US2617954A - Pickup tube - Google Patents

Description

A. ROSE PICKUP TUBE Nov. 11, 1952 Filed Dec. 27, 1950 INVENTOR ALBERT ROSE A41; NEY

Patented Nov. 11 1952 PICKUP TUBE Albert Rose, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December 27, 1950, Serial N 0. 202,953

4 Claims.

' This invention relates to cathode ray tubes and more particularly to improved means and method for increasing the sensitivity of television pick- .up tubes, especially tubes of the image orthicon type.

Pick-up tubes of the type known in the art as image orthicons have an image section consisting of a photocathode electrode formed on the end plate of the tube envelope and a thin glass target electrode spaced from the photocathode. An optical image focused upon the photocathode causes photoemission, and the resulting photo electrons are accelerated and focused so that they strike the thin glass target electrode with an energy sufficient to cause a secondary emission from the glass surface greater than unity. The secondary emission leaves a positive charge pattern on the glass target surface corresponding to the optical image focused upon the photocathode. The opposite side of the glass target is scanned by anelectron beam which is caused to approach the target at close to zero velocity. Due to the extreme thinness of the glass target,

' the electrostatic image on the one side of the target sets up the same image potential on the scanned side of the target. Electrons from the scanning cathode ray beam will be drawn to the positive areas of the target surface and will be deposited on the target to neutralize the positive potential pattern on the glass. The deposited beam electrons discharge the target surface to cathode potential at which point the remainder of the beam is reflected back along the path and is collected to form video signal of the tube. A focusing coil normally surrounds the tube and extends from a point near the source of the electrons of the scanning beam to a point slightly beyond the photocathode end. This coil, when energized, produces a uniform magnetic field within the tube for controlling the path of electrons therein.

However, it has been found that unsatisfactory results are obtained if the scene, the image of which is focused upon the photocathode, has a very low light intensity. Under such conditions, the positive charge pattern formed on the glass target is insufficient to produce a satisfactory modulation of thescanning beam. When the scanning beam is thus undermodulated, the signal to noise ratio in the resulting video signal is excessive.

An example of a scene or subject having such a low light intensity is a fluoroscopic X-ray screen. In order to observe the dynamic conditions existing in a member, for instance, a portion of the human body, an image of the luminous screen must be focused on the photocathode. The light intensity of such luminous screens is so low that, under ordinary conditions the cathode beam of the pick-up tube will not be sufiiciently modulated to produce an intelligible video signal.

It is, accordingly, an object of the present invention to provide an improved television pickup means.

Another object of the present invention is the provision of a television pick-up device which is suitable for use with scenes having a very low light intensity.

It is a further object of the present invention to provide a television pick-up device, the response characteristics of which permit its use with such low-light intensity scenes as fluoroscopic X-ray screens.

In accomplishing these and other objects, I have provided an image orthicon type pick-up tube which has been modified to increase the distance between the photocathode and the glass target. Further, the focus coil which normally extends slightly beyond the photocathode end of the tube is shortened so that it extends to a point between the photocathode and the target whereby the converging magnetic field set up at the end of the coil' concentrates the stream of electrons flowing from the photocathode to the target. The image formed on the target is thus reduced in size and correspondingly intensified.

A better understanding of the present invention may be had from the following detailed description when read in connection with the accompanying drawing in which:

Figure 1 is a side view, partly in cross-section, of one form of the present invention, and

Figure 2 is a side view, reduced in scale, partly in section, partly diagrammatic, showing a typical magnetic field set up by an energized solenoid. V

Referring more particularly to the drawing, there is shown in Figure 1 a television picture tube 2 of the image orthicon type. This tube includes a photocathode 4 formed for example by depositing a photoemissive material on the inner surface of one end of the glass envelope 6. When a scene is optically focused on the photocathode, an electron image is formed in the pattern of the optical image by photo electrons released by the light falling on the photocathode. The released electrons are accelerated by a pair of accelerating electrodes 8 and I0 so that they strike a thin glass target i2 with sufiicient energy to produce secondary emission from the target. The secondary electrons emitted by the target are collected on a fine mesh grid H3. The emission of the secondary electron from the target leaves a charge pattern on the target which is an image of the optical image that was focused on the photocathode, the more positively charged areas being those emitting the greater number of electrons. Since the target is very thin glass the same charge image is produced on the side of the target away from the photocathode. The target I2 is scanned by an electron beam Hi which is produced in an electron gun [8 located at the opposite end of the tube 2 from the photocathode 4. As the electrons of the beam approach the target they are decelerated to substantially zero velocity by a decelerating electrode 2%. A portion of the electrons of the beam l6 will continue on their path to neutralize the positive charges on that portion of the target instantane- -ously being scanned. The remainderof the electrons of the beam are returned to the vicinity of 'the'gun where they are amplified and collected in a conventional dynode multiplier structure 22 to form video signals.

The ratio of the portion of electrons of the beam required to neutralize the charge on the target to the portions of electrons of the beam returned to the dynode structure determines the degree of modulation of the electron beam.

As in conventional arrangements, the electron beam H5 is caused to scan the target by a defiection yoke, represented in the drawing by the inner coil 24. Surrounding thedefiection yoke there is a field coil or solenoid 26. This coil produces a uniform magnetic field within the tube, between the electron gun is and the target l2,

which focuses the scanning beam on the target.

As previously mentioned, the ratio of the portion of the electrons of the scanning beam l6 required to neutralize the charges on the target l2 to the portion of the electrons of the beam returned to the dynode structure 22 determines the degree of modulation of the beam. However, if the brightness of the scene focused on the photocathode of the tube is very low (say, substantially below l0 foot lamberts), the corresponding charge on the target will be so slight that the degree of modulation of the scanning beam will be insufficient to produce a satisfactory video signal.

To solve this problem, the spacing between the photocathode l and the target i2 is increased to provide a greater degree of flexibility in controlling theelectron image. The field coil or solenoid 26, whichnormally extends slightly beyond the face of the tubebearing the photocathode, in accordance with the present invention terminates in a plane through the tube between the photocathode and the target, preferably midway. Thus the electrons of the electron image emitted by the photocathode are subjected to a converging magnetic flux produced at the end of the solencid (shown schematically in Fig. 2), thereby compressing or concentrating the electron image (represented by the broken lines 30 of Fig. 1). This compression or concentration results in a smaller but more intense image striking the target. Consequently, the charge image formed on the target is smaller but intensified. With the intensified charge image on the target, the scanning beam may be made to traverse the entire target or merely the portion excited by the image; the improved signal to .noise ratio would be the same in either case. Thus intensified, the charge image on the target produces sufficient modulation of the scanning beam to generate satisfactory video signals at scene brightnesses that heretofore have been far below a useful level.

A modification of this invention is shown in Figure 2, wherein an auxiliary coil 32 is supported coaxially with the solenoid. The auxiliary coil is mounted to permit of axial movement. Such movement of the auxiliary coil causes the magnetic field pattern in the space between the target and the photocathode to vary, producing a corresponding variation in the degree of concentration of the electron image. With such a modification, the degree of concentration (and hence, th intensification) of the electron image may be conveniently adjusted to produce optimum results for each different low light intensity scene to be televised, depending on the light intensity of the selected scene.

Thus, it may now be seen that there has been provided convenient, simple means for televising scenes having very low light intensity such as fluoroscopic X-ray screens.

What is claimed is:

1. An electron discharge device of the image type, including a photocathode for emitting electrons in accordance with a light image focused thereon and a target spaced therefrom receiving said electrons to form a corresponding charge image therein when secondary electrons are generated thereby, and means for concentrating the electrons emitted from said photocathode onto said target, said means comprising a solenoid surrounding said device for producing an electromagnetic focusing field within said device extending from said photocathode to said target, said solenoid extending from one end thereof lying in a plane intermediate said target and said photocathode to a point on the other side of said target, the concentration of said electrons from said photocathode being effected by the convergence of the magnetic-flux at said end of said solenoid.

2. A pickup tube, said tube including a photocathode for forming an electron image and a target upon which electrons from said electron image imping to produce a corresponding charge image, said target being spaced from said photocathode, and means for converging electrons from said photocathode onto said target, said means comprising a solenoid surrounding'saidtube for producing an electromagnetic focusing field within said tube extending longitudinally of said tube between said photocathode and said target, said solenoid extending from one end thereof lying in a plane normal to said longitudinal axis and intermediate said target and said photocathode to a point on the other side of said target, the convergence of said electrons being effected by the convergence of the magnetic flux at said end of said solenoid.

3. A television pickup tube including an envelope, a photocathode within said envelope for emitting photoelectron in accordance witha light image impressed thereon, a target spaced from said photocathode and upon which said electrons impinge to produce a charge pattern on said target similar to said light image, and-means for converging said electrons on said target whereby said charge image is reduced in size compared .to said light image, said means including acoil surrounding said envelope and. producing. a magnetic focusing field within said tube, extending from said photocathode to said targetsaidcoil extendmg from a point-intermediate .saidphotocathode and said target to a point on the opposite side of said target whereby said field converges from said photocathode to said target.

4. A pickup tube, said tube including an envelope, a photocathode within said envelope for forming an electron image in accordance with a light image impressed thereon and a target upon which electrons from said electron image impinge to produce a corresponding charge image, said target being spaced from said photocathode, and means for converging said electrons emitted from said photocathode onto said target, said means comprising a solenoid surrounding said tube for producing an electromagnetic focusing field within said tube extending longitudinally of said tube between said photocathode and said target, said solenoid extending from one end lying in a plane intermediate said target and said photocathode to a point on the opposite side of said target, and an auxiliary coil supported for axial movement coaxially to said solenoid and REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,159,534 Ruska May 23, 1939 2,403,239 Rose July 2, 1946 2,513,221 Webb June 27, 1950 2,520,244 Iams Aug. 29, 1950 2,540,632 Rose Feb. 6, 1951 2,550,316 Wilder Apr. 24, 1951

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