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1945 Theatre Catalog, 4th Edition, Page 323 (299)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 323
Page 323

1945 Theatre Catalog, 4th Edition, Page 323

lenses Improved by low-Reflection Films

Perfection of Process for Coating Lenses A Great Development in Proieciion Optics

Lens coating, perhaps one of the great optical developments of the past half century, is bound to be more and more an important factor in the making and showing of better motion pictures. Basically the idea is to treat a lens by special means to allow more of the incident light to pass through the optical system and thus put more usable light on the film in the camera and on the screen in the theatre. This effect is attained by means of coatings, referred to as low-reflection films, since the essential property of the films is that very little light is reflected. The term transmission film* is also used, because, by reducing the reflection, the transmitted light is increased. However, on the theatre supply shelves, projection and camera lenses provided with such films will probably be referred to as plain ifcoated lenses."


It is a matter of common knowledge that the clean surface of a piece of glass is a rather good reflectoreand the polished surface of a lens is no exception. Regarded from the viewpoint of light economy, the refiections from a lens are highly undesirable, since the light which is reflected is obtained solely at the expense of the transmitted beam, thereby weakening it.

The amount of light lost when it strikes a glass surface increases with the refractive index (roughly a measure of density) of the glass. For ordinary winlow glass, the loss is 4.2 per cent at each surface and for the heavier flint glasses, it may amount to as much as 7 per cent for each surface passed.

The refiection, of course, at a single lens surface may not be of much consequence, but the cumulative effect with many surfaces may become extremely high. To illustrate, consider a standard 7x50 prismatic binocular, in which the light crosses boundary surfaces ten times in its passage through the instrue merit. The transmission is approximately 55 per cent, the other 45 per cent being lost by absorption within the glass, and reflection at the ten air-glass surfaces. Although only about 12 per cent is accounted for by absorption within the glass itself, the remaining 33 per cent represents the total of all refiection losses. Many complicated optical systems often involve twenty or more airglass surfaces, and even the usual motionspicture projection lens-like the Bausch and Lomb Super Cinephor, has eight air-glass surfaces.

f The terms Hlow-refiection film" and "transmission film" are used synonymously, with, however. the prevailing usage inclining to the former term as the one more accurately expressing, optically. the function of the film. However, decreasing refiection by any of the means herein described naturally increases the transmission.



Optical engineers have, of course, long been aware of these losses, but the reduced transmission had generally been accepted as a necessary evil, since corrective measures were not known.

Attempts to reclaim a major portion of this redected light by preventing its formation have only recently been sufficiently successful to warrant commercial application. And the solution was quite simple: Merely put a new surface on the glass!

The great amount of publicity that lens coating received five or six years ago was doubtless due in a large measure to its importance in the optical field. But it must be granted that this was aided by the perhaps unfortunate (but nonetheless appealing) popular appellation of itinvisible glass."

Under ordinary conditions, of course, there is really no difficulty in seeing a piece of ffinvisiblen glass, and hence, the nickname oughtfnow to be dropped, for it has outlived its purpose in the popularization of the subject. It seems best to speak simply of surface-treated glass or of glass provided with low refiection or transmission films, since the treatment to increase transmission consists in providing the glass with a surface film of definite characteristics.


There are now known several methods of obtaining low-reflection films on a lens or other glass surface, among which are (1) leaching; (2) by building up a film

of successively-applied monomolecular layers of certain metallic soaps; (3) by thermal evaporation of a salt, generally a duoride, in vacuo, and allowing the vapor stream to condense on the optical surface; (4) by chemical deposition from a bath or application directly to the glass; and (5) by etching the surface of the glass with the vapor of hydrofiuoric acid. The subject of filming is so active, however, that further methods may be expected to be announced.

Historically, the chemical treatment of surfaces is oldest, dating back to 1892, when the English lens designer, H. Dennis Taylor, observed that lenses which displayed the tarnish produced by weathering actually had less redection loss than freshly polished lenses. While this observation was contrary to popular, prevailing opinion, Taylor demonstrated that the tarnish was a benefit and not a detriment, and showed experimentally that an artificially accelerated weathering effect could be induced by applying chemicals and that lenses so treated have less than normal reiiection.

Tayloris observations were buried in the scientific literature of optics until 1916, when the idea of chemically leaching certain kinds of glass was resurrected and made the subject of further investigation by Frederick L. G. Kollmorgen, who before he became vice-president of the Kollmorgen Optical Corporation in 1915, had been associated with Cark Reichert of Vienna, Ross,iLtd., of London, and Keuffel and Esser of Hoboken, serving all as an expert optical adviser.

COATED AND UNCOATED lENSES side by side show clearly the advantage to be obiained from adding lowrefleciion films at the air-glass boundaries. Here are seen iwo Balfar camera lenses, the uncoafed Iens on the left, ihe coated on the right. Note particularly how, with the ordinary lens, the reflecfions ulmosi prevent one's being aware of the background, while with the coated lens the details are seen. (Bousch and Lomb photograph.)

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1945 Theatre Catalog, 4th Edition, Page 323