US2634217A - Luminescent screen for cathode-ray tubes - Google Patents

Description

fatigue. A contrast by increased voltage or other known Patented Apr. 7, I953 Ulises A. Sanabria, Warren G. Taylor, and Earl L; Browning, Chicago, Ill., assignors to American Television, Inc., Chicago, 111., a corporation of Illinois No Drawing. Application May 19, 1949, Serial No. 94,258

3 Claims.

r This invention relates to a screen for cathode ray tubes and similar devices dependent upon luminescence for their action, and particularly tubes adapted for the reception of television images. I

An important object in the development of the-television type of cathode ray tubehas been that of obtaining acceptable definition of the picture elements together with minimizing of fatigue of the optical muscular system. Heretofore suitable corrective measures insuring against eyestrain, so essential to relaxation on the part of the observer, have not been incorporated in the television image.

In television tube screens the luminescent powder forming the basis of the screen comprises a crystalline material applied in discrete particles, which, inter se, reflect and disperse light resulting from the luminescence excited by the electron beam. In addition to the luminescence resulting, from primary bombardment, there is believed to be substantial luminescence due to secondary electron emission, these electronsimpinging upon adjacent crystals to produce further reflection and dispersion of the light. Regarded specifically, the foregoing has the effect of reducing the intensity of the blacks and greys,

and concomitantly the contrast between these darker shades and the whites. Accordingly the observer, in attempting to reconstruct these degraded color values to normal incurs optical If measures are taken to heighten the means, the result is whites which are too white, and consequently undesirable glare. Attempts of the viewer to justify and rationalize the image to secure satisfactory mental synthesis of the picture are accordingly physiologically deletenous.

' The principal object of our invention is to provide improvements in the screen of a television or other type ofluminescent tube which shall have the effect of reducing dispersion and reflec- -tion of light between adjacent crystals of the screen which affect the true light values, and which may be due ta 'o optical or electronic causes.

Basically, the invention comprehends the provision of a barrier surrounding each of the luminescent particles to imprison the light created by the electron beam in order that lateral diffusion thereof maybe arrested, and which barrier serves also to localize the efiects of secondary electron'emission and the initiation thereby of alien light values in adjacentparticles of the screen. Thus contrast is greatly increased without the needfor exaggerating the intensity of any of the shades of the picture elements. Stated otherwise, a transmitted black element will be black upon reception, and not gray, and similarly for the shades intermediate white and black.

So marked is theimprovement in tbeimage that when viewed by the average observer there is a total absence of the fatigue which has hitherto accompanied screens of conventional construction.

Inclusion in a conventional screen formula of an opaque substance designed to form the barrier is satisfactory, provided that the substance is not detrimental to the operation of the tube, i. e. does not release gas or enter into undesirable chemical combination or affinity with the remaining constituents of the screen. Such substances are hereinafter denoted as being stable under electron bombardment. Metallic barriers are desirable. While we have found that the practice of Y our invention yields the desired result, it is yet not certain whether such result is reduction 01' secondary electron emission or of light dispersion or of both. We believe that both factors contribute to the result. It would appear that the confinement of the light to the particular elements of the screen intended to be primarily affected thereby is of obvious benefit, while the effects of confining secondary emission remain to be fully analyzed. However, tests have shown that a substantial improvement in the image has been attained with the screen of this invention.

In carrying out the invention in one form, e. g. to produce a white screen, i. e. one for television receiving tube uses, we employ a preferred process and formula as follows: Beginning with a clean glass cathode ray tube envelope, the same is positioned with its outer face lowermost and a so-called cushion poured therein. Such cushion is constituted of water containing 0.5% by volume of phosphoric acid, and provides a medium into which the screen material proper may be poured to settle subsequently on to the inner face of the tube. The phosphoric acid serves to jell the potassium sillcate comprising part of the screen. The tube and cushion are permitted to stand for a sufficient time until completely quiescent.

One preferred formula for a screen material embodying our invention is prepared by grinding in a ball mill a formula comprised as follows, based on the requirement for one 10" tube:

0.5 gram of sodium metasilicate 0.4 gram of manganese dioxide For convenience a large quantity of the aforesaid mixture may be ground at one time, say 36 grams, and with the use of /2" Pyrexballs V for from 6 10.minutes. 55-

,After being milled the total quantity of 0.9 gram of sodium metasilicate and manganese dioxide is combined. with a mixture of materials ordinarily employed for imparting white luminescense to the screen upon bombardment and may for example comprise cadmium sulphide, zinc sulphide and activators in the proportions customarily used for a white screen, there being used approximately 1.1 grams thereof which, to-

gether with the sodium metasilicate and man- 2 grams powder 170 cc. of 10% solution of potassium silicate 180 cc. of water and the whole shaken well together and passed through a 325 mesh stainless steel strainer.

The thus prepared formula is gently poured into the cushion and allowed to settle for about -8 minutes, during which time the powder precipitates on to the inner face of the tube, with the potassium silicate and sodium metasilicate serving as a binder.

The liquid is then slowly decanted and the screen dried in any usual manner.

.In general the foregoing steps follow conventional practice, the inclusion of the manganese dioxide or other substantially black, i. e. opaque, material constituting the novel step of the invention. The manganese dioxide or other suitable opaque material provides a barrier between the particles of luminescent material and if the quantities and proportions are properly chosen ,a screen of substantially single-crystal thickness is attained. Thus the luminescent crystals remain exposed on the back thereof for impingevment by the electron beam, and on the front thereof for suitable luminescence in the formation of the image. The opaque material acts to confine the fluorescence or phosphorescence of each crystal to the crystal itself, so that a white picture element has no tendency to lighten the grays or blacks. As noted heretofore some advantage may reside in combating the effects of secondary electron emission.

Should it be found desirable to increase the thickness of the screen, and thus in some measure detract -from the advantages of a single crystal layer, it is not difficult to increase the intensity of the electron beam to enable the same to penetrate any rear barrier set up by the opaque material.

-In lieu of manganese dioxide any stable, nongas forming metal, metal hydride, metal oxide or metal sulphide may be employed, e. g. zirconium, zirconium hydride, titanium sesquioxide, etc. By using zirconium advantage is taken of its gas-getting ability, thereby assisting 1n maintaining the hardness of the tube inasmuch as this metal has the ability to absorb residual gas when subjected to electron bombardment. The principal desideratum is the attainment of a screen which comprises crystals of the luminescent material set into a light-blocking matrix,

and in such manner that the rear and front faces of the crystals are optically and electronically free.

A satisfactory proportion of the opaque material depends largely on the character of the screen desired, and may vary from say to 100% by weight, based on the fluorescent material.

Alternatively we may employ glass-like materials capable of being bonded to the face of the tube by fusion, e. g. cobalt glass having a lower melting temperature than the envelope,

4 and providing a binder in which the individual luminescent crystals are embedded but with the rear and front faces free for bombardment and observation respectively.

In referring herein to fluorescence and phosphorescence we intend by the former to refer to the portion of the light emitted by a substance during the period of excitation, but absent immediately upon removal of the excitation, and by the latter we refer to the total light emitted by the substance subsequent to the removal of excitation. Luminescence, of course, includes the light of both fluorescence and phosphorescence.

While We have described particular embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

1. A fluorescent screen assembly for a cathode ray tube having a glass support provided with a coating thereon carried in a matrix, said coating comprising a mixture containing finely divided particles of fluorescent material and finely divided particles of at least one member selected from the group consisting of manganese dioxide and titanium sesquioxide which are opaque and which are stable under electron bombardment, the members of said last-mentioned group being present in amounts between about 5% and about by weight, of said fluorescent material.

2. A fluorescent screen assembly for a cathode ray tube having a glass support provided with a coating thereon carried in a matrix, said coating comprising a mixture containing finely di vided particles of fluorescent material and finely divided particles of manganese dioxide, said manganese dioxide being present in amounts between about 5% and about 100%, by weight, of said fluorescent material.

3. A fluorescent screen assembly for a cathode ray tube having a glass screen provided with a coating thereon carried in a matrix, said coating comprising a mixture containing finely divided particles of fluorescent material and finely divided particles of titanium sesquioxide, said titanium sesquioxide being present in amounts between about 5% and about 100%, by weight, of said fluorescent material.

ULISES A. SANABRIA. WARREN G. TAYLOR. EARL L. BROWNING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Froelich Sept. 14, 1950

Claims (1)

1. A FLUORESCENT SCREEN ASSEMBLY FOR A CATHODE RAY TUBE HAVING A GLASS SUPPORT PROVIDED WITH A COATING THEREON CARRIED IN A MATRIX, SAID COATING COMPRISING A MIXTURE CONTAINING FINELY DIVIDED PARTICLED OF FLUORESCENT MATERIAL AND FINELY DIVIDED PARTICLES OF AT LEAST ONE MEMBER SELECTED FROM THE GROUP CONSISTING OF MANGANESE DIOXIDE AND TITANIUM SESQUIOXIDE WHICH ARE OPAQUE AND WHICH ARE STABLE UNDER ELECTRON BOMBARDMENT, THE MEMBERS OF SAID LAST-MENTIONED GROUP BEING PRESENT IN AMOUNTS BETWEEN ABOUT 5% AND ABOUT 100%, BY WIGHT OF SAID FLUORESCENT MATERIAL.