International photographer (Jan-Dec 1941)

coatecI Lenses Reprinted from the Journal of the Society of Motion Picture Engineers, September, 1941, page 265. RECENT IMPROVEMENTS IN NON-REFLECTIVE LENS COATING By William C. Miller Summary. — As early as 1892 it was known that the reflectivity of polished glass surfaces lias reduced and the light transmission increased when a suitable film teas present on the surface of the glass. Many efforts to produce such a film artificially met with only partial success. In the past five years, two different methods have been discovered that achieve the desired results. Only one of the processes, however, was satisfactory for commercial application. Great improvements have been made in the durability and weather resistance of the thin films deposited upon the lens surface by this method. Lenses coated by this improved process require no more careful handling than any good lens is entitled to; fingerprints and dust can be removed without detrimental effects to the coating. The thin films can not be scratched with anything less hard than a metal point. By this process, reflectivity can be reduced from an average of 5 per cent for untreated polished surfaces to as low as 0.5 per cent for treated ones. Experiments show thai even greater reductions are possible and should be available in the near future. The general application of the lens-coating process to studio optical equipment is now just one year old. In view of the wide interest and attention that this process has aroused, a discussion of the results and a report of the improvements made in the process will be of interest. Unfortunately, time has not permitted the accumulation of exhaustive data. However, those that are available show that the new process is of vital importance in many fields and is already quite indispensable. HISTORICAL Although it had been known for many years that certain types of glass developed a tarnish after prolonged exposure to the air. it apparently was not until 1892 that any careful study of the effects of such tarnish was made. At that time H. Dennis Taylor, famous lens designer, made careful measurements upon several tarnished lenses that had come to his attention. The tarnish had the appearance of a metallic sheen and had always been considered to be highly detrimental. The results of Taylor's measurements and tests, however, showed that the tarnished lenses reflected less light from their polished surfaces than did identical new ones. This of itself was of great importance, but of still greater importance was the fact that the light that was no longer reflected by the polished surfaces was transmitted by the lenses. The tarnished lenses produced images measurably brighter than did identical new and untarnished lenses. Taylor was so impressed with the potentialities of the discovery that he made extensive experiments to find means of producing this tarnish artificially on the surfaces of new lens elements. Unfortunately he met with only partial success, for the types of glass that he was able to treat proved to be limited. Furthermore, the reduction in reflectivity obtainable with many of the glasses was too slight to be of commercial value. Many efforts were made in subsequent years to discover methods of artificially producing the desired results, but with only moderate success. Kollmorgen, Kellner, Wright, and Ferguson all made contributions to the art, but certain types of glass resisted all attempts to produce a tarnish of the desired nature. All the processes developed up to that time were of the chemical type; that is, they depended upon the action of chemical solutions or concentrated salts upon the surface of the glass to produce the desired tarnish. Since this reaction took place with the glass itself, it was impossible to remove the effects of the treatment without completely refinishing the optical surface, a costly and time-consuming procedure. The greatest care was therefore necessary in the treatment of optical elements to insure satisfactory results, since an error meant refinishing the surface or making a new element. This treatment could not be safely attempted by anyone other than the makers of the original optical parts. Since many varieties of glass are employed in the lenses in common use, and many of these glasses either could not be treated at all or could be treated with only moderate success, the application of the process was not widespread. What was required to make the theory universally practical and applicable was a method of producing the tarnish upon lens surface irrespective of the type of glass from which the lenses were made and would yield reductions in reflectivity sufficiently great to justify the trouble and expense of application. In view of the many years that elapsed with little or no successful development of the art, it is remarkable that two independent processes of quite a different nature should be announced within the short period of three years. The first announcement came in 1936 of a process discovered by Dr. John Strong1 of the California Institute of Technology. Strong's process consisted of the deposition of a thin film of (Continued on Page 16) lrj'^1. 1/ ■ ^§>T JL $&' \ < Vr #i'\N I f « *^ A *< up fr -V j i V l , M 1 \ Stillman Ted Weisbarth's idea of "Leg Art"" 12