Electric lamp and discharge devices – Photosensitive – Secondary emitter type
Reexamination Certificate
1999-05-07
2002-11-19
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Secondary emitter type
C313S542000, C313S1030CM, C313S1050CM, C313S530000, C313S544000, C250S2140VT, C250S207000
Reexamination Certificate
active
06483231
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of night vision devices. More particularly, the present invention relates to a night vision device which uses an image intensifier tube to amplify light from a scene. This light may be too dim to be seen with natural human vision, or the scene may be illuminated substantially only by infrared light which is invisible to human vision. The image intensifier tube both amplifies the image from the scene and shifts the wavelength of the image into the portion of the spectrum which is visible to humans, thus to provide a visible image replicating the scene. Still more particularly, the present invention relates to such an image intensifier tube having a unitary ceramic body portion, as well as a photocathode and a microchannel plate spaced from one another to define a spacing dimension, this dimension being established by structure extending axially between the photocathode microchannel plate, and establishing this spacing dimension independently of tolerances and variability's of the other components of the image intensifier tube. Methods of making of operating such an image intensifier tube are presented.
2. Related Technology
Even on a night which is too dark for natural human vision, invisible infrared light is richly provided in the near-infrared portion of the spectrum by the stars of the night sky. Human vision cannot utilize this infrared light from the stars because the infrared portion of the spectrum is invisible to humans. Under such conditions, a night vision device (NVD) of the light amplification type can provide a visible image replicating a night-time scene. Such NVD's generally include an objective lens which focuses invisible infrared light from the night-time scene through the transparent light-receiving face of an image intensifier tube (I
2
T). At its opposite image-output face, the I
2
T provides a visible image, generally in yellow-green phosphorescent light. This image is then presented via an eyepiece lens to a user of the device.
A contemporary NVD will generally use an I
2
T with a photocathode (PC) behind the light-receiving face of the tube. The PC is responsive to photons of visible and infrared light to liberate photoelectrons. Because an image of a night-time scene is focused on the PC, photoelectrons are liberated from the PC in a pattern which replicates the scene. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate having a great multitude of microchannels, each of which is effectively a dynode. These microchannels have an interior surface at least in part defined by a material liberating secondary-emission electrons when photoelectrons collide with the interior surfaces of the microchannels. In other words, each time an electron (whether a photoelectron or a secondary-emission electron previously emitted by the microchannel plate) collides with this material at the interior surface of the microchannels, more than one electron (i.e., secondary-emission electrons) leaves the site of the collision. This process of secondary-electron emissions is not an absolute in each case, but is a statistical process having an average emissivity of greater than unity.
As a consequence, the photoelectrons entering the microchannels cause a geometric cascade of secondary-emission electrons moving along the microchannels, from one face of the microchannel plate to the other so that a spatial output pattern of electrons (which replicates the input pattern; but at an electron density which may be, for example, from one to several orders of magnitude higher) issues from the microchannel plate.
This pattern of electrons is moved from the microchannel plate to a phosphorescent screen electrode by another electrostatic field. When the electron shower from the microchannel plate impacts on and is absorbed by the phosphorescent screen electrode, visible-light phosphorescence occurs in a pattern which replicates the image. This visible-light image is passed out of the tube for viewing via a transparent image-output window.
The necessary electrostatic fields for operation of an I
2
T are provided by an electronic power supply. Usually a battery provides the electrical power to operate this electronic power supply so that many of the conventional NVD's are portable.
However, the electrostatic fields maintained within a conventional image intensifier tube, which are effective to move electrons from the photocathode to the screen electrode, also are unavoidably effective to move any positive ions which exist within the image intensifier tube toward the photocathode. Because such positive ions may include the nucleus of gas atoms of considerable size (i.e., of hydrogen, oxygen, and nitrogen, for example, all of which are much more massive than an electron), these positive gas ions are able to impact upon and cause physical and chemical damage to the photocathode. An even greater population of gas atoms present within a conventional image intensifier tube may be electrically neutral but also may be effective to chemically combine with and poison the photocathode.
Conventional image intensifier tubes have an unfortunately high indigenous population of gas atoms within the tube—both those gas atoms which become positive ions and those much more populous atoms that remain electrically neutral but are possible of chemically reacting within the tube. Historically, this indigenous population of gas atoms resulted both in the impact of many positive ions on the photocathode, and in chemical attack of the photocathode. With many early-generation I
2
T's, this resulted in a relatively short operating life.
As those ordinarily skilled in the pertinent arts will understand, later generation I
2
T's of the proximity focus type have partially solved this ion-impact and chemical reaction problem by providing an ion barrier film on the inlet side of the MCP. This ion barrier film both blocks the positive ions and prevents them form damaging the PC, and inhibits the migration of chemically active atoms toward the PC. However, the ion barrier film on a MCP is itself the source of many disadvantages.
A recognized disadvantage of such an ion barrier film on an MCP is the resulting decrease in effective signal-to-noise ratio provided by the MCP between a PC of an I
2
T and the output screen electrode of the tube. That is, although the material of the ion barrier film itself acts as a secondary emitter of electrons, but only for those electrons of sufficient energy. Electrons of lower energy may be absorbed by the ion barrier film, so that this ion barrier film acts to prevent these low energy electrons from reaching the microchannels of the MCP. Secondary-emission electrons typically have a comparatively low energy. Recalling that about 50% of the electron input face of a MCP is open area, and about the same percentage is defined by the solid portion or web of the microchannel plates, it is easily appreciated that about half of the photoelectrons impact on the web of the MCP. Moreover, these photoelectrons which impact the web of the MCP result in the production of secondary emission electrons closely adjacent to the open areas of the MCP, and with low energies. These low-energy electrons lack the energy to either penetrate the ion barrier film, or to cause this film to liberate secondary electrons. So these low energy electrons are absorbed by the ion barrier film. The result is that in some cases, as much as 50% of the electrons that would otherwise contribute to the formation of an image by the I
2
T are blocked or absorbed by the ion barrier film and do not reach the microchannels to be amplified as described above. Thus, about the same percentage of the image information which theoretically could be provided by the tube is lost.
Another disadvantage of the ion barrier film is that it contributes to halo effect in the image provided by the conventional image intensifier tube. This halo effect may be visualized as photoelectrons incident on th
Litton Systems Inc.
Marsteller & Associates, P.C.
Patel Ashok
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