US 2680205 A
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June 1, 1954 A. L. BURTON CATHODE-RAY TUBE AND METHOD OF MAKING SAME Filed NOV. 17, 1950 ALLAN L.
INVENTOR BURTON ATTORNEY Patented June 1, 1954 UNITED STATES PATENT OFFICE CATHODE-RAY TUBE AND METHOD OF MAKING SAME Allan L. Burton, Thompson,
American Optical Mass., a. voluntary ass Application November 17, 1950, Serial N 0.
method of improving the observation qualities of such images.
or effectively introducing no change as to contrast discrimination of the image produced by the inner phosphor layer of such a tube.
Another object is to provide a cathode ray tube or the like with Another object is to provide a tube of the above character with a light diffusing surface substantially in the plane of the image-producing surface whereby the definition of the image visible through the diifusing surface will be effectively unaltered.
Cnn., assignor to Company, Southbridge, ociation of Massachusetts sion has been the presence of annoying surface reflections as well as specular reflections of outside sources of illumination from the outer and inner surfaces of the tube.
Many attempts have been made to obviate such reflections as, for example, by
the combining of an outer reflection-reduction coating with an inner light-diffusing coating to overcome the difiiculties of the prior art as set forth above and which have been found to provide very sim ple, eflicient and economical means for accomplishing the desired results while introducing effectively no change as to the definition, discrimination and contrast of image.
Referring to the drawings wherein like characters of reference designate like parts through" out the several views, in Figs. 1 and 2 there is illustrated a cathode ray tube embodying a transparent face portion 19, conical side walls ii and a neck portion 12. The side walls ll may be formed either of glass or metal as desired. The neck i2 contains a conventional electrical discharge device (not shown) which is adapted to direct a moving beam of electrons E toward the face portion of the tube it so that the electrons will engage a fluorescent screen 13 deposited by conventional methods upon the inner surface i l of the face of the tube. The fluorescent screen 13 is formed of a plurality of tiny phosphor particles each of which is capable of fluorescing when contacted by the electron beam E.
in following the teachings of the present invention, the inner surface ill of the face of the tube iii is initially provided with a light diffusing surface whether by grinding, sand blasting, acid etching, or by any other known method prior to the depositing of the phosphor coating l3 thereon. The light dilfusing surface it, therefore, is in substantially the same plane as the image produced by the phosphor coating l3 when bombarded by the electron stream E and therefore avoids deterioration of the definition of the resultant image as would be the case if the light diffusing surface were placed on the outer surface of the face it and spaced considerably from the plane of the image by the inherent thickness of the material of the face. This light diffusing surface eliminates spherical reflections which might result from light from an outside source.
To further eliminate specular reflection as well as other surface reflections from outside sources, the outer surface of the face ill is provided with a reflection-reduction coating l5. This coating may be formed as follows:
By evaporation of calcium or other fluorides on the outer surface of the glass face ill or by wetting the face with silicic acid and treating with hydrochloric acid to precipitate the silicic acid; another type is made as a film of barium stearate which is produced by dipping the face in water containing barium salts and having a layer of stearic acid on top; others are films of magnesium fluoride, calcium fluoride, sodium fluoride or sodium aluminum fluorides; still others are chemical decompositions of salts.
The preferred reflection-reduction coatings,
however, are formed in accordance with the teachings of Moulton Patent No. 2,432,484, issued December 9, ran, and Moulton application Serial No. 739,545, flled April 5, 1Q47, and comprising compositions consisting of colloidal suspensions containing from about 0.1 to 6.0 per cent by weight of submicroscopic, microgranular, discrete particles of solid anhydrous transparent material such as silica, magnesium fluoride, lithium fluoride, strontium fluoride, calcium fluoride or cryolite substantially uniformly dispersed in a volatile liquid inert to the particles, with the particles being approximately spherical in shape and substantially less than one-quarter of the wave length of light in diameter.
The glass surface can be provided with the reflection-reduction coating by applying-to the surpriate index face a thin layer of the above composition and causing it to dry, leaving a dry coating of very minute ultra-l icroscopic particles on the surface.
It is desirable to control the concentration of the particles in the suspension to produce a reflection-reduction coating having a resultant thickness of approximately one-quarter wave length of light.
The reflection-reduction coating thus formed will comprise sub-microscopic, discrete, microgranular, transparent solid particles which are so deposited on the surface as to form minute projecting irregularities on said surface, the concentration of the particles in the irregularities decreasing from the surface of the glass outwardly and the material of the particles being such that the effective index of refraction of the reflectionreduction coating varies from substantially unity at the layer-air interface to an index value which progressively increases as it approaches the surface of the glass where it substantially approximates the index of refraction of said glass.
in order to render the above coating more resistant to abrasion, a small amount of tetraethylorthosilicate may be incorporated in the colloidal sub-microscopic suspension.
Other reflection-reduction coatings can be formed by following the teachings of Cartwright et al. Patent Nos. 2,207,656 issued July 9, 1940, 2,281,474 issued April 28, 1942, and 2,281,475 issued April 28, 1942.
For example, a reflection-reduction coating suitable for the purpose can be formed in accordance with the above patents by applying to the glass surface a layer of a suitable substance having an index of refraction intermediate the index of refraction of the optical element and the index of refraction of air and having an optical thickness approximately 122/4 of the wave length of preselected light, a: being a positive odd integer not greater than 9 and preferably being 1. A film of lithium fluoride, sodium fluoride, sodium aluminum fluoride, calcium fluoride, or the like, of the appropriate optical thickness and approof refraction, on the surface of the glass will very greatly diminish. light reflection from the surface. Such a film can be deposited by evaporation onto the. glass, with the evaporation being controlled so as to produce a layer with an optical thickness of approximately, onequarter of a wave length of the light and with an eifective index of refraction approaching the square root of that of the glass to which it is applied. The fllm can be improved with respect to ruggedness and tenacity by subsequently baking at a temperature between 390 C. and 500 C. for a period of time sufficient to effect the desired improvement.
Still another method for reducing reflections from the surface of the glass isthe skeletonizing process wherein. the surface is treated with a solution of fluosilicic acid having a quantity of silica dissolved thereinranging from saturation to about 3 millimoles supersaturation per liter, the treatment being for such a length of time as will produce on the surface a skeletonized fllm of substantially pure silica of desired depth and index of refraction, whereby the desired reduction in reflection is accomplished, the depth being indicated by the wavelength of light predominantly reflected by the surface. The treatment is pref; erably accomplished by immersing the glass surface in the solution while the solution is main.- tained under agitation.
Another method of forming areflectionrreduce tion coating is by providing a solution of fluosiliclc acid having a The above reflection-reduction coatings, while being efficient as to the elimination of specular reflections and th reflections from outside mally non-fluorescing particles of the phosphor will be illuminated and result in the introduction of halation. The reflection-reduction coating,
Robert Bowling, Barnes.
Although light difiiusing surface may and the image forming characteristics thereof destroyed.
reflection-reduction coating deteriorate the same, results in a tube a better overall performance.
It might be said that slight deterioration of the image which external sources.
the observer is obtained.
2. A cathode ray tube of the character describedhaving a light-difiusing surface on the from externally of the tube, a layer of fluorescent material disposed over said light-diffusing surouter side in the' form of minute projectingirregularities with the material of the particles being such that the effectiveindex of refraction of the reflection-reduction coating formed thereby varies from substantially unity at its air interface to an index value which progressively increases as it approaches the outer surface of said face where it substantially approximates the index of refraction of said face whereby simultaneously transmission of said luminous image through the outer side of the face of the tube is increased and the amount of light from said luminous image reflected back is decreased, and with the amount reflected back being diflused by said light so as to substantially increase contrast and definition the image when viewed by an observer, and said coating on the outer side of the tube face simultaneously reducing the reflection of light, by said surface, from exteriorly of the tube.
3. A cathode ray tube of the character described having a layer of fluorescent material disposed. on its inner surface in intimate contact therewith and adapted when bombarded by an electron stream to emit a luminous image for transmission through the face of the tube, said inner surface in intimate contact with the layer of fluorescent material embodying a plurality of @ontiguously related irregularities forming lightdiffusing means located substantially in the same plane as said image emitted by the fluorescent material for breaking up specular images normally reflectedby said side-from exteriorly of the tube while not substantially affecting said image aslit is transmitted therethrcugh, and a transparent coating on the outer side of the face of the tube, said transparent coating being of acharactor and having an optical thickness such as toincreasethe transmission of said luminous image through the outer side of the face of the tube and said lightaiiifusing meansbreaking up the portion not transmitted and reflected back whereby the definition of the luminous image emitted. by the. fluorescent material is substantially improved, and said outer coating simultaneously' reducing the reflection of light from ex!- teriorly of the tube.
4. A cathode ray. tube of the character described having an acid etched, surface on the inner side of the. face thereof forming light-difiusing' means for breaking up specular images norially reflected by said side from externally of the tube, a layer of fluorescent material disposed over said light-diffusing surface in intimate con tact therewith and adapted when bombarded by an electron stream to emit a luminous image for transmission through the face of the tube, said light-diffusing surface by reasonof its'close proximity to the fluorescentm rial having substantially no effect upon the de nition of said image as it is transmitted therethrough, and a transparent coating on the outer side of the face of the tube, said transparent coating having an effective optical thickness and refractive index such as to increase the transmission of said luminous image through the outer side of the face of the tube, and said light diffusing surface breaking up the portion reflected back by said outer surface whereby improved contrast and definition of the luminous image emitted by the fluorescent material is had and said transparent coating further functioning to simultaneously reduce the reflection of light from exteriorly of the tube.
References Cited in the file of this patent UNITED STATES PATENTS