US2251573A - Electronic tube - Google Patents

Description

Aug. 5, 1941. c. A. MORTON ELECTRONIC TUBE Filed May 15, 1937 PIC Tums ou'l'Pu'r W ELECTRON MUL '7'IPL [ER 2 Sheets-Sheet l 3maentor (Ge urge H. Morton Bu Ctttorngg.

1941- G. A. MORTON 2,251,573

' ELECTRONIC TUBE Filed May 15, 1937 2 Sheets-Sheet 2 Snventor George H.Mo rt0n (Ittorneg Patented Aug. 5, 1941 UNIT STATS AT FFICE ELECTRONIC TUBE Delaware Application May 15, 1937, Serial No. 142,748

9 Claims. (Cl. 259-450) My invention relates to electronic tubes and particularly to cathode ray tubes of the type employed for television transmission.

Specifically, my invention relates to electronic tubes of the type described and claimed in Flory and Morton Patent No. 2,093,166, issued September 14, 1937, and assigned to the Radio Corporation of America. Tubes of this type comprise a photoelectric screen upon which is projected the image of a subject to be transmitted. The resulting electrical image appearing on the screen is scanned by a cathode ray whereby secondary electrons are released from successive points on the screen, the secondary electrons released from a particular point depending upon the intensity of the light image at that point.

The secondary electrons, as they are released from the screen, are pulled into an electron multiplier which multiplies them and supplies them to an output circuit.

An object of my invention is to increase the sensitivity of an electronic tube of the abovedescribed type.

Broadly speaking, it is an object of my invention to provide an improved cathode ray transmitter tube of the desired sensitivity for picture transmission.

A further object of my invention is to increase the picture signal to noise ratio of a cathode ray transmitter tube.

In the above-described combination of cathode ray transmitter tube and electron multiplier, the shot noise in the scanning beam imposes a limit to the tube sensitivity. In a tube of conventional design it is impossible to reduce the beam current, and, thus, the shot noise, because the capacity of each element or globule of the photo-electric screen is so large that the elements will not reach equilibrium if the beam current is reduced. By equilibrium is meant the condition where the number of primary electrons reaching an element is equal to the number of secondary electrons leaving it. Therefore, if the beam current is reduced, the signal output is also reduced.

In accordance with my invention, I so construct the photo-electric screen that the capacity of each photo-electric element is greatly reduced whereby the beam current may be reduced without decreasing the signal output. Thus, the shot noise is reduced and the sensitivity of the tube increased.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a view of one embodiment of my invention, and

Figure 2 is a view of one of the preferred embodiments of my invention.

Referring to Fig. 1, the transmitter tube comprises an evacuated envelope I having therein an indirectly heated cathode 2, a control electrode 3, a first anode 4 and a second anode 6. These electrodes generate an electron beam, which is electrostatically focused, and direct it towards a photo-electric screen 1. The beam is caused to scan the screen 1 by any well known means such as deflecting coils (not shown).

The screen structure 1 comprises a sheet or plate 8 of insulating material such as glass or mica which has on the front side thereof a mosaic of minute photo-sensitive elements or globules 9 which are substantially insulated from each other. The mosaic of elements 9 may be formed in any one of several ways as by ruling a coating of silver which has been evaporated upon the supporting sheet 8, and then sensitizing the ruled silver with an alkali metal such as caesium. Preferably, however, the elements 9 are photo-sensitive silver globules formed thereon by the method described in Patent No. 2,065,570, issued December 29, 1936, in the name of Sanford E. Essig, and assigned to the Radio Corporation of America. When formed by this method, the globules are microscopic in size.

The screen structure I may be supported in any suitable manner as by means of a yoke II. It will be noted that in the specific embodiment of the invention illustrated in Fig. 1, the screen structure 1 has no back-plate or metal coating on the back side of the member 8.

In transmitting a picture, an image of the sub ject to be transmitted, indicated at I2, is projected upon the screen 1 whereby each of the photo-sensitive capacity elements 9 acquires a charge depending upon the intensity of light striking that particular element. Thus an electric image of the subject I2 is produced on the screen 1.

Now when the screen I is scanned by the electron beam, secondary electrons are released from each of the elements 9, the number of secondary electrons leaving each element depending upon the charge or potential of that element. These secondary electrons are pulled into an electron multiplier indicated at 14 where they are multiplied or amplified and supplied to an output conductor [6.

The electron multiplier is in the same envelope l as the electron gun and photo-electric screen as described in the above-mentioned Morton and Flory patent, and the multiplier structure and connections may be the same as described in this patent. If preferred, the multiplier may be the same as that described hereinafter in connection with Fig. 2.

Since the screen I has no back-plate, the capacity of each element 9 is very small and the intensity of the electron scanning beam may be greatly reduced whereby the shot efiect is reduced and the sensitivity of the tube increased. The word sensitivity is used here in the broad sense that the tubes ability to transmit good pictures when the subject is not strongly illuminated is improved. Decreasing the photosensitive element capacity and at the same time decreasing the beam current does not change the signal sensitivity appreciably but it does reduce the noise without reducing the picture signal current. Therefore, the picture signal to noise ratio of the signal output of the tube is greatly improved.

As will be more fully explained in connection with Fig. 2, it is not necessary that the backplate" be omitted from the screen structure I as the capacity of the elements may be sufiioiently reduced by making the supporting plate 8 considerably thicker than the plate employed in a tube of more conventional design.

Referring to Fig. 2, one of the preferred embodiments of my invention is illustrated, the screen structure indicated at 20 being shown with a thick insulating plate 2| and a back-plate" 22. It will be understood that the back-plate 22 may be omitted as shown in Fig. 1, this being the limiting condition for reducing the capacity of the photo-sensitive elements.

Except for the screen structure 20 and the adjustment of the electron beam current, the tube and circuit shown in Fig. 2 are the same as described and claimed in application Serial No. 122,690, filed January 28, 1937, and in application Serial No. 128,050, filed February 27, 1937, both filed as joint inventions of Leslie E. Flory and George A. Morton, and assigned to the Radio Corporation of America.

In this embodiment of the invention, the picture transmitting tube comprises an evacuated envelope 23 having therein the mosaic screen structure 29 which corresponds to the screen 1 in Fig. 1 and which, like screen I, is set at an angle to permit the projection of a light image thereon. In order to simplify the drawing, the screen 20 is not illustrated at this angle. The envelope also includes an electron gun 24 and two electron multipliers 26 and 21. A second anode which is provided for accelerating and focusing the electron beam is in the form of a continuous silver coating on the envelope 23 as indicated at 28, this silver coating extending around the necks of the electron multipliers as indicated at 28a and 28b.

The electron multipliers 26 and 21, which are so spaced as to collect secondary electrons uniformly from the screen 20, are identical in construction. Referring to the multiplier 21, it comprises a collector plate 3| and a truncated cone electrode 32 which pull the secondary electrons into the multiplier where they strike the T- shaped element 33. Secondary electrons are released from the inner surface of element 33 and pulled into T-shaped element 34 where more secondary electrons are released. These are pulled into element 36 where they are further multiplied. The final output appears on the plate 31 and is supplied to the input circuit of an amplifier tube 38 where the picture signal is further amplified. It will be noted that the elements 3|, 32, 33, etc., are at successively higher potentials, being supplied with voltage from a voltage divider 40.

As shown, the multiplier 26 is connected in parallel with the multiplier 21. Preferably, the collector plates 3| and 3| of the two multipliers are connected to the voltage divider 40 through separate taps whereby their voltages may be separately adjusted.

The back-plate 22 and the second anode 28 are connected to the negative terminal of the voltage divider 40.

Referring more specifically to the screen structure 20, it has on its front surface a mosaic of photo-sensitive elements 4| which maybe formed as described in connection with Fig. 1. In this screen structure the capacity of elements 4| is reduced by making the insulating plate 2| very thick as compared with usual practice. For example, the plate 2| may be 10, 20 or 30 mils thick as compared with a plate 1 mil thick in a standand mosaic screen structure.

The beam current usually should be substantially inversely proportional to the thickness of the insulating plate 2|. For example, in a standard tube the beam current is about one-half micro-ampere, while the mica plate of the mosaic screen is about 1 mil thick. In the tube shown in Fig. 2, however, if the plate 2| is 30 mils thick, the beam current preferably is reduced to approximately micro-ampere. Or, if the plate 2| is- 15 mils thick, the beam current is reduced to the order of micro-ampere.

In practice, when adjusting one of my improved tubes for transmitting a picture, the electron beam current is reduced to a value such that any further substantial reduction in beam current would impair the quality of the transmitted picture. In a preferred tube, the capacity of the effective picture element (that is, the area of the mosaic under the electron beam) is reduced to a value such that the beam current may be reduced to a value not greater than ten times the photoelectric current from the mosaic to obtain a signal to noise ratio which is at least ten. The beam current should never be less than the photoelectric current.

I claim as my invention:

1. A cathode ray picture transmitting tube comprising an evacuated envelope having therein an electron gun for producing an electron beam, a screen positioned to be scanned by said beam, said screen comprising a sheet of insulating material having on the front side thereof facing said gun a mosaic of secondary electron emissive elements and having on the back side thereof an electrically conducting sheet or backplate, said elements being insulated from each other and said insulating sheet being at least 10 mils thick, and an electron multiplier so located in said envelope as to receive the secondary electrons which are released from said mosaic when scanned by said beam, the beam current being of the order of from micro-ampere to micro-ampere.

2. The invention according to claim 1 characterized in that said photo-electric elements are microscopic photo-sensitized silver globules.

3. A cathode ray picture transmitting tube comprising an evacuated envelope having therein an electron gun for producing an electron beam, a screen positioned to be scanned by said beam, said screen comprising a sheet of insulating material having on the front side thereof facing said gun a mosaic of elements and having on the back side thereof an electrically conducting sheet or back-plate, said elements being insulated from each other and said insulating sheet having a thickness of the order of 20 mils, and an electron multiplier so located in said envelope as to receive the second electrons which are released from said mosaic .when scanned by said beam, the beam current being of the order of from micro-ampere to micro-ampere.

4. The invention according to claim 3 characterized in that said photo-electric elements are microscopic photo-sensitized silver globules.

5. A cathode ray picture transmitting tube comprising an evacuated envelope having therein an electron gun for producing an electron beam, a screen positioned to be scanned by said beam, said screen comprising a sheet of insulating material having on the front side thereof facing said gun a mosaic of elements and having on the back side thereof an electrically conducting sheet or back-plate, said elements being insulated from each other and said insulating sheet having a thickness of the order of 30 mils, and an electron multiplier so located in said envelope as to receive the secondary electrons which are released from said mosaic when scanned by said beam, the beam current being of the order of from micro-ampere to micro-ampere.

6. The invention according to claim 5 characterized in that said photo-electric elements are microscopic photo-sensitized silver globules.

7. A cathode ray picture transmitting tube comprising an evacuated envelope having therein an electron gun for producing an electron beam, a photo-electric screen positioned to be scanned by said electron beam, said screen comprising a sheet of insulating material having on the front side thereof facing said gun a mosaic of photoelectric elements and having no back-plate, said elements being insulated from each other, and an electron multiplier so located in said envelope as to receive the secondary electrons which are released from said mosaic when scanned by said beam, the beam current having substantially the minimum value for causing said elements to reach equilibrium potential.

8. A cathode ray picture transmitting tube comprising an evacuated envelope having therein an electron gun for producing an electron beam, a photo-electric screen positioned to be scanned by said electron beam, said screen comprising a sheet of insulating material having on the front side thereof facing said gun a mosaic of photo-electric elements and having on the back side thereof a back-plate, said elements being insulated from each other and said insulating sheet being at least 10 mils thick, and an electron multiplier so located in said envelope as to receive the secondary electrons which are released from said mosaic when scanned by said beam, the beam current having substantially the minimum value for causing said elements to reach equilibrium potential.

9. A picture transmitting tube comprising an electron gun for producing a beam current, a mosaic of elements positioned to be scanned by said beam, and an electron multiplier positioned to receive electrons from said mosaic and to supply them in amplified form to an output circuit, the capacity of the effective picture elements of the mosaic being at least as small as when the mosaic has a back plate spaced from said elements by an insulating sheet 10 mils thick, and the ratio of the beam current to said capacity being such that the beam current has substantially the minimum value for causing said elements to reach equilibrium.

GEORGE A. MORTON.

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