Technology. He demonstrated that reflection losses could be reduced considerably by covering the glass surface with a very thin layer of material having a low refractive index. The Strong method was to evaporate calcium fluoride in a high vacuum. The vaporized fluoride, streaming out in all directions within the vacuum chamber, was deposited as a thin film upon all surfaces in its path. This is the method which was used almost universally throughout the war period during which hundreds of thousands of optical elements were coated.

A fourth method, involving the deposit of a coating from a bath, has been developed by the American Optical Company, a project with which R. H. Moulton, assistant director of research, has been prominently associated. The method

utilizes spinning of the elements to be coated at very high speeds and under exact conditions of temperature and humidity. As the elements are spun, a solution containing the coating material is dropped to the plane of rotation and, through centrifugal force and surface tension, a layer of the proper thickness is spread over the optical parts. A variation of this procedure, for use with Hat glass, is to raise the glass from the bath at a slow, regulated rate, so that the liquid can drain off and leave a uniform coating. This method was developed too late to be of much service during the war but it may assume a more important role in the future.

The last method is one which has been developed by the Radio Corporation of America. Certain glasses are treated with hydrofiuoric acid vapor of low concentration which acts to form, quite contrary to expectations, a surface layer of sponge-like and porous silica. This process is somewhat analogous to the leaching process described above, in which certain components of the glass are removed and the silica remains. This method, as in the preceding, may become important in future application but did not play an active part during the war.

Credit for the earlier commercial process should go, in a large measure, to the researches at the Massachusetts Institute of Technology of Dr. C. Hawley Cartwright presently at the California Institute of Technology, and Dr. A. F.

light reflected from the optical parts change color (right)-the changing color being an index of the thickness of the coating-the process may be terminated at the moment the film becomes a quarter-wavelength thick. By strict attention to the length of time, the degree of temperature, and the amount of vacuum, the modern, hard low-reflection film is obtained. (Bausch and Lamb photographs.)


1945 Theatre Catalog, 4th Edition, Page 324