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1945 Theatre Catalog, 4th Edition, Page 325 (301)

1945 Theatre Catalog, 4th Edition
1945 Theatre Catalog
1945 Theatre Catalog, 4th Edition, Page 325
Page 325

1945 Theatre Catalog, 4th Edition, Page 325

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FIGURE l.-Light striking the surface of glass sets up an infinite series of reflections within the glass itself, as shown here. The more incident light that is reflected, the less there is to become transmitted.

Turner, now staff member of the Bausch and Lomb Optical Company.

The transmission films produced by Doctor Cartwright and Doctor Turner possessed sufficient durability combined with optical efliciency to make them suitable for some commercial applications. While they did not fare too badly in spite of the normal abuse and other encounters of military parts, a still higher degree of durability was deemed necessary for the purposes of the Army and Navy.

After considerable research and experimentation in the Optical Shop of the Naval Gun Factory, Dr. Dean A. Lyon perfected an improved heat treatment for the magnesium fluoride films which was more convenient, economical, and efficient than the previous baking procedures and resulted in an evaporated film which was far superior to any then known.

The Lyon method was successful-and, being acceptable to the Armed Services, the process was made available to all companies engaged in making optical instruments for the Services.

The Lyon method in brief comprises the heating in a high vacuum of the optical parts to be coated. When the desired temperature is attained, magnesium fluoride, in a crucible in the vacuum chamber, is evaporated by the heat of a small electric coil immediately above. By attention to the conditions of time, temperature, and vacuum, the hard film is obtained on the optical elements.

Thus, a new commercial process came into being which, in three short years, has grown into a thriving new industry having a capital investment of several million dollars.


In determining where reflection will occur, it is an inflexible law of Nature that light will be reflected whenever the light goes from a glass of one optical density into a glass of a different optical density. Since the itindex of refraction" is a measure of the optical density, it follows that light will be reflected wherever the index of' refraction changes. It does not matter whether this change of index takes place gradually or occurs



suddenlyeas at the surface of a piece of glass. Since homogeneous, transparent materials-such as glass, quartz, liquids, etaein contact with each other or with air are practical cases, the index of refraction changes suddenly at the boundary surfaces and is the only case which needs to be considered.

Since a change in index is the condition necessary to cause reflection, it follows that reflection will occur if light goes from a less optically dense into a more optically dense (transparent) medium, and vice versa. Renection will therefore occur when light goes from air (index equals 1) into glaSs (index equals 1.51 or more) and also when the light goes from the glass into the air.

Theoretically, every reflection, except the very first, gives rise to an infinite series of reHections, as indicated in Figure 1 for a single sheet of glass.

Referring to Figure 2, it is reflection B plus the minor part represented by the multiple reflection which cancels reflection A. Similarly, reflection E plus the minor part presented by the multiple reflections cancels reflection D.

If there is optical contact between the two pieces of glass, the reflections reduce to a single one (C, in Figure 2). No matter how perfect this contact, a reflection will always exist because light goes from glass 1 of index V to glass 2 of index W, or vice versa. If the indices of the two glasses are identical, then no reflection occurs, as it is equivalent to a single thickness of glass. In order to eliminate the reflection at the cemented surface for any specified wavelength, the cementing medium (balsam or resin) would have to be one-quarter wavelength

.thick and have an index of refraction

equal to the square root of the product of the two indices of refraction. In practice, no effort is made to attain these ends, because the reflection at the cemented surface is small and inconsequential.

Thus, it is seen how, when light strikes a plate of glass, it is slowed up and a portion of the energy fails to penetrate the surface. The surface of the glass acts like a bump and, in passing over it, some of the energy of the light is jarred loose, and goes to form a reflected beam. Now, it seems plausible to try smoothing out this refractive-index bump, by letting the light penetration take place in steps. Over the surface of the plate of glass is placed a layer of special material of lower index. Then the initial loss at the






FIGURE 2.-While, with coated glass, reflections do occur (compare Figure l), certain pairs of reflections, being half-way out-of-phose, cancel. Thus reflections A and B being canceled, as are D and E, thus, there is an increase in the amount of transmitted light.

new surface will be lower than if the light struck the bare surface of the original plate directly, because the index of the new surface is smaller. Of course, there has been added a new reflecting surfaC%the inner face between the surface layer and the plateebut it turns out that the combined light loss at the two surfaces is less than the light loss by one reflection from the bare surface of the untreated plate if the film thickness is properly chosen.

Even though complete extinction of reflection cannot be obtained, the practical results are remarkably good. Using magnesium fluoride, a crOWn glass (a1kali-lime glass) having a refractive index of 1.52 will have its normal reflectance of 4.2 per cent reduced to 1.3 per cent. A dense flint glass (heavy, brilliant glass containing lead), with an index of refraction of 1.65, will have its normal reflection of 6 per cent reduced to 0.6 per cent.

The maximum sensitivity of the eye lies within the wavelength range between 510 and 555 millimicrons, the lower value (the blue end of the spectrum) being reached when the eye is dark-adapted and the higher value (the red end) when it is adjusted to the high-level illumina THE LIGHT LOSS DUE TO REFLECTION at the airvto-gloss surfaces in o Cinephor lens without (left) and with (right) low-reflection films is graphically shown. The width of the black bond, as it increases, passing through the lens from left to right, indicate the cumulative loss of light. Note that there is an increase in light loss at every uir-to-glass boundary and how coating greatly reduces the loss. (Bousch and Lomb photograph.)
1945 Theatre Catalog, 4th Edition, Page 325