GB717057A - Improvements in cathode ray tubes - Google Patents

Abstract

717,057. Cathode-ray tubes. ABBEY, A. (American Optical Co.), March 5, 1952, No. 5765/52. Drawings to Specification. Class 39(1) A cathode-ray tube having a fluorescent screen disposed adjacent the inner side of the tube face, has a non-absorbent transparent film on the outer surface of the tube face which is designed to reduce reflection of light rays emitted by the fluorescent screen and striking the outer surface of the tube face at angles less than the critical angle in the absence of the film. The amount of light transmitted through the tube face is increased and halation effects are reduced. The film has an optical thickness which is an odd multiple of a quarter wave length of incident light and a refractive index which is lower than that of the material of the tube face. The optical thickness may be in the vicinity of a quarter wave length and the refractive index may approach the square root of that of the material of the tube face or vary from a value of approximately one at the air interface to a value approaching that of the material of the tube face. A similar film may be provided between the fluorescent screen and the tube face. The film may comprise discrete, microgranular solid particles forming minute irregularities on the outer surface of the tube face. The film may be formed by the evaporation of calcium or other fluorides on to a glass surface or by wetting the glass with silicic acid and treating with hydrochloric acid to precipitate the silicon dioxide. Another type of film is made as a film of barium stearate which is produced by dipping the glass in water containing barium salts and having a layer of stearic acid on top; others are films of magnesium fluoride, calcium fluoride, sodium fuoride or sodium aluminium fluoride. Other films are decompositions of salts. One film may be formed of a composition consisting of a colloidal suspension containing discrete microgranular solid particles of anhydrous magnesium fluoride, lithium fluoride, strontium fluoride, calcium fluoride, barium fluoride or cryolite dispersed in a volatile liquid. This composition is then applied to a glass surface and subjected to heat to evaporate the liquid and to leave a dry coating on the surface. To avoid halation effects at the edges of the fluorescent screen due to fluorescent material being deposited on the side walls of the tube adjacent their junction with the tube face, a ring member may be inserted in the tube for limiting the image area, In addition, the outer edge of the tube face may be highly polished or coated with light-absorbing material such as a suspension of graphite particles in oil or cement such as Canada balsam, or black lacquer.