US2727182A - Image transformer with electronoptical image projection - Google Patents

Description

Dec. 13, 1955 J. c. FRANCKEN IMAGE TRANSFORMER WITH ELECTRON-OPTICAL IMAGE PROJECTION INVENTOR Jun Cure] Froncken By ent Filed Oct. 19, 1951 United States INIAGE TRANSFORMER WITH ELECTRON- OPTICAL IlVlAGE PROJECTION Jan Carel Francken, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application October 19, 1951, Serial No. 252,071

Claims priority, application Netherlands November 6, 1950 3 Claims. (Cl. 315-) This invention relates to an image transformer with electron-optical image projection and in particular to a scanning tube in which an image formed on a photoelectric cathode is projected onto a collecting screen.

Attempts have been made to control the magnification of the image produced on the collecting screen in such devices in order to avoid the use of different optical lens systems heretofore required for magnified reproduction of details of the image.

Magnification of the photo-cathode image electron optically can be controlled by means of a combined magnetic and electron-accelerating field of rotation symmetry, using a disc of ferromagnetic material which is arranged at right angles to the axis of symmetry of the lens and provided with an aperture for the passage of the electron paths and which can be shifted in the direction of the electron paths. Shifting of the disc has the elfect of varying the strength of the magnetic lens and hence the image size, it being necessary to alter the energising current in the magnet coil to retain the definition of the image on the collecting screen, this resultant variation in the magnification being a factor of 2.

It is an object of the invention to permit control of the magnification without shifting any part of the electronoptical lens in an image transformer.

Another object of the invention is to provide means for continuous control of magnification in an image transformer.

According to the invention, an image transformer having a photoelectric cathode and using combined magnetic and electron accelerating fields, the two co-operating fields are directed substantially at right angles to the surface of the photoelectric cathode and the electron accelerating field is caused to diverge to a greater extent than the magnetic field. The magnification can then be controlled by varying the divergence of the magnetic field.

In a preferred embodiment, the image transformer comprises an electron discharge tube having a photoelectric cathode, an image collecting screen and an energising coil for developing a magnetic field which extends in the axial direction of the tube. The energising coil is arranged so that the magnetic field at the photocathode is substantially uniform. In addition, the electron discharge tube includes an electrode positioned to apply an electric voltage with relation to the photocathode which extends over the electron path down to a short distance from the photocathode. A second energising coil is also provided the field of which acts on the divergence of the field developed by the first energising coil. In order to provide the desired variation of the electric field with relation to the magnetic field it is generally necessary for the distance between the photocathode and the acceleration anode to be bridged by the last mentioned coil. I

When the cathode image is reproduced using an electron-optical image producing device of the type in which a combined magnetic and electron accelerating field is used rotation of the image is produced. Accordingly, the tube is generally rotated through a similar angle in the opposite sense. However, the rotation varies according to the variation in magnification and experiments have shown that. with a device according to the invention the image rotation increases substantially linearly with decreasing magnification. Thus, the device can be constructed so that the control of the magnification is efiected while at the same time position-Variation of the reproduced image by rotation of the tube is suppressed, only one adjusting member being required.

It is also possible to counteract the variation of the image rotation using three instead of two energising coils. In this case, the two energising coils mentioned hereinbefore are so close to the cathode that their fields are operative largely in the region in which the electric field is operative, whereas the third coil is operative chiefly in a region which is free from any electric field and hence is arranged between the two first-mentioned coils and the collecting screen.

The invention will now be described with reference to the accompanying drawing, in which Fig. 1 shows an image transformer according to the invention; Fig. 2 illustrates the electron paths occurring in the tube; Fig. 3 shows a television scanning tube with a control device according to the invention; Fig. 4 shows a tube of the type shown in Fig. 1 including a further coil to compensate for the image rotation; and Fig. 5 shows an alternative form of the coils shown in Fig. 1.

The image transformer shown in Fig. 1 comprises a discharge tube 1, which is constituted by a hermetically sealed glass vessel of circular cross-section, a photoelectric cathode 2 which, at the end of the narrow cylindrical part closed by a flat well, is arranged in a conventional manner on thebottom coated with a thin transparent conductive layer. A collecting screen 3 is positioned at the other end of the tube which has a larger diameter and is closed by a slightly convex wall. The screen 3 is formed by applying a layer of luminescent substance to a transparent carrier, for example a small mica plate, the latter being secured to the wall by means of a stay member 4. The inner surface of the glass wall of the tube 1, except for a portion in the proximity of the photocathode, is coated with an electrically conductive layer 5 which is electrically connected to an anode 6 extending to within a short distance of the photocathode 2. A potential difference is applied between the photocathode 2 and the anode 6 by means of supply conductors 7 and 8 sealed in the tube wall and electrically connected respectively to the conductive layer to which the photocathode 2 is applied and the internal conductive coating 5. The voltage required is about 1000 to 6000 volts.

Cathode 2 and anode 6 together constitute a negative accelerating electron lens and the form of the electric field in front of the cathode is denoted by broken lines 9 for several points of the cathode surface as shown in Fig. 2.

A coil 10 surrounds the tube and is arranged so that the cathode is positioned in a substantially uniform magnetic field. A second coil 11, the field of which cooperates with that of the coil 10, is located a greater distance from the cathode and is wound on a common coil former 12 with coil 10 jointly enclosed by a housing 13 of ferromagnetic material. The poles of the magnetic lens field are formed by flat head plates 14 to 15 of the housing 13. Energisation of the magnet coils results in development of a field, the variation of several lines of force of which is shown by 16 in Fig. 2.

The image impressed on the photocathode 2 is depicted on the collecting screen 3 by means of small electron beams formed by electrons which are emitted from an object point on the photocathode. These beams are centrally symmetrical about their main path, i. e., the path of an electron leaving the cathode without initial velocity.

Electrons which leave the object point with a radial or tangential velocity, but have no axial initial velocity, move helically about the main path which they finally intersect. In the case of a sharply defined image these intersections of all the main paths are located in the image surface. More intersections may be formed along the main paths. The distance of these intersections from the cathode can be controlled by altering the field intensity of the depicting coil and permits focussing several images of the photocathode onto the image surface. This also enables control of the magnification, but only stepwise since a sharply defined image is obtained only with several adjustments of the energising current in the depicting coil.

The electrons having an axial velocity component are responsible for the so-called chromatic aberration, an image fault which always becomes manifest with such image projecting devices and which will be left out of consideration hereinafter.

The size of the image and the orientation with relation to the object image on the cathode are therefore determined by the variation of the main path. Referring to Fig. 2, the main path associated with the point 13 of the photocathode 2 is designated 17. Even though magnetic lines of force at the cathode are parallel to the axis the diverging action of the electrostatic forces results in the production of a component of the magnetic field strength which is at right angles to the main path. The latter results in a Lorenz force which brings about a rotary movement of the main path. The tangential velocity component of this movement brings about a secondary Lorenz force which is directed radially towards the axis 19 tending to move the electron towards the axis through a distance a as in Fig. 2. As a result of the divergence of the magnetic field the angle between the direction of movement of the electron and the magnetic field lines will reverse so that the Lorenz force reverses its direction. The tangential velocity therefore diminishes and then reverses its direction and the rotary movement of the main path becomes opposite to the original direction of rotation. This results in the secondary, radially directed Lorenz force being now directed away from the axis. This Lorenz force directed away from the axis is further assisted by the centrifugal force which naturally is also directed away from the axis. The main path finally strikes the collecting screen at the point 20 which owing to the predominant influence of those two forces is located farther away from the axis than the point 18 at the photocathode.

It has been found that the rotation of the main path in the last-mentioned sense is considerably greater than that which the path performs in the converging part of the electron lens so that the image which is finally formed on the collecting screen is rotated with relation to the cathode image. The tube may be so arranged that the viewed image is nevertheless properly oriented.

It becomes more difiicult however, when the magnification is to be controlled. For this purpose the current in the second energising coil 11 may be altered so that the divergence of'the magnetic field varies. As the current weakens the magnification increases, and vice versa. However, by varying the field of the depicting coil 10 in the opposite sense it is still possible for the image to be focussed with any magnification. This provides constant controllability between 2.5 to 75-fold magnification. In order to eleminate the disadvantage that the image rotation is thus also varied, the device shown in Fig. 3 may be employed.

The discharge tube shown in Fig. 3 is a televisionscanning tube, the construction of the part in which the image intensification occurs being similar to that of the tube shown in Fig. 1. In this tube, collecting electrode 21 is constructed as a thin insulating layer, coated with a layer of secondary emission substance which is applied to a metal support which is referred to as the signalling plate.

Discharge tube 22 is provided with a hollow cylindrical extension 23, for example, of glass (shown only in part) which houses a conventional electrode system for developing and deflecting a directional electron beam, referred to as a scanning beam. This system usually includes an electron gun and electrostatic and/or electromagnetic deflecting means for causing an electron beam generated by the gun to scan the signalling plate 21 in the wellknown manner.

The coil combination 13 is arranged at the end of the tube 22 remote from the collecting screen 21 and is energised by two potentiometers 24, 25 which control the energising current, the current in the depicting coil 10 being adjusted by means of potentiometer 24, whereas the magnification is controlled by the potentiometer 25. The depicting coil focusses the image at any magnification. With a proper choice of resistances, these adjustments may be effected simultaneously and for this purpose rotary contact levers 26 to 27 are fitted on the common shaft 291. A transmission ensures that rotation of the shaft 28 also results in rotation of the tube 22 through an angle which is opposite and equal to the image rotation brought about by variation of the magnification. The transmission is formed by a disc 29 which is mounted on the shaft 28, the movement of which is transmitted to an annular track 3% arranged to surround the tube. Adjustment is effected by turning knob 31 to alter the adjustment. The annular track also serves to support the tube, which is adapted to rotate between rollers 32.

Tr 6 current for the depicting coil It) and the control coil 11 is supplied from the direct current source 33. The coils and resistors are connected so that upon an increase of the current in the depicting coil the current in the control coil decreases, and vice versa.

Pig. 4 shows a device similar to that shown in Fig. l but provided with a correction coil 34 which serves to render the image rotation independent of the variation of the magnification.

instead of using the two cylindrical coils for focussing and control of the magnification of the reproduced image, coils having a number of turns extending in the opposite direction and being wound one over the other may be used as shown in Fig. 5, in which for example the coil 35 acts to focus the image and the coil 36 to control the magnification. It has been found that by a proper choice of the number of turns in each coil, the variation of the ampere-turns and the variations in current strength this combination yields about the same results as far as the control of the magnification is concerned as in the use of cylindrical coils.

While the invention has thus been described in connection with specific embodiments and applications thereof other modifications will be readily apparent. to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. An image transforming system comprising an electron discharge tube including a planar photoelectric cathode, an anode, and a collector electrode, on which an image of the cathode is formed, disposed in that order within the tube and spaced from one another; a first coil winding surrounding the tube in proximity to the space between the cathode and the anode; a second coil Winding surrounding the tube in closer proximity to said cathode than said collector; means to adjustably pass current through said first and second coil windings to thereby vary the magnification of the image on the collector; means to apply a potential between said cathode and said anode to thereby produce an electric field between the cathode and the end of the anode adjacent thereto; a third coil winding disposd completely between aid first and second coil windings and said collector and outside said electric field; and means to adjustably pass current through said third coil winding to compensate for image rotation produced by changes in image magnification.

2. An image transforming system as claimed in claim 1 in which a single ferromagnetic core member surrounds all three coil windings.

3. An image transforming system comprising an electron discharge tube including a planar photoelectric cathode, an anode and a collector electrode disposed in that order within the tube and spaced apart from one another, means to apply a potential between said cathode and anode to thereby produce an electric field in the space therebetween, first and second adjacent coil windings surrounding the tube and the space between the cathode and anode, means to adjustably pass current through said first and second windings to produce magnetic fields cooperating with the electric field to magnify an image and to vary that magnification, a third coil winding surrounding the tube and disposed between the first and second windings and the collector and outside the space of the electric field, and means to adjustably pass current through said third winding to compensate for image rotation produced by changes in image magnification.

References Cited in the file of this patent UNITED STATES PATENTS

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