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

:SllAUl)VV I'KTURES OF 200, 500, 1000, AND 1500 MESH SCREENS ENLARGED 200 TIMES BY ELXCTRON MlCROSCOrE. MAKING FINE MESH SCREENS Metallic Gossamers of Extreme Fineness Arc Made for Image Orthkon Tubes by Process Developed at RCA Laboratories by electrolysis. They had about 200 holes per linear inch, or 40,000 openings to the s()iiare inch. How- ever, these metallic gossamers passed less than 40 percent of the electron imape and, in addition, were non-uniform in the arrange- ment of openings. Although these were the screens that had to be used in early models of the image orthicon, it was imme- diately obvious that they would seriously restrict picture quality. Because of the possible value of the image orthicon in military ap- plications, a search was started for a method of making a high trans- mission, uniform screen of 500 mesh or more. Out of this war- intensified activity came a proce- dure that, on a small scale, delivered very uniform screens up to 1.500 mesh, three times the goal. On the opposite page is an at- tempt to illustrate the fineness of a .'iOO-mesh screen. A small section of screen was laid over a period of the size which ends this sentence and the combination was enlarged about 70 times. Small as the dot appears to the naked eye, neverthe- less, the photomicrograph reveals that 66 perfectly-formed, conii)lete openings of the .>icreen are included within the circumference <if the period. Production of fine mesh scre<'ns by the i)rocess developed at RCA Laboratories reallv beirins with a sheet of highly polished plate glass By Dr. Harold B. Law RCA Laboratories Division PRODUCTION of a copper screen with 250,000 openings to the square inch was one of the prob- lems faced in developing the sensi- tive image orthicon television cam- era tube now in common use at most television studios. Because the electron image of the scene to be televised is focused on this screen, the mesh must be extremely fine, otherwise it would be visible in the picture when viewed at the receiver. Despite formidable difiiculties, the problem was not only solved but in the research a method of manu- facture Was devised by which a screen could be made with the holes constituting more than 50 percent of the screen area. The finest prewar mesh .screens were made <if wnvrn wire or formed [18 RADIO AGE coated with a layer of material re- sistant to acid. An exceedingly ac- curate ruling engine, similar to those used in making optical grat- ings, scores the desired pattern through the resistant layer, and then the lines are etched into the glass by submersing the entire plate in a hydrofluoric acid. The metal which will form the screen is applied to the master by a process called sputtering. In this, the master is e.xposed to a fine mist of a special palladium-gold prep- aration until a thin semi-transpar- ent layer of the metal covers the surface of the glass. The master is then placed in a shallow dish of Water while the surface is rubbed with a thin piece of rubber. This removes the thin metal on the sur- face, without affecting the metal in the grooves. A corner is left unrubbed in order to provide elec- trical contact for the next step, that of copper-plating the metal in the grooves. Since the metal in the grooves is SECTION OF 200-MESH GLASS "MASTER", ENi.ARCEn ABorr 400 TiMt:s.