Improvements in or relating to films for reducing or altering surface reflection from light-transmitting and other articles

Abstract

Abstract 538,273. Altering and reducing surface reflection. BRITISH SCIENTIFIC INSTRUMENT RESEARCH ASSOCIATION Dec. 22, 1939, No. 32794 Convention date, March 20, 1939. [Class 97 (i)] Films altering or reducing, by interference effects, the reflection from a surface are formed by successively evaporating on to the surface dissimilar light-transmitting materials having different indices of refraction ; a layer a few molecules thick may be first applied to the surface to increase adhesion of the film. The thicknesses of the film may be controlled by use of a " crossed wedge " formed by evaporating one substance on to a glass plate inclined to a line joining the heater and the plate, so as to produce a deposit of gradually changing thickness in one direction, and then turning the plate through 90 degrees in its own plane and evaporating the second substance on to it to produce a deposit having a thickness gradually changing in a direction at right angles to the direction of change of the first deposit. The thicknesses necessary to produce the desired reflecting power are determined by the " crossed wedge," and deposition may be controlled photometrically during the evaporations on to the surface to be treated. The surface may be first coated with a chromium layer a few molecules thick which is oxidized by exposure to air and increases the adhesion of the film ; a film of aluminium oxide slightly thicker than quarter of a selected wavelength may then be produced by evaporation, and a film of quartz having a thickness a little less than the quarter wavelength may be produced thereon by. further evaporation. Such a film produces a reduced reflectivity in the spectral region to which the eye is most sensitive. Examples of other combinations of substances are given in the Specification, of which one produces selective increased reflectivity, and all are sufficiently rugged to withstand vigorous brushing with a soft brush. Ruggedness may be increased by baking the films.

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