US2212923A - Picture transmitter - Google Patents

Description

g- 27, 1940- H. MILLER 2,212,923

PICTURE TRANSMITTER Filed Aug. 12, 1936 2 Sheets-Sheet l INVENTOR HAROLD MILLER ATTORNEY Aug. 27, 1940.

H. MILLER PICTURE TRANSMITTER 2 Sheets-Sheet 2 Filed Aug. 12, 1936 INVENTOR HAROLD MILLER ATTORNEY Patented Aug. 27, 1940 SATS A'lEiNT OFFICE PICTURE TRANSMITTER of Great Britain Application August 12, 1936, Serial No. 95,517

In Great Britain September 10, 1935 6 Claims.

The present invention relates to cathode ray tubes adapted for use in television transmission apparatus and comprising a screen adapted to rec'eive an image of an object to be transmitted and to be scanned by a cathode ray to yield electrical picture signals.

Such tubes at present in use, of which the Iconoscope of Zworykin is a well-known example,

u depend for their operation on the emission of phot0-electrons from a photo-electrically-active mosaic surface on which an optical image is projected. This surface thus acquires an electrostatic image, and the potentials of the mosaic elements are periodically restored to a datum Value by a scanning cathode ray, the changes of potential of the elements during scanning serving to induce corresponding charges in a signal electrode capacitively associated with the mosaic elements. These known transmitting tubes suffer from certain difficulties in construction and operation. Firstly, they involve the preparation of photosensitive surfaces in a vacuum, which is a delicate and relatively expensive process. Secondly, it has been found impossible to maintain a substantial potential gradient normal to the scanned surface of the screen, and since in operation the mosaic elements acquire between successive scans a potential which is positive relative to their equilibrium potential, the proximity of highly charged mosaic elements to an element being scanned by the cathode ray causes a potential gradient parallel to the scanned surface, with the result that secondary electrons produced by the scanning ray are attracted to and partly discharge illuminated elements before they are scanned, so that picture signals are weakened and subjected to a form of distortion known as tilt. Furthermore, when a transient change occurs in the average brightness of illumination of the object being transmitted, there is a corresponding sudden change in the photo-electric current, and since this current flows through the impedance across which the picture signals are developed, it causes an unwanted surge in the signals.

Objects of the present invention are to overcome or reduce some or all of these difliculties.

The present invention utilises the known photo-voltaic efiect, which may be described as follows. An electrode system, consisting of two electrodes, one of which is of high electrical conductivity, and the other of which is a semi-conductor in which electrons can be liberated by the action of light, these two electrodes being separated by a blocking layer of relatively high resistance, is photo-electrically sensitive. When the boundary layer of the semi-conductor next to the blocking layer is illuminated, electrons released in the semi-conductor pass into the blocking layer and reach the high-conductor through it. Thus an electromotive force is generated upon illumination. If the electrodes are connected by an external circuit, the electrons travel back to.

the semi-conductor some through the external circuit and the remainder through theblocking.

layer. The potential difference developed by such a system under ordinary illumination may be.

a considerable fraction of a volt.

According to the present invention, a method of television transmission comprises the steps of forming an optical image on a junctionbetween a semi-conducting electrode and a blocking layer of relatively high resistance to cause a photo-voltaic flow of electrons from the semi-conducting electrode to an electrode which is capacitively associated therewith, scanning one of:

screen which has a blocking layer of relatively high resistance bearing on opposite sides thereof respectively an electrode of high conductivity and an electrode of semi-conducting material, one of said electrodes being of mosaic character and the other of said electrodes being electrically continuous, and an electron gun having an anode and adapted to scan said mosaic electrode, means for forming an optical image on the junction surface between said blocking layer and said electrode of semi-conducting material, and a circuit connected between said electrically-continuous electrode and said anode, said circuit including a signal impedance and a source of potential difference which serves to maintain said electrically continuouselectrode at a negative potential with respect to said anode. The electrode of semiconducting material is preferably the one that is electrically continuous. I

Television transmission apparatus according to the present invention in another aspect 'com-, prises. a mosaic screen which has an opaque blocking layer of relatively high resistance bearing on opposite sides thereof respectively a photo-electrically sensitized mosaic electrode of high conductivity and an electrically-continuous electrode of semi-conducting material which is at least partly transparent, an anode disposed on the side of said screen bearing said mosaic electrode, said screen and said anode being mounted in spaced relationship Within an evacuated envelope, meansfor projecting a light beam on to said mosaic electrode and deflecting said beam to scan said mosaic electrode, means for projecting an optical image through said semi-conducting electrode on to its junction surface with said blocking layer, and a circuit connecting said semi-conducting electrode and said anode, said circuit including a signal impedance and a source of potential difference which serves to maintain said semi-cone ducting electrode at a negative potential with respect to said anode.

The invention will be further described with reference to the accompanying diagrammatic drawings, in which:

Fig. 1 shows a cathode hay tube, and some of the associated electrical circuits, of one form of the improved television transmission apparatus,

Fig. 2 is a modification of the apparatus'shown in Fig. 1.

Fig. 3. is a further modification, in which the scanning beam is a ray .of light, and

Fig. 4 is a detail of Fig. 3 shown to a greatly enlarged scale.

Equivalent parts in the various arrangements shown in the drawings are denoted by the same reference numerals. Y

The apparatus shown in Fig. 1 includes a cathode ray tube, disposed within the envelope 5 of which is a mosaic screen consisting of a metal signal plate 6 covered with a continuous layer 1 of semi-conducting material which in turn is covered with a transparent or partly transparent blocking layer 8 of high-resistance material. On the blocking layer is formed a mosaic of transparent or partly transparent metal elements- 9 insulated from each other except for the comparatively high-resistance path through the blocking layer.

The mosaic screenv may be -made as follows: A copper plate forming the signal plate 6 is oxidised in air at a high temperature, and, after it has cooled, the outer black cupric oxide layer is removed by washing in sodium cyanide, so as to leave exposed a red cuprous oxide layer about 0.01 mm. thick deposited on the copper. This forms the semi-conducting electrode I. The blocking layer 8 may be formed on the cuprous oxide layer by bombarding it with electrons by means of an electron beam of several kilovolts energy. which scans. the cup-rous oxide surface. The blocking layer 8 can also be produced by subjecting the cuprous oxide layer to ionic bombardment due to a, gaseous discharge in a sputtering chamber. As a further alternative the blocking layer may be produced by coating the cuprous oxide surface with a thin layer of a resistance material such for example as a resin deposited from a solution or a material deposited by sublimation in a high vacuum. Thus calcium fluoride or quartz may be sublimed from a heated tungsten filament. A mosaic of transparent silverelements 9 is then evaporatedon to the block'- ing layer through a grid.

An untreated cuprous oxide layer covered with an evaporated layer of metal does not provide a photo-voltaic cell; but if the oxide layer is previously bombarded in the way described, or

if the metal is deposited on to the oxide layer by cathodic sputtering instead of by evaporation, a blocking layer is formed between the cuprous oxide and the metal deposited thereon.

A lens system Hi is arranged to project on to the side of the screen bearing the transparent mosaic elements 9 an optical image of an object to be transmitted. An electron gun having .a cathode H, focusing electrodes l2 and i3, and an anode M, is disposed with its axis lying obliquely to the screen and is associated as shown with electrostatic deflecting plates l5 and H5 or electromagnetic scanning means may be used for causing a cathode ray emitted by the gun to scan the screen on the side bearing the mosaic elements 9. The electrical circuits for biasing the focusing electrodes and for applying the scanning potentials to the deflecting plates, which are not shown in Fig. 1, are arranged in known manner. The signal plate '5 of the mosaic screen is connected to'the anode M of the electron gun through a signal resistance 11 and a battery [8 whichmaintains the signal plate at a negative potential with respect to the anode I4. The anode J4 is connected to earth by a conductor 19, and the cathode H of the electron gun is maintained at a high negative potential with respect tov earth by a battery 20.

This apparatus operates as follows. When it is inoperative, the mosaic elements 9 have the same potential as the signal plate 6. If now the mosaic, screen is scanned by the cathode ray, but is not illuminated, the mosaic elements 9, on being scanned, acquire an equilibrium or datum potential which is positive relative to the potential of the signal plate, since the velocity of the, scanning ray is such that the elementswhen scanned emit under the impact of the scanning ray more secondary electrons than the number of primary electrons received from the ray. The fact that the path through the screen between the signal plate and the mosaic elements is partly conductive enables the equilibrium potential of the mosaic elements to be substantially lower than the potential of the anode {4. Consequently the secondary electrons are collected by the anode I4.

If now certain parts of the screen are illuminated, light passes through the transparent mosaic elements on these parts and illuminates the boundary surface between the semi-conducting layer I andthe blocking layer 8. Then between successive passages of the scanning ray the mosaic, elements on these parts collect electrons from the semi-conducting electrode 7 and consequently become more negative than neighbouring elements in dark parts of the screen. Under the impact of the scanning ray both light and dark elements are restored to the same equilibrium potential, more electrons-being lost during the process, from an illuminated element than from a dark element; Signals are capacitively transmitted across the blocking layer 8 and semiconductinglayer l to the signal plate 8, and the potential differences consequently developed across the signal resistance 51 are employed to develop picture signals in known manner.

This action is different from the action in an Iconoscope where the illuminated photo-electric mosaic elements lose electrons during the interval between scans. In the present arrange ment, between scans the illuminated elements acquire electrons across the blocking layer; consequently the signals obtained will be reversed, as compared with Iconoscope signals.

- 21, 1939 to Lubszynski and McGee.

Since with the present device the equilibrium potential of the elements 9 can be much more negative than the potential of the anode Id, the potential gradient normal to the screen can sub-- stantially exceed any potential gradient occurring parallel to the screen, so that secondary electrons tend to flow to this anode rather than spread to parts of the screen adjoining the scanned spot, thus eliminating various injurious efiects, for example, tilt distortion.

Since the illuminated elements are negative with respect to the dark elements, an illuminated element will not tend to collect secondary electrons during the interval between scans, as occurs with the known Iconoscope.

Again, a transient charge of average intensity of illumination does not cause a sudden change in the potential of the signal plate, such as occurs with the known Iconoscope upon the sudden emission of photo-electrons.

An alternative construction according to th present invention is similar to that just described, except that the mosaic elements are of semiconducting material and are separated from a metal signal plate by the blocking layer. A mosaic screen of this kind may comprise an aluminium signal plate provided electrolytically with a thin layer of oxide forming a blocking layer which in turn is sprayed with small particles of a suitable semi-conducting material such for example as cuprous oxide, sufficiently transparent for light projected on to the mosaic side of the screen to penetrate to the junction between the mosaic elements and the blocking layer. When such a screen is illuminated, electrons released in the mosaic elements pass through the blocking layer to the signal plate, so that at the moment of impact of the scanning ray illuminated elements are more positive than dark elements. Signals are consequently in the same sense as with the known Iconoscope.

Both of the arrangements according to the present invention as hereinbefore described may be modified to enable the optical image to be projected on to the screen on the side thereof opposite to that which is scanned.

Fig. 2 shows such a modification, in which the lens system ID is co-axial with the electron gun, the lens system and the electron gun being arranged on opposite sides of the screen. The signal plate 6 is a semi-transparent metal layer, deposited on a mica support sheet 2|, as described in U. S. Patent #2,150,980 issued March The semiconducting electrode 1 is formed by settling on the signal plate 6 a layer of cuprous oxide crystals thin enough to be fairly transparent to red light. A blocking layer 8 is formed on this layer '1 by any of the methods hereinbefore described, and finally a mosaic of silver elements 9 is deposited on the layer 8.

In a modification of the arrangement shown in Fig. 2 the screen is formed by oxidising a thin copper plate right through and dissolving the faces thereof until the remaining cuprous oxide sheet is thin enough to be fairly transparent and yet not too fragile to be handled. A semitransparent metal film is evaporated on to one side of this plate, and on the other side, which is to be scanned, a blocking layer is formed on which is deposited a mosaic of silver elements. This assembly may be secured to a mica supporting sheet placed adjacent to the signal plate.

In the last two arrangements described the optical image is projected by the lens system through the signal plate and the semi-conducting layer to the junction of this layer with the blocking layer.

In a further modification of the arrangement shown in Fig. 2, a semi-transparent metal signal plate is deposited on a mica support sheet by sputtering or evaporation. This is covered with a semi-transparent blocking layer which may be a thin layer of resin or a sublimed layer as hereinbefore described. The blocking layer is covered with a mosaic of semi-conducting elements such as a sprayed layer of cuprous oxide crystals. The optical image is projected by the lens system through the signal plate and the blocking layer to the junction of this layer with the semi-conducting mosaic elements.

In the arrangement shown in Figs. 3 and 4, the mosaic screen generally denoted by 23 in Fig. 3, is mounted in an evacuated glass envelope 5a,, and is spaced from a ring-shaped anode [4a. The mosaic screen, shown to a larger scale in Fig. 4, includes a semi-transparent cuprous oxide sheet I on one side of which is deposited a semitransparent metal signal plate 6 which is connected to the end of the signal resistance I! that is remote from the earth connection I9. On the other side of the cuprous oxide layer is formed an opaque layer 8 of highly resistive material, and on the layer 8 is formed a mosaic of silver elements 9, which are oxidised and photo-sensitized With caesium in known manner. A cathode ray tube M of the kind having a fluorescent screen 25 which can be scanned by the cathode ray is placed with its axis normal to the middle of the mosaic screen 23 and with the fluorescent screen 25 facing the mosaic elements 9. A lens system 26 serves to project an optical image of the fluorescent screen 25 through the anode Ma on to the mosaic elements 9.

The image of an object to be transmitted is projected by the lens system I!) through the signal plate 6 and the cuprous oxide layer 1 to the junction of this layer with the blocking layer 8. Owing to the photo-voltaic efiect, the mosaic elements 9 opposite illuminated parts of the image become negative, to an extent proportional to the intensity of illumination, relative to mosaic elements opposite dark parts of the image. The fluorescent screen 25 is meanwhile scanned by the cathode ray, with the result that a bright luminous spot executes a scanning movement over the outer side of the screen 25, and the lens 26 causes an image of this spot to scan the mosaic elements 9. As each element is struck by the scanning spot, it emits photo-electrons which are collected by the anode Ma, the common equilibrium potential to which all mosaic elements are successively restored by the scanning operation being substantially lower than the potential of the anode Ma, owing to the presence of the battery l8 and the partly conductive path between the mosaic elements and the signal plate.

Except for the mode of scanning, this device operates similarly to the apparatus shown in Fig. 2.

I claim:

1. A cathode ray tube for use in television transmission comprising means for generating a beam of electrons, said means including an electron accelerating anode, an electrode in said tube adapted to be scanned by said beam of electrons comprising a metallic signal plate of high conductivity, a light responsive layer of semi-conducting material on said signal plate, a blocking layer of high resistance material positioned on said signal plate is sufiiciently thin to permit the transmission of an optical image therethrcugh,

4:. Alight responsive electrode for. a cathode ray television transmitting tube comprising a copper signal plate, a thin layer of cuprous oxide positioned on one side of said signal plate, a transparent or translucent layer of high resist-1 ance blocking materialv positioned on said oxide layer and a mosaic of discrete silveryelements on the surface of said blocking layer. a

5. Apparatus as claimed in claim 4, wherein said layer of cuprous exide provides a semiconducting surface. s

6. Apparatus as claimed in claim 4 wherein the thickness of said copper signal plate is so reduced as to permit the passage of light therethrough.

HAROLD MILLER.

Images