US1970036A - Photoelectric apparatus - Google Patents

Description

Aug. 14, 1934. P. T. FARNswoRTH 1,970,036

` PHOTO ELECTRIC APPARATUS Filed aan. 9, 192s HIS ATTOle/YEY Patented Aug. 14, 1934 UNITEDv STATES PATENT OFFICE by mesne assignments, to

Francisco, Calif., a corporatories, Inc., San tion of California Television Labora- Application January 9, 1928, Serial No. 245,334 14 Claims. (C1.25027.5)

My invention relates to television; and this application is a continuation in part of my copending application, Serial No. 159,540; filed January '7, 1927. The broad object of the invention is to provide a generator of picturemodulated electric currents.

An object of my invention is to provide a photo-electric tube in which picture analysis is 1o accomplished.

, Another object of my invention is to provide a photo-electric tube inV which the eiect of lightsensitive material other than that on the cathode is minimized. A further object is to provide a tube giving a high degree of definition. A

Another object is to provide a photo-electric tube in which a simple optical system may be used to focus an image directly upon the active light-sensitive surface.

Still another object of my invention is to provide a photo-electric apparatus of high sensitivity.

My invention possesses other objects and valuable features, some of which will be set forth in the following description of my invention which is illustrated in the drawing forming part of the speciiication. It is to be understood that I do not limit myself to the showing made by the said description and drawing, as I may adopt varying forms of my invention within the scope of the claims.

Referring to the drawing:

Fig. 1 is a sectional view of my apparatus, the plane of section being normal to the cathode and parallel to the axis of the tube. The optical system is shown schematically.

Fig. 2 is a sectional view on the line 2-2 of Fig. l. The optical system is not shown in this figure.

Fig. 3 is a For clearance in ecting coils and omitted. 45 Broadly considered, my invention comprlses a plane cathode coated with light sensitive material, such aspotassium hydride, in an evacuated bulb. Placed as closely as is practical to the cathode is a box-or-prism-shaped hollow anode. The surface of the anode nearest the cathode` is of wire mesh or similar conductive material permeable to light, and is parallel to the cathode. The opposite face is ot similar material and is apertured at the center.

55 Behind this aperture, as viewed from the cathode, is an auxiliary electrode or target, which diagram of the electrical circuits.

the drawing one pair of detheir exciting oscillator are is completely shielded from electron discharge with the exception of a single small area which isexposed to discharge passing thru thev aperture. The auxiliary electrode is preferablyof nickel, or other material which emits electrons readily under cathode ray bombardment.

Optical means external' to the tube form an image on the cathode, focusing it thru' the screen faces of the anode. As the elements of these screens lie without the focal plane they cast no 55j deiined shadows on the cathode. An external source provides a positive potential upon the anode, which accelerates the electrons liberated from the cathode by the light forming the vimage thereon. The major portion of these electrons pass thru the screen face of the anode to enter the equipotential space within it, traveling toward the opposite face at constant velocity.

Means are provided for deilecting the electrons within the anode in two dimensions and in accordance with a predetermined time schedule, so as to pass every part of the beam successively over the aperture behind which is located the target.

The electrons entering the aperture impinge upon the target, causing it to liberate further electrons by secondary emission. An external circuit contains a source of potential which maintains the auxiliary electrode'negative with respect to the anode, and the secondary emission is thereby drawn to the anode, causing a current to flow in the external circuit. As this current corresponds in intensity to the illumination of that part of the cathode whose emission is at the instant falling upon the aperture, it is in fact. a picture current, and may be used as such in television systems.

In terms of greater detail a preferred embodiment of my invention employs an evacuated glass bulb or tube 6 having a stem 'I of the usual type, a side seal 8, and the customary evacuating tip 9.

The side seal 8 comprises a short tube sealed into the side of the bulb. Within this tube is tted a roll 11 of spring tempered metal, usually nickel, carrying a wire frame 12 which supports 10 the clips 13. These clips grasp the rectangle of plate glass 14, and make contact with a silver lm on which is deposited the light sensitive coating 16 whichacts as-the cathode of the tube. The entire structure is connected to its external circuit by the wire 17 which is connected to the frame 12 and passes thru the side seal.

The deposit of light sensitive material upon the cathode is accomplished during the process of evacuation in the customary manner. Metallic potassium is -distilledl from a Ireservoir in the exhaust line, and is allowed to condensev on the cathode surface. The anode, and the walls of the bulb, except adjacent the cathode, are heated, so that little condensation takes place on these parts. Hydrogen is then admitted to the tube, and in this atmosphere a glow discharge is passed between cathode and anode, converting the potassium to the much more photo-sensitive hydride.

The anode is supported by the stem 7. The wire lead 19 passing thru the seal, and the "dummy 18, which is held by the seal but does not pass thru it, are bent to embrace the anode and are welded thereto. The anode is preferably in the form of a box or rectangular prism. The opposite sides or walls 20 and 21 are of fine mesh screen, the side 20 being parallel to the cathode and as close thereto as is structurally practical. The two adjacent walls 22 are sheet metal. They are preferably of low magnetic permeability and electrical conductivity to limit eddy currents as far. as possible. Wire frames 23 and 24 around the top and bottom of the anode give it rigidity.

Behindy the center of the screen 21 is the auxiliary electrode or target 26 of nickel or other material which is a good secondary emitter of electrons. This is completely shielded with the lexception of the single area behind the small aperture 27 in the housing 28 which surrounds the target. yThe housing 28 is electrically connected to the metallic sheath 29, surrounding the lead 31 from its connection with the electrode 26 the light sensitive cathode.

not in thefocal plane they cast no discrete shadows, altho they do rintercept a certain proportion to its seal in the stem 7, and insulated from the lead preferably by a tube 32 of boro-silicate glass. The shielding system, comprising sheath and housingis connected to the anode.

Means are provided for deflecting the electrons emitted from the cathode. Coils 33 and 34, co-axial with the bulb, set up a magnetic field which bends the electron beam transversely of the tube. A second set of coils 36 and 37, is parallel with the anode walls 22. Currents flowing in these coils cause a. magnetic field which bends the beam in a direction parallel to the axis of the tube. By passing currents of different frequency thru the two sets of coils the beam may be oscil` lated so that each element of its cross section in turn traverses the aperture 2'7, as is described in my co-pending application No. 159,540 above mentioned.` The oscillators 38, for accomplishing this may be of any suitable character, altho I prefer to use the type there described.

A source of potential, conveniently the battery 41, imposes a high positive voltage on the anode with respect to the cathode. I The auxiliary electrode is maintained slightly negative to the anode, being connected to an intermediate point on the battery thru the high resistance 42.

An optical system, indicated by the lens 43,

focuses an image of the picture or view to be reproduced thru the anode screens 21 and 20 upon As the screens are of the rays.

The cathode emits electrons from each elementary area of its surface at a rate proportional to the illumination of the area.` These electrons leave the cathode surface with small velocities in random directions. They immediately fall under the influence of the strong electrostatic field from the anode, yvhich accelerates them in a direction normal to the cathode surface until they pass thru the screen 20 as an electrical image, or beam of electrons whose intensity in a plane normal 't0 the path of the beam is at all points substantially proportional to the corresponding point of the optical image. Beyond this screen'they continue to Atravel with the high velocity thus acquired. While traveling at this velocity they are deflected by the fields from the two sets of coils 33, 34 and 36, 37, and are separated into two trains. The first and by far the smaller of these trains comprises those entering the aperture 2'7 to strike the auxiliary electrode. The larger train eventually strikes either the wall of the bulb or the anode. This train serves no further useful purpose.

The first train liberates a secondary electron discharge from the target. The electrons thus liberated are attracted to the more positive anode or to the housing 28, carrying a current which flows thru a portion of the battery 4l and the high resistance 42 back to the target. This of course refers to the actual electron flow and not to the conventional current flow. There is thereby set up across the terminals of the resistor 42 a potential which corresponds to an analysis, in accordance with a predetermined time schedule, of the illumination of the cathode. An amplifier 44 connected across the resistor 42 is activated by this potential and its output supplies the picture current for television transmiss1on.

The construction thus set forth has a number of highly advantageous features. The target is shielded from all electron bombardment except that originating from the cathode. This makes the accidental deposition of light-sensitive material on other parts of the tube relatively unimportant. As such deposition is almost certain .to Occur to some extent this is a valuable property. The light-permeable anode screens permit the image to be formed directly upon the active surface of the cathode, thus escaping many difficulties which are usually encountered. The use of 115 secondary emission gives a current which is from five to twenty times as great as can be obtained by photo-emission alone.

In addition to these advantages, my apparatus gives greatly improved definition. There are two causes for the electrons which leave a small area spreading to impinge upon a larger area of the target, causing blurring or diffusion. The first is their mutual repulsion. Where a narrow pencil is used as in the more usual type of apparatus, this has its maximum effect. In my apparatus the space charge in the beam outside of the portion whichr is momentarily active counteracts the charge within that portion, and largely negatives this effect. The second cause of diffusion is the 130 random initial velocity with which the electrons leave the cathode. 'I'his velocity in general has a component normal to the desired electron path. To render the diffusion due to the normal components negligible the velocity along the path must be made very high in comparison, and this is done by using a high anode potential. The velocity at which the electron travels is proportional to the potential thru which it has fallen, and its mean velocity up to the time it reaches the anode is one half its final velocity. In my device the final velocity is acquired by the time the electron reaches the anode screen 20, and its travel thru the substantially equipotential space between the screens is at this final velocity. Its mean velocity from cathode to target is therefore almost twice as great as it would be if the screen 20 were omitted, and diffusion is therefore reduced to about half what it would otherwise be. The result is the same as tho the anode voltage 150 ode.

were almost doubled, and this without the manifest dangers that doubling the voltage would involve.

In using the term electrically permeable" in this specification, I mean permeable to cathode ray or electronic discharge, and do not refer to Iclaim'L" i 1. A/phot'o-elect'rictube comprising an anode permeable to light and a photo-sensitive cathode, and an auxiliary electrode having all but a minute area shielded from said cathode and positioned to receive a predetermined portion of the discharge therefrom.

2. A photo-electric tube having a cathode, and an anode comprising screens permeable to light and dening a substantially equipotential space.

3. A photo-electric tube having a cathode, and an anode comprising parallel screens permeable to light and parallel to the cathode and defining ,a substantially equipotential space. i

4. A photo-electric tube comprising an electron emitting surface, an electrode parallel to said surordinary/A, etallic conduction.

`face and permeable to electrons, a second surface parallel to said electrode and electrically connected thereto, and means for deecting electrons passing between said electrode and said second surface.

5. A photo-electric tube comprising an electron emitting surface and an electrode permeable to electrons, a second surface electrically connected with said electrode, and means for deecting electrons -passing between said electrode and said second surface, said electrode and second surface being permeable to light.

6. A photo-electric tube comprising an anode screen, a light sensitive cathode positioned in front of said anode screen, and an auxiliary electrode positioned behind said screen, and having all except a minute area shielded from said cath- 7. A photo-electric device comprising an anode screen, a light sensitive cathode positioned in front of said screen, means for forming a substantially equipotential space behind said screen, and an auxiliary electrode positioned to intercept a portion of a discharge entering said equipotential space from said cathode.

8. A photo-electric apparatus comprising means for forming an optical image, means for forming a beam of electrons constituting an electrical image corresponding to said optical image, an electrode for intercepting a portion of said beam, and means for varying the portion of the beam intercepted.

v 9. A photo-electric apparatus comprising means for forming an optical image, means for forming a beam of electrons constituting an electrical image corresponding to said optical image, an electrode for intercepting a portion of said beam, and means for deecting the beam to vary the portion intercepted by said electrode.

10. Av photo-electric device comprising an anode screen, a light sensitivel cathode positioned in front of said screen, means for forming a substantially equipotential space behind said screen, an auxiliary electrode positioned to intercept a portion of a discharge entering said equipotential space from said cathode, and means for deecting the discharge Within said space to vary the portion thereof intercepted by said electrode.

11. A photo-electric apparatus comprising a cathode capable of emitting electrons in proportion to its illumination and positioned to permit the projection of an optical image thereon, an electrode positioned to receive the discharge from said cathode, and means for restricting the received discharge to that emitted from a minute area of said cathode.

12. Photo-electric apparatus comprising a light-sensitive cathode positioned to permit the focusing of an optical image thereon, and a plurality of anode screens positioned in front of said cathode, whereby electrons from said cathode are accelerated to form an electrical image corresponding to said optical image.

13. Photo-electric apparatus comprising a light-sensitive cathode positioned to permit the focusing of an optical image thereon, a plurality of anode screens positioned in front of said cathode, whereby electrons from said cathode are accelerated to form an electrical image corresponding to said optical image, and means for deecting said electrical image between the screens.

14. Photo-electric apparatus comprising a light-sensitive cathode positioned to permit the focusing of an optical image thereon, a plurality of -anode screens positionedin front of said cathode, whereby electrons from said cathode are accelerated to form an electrical image corresponding to said optical image, and an auxiliary electrode having all but a minute area shielded from said cathode and positioned to intercept an elementary area of said electrical image.

PHILO T. FARNSWORTH.

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