International photographer (Jan-Dec 1941)

CoatecI Lenses (Continued from Page 12) suitable material upon the surface of optical elements in a high vacuum. This thin film, when deposited under the correct conditions and to a specified thickness, effected reductions in the surface reflectivity as great as 85 per cent. The second announcement came in 1939 of a process discovered by Miss Katherine Blodgett2 of the General Electric Laboratories. Miss Blodgett's process consisted of the formation of a soapy film of the required characteristics upon the surface of optical elements. Although the reductions in reflectivity achieved by this process were great, the extreme fragility of the film made the process impracticable for general use. THEORETICAL The theory of the reduction of surface reflection has been dealt with so thoroughly and competently by others in the literature1 ~ 3 4 that it will be necessary to give only the general principles of the phenomenon here. The quantity of light reflected from the polished surface of a transparent material and, therefore, lost from the transmitted beam, depends upon such factors as the index of refraction of the material and the angle at which the light strikes the surface. If the angle of incidence is kept constant, then the index of refraction is the determining factor, and the higher the index the greater is the percentage of light reflected. Light can be considered as traveling in a wave form. When a beam of light is reflected from two parallel polished surfaces of a transparent material, the light-waves can be made to supplement or oppose each other in the reflected beams by suitable adjustment of the separation of the reflecting surfaces. When these have an optical separation of Y^ of a wavelength, the waves in the two reflected beams oppose each other and cause destructive interference. The total intensity of the reflected beam will be zero when, and only when, the two components are of equal intensity. If we wish to reduce the reflectivity of the polished surfaces of an optical element and thereby increase their transmission, it can, therefore, be done by providing over the entire element two reflective surfaces separated by -/4 wavelength, both surfaces reflecting an equal amount of light. Under these conditions, the two beams will cancel each other. Although it was not clearly understood until the time of Dr. Strongs work, it was this interference phenomenon that accounted for the effects observed by Taylor and the others. The most satisfactory method of producing the two reflective surfaces separated by the correct distance is to form upon the surface of an optical element a film of transparent material of such nature and of such refractive index that the light reflected from the contact surface where the film touches the glass equals that reflected from the upper surface. This index can be found with little trouble to be equal to about 1.25. The effects that Taylor observed first were due to the formation of a film of approximately the required characteristics by the chemical action of the air with some of the constituents of the glass. The chemical methods that were subsequently developed all aimed at the artificial stimulation of such a film. The failure of the methods to produce more satisfactory results was due to the fact that a film of the required index could not be formed on all types of glass. Even the process developed by Strong missed perfection in that particular respect, for there is no suitable substance that can be applied in the form of a film having an index as low as the required 1.25. All the processes — the chemical by Taylor, Kollmorgen, Kellner, Wright, and Ferguson: the evaporation by Strong; and the one by Miss Blodgett — fail in one other important respect which offers such natural obstacles that it may never be surmounted : that is, the thickness requirement. The film can be made of the required thickness for only one wavelength at a time and is, therefore, wrong for all others. Consequently, when white light is used, the re FAXON DEAN INC CAMERAS, BLIMPS-DOLLYS FOR RENT No. 22184 4516 Sunset Boulevard Night, SUnset 2-1271 duction of reflectivity can be made a minimum for only one color; all others suffer greater amounts of reflection. Fortunately, the difference for other colors is not great, but it is sufficient to give treated surfaces a colored hue when viewed by reflected light. If all colors were reduced equally, the remaining small amount of reflected light would not display any predominant color. Optical systems designed to work with light of some certain wavelength should be treated to give maximum transmission for that wavelength. Complying with this rule there are in use in the studios many violet recording systems that have been treated for maximum transmission at about 4000 A. At the writing of the previous paper5 on this subject in April, 1940, the process had been in use experimentally for only a few months, but such great interest was shown in the possibilities of the process that a report was considered desirable at that time. Due to the newness of the process, however, little definite information based on actual production results could be given. At the present writing, however, some very interesting data are at hand, supplied through the courtesy of several of the studios in Hollywood. Sound-recording systems consisting of ten air-glass surfaces have been treated both for violet and unfiltered light. A gain in transmission of 50 per cent was measured in nearly all cases. Since the tungsten recorder lamps are of necessity burned at or near their peak capacity, this 50 per cent increase in transmission in the optical train has made it possible to relieve the load on the lamps and thereby considerably increase the lamp life. In some instances the gains obtained by treatment of the lenses have been utilized, not to save current or lamp life, but to make possible the use of slower, finer-grained films. A large number of motion picture camera lenses has been treated during the past year. Careful measurements made at one of the major studios on a 3-inch focus Cooke Speed Panchro lens at //2.0 showed the transmission of the untreated lens to be 69.5 per cent. The transmission of the lens when treated was 95.1 per cent. In other words, the light loss had been reduced from nearly 30 per cent to less than 5 per cent. Another studio reports measurements showing a gain of 32 per cent due to treatment of another type of lens. Of even greater interest than the increase in transmission is the improvement in the image quality due to this treatment. The increase in contrast and brilliance of pictures made with treated lenses is very noticeable. In work where the utmost in image quality is required, such as in process projection keys, the treatment is of great value and is widely used in several studios. (Concluded in January Issue) 16