GB898714A - Improvements relating to semi-reflecting optical devices - Google Patents

Abstract

898,714. Coating by vapour deposition. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Feb. 2, 1961 [Feb. 10, 1960], No. 4742/60. Class 82(2). [Also in Groups XX and XL(b)] A semi-reflecting optical device comprises a transparent base as of glass or plastic, having thereon a first layer of silicon monoxide, a second layer of silicon dioxide and a third layer of silicon monoxide, each layer having an optical thickness which is equal to onequarter of the mean wavelength of the light (e.g. 5500Š) with which the device is to be used. This device may be made by placing a glass base in an evacuated chamber and evaporating silicon monoxide from a tantalum container which is heated by the passage of a current therethrough. Thereafter oxygen is allowed to flow into the chamber and the silicon monoxide is again evaporated so that its vapour reacts with the oxygen to form a deposit of silicon diocide. A further layer of silicon monoxide is then deposited. The thickness of each layer is controlled photo-electrically. For this purpose, a monitor glass 1 is located in the chamber 2 adjacent to the surface of the transparent base member so that similar deposits are formed on surface 3 which is partially covered by a screen 4. Light from a source 7 is passed through a window 8 and reflected on to the monitor glass 1 and then through a window 9 to a photosensitive device 10. As the thickness of the silicon monoxide builds up, the output from the photosensitive device 10 increases to a maximum at which point the evaporation of silicon monoxide is cut off. When depositing silicon dioxide evaporation is cut off when the output from the photosensitive device decreases to a minimum. In an alternative a layer # inch thick is first deposited on a monitor glass and then vapour is allowed to fall thereon and on the transparent base member until a layer having the thickness of # inch is formed on the monitor glass.