US3491233A - Image intensifier devices - Google Patents

Description

Jan. 20; 1970 B. M A-NLEY iMAGE INTENSIFIER DEVICES Filed April 29, 1968 sl F0 P29 1 E2 s2 INVENTOR. BRIAN W MANLEY AGE T United States Patent 3,491,233 IMAGE INTENSIFIER DEVICES Brian William Manley, Burgess Hill, England, assignor, by mesne assignments, to US. Philips Corporation, New York, N.Y., a corporation of Delaware Filed Apr. 29, 1968, Ser. No. 725,045 Claims priority, application Great Britain, June 16, 1967, 27,917/ 67 Int. Cl. H01j 31/26, 31/50 U.S. Cl. 31510 5 Claims ABSTRACT OF THE DISCLOSURE An image intensifier employs two image intensifier stages, the first of which is of the direct type employing a first photocathode and a first luminescent screen. The output (luminescent screen) of the first stage is optically coupled to the input (photocathode) of the second stage which includes a channel intensifier (a secondary emission electron multiplier) employing a secondary emissive resistive matrix, the opposite surfaces of which are provided with electrically conductive coating constituting input and output electrodes connected together by a plurality of longitudinally extending parallel passageways, and an output luminescent screen. The resulting combination provides a high-signal-to-noise ratio and a high gain in a short length.

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.

A channel intensifier device is a secondary-emissive electron multiplier device which comprises a resistive matrix in the form of a plate the major surface of which constitute the input and output faces on 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-section 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 sufficiently similar to that of any other unit area for the imaging purposes envisaged.

Channel intensifier devices (also referred as channel plates) can provide very high gain While occupying only a very small depth in the direction of the optical axis. The potential advantage of small depth is achieved when the device is incorporated 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-optical focusing means.

On the other hand channel intensifier devices as they are conventionally used in image intensifiers have relatively poor signal-to-noise characteristics.

There are three reasons for this. First, a photoelectron striking the channel plate may fail to produce a secondary electron which successfully undergoes multiplication in the channel. This may arise because the photoelectron does not enter the channel but strikes the face of the channel plate between the channels, or because the photoelectron fails to produce a secondary electron, or because the secondary electron or electrons produced themselves fail to produce secondaries at a subsequent collision. Secondly, the photoelectron may traverse the channel without colliding with the channel wall. Thirdly, the statistical variation of the yield from a sequence of photoelectrons entertlce ing the channel plate may be large. Each of these eifects has the practical result of reducing the information content associated with the original photoelectrons and hence, in electrical and visual terms, worsens the signal-to-noise ratio.

As an alternative to imaging tubes employing a channel intensifier device, there are more conventional image intensifier systems which may be used as imaging tubes or stages and which employ a photo-cathode and a luminescent screen or other target separated by a vacuum space which does not contain any dynode or equivalent secondary-emissive structure. The action of such a system is direct in the sense that the electrodes landing on the target are electrons emitted by the photo-cathode.

They may be of the well known type which employs a photo-cathode, a luminescent screen, and an electron-optical imaging system between the photo-cathode and screen, the electron-optical system having rotational symmetry and an electron-optical axis coincident with its axis of rotational symmetry, which electron-optical axis is normal to the surfaces of the photo-cathode and screen at the respective points of intersection.

Tubes of the latter type are frequently of the image-inverting kind known as electron-optical diodes and described in Philips Research Reports, vol. 7, pp. 119-130 (1952). In contrast with tubes containing a channel intensifier device, these tubes are only capable of a much lower light gain, but they possess a better signal-to-noise characteristic since they suffer none of the causes of information loss which are present in channel devices.

It is an object of the invention to provide an improved imaging system which permits the favourable characteristic of both the channel and direct types of tube to be exploited. A further object is to achieve this in a device 0ccnpying little more volume or axial length than a singlestage electron-optical diode used alone.

The invention provides an electronic imaging system comprising a first image intensifier stage of the direct type employing a first photo-cathode and a first luminescent screen, an optical coupling element coupling said screen to a second photo-cathode, and a second image intensifier stage comprising the said second photo-cathode together with a channel intensifier device as herein defined and a second luminescent screen on the output side of said channel intensifier device (the term image intensifier is used herein to cover also applications which could be referred to as image converter applications in the sense of the primary purpose being a change in the wave-length of the image).

By using such an arrangement the favourable signal-tonoise characteristics associated with the first (or pre-amplifier) stage determine the signal-to-noise ratio of the total system, while the channel intensifier stage contributes high gain in a short length. In addition, in practical cases the gain may varied from, say, 1 to 10 without varying the resolution, this being achieved merely by changing the channel plate voltage through a range such as 400 to 1200 volts.

By contrast with a known system composed of three fiber-optically coupled single-stage image intensifiers of the electron-optical diode type, it is necessary to change the voltage on one tube from about 1000 to 15,000 volts to encompass the gain range 1 to 5x10 and at the lower voltages the resolution is impaired.

According to the invention it is possible to construct arrangements which have the following advantages compared with such a three-tube coupled system:

1) Lower operating voltage (20 kv. compared with 45 kv.)

(2) Higher gain capability (10 compared with x10 (3) Shorter assembly (about half the length) (4) Saturating light transfer characteristic (5) Easy gain control.

With reference to the first advantage, the normal 3- tube system is run at 45 kv. to achieve the necessary gain, so that the first tube has 15 kv. applied to it. By contrast, it may be adequate to run the preamplifier at less than 15 kv. (say kv.). At this lower voltage image tubes are much less prone to background effects due to field emission and ion effects so that the signal-to-noise ratio may even be better than the 3-tube system.

The saturating light transfer characteristic of the channel intensifier occurs because the channel can supply no more output current than is carried in the walls of the plate by condition. Thus as the light into the channel intensifier device is increased, a level is reached at which further increase of input light produces no further increase in the output intensity. The advantage of this is that if very intense light is present in the field of view, it does not cause a correspondingiy intense illumination of the output screen, which would prevent effective observation due to viewing discomfort and halation.

From an alternative comparison it can be seen that, by using a first stage of the image-inverting type, the invention can provide systems having the following advantages over a channel intensifier used alone:

(1) Improved signal-to-noise characteristic (because of the relatively poor noise performance of channel plates).

(2) Correct image orientation (the objective lens or equivalent of the type normally used will invert the image, hence the image intensifier system is required to reinvert the image).

If the two stages are in separate envelopes, the first or pre-amplifier stage can be made to suit the viewing requirements in terms of angle of view, photocathode spectral sensitivity and image magnification, while a standard modular channel intensifier can be prepared to couple to various pre-amplifiers. Alternatively, the two stages may be in a common envelope in which case the fibre-optic is in the vacuum and does not have to be air-tight.

The invention will now be described with reference to the accompanying drawing on which FIG. 1 is an axial sectional view of an image intensifier according to the invention; FIG. 2 is an axial sectional view of another embodiment; and FIG. 3 is an axial sectional view of still another embodiment.

FIGURE 1 shows in axial section an electronic imaging system comprising a first image intensifier stage employing a first photocathode P1, a first luminescent display screen S1 and an electron-optical focusing system located between said photocathode and screen which electron-optical system has rotational symmetry and has an electron-optical axis ZZ coincident with its axis of rotational symmetry. A fiber-optic plate FO couples screens S1 to a second photocathode P2. A second stage is provided coaxially with the first and is formed as a channel intensifier stage comprising the said second photocathode, a channel intensifier device I and a second luminescent display screen $2 on the output side of said device I. The first stage has an electron-optical system of the image-inverting type and is shown as a system of the electron-optical diode type employing a conical or approximately conical anode A connected to a conductive layer forming part of screen S1 and also to a source of anode voltage shown schematically at Ed.

The first and second stages may both be enclosed in a common evacuated envelope which also encloses the fiber-optic plate, in which case the latter need not be vacuum tight. Alternatively, the first stage may be a separate rchang b e t be as aforemen on d, with P0 split at d, Y

In the arrangement of FIGURE 1 the first screen S1 is concave as viewed from the first photocathode P1, the elements P2IS2 of the second stage are planar and the fibre-optic plate FO has a concave input face to determine the curvature of the first screen and a planar output face. In addition, the first photocathode P1 is concave as viewed from the first screen S1.

External radiation is directed from an object O on to the photocathode P1 so as to form an image thereon. Photo-electrons are liberated simultaneously from all parts of the photocathode with varying local intensities dependent upon the image formed.

The photocathode P1 co-operates with the conical or substantially conical anode A to form the electron-optical diode. The electron-optics 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 F) which is in effect a focal or plane of best focus and has considerable curvature (as shown) in spite of the conventional term plane.

The anode A has a cylindrical skirt portion by which it is connected to the screen S1.

The channel plate comprises an electrically resistive matrix I through which the channels are formed, an input electrode E1 and an output electrode E2. The elements of the channel plate may employ voltage sources as shown schematically at B0, B1 and B2. However, the photocathode P2 need not be spaced apart from the electrode E1. Instead, P2 may be in contact with said electrode and/or in contact with the inner surfaces of the channels as is known already.

Although the cone A and screen S1 have been described as being connected together, it may in some cases be desirable to modify this diode arrangement by separating A from S1 so that different potentials may be applied to these two elements. Such a modification produces, in effect, an equivalent triode structure and the nominal cross-over point Z0 (FIGURE 1) may become a virtual cross-over. Such arrangements permit, for example, variable magnification.

In a practical example suitable for an image intensifier based on the arrangement of FIGURE 1, the dimensions of the system may be approximately as follows:

TABLE Diameter of matrix I cm 2.5 Diameter of channel 15 Length of channel mm 1 Distance between E2 and S2 mm 1 Distance between centers of S1 and P2 mm 5 Distance between centers of P1 and S1 mm 50 Without affecting the above dimensions, the channels may all be parallel to the axis ZZ or they may all be tilted.

For use with visible light, the photocathodes may be of the known S20 tri-alkali type combining Cs, Sb, K and Na and the phosphor of screen S1 should be chosen to match this type of photocathode, that is to say to give the maximum number of photoelectrons per unit current into the phosphor S1 (a suitable phosphor is the type known as P20). An advantage of arrangements according to the invention is that there is a degree of freedom in the choice of the phosphor of the first screen S1, in that it can be chosen so as to obtain either (a) optimum coupling to P2, or (b) optimum decay-time or lag characteristics for the system. This freedom is absent, or very restricted in many conventional systems.

If two separate envelopes are used for the two stages, then the fiber-optic F0 will, of course, be plit into two 5 closely-coupled plates as indicated at d (FIGURE 1) each of which forms part of the respective envelope.

Referring to FIGURE 2, a first or preamplifier stage is arranged in which the elements P1 and S1 are placed very close to each other so as to avoid undue loss of definition. In this way the overall axial length of the system can be reduced (as compared with FIGURE 1) in cases where image inversion is not required, e.g. when the screen S2 is coupled to a camera tube. However, if inversion is required, it may be effected by an optical output system shown schematically as an optical system OI. The latter may, for example, be a fiber-optic element of known type having a twisted bundle of fibres.

For simplicity, the optional source Bo (explained with reference to FIGURE 1) has been omitted from the drawing of FIGURE 2.

If both the stages of FIGURE 2 are planar then the intermediate fiber-optic plate F0 can readily be replaced by a simple thin membrane of glass or mica, but this not desirable in all cases since it necessitates a single envelope for the two stages.

In the arrangement shown in FIGURE 3 the first stage is of the same type as in FIGURE 2 but the second stage employs an electron-optical system of the image-inverting diode or equivalent type between the photocathode P2 and the channel plate I. If a fiber-optic plate is used as the intermediate coupling then the photocathode P2 can be made concave (as shown) which in turn facilitates the use of a planar channel plate.

What is claimed is:

1. An electrode imaging system comprising a first irnage intensifier stage employing a first photocathode and a first luminescent screen, an optical coupling element coupling said screen to a second photocathode and an image intensifier stage comprising the said second photocathode together with a channel intensifier device comprising a body of secondary-emissive electrically resistive material provided with a plurality of parallel longitudinal passageways connecting opposite surfaces which are provided with conductive coatings constituting input and output electrodes respectively, and a second luminescent screen on the output side of said channel intensifier device whereby a high signal-to-noise ratio and a high gain in a short length is obtained.

2. A system as claimed in claim 1 wherein the first stage has an electron-optical system of the image-inverting type having rotational symmetry and an electron-op tical aXis coincident With its axis of rotational symmetry.

3. A system as claimed in claim 2 wherein the electronoptical system is of the electron-optical diode employing an approximately conical anode.

4. A system as claimed in claim 1 wherein the optical coupling element between the first and second image intensifier stage is a fiber-optic plate.

5. A system as claimed in claim 4 wherein the first luminescent screen is concave as viewed from the first photocathode, the elements of the second stage ar planar and the fiber-optic plate has a concave input face to determine the curvature of the first screen and a planar output face.

References Cited UNITED STATES PATENTS 2,942,133 6/1960 McGee 313-105 X 3,345,534 10/1967 Charles 250-213 3,356,851 12/ 1967 Carlson 250-213 3,369,125 2/1968 Dueker 250-213 3,374,380 3/1968 Goodrich 313-105 XR RODNEY D. BENNETT, JR., Primary Examiner JEFFREY P. MORRIS, Assistant Examiner US. Cl. X.R.

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