US2339833A

US2339833A - Television pickup tube

Info

Publication number: US2339833A
Application number: US384122A
Authority: US
Inventors: Alda V Bedford
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1941-03-19
Filing date: 1941-03-19
Publication date: 1944-01-25
Anticipated expiration: 1961-01-25

Description

raLavrsroN rrcxnr z 1 Aida V. Redford, Goswood, N. 5., assor to dio Corporation of America, a corporation of Delaware Application ch 19, 1941, Serial No. 384,122

v 4 Claims.

My invention relates to cathode ray pickup tubes for picture transmission and, particularly, to pickup tubes of the type employing a low velocity scanning beam.

Pickup tubes of this type include a mosaic screen similar to that found in high velocity beam tubes, such as the Icono'scope. They also include means for producing a scanning beam in which the velocity of the electrons is so low at the mosaic screen that they do not release many secondary electrons upon striking it. Proper operation of a low velocity beam tube requires that the ratio of secondary electrons be less than unity. If, as the electron beam scans an area or spot on the mosaic, this ratio exceeds unity, the picture image for this area is not converted to a corresponding picture signal and this portion of the reproduced picture is "washed out. This is referred to as the tube going positive and may occur when an area on the mosaic is illuminated too strongly.

An object of the present invention is to provide an improved pickup tube of the low velocity beam type which has less tendency to go positive than other tubes of this type.

Stated more broadly, an object of the invention is to provide an improved means for and method of converting a light image into corresponding electrical signals.

In a preferred embodiment of the invention, the scanning beam is given such shape in cross section that the shape of the scanning spot on the mosaic screen is trapezoidal, or approximately so, with the long dimension of the spot in the direction of line scanning and with the front edge of the spot wider than the back edge. When the mosaic screen is scanned by such a spot, the picture detail over that area of the picture which is moderately illuminated is determined by the front edge of the scanning spot,

since the front edge contains enough electrons to bring the mosaic elements back to their initial potential corresponding to black or no picture. However, the spot contains enough electrons behind the front edge to bring any brightly lighted mosaic areas, also, back to their initial or no Figure 1 is a side view of a cathode ray pickup bodying my invention;

Figure 2 is an end view on an enlarged scale tube of the low velocity scanning beam type em- Referring to Figure l, there is shown a pickup tube 8 of the low velocity beam type described in United States Patent No. 2,213,175, issued August 27, 1940, to Iams and Rose. The particular tube illustrated is the same as described in this patent except that it is designed to have a. trape- 15 zoidal scanning spot. A spot of this shape may be obtained by making the aperture in one of the gun electrodes trapezoidal in shape, as shown in the enlarged view of Figure 2.

The pickup tube 8 comprises a highly evacuted envelope having therein an electron gun which consists of an indirectly heated cathode 2, a control electrode 3, and accelerating electrodes 4 and 5. Suitable biasing and accelerating voltages are provided by the batteries 7 and 8, negative bias being applied to the control electrode 3 through a grid leak resistor it.

By means of a focusing coil 9, the electron beam leaving the electron gun is focused to a small spot on a mosaic screen H. The screen I I is of the well-known type commonly employed in Iconoscopes with the exception that it is made translucent whereby the image of an object H to 4 be transmitted may be projected on the photoelectric elements of the screen from the back side by means of a suitable optical system (not shown).

The electron beam is causedto scan the screenby means of deflecting plates l4 and deflecting coils IS, the plates 96 preferably being curved as described in theabove-mentioned, Iams and Rose patent for the purpose of eliminating, distortion in. scanning.

An electrode I! having a slot l8 therein is provided for electrostatic shielding purposes. A

is is provided to which the electrons of the beam and by which they are collected.-

As explained in the Iams and Rose patent, the electrons of the electron beam are directed upon the mosaic screen It with a very low velocity, that is, a velocityapproaching zero velocity at the point of impact therewith. The importance of this feature is that the electrons of the beam approach the mosaic of the point of impact with a velocity such that the ratio of secondary electrons to primary electrons is less than unity.

In operation, elemental areas of the mosaic e0 electrode M acquire electrostatic potentials procentrally apertured electron collecting electrode not reaching the mosaic screen II are directed on. Particles or elements of the mosaic which are more highly illuminated acquire the most positive electrostatic charge with respect to the unilluminated particles. The positive charges representing an electrostatic-image of a picture to be transmitted are neutralized by the scanning 3 beam electrons. When the electrons of the scanning beam are directed toward those elements of the mosaic which are negative with respect to the cathode, they cannot reach those elements because of their low velocity. These electrons, since they are prevented from impinging upon the mosaic, are returned to an electron collecting electrode adjacent the electron gun. The scanning means which cause the electron .beam to scan over the mosaic electrode or target are so chosen as to prevent those electrons of the beam which do not reach the target from returning to the electron gun but rather causes them to be redirected along paths other than those followed by those electrons in traversing the distance be.- tween the electron gun and the target.

A tube of this type has a linear characteristic between light input and signal output. It is so adjusted that, when the mosaic screen H is dark,

all of the electrons of the electron beam are reassume. This neutralizing current is the picture current. Preferably, it is taken off the back or signal plate of the screen H and supplied to a picture amplifier (not shown). In accordance with one specific embodiment of my invention,

the electron gun of the tube l is so designed as to make the scanning beam trapezoidal in crosssection at the mosaic screen H. This may be accomplished by providing the accelerating electrode 4 with an aperture 4a which is trapezoidal in shape, as shown in the enlarged view of Fig. 2.

It will be understood that I have described one specific tube and one particular method of obtaining the desired spot shape merely by way of example and .that my invention is not limited to thist particular tube or method of shaping the P In Fig. 3, I have indicated the scanning path or line traced on the mosaic screen II by successive scannings in the tube oi Figure 1. In Figure 3,

' the scanning line pitch is indicated at p. Three scanning lines are shown which have been traced by a scanning beam having the trapezoidal spot shape illustrated.

Preferably, there is a slight amount of overlap 3f she scanning lines such as that represented One of the main characteristics of the scanning spotis that it is-long (in-the direction of line scanning) compared with its width. As illusin the event the front edge does not contain suflicient electrons to accomplish this. The result is that, while the picture detail for such brightly illuminated areas may be reduced somewhat, as

compared with the-use of a shorter spot, it is portional to the intensity of light incident theremuch better to have such reduction in detail for these areas than to have these areas going positive every now andthen, whereby all detail for them is lost.

It may be found in some cases that a rectangular spot which is long in the direction of scanning, as shown in Figure 4, may be utilized rather than a trapezoidal spot. The reason the trapezoidal spot is preferred is that it is diflicult to shape and deflect an electron beam with such exactness that the front edge of a rectangular spot would always scan ahead of the rest of the spot.

As a practical matter, the trapezoidal shape spot would probably be rounded ofi at the corners and have an appearance at the screen ll similar to that shown in Figure 5. All of the spot shapes illustrated, however, have in common the characteristic of providing a large number of beam electrons for a given mosaic element after the front edge of the spot has scanned this element. Thus, the tendency for the element to go positive" is reduced.

I claim as my invention: 1. The method of reducing the tendency of a low velocity beam pickup tube to go positive which comprises scanning the mosaic screen of said tube with an electron beam of such cross section that the scanning spot on the mosaic is at least ap- 80 proximately trapezoidal in shape with the greatest dimension in the direction of line scanning and with the maximum width at the front end of the spot.

- 2. A cathode ray pickup tube comprising a mosaic screen having a multitude of elements each capable of storing an electric charge corresponding to the light intensity of a picture element, and means for producing an electron scanning beam having a certain shape in cross 40 section at said screen and for directing it against said elements with such low velocity that the ratio of secondary electrons to primary electrons is normally less than unity, said shape being such that the length of the scanning spot on the screen in the direction of line scanning is greater than its maximum width.

3. In a picture transmitting system, a cathode ray pickup tube of the type having a mosaic of electron emissive capacity elements and having means for producing an electron beam and directing it against said mosaic to form a scanning spot thereon having a certain shape, the electrons in said beam approaching zero velocity at the point tially zero velocity in the region of said mosaic when it is dark, said spot having greater length in the direction of line scanning than width and its width near the front edge being greater than the width near the back edge.

4. In a picture transmitting system, a cathode electron emissive capacity elements and having means for producing an electron beam and directing it against said mosaic to form a scannin spot thereon having a certain shape, the electrons in said beam approaching zero velocity at the point of impact on said mosaic and having substantially zero velocity in the region of said mosaic'when it is dark, said spot being at least approximately trapezoidal in shape with the front edge wider than the back edge.

ALDA V, BEDFORD.

of impact on said mosaic and having substanray pickup tube of the type having a mosaic of