US3497759A - Image intensifiers - Google Patents

Description

IMAGE- INTENSIFIERS Filed May 14, 1968 s Sheets-Sheet 1 INVENTOR BRIAN WILLIAM MANLEY ZQ-AZ I I Fab. 24,1970 I I BQW MANLEY 7 $497,759

' IMAGE INTENSIFIERS I mea'ua 14, 1968 '3 Sheets-Sheet 2.

INVENTOR BRIAN WILLIAM MNLEY Filed May 14, 1968 3 Sheets-Sheet 3 United States Patent 3,497,759 IlVIAGE INTENSIFIERS Brian William Manley, Burgess Hills, England, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed May 14, 1963, Ser. No. 729,126

Claims priority, application Great Britain, May 15, 1967,

22,339/67 Int. Cl. HOlj 31/48 US. Cl. 315-11 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electron multiplier and image intensifier devices. More particularly the invention relates to channel intensifier devices and to electronic tubes employing such devices.

A channel intensifier device is a secondary-emissive electron multiplier device 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.

In the operation of such intensifier devices a potential difference is applied between the two electrode layers of the matrix so as to set up an electric field to accelerate the electrons, which field establishes a potential gradient created by current flowing through resistive surfaces formed inside the channels or (if such channel surfaces are absent) through the bulk material of the matrix. Secondary-emissive multiplication takes place in the channels.

With such devices the distribution and cross-sections of the channels and the resistivity of the matrix are such that the resolution and electron multiplication characteristics of any one unit area of the device is sufiiciently similar to that of any other unit area for any imaging purposes envisaged.

If such a device is used in an imaging tube or system, the latter will be referred to for convenience as an image intensifier tube or system rather than as an image converter tube or system even in applications where the primary purpose is a change in the wavelength of the radiation of the image.

British patent specifications 1,064,073, 1,064,074 and 1,064,076 describe examples of a channel intensifier device used in conjunction with a photo-cathode spaced from the input electrode and with a suitable target, for example a luminescent screen so as to form an arrangement suitable for an image intensifier tube, for example for viewing scenes at low illumination.

In further arrangements described in French patent specification 1,404,980 the photo-cathode is no longer 3,497,759 Patented Feb. 24, 1970 spaced from the channel intensifier device. Such arrangements employ the channel intensifier device in combination with photo-emissive surface areas in contact with the input electrode of the device.

The photo-emissive surface areas may substantially all be formed on the input electrode of the matrix and they may constitute an electrically continuous apertured layer, which can be represented as the layer P in FIG- URE 1 of the accompanying diagrammatic drawings. An object O is shown imaged by an optical system on to the photo-cathode P. Photo-electrons are liberated simultaneously from all parts of the photo-cathode with varying local intensities dependent upon the image formed thereon. Secondary electrons emerging from channel intensifier device I are accelerated towards a luminescent screen S.

More particularly, the channel intensifier device I is traversed by a regular array of channels. The matrix of the device may be of glass and its input and output faces carry first and second conductive electrode layers E1-E2 respectively.

In each of the channels that receives primary electrons at any given instant, multiplication takes place and the necessary electric accelerating field is set up by connecting the electrodes El-EZ to a source shown schematically at B2. A further accelerating field is provided by a source shown schematically as a unit B3 connected between E2 and a conductive coating (e.g. aluminum) associated with luminescent screen S.

Photo-electrons are emitted in a direction away from the matrix and input electrode E1 and such electrons require a field to turn them back towards the channels. Means for producing such a field are represented diagrammatically by a source B1 applying a voltage between the input electrode E1 and a transparent electrode E0 formed e.g. on the envelope. In practice it is found that the field configuration existing at the entrances to the channels due to the elements E1-E2B2 alone can be sufficient to draw back the photo-electrons without the need for the electrode E0 and source B1.

As an alternative to location on the input electrode, the photo-emissive surface areas is shown to be formed substantially entirely within the channels of the matrix. In this case the accelerating field set up by source B2 between electrodes El and E2 is clearly suflicient to accelerate the electrons in the channels without the need to have an electrode corresponding to E0 with its source B1.

As a further alternative the photo-emissive surface areas have been laid partly on the input electrode on the matrix and partly inside its channels.

There is a problem associated with imaging tube constructions which operate with an electron accelerating field to direct electrons from points on the photo-cathode to corresponding points on the input electrode. This problem is that the accelerating field tends to direct the electrons into the channels at high speed in a direction more or less parallel to the axes of the channels. Consequently there is a tendency for the electrons to fail to strike the channel walls at any early stage so that less multiplication steps occur and the total multiplication effect is reduced. A second problem exists in that electrons emitted from any given picture element on the photo-cathode are apt to spread and enter more than one channel.

According to the invention with a channel image intensifying device for electrons comprising a thin plate of glass of high electrical resistance or of a ditfe r ent kind of similar material, which plate is provided on the two major surfaces with an electrically conductive layer and with closely adjacent channels interconnecting the two surfaces and with a photo-electric cathode in contact with one of the two surfaces, the photo-electric cathode closes the passages at the entrances to the channels and is sufliciently permeable for rays of those wavelengths to which the photo-cathode material is sensitive.

The entrances to all the channels are closed by photoemissive areas, and the device can operate satisfactorily even if some parts of the input electrode are not quite in direct physical contact with the photo-emissive layer owing, say, to irregularities in said layer or in said electrode or to the presence of dust particles. This is true provided that such photo-emissive layer is in electrical contact with the input electrode even though they are not in physical contact, and this can readily be achieved by forming the photo-emissive layer as an electrically continuous layer having sufficient conductivity to maintain all the areas effectively at the same potential as the input electrode. In practice it is possible to carry out such a construction with such accuracy that areas of imperfect physical contact between input electrode and photo-emitter cause only negligible local losses of resolution due to a few electrons entering the wrong channels.

The invention also overcomes the aforesaid second problem which exists in previous arrangement of the proximity type in that electrons from each picture element on the photo-cathode are constrained to enter the appropriate channel, and this of course produces maximum definition for a given channel density or for a given total number of channels.

Embodiments of the invention will now be described by way of example with reference to FIGURES l to 10 of the accompanying diagrammatic drawings in which FIGURE 1 illustrates the prior art,

FIGURE 2 represents schematically an image intensifier tube according to the invention,

FIGURES 3 to 5 illustrate 3 embodiments of the invention,

FIGURE 6 illustrates a modification of the invention,

FIGURE 7 illustrates a method of manufacture,

FIGURES 8 and 9 illustrate in a simplified manner the action of symmetrical and asymmetrical (i.e. tilted) lenses respectively,

FIGURE 10 illustrates a further embodiment.

In the generic representation of an imaging tube employing a channel-intensifier photo-cathode combination according to the present invention as shown in FIGURE 2 the input voltage supply B1 and the separate electrode E0 of FIG. 1 have been omitted. Though the photoemissive material has a degree of conductivity of its own the original input electrode E1 is retained as part of the channel intensifier device. Electrode E1 communicates its own potential to the photoemissive areas since they are in contact with it as explained above. Thus the photoemitter and input electrode can be indicated as being connected together to one end of the supply B2.

As shown in an enlarged schematic manner in FIGURE 3 a continuous photo-emissive layer P is brought up to, and placed in contact with, the input electrode E1. The parts of the photo-emissive layer which correspond to the electrode E1 perform no photo-emissive function in the device. In some cases it may be the easiest and cheapest method of construction by depositing layer P on a glass plate W which may be the window of the envelope since the photo-emissive surface areas can be produced as a continuous layer before assembly. The areas of the photo-emitter which correspond to the channels C are the operative areas and they emit photo-electrons directly into the channels. This permits the electrons to initiate their travel in the channels at a lower energy so that they can more easily be directed towards the channel walls at an early stage. This can be done in different ways and these alternative arrangements are illustrated in FIG- URES 4- and 5. In FIGURE 4 the input electrode E1 is extended some way into each channel entrance so as to create an electrostatic lens effect which deflects the electrons outwardly towards the channel Walls. This lens action will be described in greater detail later.

In the alternative arrangement of FIGURE 5 the channels are tilted so as to cause early collisions as shown schematically. It is possible to combine the configurations of FIGURES 4 and 5 (although this is not normally necessary) or the lenses themselves may be tilted as will be explained.

In any of the arrangements of FIGURES 3 to 5 it is possible in principle to eliminate those parts of the photoemissive layer P which correspond to the input electrode E1 and are therefore not utilised for photo-emission. However, this is extremely difficult to carry out with present techniques and requires sufiicient precision of manufacture for substantially all the photo-emissive areas to be individually in physical contact with the input electrode since otherwise charge deposition and like phenomena will disturb the operation of the device.

A modification of the present invention consists in omitting the input electrode E1 and relying solely on the conductivity of the photo-emissive areas to act as an input electrode. For this reason areas must, as in the arrangements of FIGS. 35, be joined together to form an electrically continuous layer which is connected to one of the supply terminals. An example of such an arrangement is shown in FIGURE 6.

It is in accordance with the definition given above of a channel intensifier device that in the arrangement of FIGURE 6 the input electrode is regarded as constituted by the photo-emissive layer wherein the input electrode is no longer traversed by the passageways provided by the channels.

A special case of the FIGURE 6 type is the case in which the concerned photo-emissive and electrode material PE is a metal adapted to act as a photo-emitter at given wavelengths of input radiation, for example gold for an ultra-violet image intensifier. However, such a metallic layer must be thin enough to be partially transparent and this may unduly limit its current carrying capacity as compared with the arrangements of FIGURES 3 to 5 wherein the electrode E1 has apertures to pass radiation and therefore can be of any desired thickness. A similar limitation may exist also when the layer PE of the arrangement of FIGURE 6 is of a material other than gold.

In manufacturing the devices described, the electrode E1 can be formed on the matrix by known methods. If it is to be extended into the channels in accordance with FIGURE 4, it can be formed by evaporating a metal (e.g. chromium) at a suitable angle 5 as shown in FIG- URE 7, such evaporation (3) being effected from a source which is rotated round the axis of the matrix so as to cause uniform penetration the channel plate is itself rotated (or relative to a fixed source). The chosen value of the angle 5 of evaporation determines the depth of the inward penetration d of the electrode material inside the channels 2. This forms both the input face layer 1 of the electrode and the extensions 4 of the electrode into the channels.

Arrangements such as those of FIGURES 3 to 5 may be made by depositing the layer P on to a substrate plate W and assembling said plate against the electrode E1 of the channel device. This must be done with sufiicient accuracy to ensure that layer P is in contact with all or nearly all the parts of the electrode, and the two steps should be carried out in vacuo. Corresponding steps can be adopted for the case of FIGURE 6 but in this case the assembly can be carried out in air if the layer PE is of gold as previously described.

The lens effect previously referred to in connection with the electrode extensions of FIGURE 4 will now be de scribed in greater detail with reference to FIGURE 8 after a brief review of the problem associated with the channel entrance conditions in prior channel image intensifiers.

Electrons produced from the photocathode (in response to light or other radiation) must acquire so much energy that, when they strike the wall of a channel, the

secondary emission coefficient will be substantially greater than unity. This requires in practice collision energies which exceed 50 ev. It is also important, however, that electrons from the photocathode do not penetrate far into the channel before collision with the wall, since the length of channel available for the subsequent gain process will be inadequate. Usually a channel plate is separated from the photocathode by a small distance (between one and ten channel diameters), and the application of a potential difife rence exceeding 50 v. (between photocathode and channel plate input electrode) to ensure that photoelectrons enter a channel with sufficient energy to produce secondary electrons on collision with the wall of the channel. To cause collision with the wall in the early part of the channel the field strength in the channel plate is made different from that in the space between photocathode and channel plate. Thus a lens action is established at the channel entrance, which will be positive if the field strength in the channel plate is the higher, or negative if it is the lower. Unfortunately, if the strength of this lens is to be adequate to ensure that electrons are directed to strike the wall in the early part of the channel, it is necessary that either the disparity in field strengths be very great, or the energy of the electron as it enters the lens be small, so that it is readily deflected. The electric field within the channel plate is normally fixed by the gain required from the plate and its geometry, and there is little or no freedom to choose the field strength to suit electron-optical requirements. Thus to achieve disparity in field strengths, either the field strength in the photocathode/channel-plate ga must be made very high (in which case field emission from the photocathode becomes a danger) or it must be made very low, in which case the electrons from the photocathode will spread before reaching the channel plate and resolution will be lost.

In arrangements according to the present invention, wherein the channel plate is in contact with the photomissive areas and said areas close the channel entrances, this ensures that the resolution is limited substantially only 'by the channel spacing rather than by electron spreading from the photocathode.

By arranging that the input electrode of the channel plate penetrates a small distance into the channel as shown in FIGURE 4, a virtually field-free region is established in the region of the photocathode. This photoelectrons enter a converging electron lens occurring at the boundary of the penetrating electrode extension. These electrons have small energy and the lens can thus be adequately strong to ensure early wall collision for most photoelectrons.

The degree of penetration of the electrode will influence the lens strength. As a limit case, no penetration will result in no lens action, and the photoelectrons will spread in straight paths from the photocathode. Deep penetration several channel diameters in extent will give strong curvature of the equipotentials and a strong lens action; however, many photoelectrons will drift to the wall in the electrode region without gaining any energy and so will be lost. The best compromise appears to lie with a penetration between A and 2 channel diameters in depth, preferably between /2 and one diameter. In this range a large fraction of the photoelectrons will gain sufficient energy (before collision) to produce secondary electrons, and few still pass far into the channel without collision.

The lens action is shown diagrammatically in FIG- URE 8. In order to reduce the fraction of axial electrons which still proceed a considerable distance into the channel before colliding, the electrode penetration forming the lens can be tilted according to FIGURE 9. This has the result of accelerating the electrodes preferentially to one side of the channel, and even a photoelectron emitted along the channel axis will collide with the wall.

Tilted electrode penetration (i.e. tilted lenses) can be produced by evaporation of metal forming the input electrode (typically Cr) at a suitable angle. Referring back to FIGURE 7 it was explained that, by rotation of the plate during exaporation, substantially uniform penetration can be achieved. For the present purpose, skew penetration like that shown in FIGURE 9 can be obtained by omitting the rotation.

For a given total matrix area and given channel diameters density, the effective photocathode area of any of the devices described can be increased by outwardly flaring or tapering the entrances to the channels so that their initial diameter is larger. This is illustrated schematically in FIG. 10.

In a practical example the dimensions of the matrix may be approximately as follows:

Diameter of matrix.3l0 cm. Diameter of channel-15 Length of a channel.1 mm.

For use in an image intensifier the source B may produce about 1000 volts.

What is claimed is:

1. An electron multiplier comprising a thin plate of electrically insulating material, said plate having an electrically conducting layer on the two major faces and being provided wi.h a plurality of parallel closely adjacent narrow secondary emissive channels interconnecting the two major faces, and a photoelectric cathode contacting one of the two major faces, and closing the entrances to the channels and sufiiciently permeable to radiation of such wavelengths to which the photocathode material is sensitive to emit electrons in response to said radiation which enter said channels.

2. A device as claimed in claim 1, wherein the photocathode is formed by a continuous layer applied to a transparent support.

3. A device as claimed in claim 2, characterized in that between the photocathode and the plate surface a conductive layer is provided which extends into the channels.

4. A device as claimed in claim 3, wherein the length of the extension of the conductive layer in the channels is equal to /2 times a channel diameter.

5. A device as claimed in claim 3, wherein the layer extension in a channel terminates in the form of a bevelled cylinder.

6. A device as claimed in claim 1 wherein with respect to the major faces the channels extend in an oblique direction.

7. A device as claimed in claim 1, the photocathode is also the electrically conducting layer.

References Cited UNITED STATES PATENTS 2,942,133 6/1960 McGe'e 3151l X 3,001,098 9/1961 Schneeberger 3l511 3,374,380 3/1968 Goodrich 313X 3,407,324 10/1968 Rome 313105 X RODNEY D. BENNETT, JR., Primary Examiner JEFFREY P. MORRIS, Assistant Examiner US. Cl. X.R. 313-105

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