Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-06-04
2001-10-02
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C313S1030CM
Reexamination Certificate
active
06297494
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to vision systems and more particularly to a method and system for enhanced vision employing an improved image intensifier with a gated power supply and reduced halo.
BACKGROUND OF THE INVENTION
Image intensifier tubes are used in night vision devices to amplify light and allow a user to see images in very dark conditions. Night vision devices typically include a lens to focus light onto the light receiving end of an image intensifier tube and an eyepiece at the other end to view the enhanced imaged produced by the image intensifier tube.
Modern image intensifier tubes use photocathodes. Photocathodes emit electrons in response to photons impinging on the photocathodes. The electrons are produced in a pattern that replicates the original scene. The electrons from the photocathode are accelerated towards a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of microchannels, each one operable to produce a cascade of secondary electrons in response to an incident electron.
Therefore, photons impinge on the photocathode producing electrons which are then accelerated to a microchannel plate where a cascade of secondary electrons are produced. These electrons impinge on a phosphorous screen, producing an image of the scene.
A drawback to this approach is that the electrostatic fields in the image intensifier are not only effective in accelerating electrons from the photocathode to the microchannel plate and from the microchannel plate to the screen, but also move any positive ions back to the photocathode at an accelerated velocity. Current image intensifiers have a high indigenous population of positive ions. These are primarily due to gas ions in the tube, including in the microchannel plate and the screen. These include both positive ions and chemically active neutral atoms. When these ions strike the photocathode, they can cause both physical and chemical damage. This leads to short operating lives for image intensifiers.
To overcome this problem, an ion barrier film can be placed on the input side of the microchannel plate. This ion barrier is able to block the ions from the photocathode. One drawback of the ion barrier is that it reduces the signal-to-noise ratio of the image intensifier. This is due to the fact that the barrier prevents low energy electrons from reaching the microchannel plates. Another drawback of the ion barrier film is that it contributes to a halo effect in the image produced by the image intensifier tube. In addition, modern image intensifier tubes have a relatively large gap between the photocathode and the microchannel plate. This gap also contributes to the halo effect problem.
Therefore, current image intensifiers require an ion barrier since current manufacturing techniques fail to remove enough gas molecules. But the presence of the ion barriers reduces the signal-to-noise ratio and contributes to the halo effect. What is needed is an unfilmed microchannel plate that has a sufficient number of gas ions removed such that an image intensifier manufactured with such a microchannel plate has a usable life.
Modern image intensifier tubes also provide automatic brightness control (ABC) and bright source protection (BSP). ABC maintains a relatively constant level of brightness in the image produced by the image intensifier tube despite fluctuating levels of brightness in the scene being viewed. BSP prevents the image intensifier tube from being damaged by high levels of current that may otherwise be generated in response to an extremely bright source.
Conventional image intensifier tubes provide ABC and BSP by adjusting the voltage level of the microchannel plate. Thus, for a brighter scene, the voltage to the microchannel plate is reduced. A drawback to this approach is that the image intensifier tube loses resolution as the voltage to the microchannel plate is reduced. What is needed is an image intensifier tube that provides ABC and BSP without a loss in resolution as the brightness increases.
SUMMARY OF THE INVENTION
In accordance with the present invention, the disadvantages and problems associated with previous image intensifiers have been substantially reduced or eliminated. In particular, the present invention provides a method and system for enhanced vision employing an improved image intensifier with a gated power supply and reduced halo.
In one embodiment, a method is provided for detecting photons and generating a representation of an image. A photocathode receives photons from an image. A power supply to the photocathode is gated such that the photocathode is switched between an on state and an off state. The photocathode discharges electrons in response to the received photons while the photocathode is in the on state. A microchannel plate is located no more than about 125 microns from the photocathode. The microchannel plate has an unfilmed input face and an output face that receives the electrons from the photocathode. The microchannel plate produces secondary emission electrons which are emitted from the output face. A screen receives the secondary electrons and displays a representation of the image.
Technical advantages of the present invention include providing an image enhancer with improved automatic brightness control and bright source protection. In particular, an image enhancer provides automatic brightness control and bright source protection without a significant loss in resolution as brightness increases. Another technical advantage of the present invention includes providing an image enhancer with reduced halo. In particular, an image enhancer provides a reduced halo by reducing the gap between the photocathode and the microchannel plate and by using an unfilmed microchannel plate.
Other technical advantages of the present invention will be readily apparent to those skilled in the art from the following figures, descriptions and claims.
REFERENCES:
patent: 4024390 (1977-05-01), Bosserman et al.
patent: 5776538 (1998-07-01), Pierle et al.
patent: 6072565 (2000-06-01), Porter
Estrera Joseph P.
Iosue Michael J.
Saldana Michael R.
Baker & Botts L.L.P.
Le Que T.
Litton Systems Inc.
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