US2612476A - Luminescent screen for image reproducing tubes - Google Patents

Description

Sept. 30, 5 A STANDING LUMINESCENT SCREEN FOR IMAGE REPRODUCING TUBES Filed Jan. 25, 1951 I N V EN TOR. 6b NE Y .5 mmzmq,

A TTORNEY-S'.

Patented Sept. 30, 1952 2 ,612;476"f v t mmssscsm SCREEN: Fort-image- L" f Rriutonuonvci v s.

sidearm-Standing, Short Hill's, Jgass Tung-Sol lse mp Works, Ine.,-NeWarli N. I 1 cor-Infra"iiioii -"o'i' Delaware This inventionrelatesto. image reproducing tubes and more particularlytb phosphor screens employed in'tubes' of this-general type that are responsive to bombardment by abeam of elec-- trons to producevisible'lightz;

Present practices used in the formation of phosphor. screens of which applicant' has knowledge consist in'the use of'various combinations of zinc sulphide and zinc cadmium sulphide because of their high efficiencies in the production of light and'their ease of application to glass surfaces. With the trend toward the use of higher voltages on image reproducing tubes which voltages are now of the order of 16 kilovolts or greater, the screens deteriorate more rapidly. This deterioration can be noticed on the screen in the form of a burn or discoloration that is in the shape of a bow-tie" or an X that grows in area as the tube life progresses, and is believed to be produced by an inferior secondary emission characteristic that encourages ion bombardment and burning of the screen, the ions largely originating from the inner conductive coating or walls of the glass envelope of the tube.

In a cathode ray'tube, the screen, the conductive coating on the tube walls and the accelerating anode are operated at the same voltage, To maintain this condition. as the screen is bombarded by the electron beam, at least one secondary electron should be released from the screen for every electron received by it. If this condition does not exist, the screen becomes more negative than the conductive coating and the resultant voltage difference results in the release of ions from tube elements that remain at a higher potential than the screen. These ions are attracted to the screen and because of their large mass a sufficient number and concentration of them will seriously burn and discolor the screen. The greater the potential difference between the screen and other high voltage elements, the more rapidly will this burning occur. It has been found that in tubes wherein the second anode is energized with approximately 16 kilovolts, the voltage difference between the anode and screen at zero life hours is about 100 volts; that is, the potential of the screen is approximately 100 volts below the potential of the second anode to which the 16 kilovolts is applied. After twenty-four hours of tube operation this potential-increases from 200 to 1000 volts.

Some added resistance to deterioration or burning of the screen can be obtained by increasing the thickness of the coating of zinc sulphide satisfactorysolution: I I

Luminescent substances other'tlian sulphides have been used for the formationofthe'se'scre'ens, and while some of these more'successifully resist deterioration because of a higher secondary emission chara'cteristim'i they are" substantially inferior to the sulphides because their efficiency in terms of light output for a given anode voltage is below that of the sulphides.

It has been found that a very substantial reduction in the potential difierence between the screen and anode can be obtained byfortifying the sulphide combinations with a silicate such as calcium magnesium silicate activated with titanium, generally known. in the art as blue silicate. The fortification of the sulphide blends in this manner for use as luminescent screens very materially improves the secondary emission characteristic of the screen and at the same time effects little or no reduction in the light output normally obtained with sulphide blends alone.

The above and other objects of this invention will become more apparent in the following description and accompanying drawing illustrating an image reproducing tube in accordance with the invention.

In the figure, l denotes a cathode ray tube of the type generally used in direct view television receivers. for the connection of the tube to the electric circuits and a pair of magnetic cells 4 and 5 for magnetically deflecting and focusing the electron beam 6 on the screen 1. The second anode for the tube is denoted by the numeral 8 and is connected by suitable spring members 9 to a graphite coating l0 on the internal surface of the tube wall forwardly of the econd anode 8. The high accelerating potential is applied to this coating and thence to the screen I and the second anode 8 by means of the terminal I I.

As pointed out above, the sulphide screen 1 by reason of its constant bombardment by the It is provided with a suitable plug 3 little or no loss in light through the introduction of thesilicate. More particularly, it was foundthat in a tube to which a second anode potential of 16 kilovolts was applied, the potential difference between the screen and anode at zero life hours was about 30 volts as compared with 100 volts for tubes wherein the screens did not have the silicate additive. After about 100 hours of life the potential .difierence in the tube with the silicate additive in thescreen was of the order of l001vo1ts orless'. This contrasts with a voltage reading of 200 to 1000 volt's after only twenty-four hours of life fora tube with a'conventional screen. a 7

These substantial reductions inpotential differenoes between the'screen and anode are obtained'with little orno loss in light output, notwithstanding the factthat calcium magnesium I 4 silicate when used alone as a tube screen is greatly inferior to the sulphide screens from the standpoint of light output at a given potential.

The above experimental data is presented to show the efiect ofa concentration of 15 per cent of the silicate when mixed with a sulphide combination.- for use at potentialsof the order of 15 to i 16 kilovolts. jLesser concentrations may of course be used with proportionately smaller resistance to deterioration and with little or no loss in efficiency. At higher voltages higher concentrations of the silicate show very satisfactory results and with the light losses very much smaller than the proportion of silicate would indicate.

,A luminescent-screen for electron beam tubes comprising about per cent zinc sulphide and zinc' cadmium sulphide and about 15 per cent titanium activated calcium magnesium silicate.

S IDNEY A. STANDING.

UNITED s'rairns rh'rnn'rs v I Date' 2,473,960 "Krog er June 21,1949

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