Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-02-22
2001-08-07
Evans, F. L. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S207000, C313S1030CM
Reexamination Certificate
active
06271511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of night vision devices (NVD's) of the light-amplification type. Such NVD's employ an image intensifier tube (I
2
T) to receive photons of light from a scene. This scene may be illuminated by full day light; or alternatively, the scene may be illuminated with light which is either of such a low level, or of such a long wavelength (i.e., infrared light), or both, that the scene is only dimly visible or is effectively invisible to the natural human vision. The I
2
T of such an NVD responsively provides a visible image replicating the scene. The present I
2
T has an optimized microchannel plate (MCP), which provides a combination of high resolution, efficiently achieved electron gain level, and lowed requirement for operating voltage. This combination was previously unobtainable in the art.
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 for 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 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 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 (MCP) having a great multitude of microchannels, each of which is effectively a dynode. That is, these microchannels have an interior surface substantially defined by a material providing a high average emissivity of secondary electrons. In other words, each time an electron (whether a photoelectron or an 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 cascade of secondary-emission electrons (which provide substantially a geometric multiplication in response to the photoelectrons) moving along the microchannels, from one face to the other of the MCP. The result is a spatial output pattern of electrons from the MCP (which replicates the input pattern; but at a very considerably higher electron density) issuing 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 photocathode impacts on and is absorbed by the phosphorescent screen electrode, a visible image is produced. This visible image is passed out of the tube through a transparent image-output window for viewing.
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, other sources of electrical power may be utilized to operate NVD's.
A goal that has long existed in the art of night vision devices is to improve the resolution provided by devices using I
2
T's. The resolution of NVD's using an I
2
T is essentially determined by the resolution of the tube itself, and this tube resolution is strongly influenced by the size and spacing dimension of the microchannels in the MCP of the I
2
T. As a result, the art has sought over many years to progressively make both the size and the spacing dimension of the microchannels in MCP's of I
2
T's smaller and smaller. However, this effort has met with only limited success prior to this invention.
Several examples of contemporary MCP's (these being of 18 mm nominal diameter), their thickness, channel size, channel spacing dimension, open area ratio (OAR), and length-to-diameter (L/D) ratio may be seen in the following table.
TABLE 1
MCP
channel
channel
L/D
Thickness
size
spacing
OAR
ratio
Example 1
16.3 mil
8.0
&mgr;
10.9
&mgr;
48.9%
51.8:1
Example 2
12.3 mil
8.1
&mgr;
9.7
&mgr;
63.3%
38.6:1
Example 3
12.3 mil
8.0
&mgr;
8.9
&mgr;
73.3%
39.1:1
Example 4
12.0 mil
4.6
&mgr;
5.8
&mgr;
57.1%
66.3:1
Example 5
12.6 mil
4.7
&mgr;
5.93
&mgr;
57.0%
68.4:1
Example 6
11.3 mil
4.86
&mgr;
5.96
&mgr;
60.3%
59.1:1
Example 7
12.3 mil
4.9
&mgr;
5.9
&mgr;
62.6%
63.8:1
As can be readily seen from the above actual examples of the conventional art, conventional MCP's with relatively low resolution by current standards (i.e., with microchannels of about 8&mgr;size) may achieve a L/D (i.e., length-to-diameter) ratio for the microchannels of about 40 or a little less, with an OAR (open area ratio—expresses as a percentage) for the MCP of from the low 60's to the low 70's. However, such MCP's do not provide the image resolution desired for future NVD's. On the other hand, conventional MCP's with improved resolution (i.e., with microchannels of about 5&mgr;size) do not have quite as good of OAR, and have excessive L/D ratios (i.e., of about 60 or higher). The L/D ratio of a MCP is also an indication of the thickness of the MCP, since the microchannels extend from one face of the MCP to the other. It is also an indication, generally, of the required operating voltage for the MCP, since this voltage increases with increased thickness of the MCP.
Those ordinarily skilled in the art will understand that reductions in both microchannel size and microchannel spacing dimensions also have desirable and beneficial effects on other image quality factors. Two of these image quality factors that are favorably affected by reductions in microchannel size and spacing dimensions are known as fixed pattern noise and dark-multi-boundary noise. These image quality factors have to do with the “graininess” and mosaic effect visible in the image produced by an image intensifier tube.
A limiting problem with conventional I
2
T's is the desirable corresponding decrease in thickness of the MCP at the same time that the microchannels are made smaller. That is, the microchannels of a conventional MCP have a length (L) to diameter (D) ratio that conventionally falls within a selected range in order for the MCP to provide a desired level of electron gain. Because the differential voltage (i.e., the operating voltage) of an MCP depends in large part on its thickness, making MCP's thinner would have a beneficial effect because their required operating voltages would be lower. However, conventional MCP's cannot be made as thin as desired because if they are so thin, then they will not survive the conventional manufacturing processes, or will not have sufficient strength to survive common effects, such as: physical shocks, vibrations, handling, and thermal cycling, all of which they are necessarily subjected to in manufacturing and in use.
Accordingly, even those conventional MCP's, which presently have a microchannel size and spacing dimension that is larger than desired for better resolution, must still be made thicker than desired, and they also require a correspondingly highe
Evans F. L.
Litton Systems Inc.
Marsteller & Associates, P.C.
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