GB2493532A - High speed resolution electro-optic imaging - Google Patents

Abstract

An electro-optic device gating arrangement adapted to gate a magnetically focused image intensifier. This comprises a plurality of electronic pulsers and axially aligned electrode rings which are used to gate the electro-optic device. A first pulser is operably connectable to a first control electrode ring or photocathode of the electro-optic device. A second pulser is operably connectable to a second control electrode ring or photocathode of the electro-optic device. The electric field of each ring or photocathode combines to form a uniform axially directed electric field.

Description

High Speed High Resolution Electro-Optic Imaging

Background

Image intensifiers are well known for their applications as military night sights where their large optical gain (i.e. x 1000) allows them to make readily visible to the human eye (or camera) objects illuminated only by starlight.

Intensifiers also may perform spectral conversion i.e. UV or near infra-red to visible.

Another significant application is in fast electronic optical shuttering where the optical output of the intensifier may be gated electronically on and off in a matter typically of a few nanoseconds or even faster. This is conventionally achieved with modern generation 2 or 3 (micro-channel plate (MCP) proximity focused) intensifiers by rapidly switching the photocathode polarity with respect to the MCP input through 200V or more using readily available semiconductor technologies. However, MCP devices although having high gain significantly degrade the image quality with a fast decay in the modulation transfer function (MTF) with increasing resolution (i.e. contrast drops rapidly with increasing resolution) and very limited output power or brightness.

Generation 1 intensifiers (electro-statically focussed but with no MCI') although running at many kilovolts may be gated with the addition of gating meshes (c.f. streak or shutter tubes) but suffer from significant image distortions due to the employment of electro-static lenses and have limited dynamic spatial resolution.

A third common device -the proximity diode, although having high spatial resolution, good MTF properties and high output power has no means to gate other than switching its total operating voltage of circa 15KV or more on/off. Not easy at high speed.

Hence all three common current technologies have severe limitations for high speed imaging or for fast optical shuttering. These shortcomings in resolution and output power are now apparent and limiting when combined with current high resolution or ultra-high speed cameras whose resolutions are typically 2000 x 2000 pixels or with continuous imaging speeds of millions of frames per second.

Modern camera sensors have also become much more sensitive such that the very high gain of MCP derived intensifiers is no longer always required.

To resolve these issues, this patent solves the gating problem of an often overlooked older image intensifier technology, the magnetically focussed image intensifier.

The magnetically focussed image intensifier is superficially similar in appearance to the electro-statically focused generation 1 intensifier -a vacuum tube body with a photocathode at one end and a phosphor screen at the other. As per the electro-static and proximity diode designs, a voltage of perhaps ÷15KV to +25KV is applied from the screen to the photocathode to accelerate photo-electrons into the screen where their increased energy is reconverted to light achieving gain.

However, unlike the electro-static design with its internal electro-static lenses (focus cones etc.) and electron cross-over points, the magnetically focussed tube is in principal a hollow tube with uniform axial magnetic field (supplied by an external solenoid surrounding the tube) and an axial electric field. Photo-electrons emitted from the photocathode corkscrew along the length of the tube into the screen. In normal operation with correct setup the magnetically focussed intensifier gives almost distortion free amplification (no photo-electron pinch points (crossover) or constrictions (i.e. the electrons maintain their spatial separation travelling through the tube) and exceptional resolution (80 to 100 lp/mm -often higher than conventional optical lenses) and all at very high MTF. Finally they are also able to operate at very high output powers or electron currents well beyond the destruction point of any MCP or proximity diode technology. Apart from the cumbersome external solenoid they are in many ways ideal devices for today's very high resolution requirements if only they could be easily gated. Unfortunately, the necessary absence (no obstructions over imaging area allowed) of any sensible gating mechanism within the tube has meant that as per the proximity diode they could only be gated by switching the total 25KV -an impractical task at high speed. Gating meshes as per the electro-static streak tube' have been tried but result in a significant and unacceptable overlay of the mesh grid visible on the output image and still require kilovolt gating.

The output phosphor screen in all intensifier examples given may be viewed by eye. Additionally, it may be lens or fibre-optically coupled into a further imaging device i.e. a camera sensor or further optical instrumentation. There are alternatives to the phosphor screen for some applications among them being direct electron bombarded CCDs or semiconductor sensors and resistive anodes but the gating mechanisms remain the same.

Description

A typical DC operating magnetically focussed image intensifier is shown in figure 1 below. Note the photocathode A, multiple control rings or flanges B to Land output phosphor (or alternative) screen M. The purpose of the control rings or flanges is to control (or ensure) a well-controlled axial (no transverse) electric field along the body of the tube with no local wall charging or secondary effects i.e. to ensure an increasing and balanced voltage potential. Normally a large positive voltage is applied to the screen (+15KV to 25KV) with respect to the photocathode A (at ground). A chain of external resistors N form a potential divider to correctly bias the multiple control or flange rings.

Note that the control or flange rings do not impinge on the open imaging cross section of the tube (typically 25mm to 40mm diameter) but extend into the tube volume only enough to dominate any wall effects (they simply define the internal axial electric accelerating field).

For simplicity we have shown +20KV and 10 equal resistors hence each control ring is set 2KV apart but need not necessarily be so.

One approach to gating the tube without dropping the total applied voltage to near zero from its operating (+20KV) condition is to attempt to un-bias a control orflange ring. However, attempts at so doing quickly reveal that even at the unusably low overall operating voltage of 6KV we would need to apply a negative bias to say ring B or C in isolation of several kilovoits to overcome the overall axial electric field and repulse (gate off) the photoelectrons. An even higher negative bias is required when the system is operating under uselul (15KV or more) conditions. Matters may be improved by un-biasing say two rings together (B and C) but this still this requires a negative bias with respect to the photocathode of perhaps 2 to 3KV -well beyond the easy' gating limitation of conventional semiconductor systems (1Kv). The reasons for these extremely large gating voltage is that we have to extend the repulsive electric field from the peripheral ring or flange into the centre of the tube i.e. as the negative bias is increased we tend to vignette the image first at the edges and then into the middle as the ring bias voltage becomes more and more negative.

The solution disclosed to overcome the unacceptably high gating voltage is to use multiple achievable' electronic pulsers and pulse one or more of the control rings but also in a reverse manner the photocathode By way of example figure 2 shows a typical embodiment of the design.

The standard magnetically focussed intensifier is arranged and biased as per normal but a 1KV negative going, +1KV positive biased nanosecond pulser P is attached to the photocathode A. Control rings or flanges B and C (shorted together here for simplicity) are attached to a positive going 1KV pulser 0, biased at a negative -1KV. Control ring or flange D is biased at the identical voltage as per the normal DC operation and all other voltages remain identical.

Tablet illustrates the tube gating conditions and rest (off) state. Note that to gate properly; both pulsers ideally switch simultaneously through their respective 1KV range.

Table 1. Gating Conditions Rhotocathode A Voltage Control Ring B & C voltage Control Ring D Condition +1KV (rest condition) -1KV (rest condition) 4KV (standard) Tube Gated OFF OV (-ye 1KV gating pulse) OV (+ve 1KV gating pulse) 4KV (standard) Tube Gated ON As can be seen, we have achieved a gating solution without switching large EHT voltages. Both pulser (rest state) bias votages are in effect added together to give a combined negative bias of 2KV when at rest with respect to the photo-cathode sufficient to hold the tube OFF. When both pulsers momentarily switch to their active states (say SOns duration for a Sons exposure) the tube transits to an acceptable equivalent DC bias condition and gates on.

In this embodiment of the design actual optimum voltages may vary with tube design, overall operating voltage and magnitude of gating voltages employed.

Patent Proposal / Company Confidential In a practical embodiment, it is likely for simplicity that the whole arrangement would be floated positively by 1KV such that the control rings B & C bias voltage (at rest) is at ground to simplify biasing arrangements. All other voltage differentials remain the same.

Utilizing the control rings in this manner does not cause any significant loss of performance -the axial electric field under active' or on' conditions is acceptable (the rings still ensure a symmetric, uniform field along the tube axis) giving near full tube resolution, distortion performance and full output power.

A further adaption of the design would be to design the gating voltages and electrode structure such that the gating on' or off period would be determined by the overlap period of two or more gating pulses. This would have great benefit in producing very short exposures. By example, taking the photocathode negative may be contrived to gate a device on followed by a separate positive pulse or pulses to a control electrode rings or flanges to gate the device off. The exposure time would be the overlap time of the independent positive and negative pulsers which may be accurately externally set. In this example, as only the rising or falling edge of the individual pulsers need to switch fast (possibly using avalanche circuits), makes for easier electronic pulser design.

Alternatively, a plurality of pulsers may be contrived to gate only the control electrode rings with the photocathode being left at a static bias voltage.

Utilizing this simple and more achievable gating systems, the magnetically focussed image intensifier is now suitable as a shuttering element in any high speed camera or imaging application i.e. the intensifier may act as a fast shuttering accessory to cameras such as the Shimadzu HPV1 or 2 (aM fps continuous) which requires very high output power or for the very high resolution high speed video cameras (many suppliers) with 2K x 2K resolution and not degrade image quality. Finally they may also be combined together in a Gatling gun' arrangement to make a very high resolution ultra-high speed camera typified by the DRS-Hadland Imacon 200 but at much, much higher resolution.

The features of the invention, according to the applicant, are set out as follows 1. A gating arrangement for an electro-optic device where a plurality of electronic pulsers are used to gate (switch on or off) the device, each pulser being connected to a peripheral control electrode ring or flange in the device design (not a mesh or focus cone) or a photocathode.

2. An arrangement as per paragraph 1 where the electro-optic device is a magnetically focussed image intensifier 3. An arrangement as per paragraphs 1 or 2 where two or more electronic pulsers pulse photocathodes and control electrode rings or flanges such that their combined electric fields act together to bias on or off an electro-optic device.

4. An arrangement as per paragraphs 1 or 2 where two or more electronic pulsers pulse control electrode rings or flanges such that their combined electric fields act together to bias on or off an electro-optic device.

5. An arrangement as per paragraphs 1, 2, 3 or 4 where a pulser pulses a photocathode negative while simultaneously an opposing pulser pulses a control electrode positive to gate an electro-optic device from an off to on state.

6. An arrangement as per paragraphs 1, 2, 3 or 4 where a pulser pulses a photocathode positive while simultaneously an opposing pulser pulses a control electrode negative to gate an electro-optic device from an on to an off state.

7. An arrangement as per paragraphs 1, 2, 3 or 4 where a pulser pulses a photocathode negative. An opposing pulser pulses a control electrode positive to gate an electro-optic device from an off to an on state.

8. An arrangement as per paragraphs 1, 2, 3 or 4 where a pulser pulses a photocathode positive. An opposing pulser pulses a control electrode negative to gate an electro-optic device from an on to an off state.

9. An arrangement as per claims 3 to 8 inclusive where the temporal overlap of a plurality of pulsers of either polarity determine an on' (exposure) or off' period.

10. An arrangement as per paragraphs 3 to 8 inclusive where a plurality of pulsers of either polarity are switched at differing times, their combined effect to gate the intensifier.

11. A gating system as described in any of the preceding paragraph where an electronic pulser is a semiconductor (by example a MOSFET or bipolar transistor) or triode valve derived electronic pulser creating (typically 250V to 1500V) positive or negative going voltage pulses.

12. A gating system as described in any of the preceding paragraph where an electronic pulser is a semiconductor (by example a MOSFET or bipolar transistor) acting in avalanche mode for the creation of fast rise or fall time.

13. A gating system substantially as described in the typical embodiment of the patent.

14. A camera, imaging or data acquisition system using one or more electro-optic devices any of which are gated in a manner as described by the preceding claims.

Claims (18)

1. <claim-text>Claims 1. An Electro-optic device gating arrangement comprising a plurality of electronic pulsers which, in use, are used to gate the Electro-optic device, a first pulser being operably connectable to a first control electrode ring or photocathode of the Electro-optic device; and a second pulser being operably connectable to a second control electrode ring of the Electro-optic device.</claim-text> <claim-text>2. An arrangement according to claim 1, wherein the arrangement is adapted to gate an Electro-optic device in the form of a magnetically focussed image intensifier.</claim-text> <claim-text>3. As arrangement according to either of the preceding claims, wherein the plurality of electronic pulsers pulse photocathodes and control electrode rings such that their combined electric fields act together to bias on or off the electro-optic device.</claim-text> <claim-text>4. An arrangement according to any of the preceding claims, wherein the plurality of pulsers pulse control electrode rings such that their combined electric fields act together to bias on or off an electro-optic device.</claim-text> <claim-text>5. An arrangement according to any of the preceding claims, wherein a first pulser pulses a photocathode negative while simultaneously an opposing second pulser pulses a control electrode ring positive to gate an electro-optic device from an off to on state.</claim-text> <claim-text>6. An arrangement according to any of the preceding claims ito 4, wherein a first pulser pulses a photocathode positive while simultaneously an opposing second pulser pulses a control electrode ring negative to gate an electro-optic device from an on to an off state.</claim-text> <claim-text>7. An arrangement according to any of the preceding claims ito 4, wherein a first pulser pulses a photocathode negative; and opposing second pulser pulses a control electrode ring positive to gate an electro-optic device from an off to an on state.</claim-text> <claim-text>8. An arrangement according to any of the preceding claims ito 4, wherein a first pulser pulses a photocathode positive; and opposing second pulser pulses a control electrode ring negative to gate an electro-optic device from an on to an off state.</claim-text> <claim-text>9. An arrangement according to any of the preceding claims 3 to 8, wherein the temporal overlap of a plurality of pulsers of either polarity determines an on' or off' period.</claim-text> <claim-text>10. An arrangement according to any of the preceding claims 3 to 8, wherein a plurality of pulsers or either polarity are switched at differing times, their combined effect being to gate the intensifier.</claim-text> <claim-text>11. An arrangement according to any of the preceding claims, wherein an electronic pulser comprises a semiconductor or triode valve derived electronic pulser creating positive or negative going voltage pulses.</claim-text> <claim-text>12. An arrangement according to any of the preceding claims, wherein an electronic pulser is a semiconductor acting in avalanche mode for the creation of fast rise or fall time.</claim-text> <claim-text>13. An arrangement according to claims nor 12 wherein the semiconductor is a MOSFET or bipolar transistor.</claim-text> <claim-text>14. An arrangement according to any of the preceding claims, wherein the first and second electrodes are flanges 15. An Electro-optic device gating arrangement as substantially hereinbefore described and/or illustrated in any appropriate combination of the accompanying text and/or Figures.16. An Electro-optic device incorporating, or forming part of a system comprising a gating arrangement in accordance with any of the preceding claims.17. A method of gating an Electro-optic device with a gating arrangement, comprising the steps of: * Gating a first control electrode ring or photocathode of an Electro-optic device with a first pulser; and * Gating a second control electrode ring of an Electro-optic device with a second pulser.18. A method of gating an Electro-optic device with a gating arrangement as substantially hereinbefore described and/or illustrated in any appropriate combination of the accompanying text and/or Figures.19. A camera, imaging or data acquisition system using one or more electro-optic devices any of which are gated in a manner as claimed in any of the preceding claims.AMENDMENTS TO CIMS HAVE BEEN FILED AS FOLLOWSCLaim 9 1. An Electrooptic device gating arrangement adapted to gate a magnetically focussed image intensifier comprising a pluraUty of electronic pulsers and axially aligned electrode rings which, in use, re used to gate the Electro-optic device, a first puker being operably connectable to a first control electrode ring or photocathode of the Eectrooptic device; and a second pulsar being operably connectable to a second control electrode ring or photocathode of the Electrooptc device; whereby the electric field of each ring or photocathode combines to form an uniform axiallydirected electric field.
2. 2. As arrangement according to either of the preceding claims, wherein the plurality of electronic pulsers pulse photocathodes and control electrode rings such that their combined electric fields act together to bias on or off the electro-optic device.
3. 3. An arrangement according to any of the oreceding claims, wherein the plurali of pulseN puke control electrode rings such that their combined electric fields act together to bias on or off an electrooptic device.
4. 4. An arrangement according to any of the preceding claims wherein a first pulsar pulses a photocathode negative while simultaneously an opposing second pulsar pulses a control electrode ring positive to gate an electro-optic device from an off to on state.
5. 5. An arrangement according to any of the preceding claims 1 to 3, wherein a first pulsar pulses a photocathode positive while simultaneously an opposing second pulser pulses a control electrode ring negative to gate an electro-optic device from an on to an off state.
6. 6. An arrangement according to any of the preceding cLaims 1 to 3, wherein a first puber pulses a photocathode negative; and opposing second pulser pukes a control electrode ring positive to gate an electro-optic device from an off to an on state.
7. 7. An arrangement according to any of the preceding claims 1 to 3, wherein a first pulser pulses a photocathode positive; and opposing second pulsar pukes a control electrode ring negative to gate an eiectrooptic device from an on to an off state,
8. 8. An arrangement accordhig to any of the precedtng Sims 2 to 7, wherein the temporal overlap of a plurality of putsers of either polarity determines an on' or off' period.
9. 9. An arrangement according to any of the preceding Sims 2 to 7, whereIn a plurality of puisers or either polarity are switched at differing times, their combined effect being to gate the intensifier.
10. 10. An arrangement according to any of the preceding ctaims wherein an electronic pulser comprises a semiconductor or triode valve derived electronic pulser creating positive or negative going voltage pulses.
11. 11. An arrangement according to any of the preceding claims, wherein an electronic pulser is a semiconductor acting In avalanche mode for the creation of fast rise or fall time.
12. 12. An arrangement according to claims 10 or 11 wherein the semiconductor isa MOSFET CSJ or bipolar transistor.
13. 13. An arrangement according to any of the preceding claims, wherein the first and second ° electrodes are flanges U)
14. 14. An Electro-optic device gating arrangement as substantially hereinbefore described P and/or ilLustrated in any appropriate combination of the accompanying text and/or Figures.
15. 15. An Electra-optic device incoiporating or forming pait of a system comprising a gating arrangement in accordance with any of the preceding claims.
16. 16. A method of gating an Electro-optic device with a gating arrangement adapted to gate a magnetically focused Image intensifier, comprising the steps of: F Cating a first control electrode ring or photocathode of an Electra-optic device withaftrstpuIser o Gating a second control electrode ring of an Etectro-optic device with a second putser and Combining the electric field of each ring or photocathode to form a unifomiaxially directed electric field.
17. 17. A method of gating an Etectro-optic device with a gating arrangement as substantially hereinbefore described and/or illustrated in any appropriate combination of the accompanying text and/or Ftgures.
18. 18. A camera, imaging or data acquisition system using one or more etectro-optic devices any of which are gated in a manner as claimed in any of the preceding claims. Cs" U) C"</claim-text>