US2887611A

US2887611A - Television camera tube

Info

Publication number: US2887611A
Application number: US444898A
Authority: US
Inventors: Schagen Pieter
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1953-08-04
Filing date: 1954-07-21
Publication date: 1959-05-19
Anticipated expiration: 1976-05-19
Also published as: NL92219C; FR1110909A

Description

l May A149, 1959 P. SCHAGEN 2,887,611

TELEVISION CAMERA TUBE Filed July 21. 1954 CcLLEcj-oe.

GLAss -r'AzGz-r gain): A mmmmmmmm INVENTOR PIETER SCHAGEN AGENT United States Patent O TELEVISION CAMERA TUBE Pieter Schagen, Eindhoven, Netherlands, assigner, by

mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Dela- Ware Application July 21, 1954, Serial No. 444,898

Claims priority, application Netherlands August 4, 1953 3 Claims. (Cl. 315-11) This invention relates to television camera tubes.

Some television camera tubes include an image electrode made from materials having a relatively high specic resistance, These materials are termed semi-conductors and have the property that, under the inuence of a potential diiierence, charge carriers are transferred from a conducting coating provided on onesurface of the electrode to its opposite side on the surface of which the luminous image is projected, or where, with a photoelectric cathode converting the luminous image into an electron image, electrons are focussed.` Furthermore, said surface is scanned by an electron beam of small cross-sectional area at its point of impact. resulting photoor secondary emission, electronsfare dislodged from the image electrode and carried oit to an adjacent collector electrode.

The present invention relates to a camera tube of the type wherein the collector is positiveA with respect to the cathode of the electrode system producing the scanning beam, and the electrons of this beam reaching the image electrode at a high velocity dislodge va number of secondary electrons larger in number than that of theelectrons of the scanning beam. The operation of such a tube is based on the potential shift, due to electron emission, ofthe electrically-heating surface of the image electrode.

The value of the electric charge perv image element of the electrode surface and the strength of thesignal obtained by means of the scanning ray, `depend .upon the quantity of electrons carried olf at the impact side of the image electrode during the time between two scans. By applying a negative voltage to the conducting coating on the rear side of the image electrode relativeto the collector, the strength of the electric field required for carrying oi said electrons to the collector can beset to a value at which all the electrons are collected. However, raising the eld strength in order to remove thev secondary electrons at the impact side of the image electrode isopposed or ofset by the fact that` under the influence of the potential ditference between the conducting rcoating and the collector electrode, image charge is lost due to the conductivity of the image electrode. Further, an increase in the collecting eld strength for the electrons means a larger shift of the surface potential at the impact side of the image electrode under the influence of the scanning beam, and, moreover, more charge losses in building up the potential pattern, which is an electric translation of the image to be transmitted.

It is known that the application of a negative voltage to the surface of the image electrode provided with a conducting coating with respect to thecollector is advantageous for avoiding disturbing signals. vThe latter are produced by secondary electrons returning to the-surface of the image electrode due to space charge phenomena and being redistributed thereover. In the known arrangement, the eiect is such that the potential uctuations produced at the image electrode surfaceby the scanning beam do not cause the surface 'potentialat the impact side of theimageelectrode to be larger-than is`'permissible''forv Due to the maintaining the field strength required for carrying off substantially all dislodged electrons to the collector. This condition limits the choice of the intensity of the scanning beam to that required to avoid the disturbing signals. Further, it has been found that there is a dependency existing between the potential setting, the exposure strength and the inertia with Which the potential setting passes over to a different value, thereby causing disturbances in reproducing rapid image variations. A rapidly moving object is not clearly reproduced in the image, and the image of a subject swiftly removed from the image eld is suppressed too slowly.

In accordance with the invention, this disadvantage is mitigated by equipping a television camera tube with an image electrode made from a material having a high specic resistance and whose thickness is at least one third of the size of an image element and is not more than three times its size, the specific resistance of the material being at least l011 ohm-cm. with the lower value of said lthickness and having a correspondingly higher value with a greater thickness.

When using the invention, the limitation with respect to the intensity of the scanning beam is dispensed with. In lthis case, the advantage that electrons from the image electrode surface struck by the scanning ray do not return to this surface is lost; however this is not a serious objection because the infiuence of said stray electrons is greatly reduced, since the disturbance which they may bring about depends upon the charge-absorbing capacity of the image elements. This is due to the fact that as the capacity of the image elements with respect to the conducting coating at the back of the image electrode is reduced, or, alternatively, as the thickness of the semiconductor increases, less charge is required to produce a given potential variation at the surface of the image electrode. Since a necessary potential Variation may be assumed as that at which no further stray-electrons are absorbed by an image element, a prerequisite is that the negative voltage set up at the conducting coating of the image electrode shall be much higher so that in producing the image charges for obtaining the potential pattern, despite returning secondary electrons to the image elements, the potential-reducing iniluence of the negative voltage set up at the conducting coating is sufcient yto bring the image elements into said state Without any appreciable delay. Due to the low capacity per image element, the charge required therefor is so 10W as to prevent any appreciable disturbing signals. The potential stabilization of the surface of the image electrode by the scanning beam has a constant Value substantially corresponding to the collector Voltage, which value is independent of the degree to which the surface is struck by image-producing rays. After each scan, during which an image element attains a xed potential, the potential is varied under the' influence of the image-producing rays, and the electron.

supply from the conducting coating and the variation during the time elapsing between two scans is a measure of the image signal strength.

The choice of the specific resistance of the material from which the image electrode is made is related with the loss of image charge due to diverting electrons through the material, the displacement of charge carriers between two image elements at different potentials also having to be considered. The last-mentioned charge losses increase with the thickness of the electrode, while the image signals should not be weakened to such a degreeas to offset the advantage realized from the potential shift.

The invention will nowbe described, by way of example, With reference to thel accompanying drawing',

which represents 'a t camera tube V.of the image-iconosccpe;

3 type according to the invention with means for supplying the required voltages.

Referring to the drawing, there is shown a substantially cylindrical, glass wall 1, of a camera tube. The larger` diameter part of the wall is closed by means of a glass` bottom 2 including a supporting insulator 3 through which a metal support 4 for the image electrode or electron image receiving electrode 5 is led.

A part of smaller diameter of the Wall 1 is closed by means of a at bottom 6 carrying a photo-electric cathode 7. The latter is made from light-sensitive material emitting photoelectrons under the influence of light rays. This photo-electric cathode may be of the usual kcom-- position so that it will not be further described.

A laterally-projecting tube S contains an electrode system 9 producing and controlling the strength of a scanning beam whose axis extends in the direction of the center of the image electrode 5. Detiection coils 10` and 11 supply the elds required for varying the path of. the scanning beam so that it may be directed to any point of `the image electrode.

The Wall of the tube is provided with a conductive coating 12 which is electrically connected to a collector- 13 arranged adjacent the image electrode S.

The image electrode 5 is made from semi-conducting material and, at the side remote from the photo-electric cathode 7 and the electrode system 9, is provided with a conducting layer 14. The specific resistance of the semi-conducting material from which the electrode is made is at least 1011 ohm-cm. Use may be made of vitreous enamel provided on a metal plate. Glass species are known which approximately have said specific resistance and can be worked to form a thin layer of uniform thickness and homogeneous composition. A glass plate having the required thickness can be made suiciently smooth by grinding and polishing. The composition of the enamel corresponds to that of a suitable glass and may be made from a glass mix or melt comprising mainly silica and sodium oxide together with the oxides of potassium and aluminum.

Glass having the desired characteristics has been made from a mix comprising the following specilic component quantities in percent by weight Silica 69.80 Sodium oxide 18.00 Aluminum oxide 2.30

Potassium oxide 8.80 Zinc oxide 0.50 Manganese oxide 0.60

The conducting 'layer 14 is connected through a resistor 15 to the negative terminal of a source 16 whereof the positive terminal is connected to the wall coating 12. The collector 13 has the same potential as the coating 12 and is thus biased positively relative to the conducting Llayer 14. A second source 17 delivers suitable voltages for producing the scanning beam and is connected to the photo-electric cathode 7. An accelerating voltage is applied to the wall coating 12 and the collector 13 so that the electrons of the scanning beam impinge at a high velocity on the surface of the image electrode S, in-as-much as the voltage supplied by the source 16 is small compared with the accelerating voltage from the source 17.

After the scanning beam has several times swept the impact surface of the image electrode, the whole surface assumes an average potential which depends upon the voltage between the conductive coating and the collector, the beam current, the resistance of the semi-conductor and the secondary-emission coetiicient of the surface struck. It is advisable to improve the electron-emission properties of the surface of the semi-conductor material and to this end an oxide of an alkalior earth-alkaline-metal, which possesses a high secondary-emission coefficient,

may be spread in the form of a thin layer overthe sur-` face. The next scans raise the potential of an image element approximately to the collector voltage, the potential reassuming its average value through electron-conduction through the image electrode 5 between two scans.

During exposure between two scans, in general photoelectric emission, and in particular for the image iconoscope shown, further secondary-emission will occur due to the image-producing rays impacting the photo-cathode 7 and releasing photo-electrons that bombard the image electrode 5, and a part of the charge carriers, supplied through conduction from the conductive coating to the image elements, will be removed. In this case, the potential shift occurs less rapidly than in the absence of light incident on the photo-cathode. During the available time period, the potential of the elements drops to a lesser degree, and this drop of potential to a greater or lesser degree is a measure of the differences in signal strength of the individual image elements. A video signal may be derived across the resistor 15 when the highvelocity scanning beam restabilizes the potential of each image element approximately at the collector potential.

As pointed out hereinbefore, the thickness of the semiconductor material is between one-third and three times the diameter of an image element, the latter of which is dependent on the scanning rates and the overall size of the image electrode. Generally, the thickness of the semi-conductor will range between 0.034 and 0.3 mm.

While I have described my invention in connection with specific embodiments and applications, other modications thereof Will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A television camera device of the image-iconoscope type comprising a photo-electric cathode at one end, an electron-image-receiving electrode at the other end, said image electrode comprising a conductive member and a semi-conductive glass material facing said cathode and having a specific resistance of at least 1011 ohm-cm., a collector disposed adjacent said image electrode and between the latter and said cathode, means for biasing said `conductive member at a relatively low negative potential with respect to said collector, means producing a high-velocity electron beam for scanning said image electrode as a plurality of discrete image elements of given size and generating more `secondary electrons than primary electrons for stabilizing the potential of said image elements at a positive value in the vicinity of the collector potential, said semi-conductive material having a thickness between 1/3 and 3 times the size of an image element on said image electrode scanned by the high-velocity beam and ysaid thickness lying in the range of 0.034 to 0.3 millimeter whereby the positive stabilized potential of each image element falls ot and approaches the negative potential of the conductive member as a consequence of continuous current flow through the semi-conductive image electrode throughout the entire time interval between the high-velocity beam scansions, the number' of photo-electrons from the cathode impacting each image element and producing further secondary electrons limiting the image elements potential drop and determining its iinal value, and means coupled to said conductive member for deriving an electrical signal therefrom depending on the potential increase acquired by each image element when impacted by the high-velocity scanning beam.

2. A television camera device of the imageconoscope type comprising a photo-electric cathode at one end, an electron-image-receiving electrode at the other end, said image electrode comprising a conductive member contacting a semi-conductive glass member facing said cathode and having a specific resistance of at least 1011 ohmcm., a layer of material possessing a high secondaryelectron-emission coefficient on said glass member, a collector disposedadjacent said image electrode and sur- 5 rounding a portion of the space between the [latter and said cathode, means for biasing said conductive member at a relatively low negative potential with respect to said collector, means producing a high-velocity electron beam for scanning said image electrode as a plurality of discrete image elements of given size and generating more secondary electrons than primary electrons for stabilizing the potential of said image elements at a positive value in the vicinity of the collector potential, said semi-conductive glass member having a thickness between 1/3 and 3 times the size of an image element on said image electrode scanned by lthe high-velocity beam and said thickness lying inthe range of 0.034 to 0.3 millimeter whereby the positive stabilized potential of each irnage element falls on. and approaches the negative potential of the conductive member as a consequence of continuous current ow through the semi-conductive image electrode in the time interval between the high-'velocity beam scansions, the number of photo-electrons from the References Cited in the tile of this patent UNITED STATES PATENTS 2,150,160 Gray Mar. 14, 1939 2,177,736 Miller Oct. 31, 1939 2,518,434 Lubszynski Aug. 8, 1950 2,747,131 Sheldon May 22, 1956 2,747,133 Weimer May 22, 1956 2,776,387 Pensak Ian. 1, 1957