Radio age research, manufacturing, communications, broadcasting, television (1941)

Splitting Light Beams A PRE-WAR DEVELOPMENT /N SOUND-FILM RECORDING, IS PUT TO WORK TO INCREASE ACCURACY OF GUNFIRE. By R. H. Heacock Engineering Products Division RCA Victor Division THE optical phenomenon that causes bright blobs of color to appear when a thin film of oil floats on a puddle was put to work by RCA scientists to help our 40mm. anti-aircraft gunners draw a deadly accurate bead on Jap planes. The principle is used by RCA in process- ing a vital element of the new U. S. Army M-10 range finder, develop- ment of which was recently an- nounced by the Eastman Kodak Company and the Rochester ( N. Y.) Ordnance District of the Army. This element is a rectangular glass plate about the size of a calling card, but i/4-inch thick, with a sub- microscopically thin chemical coat- ing. Its function is to "split" the light striking it, transmitting a band of light with one color pre- dominating, at the same time re- flecting a band with the complemen- tary color, both functions being car- ried out without absorbing any light within the reflector. This reflector is a direct out- growth of a pre-war development made by G. L. Dimmick of RCA's Indianapolis Engineering Depart- ment for the solution of a Holly- wood sound-film recording problem. At that time Mr. Dimmick devel- oped a color selective reflector for separating out from the modulated light-beam a band which is not very useful for photographic exposure but which is highly efficient for operating the phototube of a moni- toring system. Thus in the film re- cording application, the reflector enables direct monitoring of the modulated light-beam without de- tracting measurably from its photo- graphic exposure values. This rep- resents a decided improvement over the type of monitor which divides the light on a purely quantitative basis and, therefore, noticeably re- LOW REFLECTION COATINGS APPLIED TO BINOCULAR LENSES INCREASE LIGHT TRANSMISSION AND DECREASE SURFACE RE- FLECTIONS. LEFT, BELOW: IMAGE FROM UNCOATED LENS; RIGHT, IMAGE FROM TREATED LENS SURFACE. f\ / WHEN WHITE RAYS (LOWER LEFT) STRIKE A COLOR SELECTIVE REFLECTOR AND ARE SPLIT, THE TRANSMITTED RAYS (right) have a YELLOW COLOR AND THE REFLECTED RAYS (UPPER LEFT) ARE BLUE. duces the light available for activat- ing the emulsion on the film. This development was described by Mr. Dimmick in the Januai-y 1942 issue of the Journal of the Society of Motion Picture Engineers. Before discussing the principal application for similar reflectors in military range finders, it may be well to review briefly the best known methods by which light may be divided on quantitative and qual- itative bases. The common method for dividing light on a purely quantitative basis is to employ a partially silvered sur- face, that is, one with such a thin silver coating that part of the light passes through and part is reflected. These are far from 100% eflJicient because the silver film actually ab- sorbs an important percent of the light it intercepts. Standard Method Inefficient A standard method for dividing light on a color selective basis is by means of colored glass or gelatine filters which are inefficient for many applications because they also ab- sorb much of the light energy. RCA has developed unique light dividers which employ the same optical interference phenomena which make low-reflection coatings possible. Reflectors which divide the light quantitatively and others which divide the light qualitatively (by color) are both made by the Company. They are phenomenally efficient because they have practi- cally no absorption. 20 RADIO AGE |