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1945 Theatre Catalog, 4th Edition, Page 326 (302)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 326
Page 326

1945 Theatre Catalog, 4th Edition, Page 326

tion. In actual practice, therefore, film thicknesses are adjusted so that maximum efiiciency in reducing reflection also occurs within this same wavelength range.

The question is often asked why a reduction in reflection necessarily means an increase in transmission. The explanation is that light is a form of energy, and, since energy cannot be destroyed, an accounting must be made of all of it. Since there is negligible absorption and scattering in the film, the energy which no longer appears as reflection must, necessarily, appear as an increase in transmission.


No matter how the films are applied, the basic principles of their behavior remain the same. The present transmission film is commonly made of magnesium fluoride, this particular material having been chOSen because its films can be made hard and durable and they will retain these properties in service. Other substances are known which are more eflicient in reducing reflection, but they lack some essential mechanical property.

Naturally enough, the haphazard application of a film to a glass surface can scarcely be expected to produce the best results in decreasing reflection. From an optical point of View, a transmission film may be characterizedby two quantities: by thickness and by refractive in AN ILLUSTRATION OF THE INCREASE IN CONTRAST obtained by using low-reflection films on ,proiection lenses is shown here. The proiectors (right) were identical, excapt that the lens of one had been provided with transmission films. A small rectangular aperture was proiected with each and the proiecfion screen

dex. Each of these quantities has a single best-value for the purpose at hand. Disregarding the index for a moment", the question arises just how thin film may be and be most effective. The rather startling answer is, practically speaking, about four millionths of an inch! The reason for this lies in a powerful optical phenomenon which becomes prominent: the interference of light.

It can be shown mathematically that, if the optical thickness of the film is made equal to one-quarter of the wavelength of the light to be eliminated, then the reflection from the glass-film boundary will be 180 degrees out of phase (ffout of step," that is) with the reflection from the film-air boundary, and then the destructive interference will result. If the amplitudes of the two reflections are equal, then complete cancellation of the reflection takes place. The film is, therefore, a way to split the incident beam into reflections which cancel each other.

Just how thick is a quarter-ewavelength film? A relative example shows that if a filmed glass 1/16 inch thick were magnified to appear as thick as Mount Washington is high (6,293 feet), then the quarter-wavelength film would show as a four-inch snowfall on the mountain top!


Thus far, single-layer films only have been discussed. It was stated earlier that

materials having sufficiently low refrac tive indices to give complete cancellation '

of reflection were not available. .Would it not be possible to approach the problem from another direction, and, by some artificial means, to increase the effective index of refraction of the glass, so that the theoretical relationship could be satisfied in practice? Yes, for when reflection from an optical element is increased by the application of a thin film having a refractive index greater than that of the glass, the result can be interpreted as equivalent to an unfilmed glass having a higher refractive index. Hence, zero reflection at any specific wavelength can be realized if the glass is coated first with a high-index material to increase the reflection and then a second film of low-index material is applied to cancel the reflection. Theoretically there is an infinite number of combinations which will give zero reflection, but the number of materials which have the required mechanical and chemical properties is severely limited.

The intensive research now going on will undoubtedly lead to the development of a satisfactory double-layer film which can be used industrially.


In the foregoing it has been more or less assumed that the low-reflection films are operating in monochromatic (onecolor) light, which, however, would be

photographed (left). The untreated lens produces a diffusely lighted background due to illumination of opaque portions of the slide by misdirected light originating at the lens surface. The treated lens leaves the background black (because lens coatings eliminate reflections), enhancing contrasts. (Bausch and Lomb photos.)

1945 Theatre Catalog, 4th Edition, Page 326