US2186393A

US2186393A - Fluorescent screen

Info

Publication number: US2186393A
Application number: US189010A
Authority: US
Inventors: Ring Friedrich; Weizs Georg
Original assignee: Individual
Current assignee: Individual
Priority date: 1936-10-26
Filing date: 1937-12-15
Publication date: 1940-01-09
Anticipated expiration: 1957-01-09
Also published as: CH204095A; NL53011C

Description

Jan. 9, 1940. F ET 2,186,393

FLUoREs'cENT ,SCREEN Fild Dec. 15, '1937 g .'/nventan:

Patentci Jan. 9, l

UITE

P 'I' l Weiss, Berlin-Banem, Germa ny 1937, Serial No.,180,010

In October 26, 1936 z cum; (01. 250-150) The invention relates to fluorescent screens and .to cathode. ray' tubes containing such screens. It is particularly concemed with screens for making visible pictures or drawings of any kind, for instance lines, diagrams` and letters. The ar rangement according to the invention is especially adapted to produce bright television images with changing contents, advertising pictures in black and white only or oscillograms. 4

Among the obiects 'of the invention are: to produce a fluorescent screen 'emitting light under the impact of electrons and in which the emission of light continues at a uniform rateuntil it is stopped by a reverse controlling force; to provide a screen structure having light emitting and dark portions adjacent to one another on the same surface, the whole surface of the screen being bombarded by electrons of the same source; to provide a self controlling fluorescent screen having deflnite areas of two diflerent potentials; to provide a cathode ray tube including a fluorescent screen with a material emitting secondary electrons under the impact of primaryelectrons and a cathode emitting electrons at a uniform rate over the whole area' of the screen; to provide a screen structure on which a fluorescent image is storedby electrlcal charge storage during the longer part of the period between successivepictures and cancelled out shortly before a new picture isstored.

Further objects are to provide a method of making a self-controlling screen and of making a screen structure from which the pictures are easily obliterated in quick succession.

Other objects of our invention will be apparent as will be speciiically pointed out in the description forming apart of the speciflcation but` we do not limit ourselves to the embodiments of the invention herein described as various forms may be adopted within the scope of the claims;

Referring to the drawing:

Flg. 1 is a sectional 'view of a cathode ray cording to the invention. i

Figs. 2 and 3 are dia explaining the operation of the tube represented in Flg. l.

Flg. eis a cross section through a screen structure according to the invention on an enlarged scale and u Fig. is a plan 'view of a part of the screen structure of Flg. 4. j

The tube of Flg. 1 contains within an envelope i a screen 2 of insulating material with a layer of fiuorescent material 8. A grid electrode 4 is arranged in front of the screen and connected. by a lead 5 to the outside. The tube contains furthermore a cathode 9 having for instance the form of a wire. spiral bent to form a ring and connected by conductors and 8 to a heating battery 9. The tube contains furthermore a cathode ID of oonventional type for producing a cath- 6' ode ray and connected to a battery l I. A control electrode |2 is arranged in front of the cathode !0. This electrode has the form of the wellknown Wehnelt cylinder. Externally of the tube j a concentrating coil |3 is situated around the 10 neck of the tube Two deflectlng' systems M and I 5 are indicated schematically and constructed in accordance with the usual practice of oscillographs and television tubes.

In the operation of the device a potential of +500 V. is applied togrid 4 with respect to the potential of cathode 6 a's zero-potential. Cathode !0 has a potential of -1000 V. against cathode 6. In order to obt'ain a stationary record on the screen the cathode ray emitted by cathode !0, modulated by electrode |2 and concentrated by coii M is deflected over the screen 2 so that a local potential distribution is produced and stored on the screen in such a manner that certain portions of the screen have a potential lying above a value of for instance 250 V. against cathode 8 and the remaining portions have a potential below 250 V. The potential of 250 V.

corresponds to the critical potential at which electrons hitting the screenliberate the same number of secondary electrons as primary electrons are arriving on the screen.- It is assu'med that the distribution of potential is' maintained approximately constant for a period of time owing to the insulating qualities of the screen 2 85 and the fluorescent layer 3.

If now the source of electrons 6 is energized the whole screen is sprayed diflusely and uniformly with electrons. These electrons are accelerated to a velocity of 500 V. by the grid 4 40 but they reach the fluorescent layer with a velocity depending upon the potential of the respective surface element of the screen. An element having a potential above 250 V. is bombarded by electrons of high velocity 'and more u secondary electrons' are liberated than primary eiectronsreach this element.` The potential of the element is made more positive by the loss of. electrons and this increase of potential is con tinued until the potential of grid I is reached.

` This is the end value which cannot ibe surpassed.

Another elemento! the screen having a potential lower than 250 V. is reached by electrons .having a comparatively low velocity and the number of secondary electrons is smaller than the u ordinate 1 gives'the ratio of the number of secondary electrons to the number of primary electrons the abscissa indicates the velocity of the imameting electrons in Volts. Fluorescent materials as ordinarily used in cathode ray tubes for instance zine sulde have ai characteristic cui-ve Oi reaching the value of I at 250 V. Ali elements oi the screen having a potential below this critical value, show a decrease in potential under the influence of the electrons emitted by cathode t as indicated by 'potential lying on the descending part of the characteristic curve in Fig. 2. The critical potential is then according to this curve 2700 V. and the positive end potential is dened by the potential ofgrid ti. v

In order to remove the image stored and appearing on screen E it is possible to bombard the whole screen by the concentrated ray from source i@ so that all parts of the screen take on the high potential and are luminating brightly. In order to control the screen in the reverse' direction and to make -it dark it is possible to lower the potential of grid i for a short period of time so that all parts of the screen assume a potential below the critical value. i

The critical value of 250 V. depends upon the material of the fluorescent layer and its treatment and can be changed in fairly wide limits by the correct selection of the material. It is 'also possible to change the potential on grid 3 and cathode i@ in wide limits. Experiments have shown, that it is possible to maintain luminating and dark portions on the screen adjacent to one another also in case the base of the screen consists of a metal plate. The insulating properties of the fiuorescent material are sufliciently good to build up local charges on the screen.

In order to obtain a stationary image remaining unchanged it is necessary to maintain the follow ng Operating conditions explained in connection with Fig. 3. The fiuorescent material !9 is arranged upon the'insulating support 8 and bombarded by electrons in the direction of arrows 20. It is assumed that the fluorescentmaterial has the potential on the right hand side of the figure as' far as it isrepresented black and has a potential of 500 V. in the left hand part. The distribution of the potential is indicated by curve Zi showing the potential in dependence from the local extension of the screen. A large difference of poten'tials ex sts at the place of arrow 22 and the slope of the local diflerence in potential depends upon the conductivity of the material. Experiments have shown that the &188398 I lines separating areas of high potential !rom those of low potential, i. e. separating the bright and dark parts of the screen are subject to a displacement depending upon the velocity of the electrons emitted by cathode G. In case the electrons arrive too slowly the dark portions are increaslng and in case the velocity of the electrons is too high the bright portions of the screen are increasing. This displacement would destroy the picture after a certain time and care must be taken to stabilise the picture. rate of the' displacement depend also from the material of the screen and the support of the screen.

According to the invention the velocity of the diflusely emitted electrons is exactly adjusted in such a manner that neither a displacement in one or the other direction occurs. In the above mentioned example of zinc suifide as screen material a voltage of about 500 V. has been found as critical for a stationary picture. It'can be assumed that the critical value of the material of corresponds to about half this voltage, i. e. to'

In case the the, screen is' used for producing moving pictures for instance for cinematographic or television purposes it is necessary to produce the picture and to remove it within a short period of time of for instance of a sec. able to extinguish or cancel the picture not momentariiy over the whole' screen surface but only on that part of the screen on which the new picture is produced immediately thereafter in order to obtain pictures of uniform brightness which are tree from fiicker and as bright as possible. A second concentrated cathode ray may be used for cancelling the pictures; this ray is defiected in a similar manner as the recording pencil but is displaced a few lines in advance of the recording pencil. It is also advantageous to divide the screen and/or the grid in front of the screen into a number of sections parallel to the lines of the picture. The cancellation is eil'ected in one section only of the screen at a time shortly before the recording The direction and' It is prefercathode ray is touching this part of the screen.

A particularly simple embodiment is represented in Figs. 4 and 5. Fig. 4 shows the screen of a tube similar to that of Fig. 1 with two cathodes. The screen is rep'esented on an enlarged scale and consists of a number of parallel wires 23 insulated from one another. The wires are partly embedded in the insulating material 24 and are covered with fluorescent material 25. The fluorescent material is covering also the exposed surface oi' the insulating material 24. Fig. 5 shows a View of the screen from the side of the cathode. The wires 23 are bent upwardly on the right hand side and are terminating in or upon the insulating material forming the carrier, of the screen. A conductor 24 is arranged at the edge of the screen. A conductor 24 is arranged at the edge of the scren. The strip of material 25 between the conductor 24 and the ends of wires 23 is semi-conducting so that all the wires 23 are connected by way of this resistance with a source of potential of 500 V. The surface of the wires is carefully cleaned between the dotted lines 26 and 21 and a strip of finely divided carbon, for instance graphite, is applied. Furthermore each pair of adjacent wires -is connected by conductingi bridges 28.

aieasee 'ment the ray impinges upon the

non-emitting strip

25, 2?. The wires crossed over by the cathode ray at this place receive more electrons than they give o so that the potential of these wires is sharply lowered. As the strip of graphite is somewhat conducting also two or three adjacent wires will attain a more negative potential. It is preferable to control the ray during this time for highest intensity and the defiecting field is controlled preferably in such a manner that the ray is remaining for a while upon the

strip

26, 2?.

It may be preferable also to make the strip broader than represented in the drawing. The quick decrease of potential of the wires serves to cancel the illumination in that part of the screen lying near thewires. The screen is therefore brought to darkness a few lines in advance of the cathocle ray. The cathode ray is then quickly re.- turned and writes the second line as indicated by arrow 38. During the return movement it touches the sections 2 s of wires which are more closely adjacent to its next path, so that these wires are made more positive thereby. During the whole time of recording, the wires brought i quickly to a lower potential are slowly made more positive by the current flowing through the semiconductor 25, so that the wires assume again the function of grid 4 of Fig. 1, the lumination near these wires being however cancelled. Only when the modulated cathode ray is charging definite parts of the line to` a more positive. potential the illumination starts at these points and will remain under the influence of the electrons emitted by cathodes 6 until the concentrated ray is touching again the upwardly bent edges of the wires between lines 26 and 21.

A screen of this type may contain separate cancelling and accelerating electrodes. It is also possible to divide the fiuorescent material into single insulated elements by cutting lines into the layer or by spraying the fluorescent material through a' grid which is removed afterwards. such a. screen consisting of insulated fiuorescent particles has the advantage that the displacement efiect described in connection with Fig. 3 is minimsed.

As the elements of the screen are luminating either brightly or not at all the pictures are only black and white without half-tones' It is however possible to obtain half-tone-pictures by changing the width of the luminating lines. The modulated ray m is therefore controlled in such a manner that chiefly the diameter or size of the spot is changed while the intensity of the ray per surface unity is kept approximately constant. The visibility of the lines can be decreased by using an interlaced scanning method.

It is preferable to use as high a potential as possible between the cathode 6 and the screen. This however is only possible in case the factor p is reached for a very high velocity of primary electrons. According to the invention a material* is used for the screen in which the value is reached above 500 V. or more.

The efiect of the cancelling strip 26, El may be increased .by `multiplying the effect of decreasing the potential by a multistage secondary emisslon multiplier for instance of the grid type.

The potential distribution maintained on the screen can also be used for controlling 'other effects. It is also within the scope of the invention to produce an electron optical image of the screen surface upon another surface and to sepa- 'rate locally the production-of the potential distribution and the luminous efifect.

We claim: i

1. In a cathode ray tube an insulating plate, a fluorescent screen on said plate, a source of electrons for uniiormly bombarding the whole area of said screen with'electrons, an accelerating electrode in front of said screen having a fixed potential above the critical value at which the number of secondary electrons emitted by the screen exceeds the number of primary electrons,

means for charging elemental areas of said screen to different potentials, and means for intermittently bringing the accelerating electrode to negative potential.

2. In a cathode ray tube a plate, fluorescent insulated elemental areas on said plate, a source of electrons for uniformly bombardng the whole area of said plate with electrons, an accelerating electrode having a fixed potential of twice the critical value at which the number of secondary electrons emitted by the plate exceeds the number of primary electrons, and means' for charging elementa areas of said plate to diflerent potenv tials.

mmnncn RING. anoaa worse.