US3487258A - Image intensifier with channel secondary emission electron multiplier having tilted channels - Google Patents

Description

Dec. 30, 1969 B. w MANLEY ETA-L [3,487,258

IMAGE INTENSIFIER WITH CHANNEL SECONDARY EMISSION Filed Feb. 20, 1968 ELECTRON MULTIPLIER HAVING TILTED CHANNELS 2 Sheets-Sheet 1 INVENTORS BQIAN W MANLEY JOHN ADAMS BY Qua-M 1?- AGE Dec. 30, 1969 5 w AN- ET AL 3,487,258

IMAGE INTENSIFIER WITH CHANNEL SECONDARY EMISSION ELECTRON MULTIPLIER HAVING TILTED CHANNELS Filed Feb. 20, 1968 2 Sheets-Sheet 2 FIGA INVENTORS BRIAN W. MANLEY JOHN ADAMS United States Patent ()fiice 3,487,258 Patented Dec. 30, 1969 US. Cl. 315-41 6 Claims ABSTRACT OF THE DISCLOSURE An electronic image intensifier or image converter employs a channel secondary emission electron multiplier with channels tilted at a uniform angle so that none of the principal rays emitted from particular object points in directions perpendicular to the surface of the photocathode is parallel to the axis of a corresponding channel. An electron-optical system having rotational symmetry and an electron optical axis coincident with its axis of rotational symmetry is provided between the photocathode and the input face of the channel multiplier so that the principal rays from the photocathode diverge between a cross-over point and the multiplier itself.

This invention relates to electronic image intensifier devices, More particularly the invention relates to channel intensifier devices and to electronic imaging tubes employing such devices. Such devices Will be defined later but, briefly, they are secondary-emissive electron multiplier devices comprising a matrix in the form of a plate having a large number of elongated channels passing through its thickness, said plate having a first conductive layer on its input face and a separate second conductive layer on its output face to act respectively as input and output electrodes.

Secondary-emissive intensifier devices of this character are described, for example, in British patent specifications No. 1,064,073, No. 1,064,074 and No. 1,064,076, While methods of manufacture are described in patent specifications No. 1,064,072 and No. 1,064,075.

In the operation of all these intensifier devices (when incorporated in electronic imaging tubes) a potential difference is applied between the two electrode layers of the matrix so as to set up an electric field to accelerate theelectrons, which field establishes a potential gradient created by current flowing through resistive surfaces formed inside the channels or (if such resistive channel surfaces are absent) through the bulk material of the matrix. Secondary-emissive multiplication takes place in the channels and the output electrons may be acted upon by a further accelerating field which may be set up between the output electrode and a suitable target, for example a luminescent display screen.

As a summary of this art, the devices referred to herein as channel intensifier devices are defined in the patent specification referred to above in a definition given in the following terms:

A channel intensifier device is a secondary-emissive electron multiplier device for an electron imaging tube which device comprises a resistive matrix in the form of a plate the major surfaces of which constitute the input and output faces of the matrix, a conductive layer on the input face of the matrix serving as an input electrode, a separate conductive layer on the output face of the matrix serving as an output electrode, and elongated channels each providing a passageway from one face of the assembly consisting of matrix and input and output electrodes to the other face of said assembly, the distribution and cross-sections of the channels and the resistivity of the matrix being such that the resolution and electron multiplication characteristic of any one unit area of the device is sufiiciently similar to that of any other unit area for the imaging purposes envisaged.

In the operation of the simpler types of such devices, various problems arise. One problem is a critical dependence of gain on the length-to-diameter (L/D) ratio of the channels. The tilted channel arrangement of the aforesaid British patent specification 1,064,073 was provided mainly to meet the problem of critical dependence on length-over-diameter ratio in a device of the proximity type.

A second problem concerns the behaviour of an electron which approaches a channel along a path which is parallel to the axis of the channel. Such an electron can pass straight through the channel without striking the channel surface and, hence, without causing any secondary emission. In an imaging tube of the proximity type (i.e., one in which the photo-cathode is placed very near to the channel intensifier device without intermediate electron-optics) this may reveal itself as an overall loss of information. As for imaging tubes (for example a socalled electron-optical diode) in which electrons are directed along diverging paths towards a channel intensifier device having all its channels parallel to the electronoptical axis, it has been found that the defect tends to cause a dark patch at a substantially central area of the image where the greatest number of electron paths are substantially parallel to the axes of the channels.

In a low-voltage device this second problem can be resolved to some extent by setting up an abrupt transition in field strength at or near the entrances to the channels (i.e., at or near the input face of the matrix) so as to set up a convergent or divergent lens field at each entrance. Alternatively, with low-energy devices this problem can be met by tilting the electron field e.g., as described in the aforesaid British patent specification No. 1,064,076. However, with high-energy or high-velocity electrons, the effect of a tilted electric field, i.e., a transverse field component, tends to be insuflicient to cause deflection of the electron trajectory.

Moreover, there are problems which result specifically from the existence of diverging electron paths in tubes such as the said electron'optical diodes.

It is an object of the invention to provide an improved imaging tube construction which permits all the above problems to be resolved to a satisfactory extent. It is a further object of the present invention to provide a compromise solution in which the channels have angles of tilt with respect to the electron paths such that, without introducing excessive astigmatism, it is possible to substantially avoid a dark patch due to electrons passing straight through the channels.

The invention provides an electronic imaging tube comprising a photo-cathode, a channel intensifer device as herein defined, and an electron-optical system located between said photo-cathode and the input electrode of said channel intensifer device, the device being so constructed that: x

(a) The electron-optical system has rotational symmetry and has an electron-optical axis coincident with its axis of rotational symmetry;

(b) The said electron-optical axis is normal to the surface of the photo-cathode and to the faces of the channel intensifier device at the respective points of'intersection;

(c) Principal rays of the photo-cathode (as herein defined) diverge between a cross-over point or area and the channel intensifier device; and

(d) The channels of said channel intensifier device are all parallel to each other and tilted at a uniform angle 95 to the second main axial plane (as herein defined) such that none of the principal rays is parallel to the axis of the corresponding channel.

In such an arrangement all the principal rays form angles of differing values with the axes of the corresponding channels.

The term second main axial plane is defined as follows. In a system comprising a matrix having all its channels tilted in a common orientation with respect to an electron-optical axis passing through the center of the matrix, one of the axial planes (i.e. the planes containing said axis) contains also the axes of those channels which it intersects. This plane is referred to herein as the first main axial plane and the axial plane normal to it is referred to (as above) as the second main axial plane.

The term principal ray is defined as follows. The electron-optical system demands that all electrons emitted from any given point of the photo-cathode be brought substantially to one point on the image or focal surface or plane. Electrons are emitted at all angles (within a wide core) from such an object point on the photocathode. The terms principal ray and principal electron are used to denote the path of that electron (or the electron itself) which is emitted from the particular object point in the direction normal to the surface of the photo-cathode.

As for the term image surface, it will be convenient to refer to it, according to normal practice, as the image or focal plane in spite of its curvature.

One result of the tilting of the channels is, in effect, to displace any potential dark patch area to one side and away from the operative image field of the electronoptical and channel intensifier systems.

As will be explained later, the angles {3 between principal rays and their corresponding channels will vary from a minimum value to a maximum value.

The minimum value of the angles ,8 must be so chosen as to be sufficient to prevent electrons passing straight through or with insufficient multiplication, and this can be done in practice by ensuring that the dark patch is displaced out of the effective viewing area. As for the maximum value of the [3 angles, this must be restricted so as to avoid an excess of the astigmatism which occurs with tilted channels and increases with increasing values of the angle It has been found that such variation of the angle ,8 need not cause any appreciable visual difference in quality between the various parts of the image. This feature also removes any need there might otherwise be for a matrix having channels with converging or diverging axes which would present extremely difficult problems of manufacture.

The electron-optical system may be of the kind used in a so-called electron-optical diode as described in Philips Research Reports vol. 7, pages 119-130 (1952). The channel intersifier device may have plane faces in spite of the curvature of the image plane of the electronoptical system if the latter exhibits a. sufiiciently large focal length. However, the channel intensifier device may alternatively have a curved form as described in application Ser. No. 706,893, filed Feb. 20, 1968, its curvature being in the same sense (and possibly the same degree) as the curvature of the image plane. These possibilities are illustrated by various embodiment which will now be described by way of example with reference to the accompanying diagrammatic drawings as applied to imaging tubes of the image intensifier or image converter type employing as the target a luminescent viewing screen.

In the drawings:

FIGURE 1 is an axial section of a tube employing a flat channel intensifier device;

FIGURE 2 is an enlarged fragmentary section of the 4 channel intensifier device of the tube of FIGURE 1, the plane of the drawing being the first main axial plane;

FIGURE 3 is a diagram illustrating the conditions which obtain in the first main axial plane of the tube of FIG- URE 1;

FIGURE 4 illustrates a method of manufacturing tilted matrices; and

FIGURE 5 is an axial section of a tube employing a curved channel intensifier device.

In FIGURE 1 external radiation is directed from an object O on to a photo-cathode P so as to form an image thereon. Photo-electrons are liberated simultaneously from all parts of the photo-cathode with varying local intensities dependent upon the image formed.

The photo-cathode P co-operates with a conical or substantially conical anode A to form an electron-optical diode of the kind previously mentioned. The electronoptics of this system are such that the emitted photo-electrons are formed into a so-called pencil of rays as indicated at R, said pencil being converged by the action of the spherical equipotential surfaces between the photo-cathode and the anode cone. On passing through the aperture in the anode cone, the pencil is made less convergent by the negative lens action at the cone aperture (this represents an increase in focal depth). The pencil of rays finally converges to a focus at the so-called image plane (indicated at P) which is in effect a focal plane or plane of best focus and has considerable curvature as shown.

The anode A has a cylindrical skirt portion which is connected to the input electrode E1 of a channel intensifier device I, said device also having an output electrode E2. The electron-optical system P-A has rotational symmetry about an electron-optical axis Z-Z which is normal to the surface of the photo-cathode P and to the faces of the channel intensifier device I at the respective points of intersection.

As will be seen from the enlarged fragmentary axial section shown in FIGURE 2, the channel intensifier device I may, as shown, be traversed by a regular array of channels C each having its axis X at an angle to the second main axial plane (i.e. the axial plane normal to the plane of the drawing which contains the axis ZZ).

Photo-electrons from the photo-cathode P are shown arriving at b in FIGURE 2. In each of the channels that receives photo-electrons at any given instant, initial multiplication takes place e.g. as shown schematically in the drawing under the influence of the electric accelerating field set up by connecting the electrodes E1E2 to a source shown schematically at B1. A further accelerating field is provided by a source B2 between electrode E2 and a conductive coating (e.g. aluminum) forming part of a luminescent screen S (FIGURE 1) situated on the output side of the device.

In the conditions illustrated in FIGURE 2 the photoelectrons b are shown for simplicity all on parallel paths normal to the matrix face and therefore approaching the channels at a constant angle (which is equal to the angle to the channel axes, although this is not a realistic situation except (as a first approximation) at the center of the device I.

In reality the' principal rays (e.g. the principal ray indicated at Rp in FIGURE 1) strike the device I at varying angles so that they form (with the corresponding channels) angles such as the angles 51- 82 8? shown schematically in FIGURE 3 (this represents conditions in the first main axial plane, i.e. the axial plane which contains the axis Z-Z and also the axes XX of those channels which lie on the plane). As will be appreciated where 3 is the nominal angle at which the principal ray Rp3 diverges from a notional cross-over point Z0 of the electron-optical system P-A (as will be appreciated, the principal rays, e.g. ray Rp of FIGURE 1 and Rp1-Rp3 of FIGURE 3, are not truly straight lines in normal practice, but they are nevertheless identified by the initially orthogonal electron path at the point of emission on the photo-cathode P). In a similar manner where 'yl is the angle of divergence of principal ray Rpl. At the center (on the axis Z--Z) the divergence of principal ray Rp2 is zero and Provided that the smallest angle 51 is sufficient to prevent electrons passing straight through the respective channel (or passing through with insufiicient multiplication), the whole of the matrix indicated in FIGURE 3 will be capable of operating effectively without any dark spot since all the other angles (52;33 etc.) are greater. However, it is desirable to limit the maximum values of angles 8, as aforesaid, since a degree of astigmatism becomes increasinglyapparent with increasing values of p.

In a practical example suitable for an application such as that of FIGURE 1, the dimensions of the tube may be approximately as follows (these proportions differ from the purely nominal proportions of the drawings which have been adopted for sake of clarity):

TABLE Diameter of matrix cm 5' Diameter of channel /L.. 30 Length of channel mm 2 Distance between E2 and S, approximately mm 4 Maximum angle 7 of divergence degrees 12 Angle of channel tilt do- 15 Angle 5 minimum do 3 Angle 5 maximum do 27 The wide discrepancy between the curved image plane F and the fiat input face of the device I will cause some loss of resolution towards the edges of the picture displayed on screen S, but this effect will be kept within acceptable limits (for many purposes) by the aforementioned large focal depth of the electron-optical system.

The matrix of the device of FIGURES 1 to 3 is flat and has its channels at a constant angle to the normal to its faces. These features render it suitable for manufacturing by relatively simple methods which include the step of slicing a channelled block by making inclined cuts W across the channels C and at an appropriate angle thereto as shown schematically in FIGURE 4.

Referring now to FIGURE 5, the same reference numerals are used to denote corresponding elements. In this case the channel intensifier device I is of curved form as described in application Ser. No. 706,893, filed Feb. 20, 1968 with its input face curved in the same direction as the curveed image plane F. These two curvatures can, in certain circumstances, be made coincident although this is not necessary in view of the large focal depth previously mentioned, and adequate resolution can be obtained all over the image displayed on the screen S. With such an arrangement employing both curvature of the matrix and tilted channels, the advantages of both systems can be obtained simultaneously, namely the resolution can be of good quality all over the picture and at the same time the dark spot problem can be overcome.

All the channels of the curved matrix have substantially the same length and the goemetry of such a matrix, and appropriate methods of manufacture, are described in the said copending application.

If desired, the effects of the curvature of the photocathode P of FIGURE 1 or FIGURE 5 and the effects of the curvature of the display screen S of FIGURE 5 can be overcome or mitigated in known manner with the aid of fiber-optics. Thus it is possible to use a fiber-optic input plate having appropriate concave curvature on one side to match the curvature of the photo-cathode while presenting a substantially flat or (as shown at F01) oppositely curved face to the incoming radiation (at the same time this permits the use of greater curvature on the photo-cathode so as to reduce the curvature of the image plane F which can then more readily be made substantially coincident with the input face of device I). As an alternative, or in addition thereto, a second fiber-optic element may be used as a face-plate on which the screen S is formed, said face-plate having an output face which may for example be flat as shown at F02 in FIGURE 5 (the screen S has in this, and in preceding; cases, a concave curvature such as to match the curvature of the electrode E2 in the sense of ensuring as far as possible equal field strength between E2 and S at all points).

Although the cone A and electrode E1 have been described as being connected together, it may in some cases be desirable to modify this diode" arrangement by separating A from E1 so that different potentials may be applied to these two elements e.g. for the purpose of obtaining an optimum energy of entry for the electrons approaching the channels (this may be done by keeping A at the same potential but reducing the E1 potential). Such a modification produces, in effect, a triode structure and the nominal cross-over point Z0 (FIGURE 3) may become a virtual cross-over.

What is claimed is:

1. An electronic image converter tube comprising a photocathode, a secondary emission electron multiplier comprising an insulating block having a plurality of parallel, closely-adjacent narrow passageways therein opening out on opposite faces of said block, a conductive layer on one surface of a said block facing said photocathode and constituting an input electrode and a conductive layer on the opposite face of said block constituting an output electrode, and an electron-optical system located between said photocathode and the input electrode of said electron multiplier, said electron-optical system having an anode which is electrically connected to the input electrode of the electron multiplier, and

(a) the electron-optical system has rotational symmetry and has an electron-optical axis coincident with its axis of rotational symmetry;

(b) the said electron-optical axis is normal to the surface of the photocathode and to the faces of the electron multiplier at the respective points of intersection;

(c) principal rays emitted from particular object points in directions normal to the surface of the photocathode diverge between a cross-over point and the electron multiplier; and

(d) the passageways of said electron multiplier being tilted at a uniform angle to a second main axial plane normal to a plane containing the axes of the channels and intersecting the electron optical axis such that none of the principal rays is parallel to the axis of the corresponding channel.

2. An image converter tube as claimed in claim 1 employing a luminescent viewing screen on the output side of the electron multiplier.

3. An image converter tube as claimed in claim 2 wherein the electron-optical system is an electron-optical diode employing an approximately conical anode electrically connected to the input electrode of the electron multiplier.

4. An image converter tube as claimed in claim 3 wherein the angle 4) of inclination of the channels is about 15 and the angles between principal rays and channel axes range from about 3 at one edge of the electron multiplier to about 27 at its opposite edge.

5. An image converter tube as claimed in claim 4 wherein the input face of the electron multiplier and the image surface of the electron-optical system are both curved and are both concave as viewed from the photocathode.

7 8 6. An image converter tube as claimed in claim 5 3,343,025 9/ 1967 Iznatoviski et a1 313105 wherein the said image surface is substantially coincident 3,345,534 10/1967 Charles 250-213 with the input face of the electron multiplier. 3,374,380 3/1968 Goodrich 313-105 X 3,400,291 9/1968 Sheldon 31368 X References Cited 5 UNITED STATES PATENTS RODNEY D. BENNETT, JR., Primary Examiner I. F. MORRIS, Assistant Examiner 2,942,133 6/1960 McGee 313-105 X 3,001,098 9/1961 Schneeberger 315-11 3,229,142 1/1966 Kalibjian 313-68 10 3,260,876 7/1966 Manley et a1. 250-213 X 250 213; 313 105 CERTIFICATE OF CORRECTION Patent No. 3487258 Dated December 13, 1969 lnv t fl B.W. MANLEY ET AL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 65, "electron imaging" should read electronic imaging-- Column 4, line 70 and Column 5 lines 5 and 10,

substitute for 6 Column 5, line 66, "goemetry" should read --geometry Signed and sealed this 28th day of y 1970 6 Anew EdmrdlLFletchnnIr.

Atwsting Officer ll I- SGHUYLER, JR.

Commissioner of Patents

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