This specification relates to the field of image intensifiers and more particularly to an image intensifier assembly with an adjustable figure of merit.
Image intensifiers for use in night vision systems commonly use a measurement called Figure of Merit (FOM) for image quality. FOM is the arithmetic product of the resolution, measured in line pairs per millimeter (lp/mm) and signal-to-noise ratio (SNR), which is unitless. Resolution typically varies in the range of 50 to 72 lp/mm. SNR typically varies in the range of 20 to 25. So FOM typically varies in the range of 1,000 to 1,800, with a higher FOM generally representing a superior overall image quality.
FOM may be important in some contexts because the United States government regulates the export of night vision systems by requiring that exported items have a FOM below a specified threshold. A common method of varying FOM is to provide a power supply that supplies a lower photocathode voltage, thus degrading both resolution and SNR.
In one aspect, an image intensifier for a night vision system may include a tube module and a power supply module. The image quality may be characterized by a figure known as the “Figure of Merit” (FOM), which is an arithmetic product of center limiting resolution and signal-to-noise ratio (SNR). Both resolution and SNR are affected by the voltage supplied by the power supply module. By providing a power supply module with an adjustable photocathode voltage, the FOM may be varied. The adjustment mechanism may then be rendered tamper resistant, thereby enabling the FOM to be permanently reduced for devices intended for export.
An adjustable FOM is useful for creating image intensifiers that are suitable for export outside of the United States. An image intensifier according to the disclosure in this specification may include a mechanically-variable adjustment mechanism such as an adjustment screw, typically as part of an adjustment potentiometer.
Generally, a tube and a power supply may be encapsulated in a single package as an image intensifier for use with a night vision system. Because of variances inherent in the production process, it is difficult to predict in advance what FOM will be achieved by joining a particular tube to a particular power supply. But a mechanically-variable power supply can be joined with any tube and the FOM can then be adjusted accordingly. If a device is intended for export, the entire image intensifier assembly can be encapsulated in epoxy to ensure that the power supply cannot be separated from the tube without damage. After the final FOM adjustment, the adjustment screw or other mechanically-variable adjustment mechanism can be disabled. For example, an adjustment screw may be set with epoxy to ensure that it is securely fastened in position. The screw head can then be ground out to prevent any further manipulation, and the adjustment port can finally be back-filled with epoxy to prevent tampering.
An image intensifier with adjustable figure of merit will now be described with more particular reference to the attached drawings. Hereafter, details are set forth by way of example to facilitate discussion of the disclosed subject matter. It should be apparent to a person of ordinary skill in the field, however, that the disclosed embodiments are exemplary and not exhaustive of all possible embodiments. Throughout this disclosure, a hyphenated form of a reference numeral refers to a specific instance or example of an element and the un-hyphenated form of the reference numeral refers to the element generically or collectively. Thus, for example, 102-1 may refer to a “pen,” which may be an instance or example of the class of “writing implements.” Writing implements may be referred to collectively as “writing implements 102” and any one may be referred to generically as a “writing implement 102.”
In this embodiment, the opt-coupler is capable of enduring any or all of the following specified parameter ratings for unlimited periods of time without any permanent degradation resulting in failure to meet the required specification:
In this embodiment, the electrical characteristics are as follows (all at an operating temperature of 23° C. unless otherwise noted):
Opto-coupler 320 is capable of meeting all requirements after being subjected to 12 temperature cycles from +85° C. to −55° C. with two hour soaks at each temperature. The transition rate from −55° C. to +85° C. and from +85° C. to −55° C. is 3° C. per minute minimum. This requirement may be satisfied through sampling a single month's production per ANSI/ANSQCZI1.4-1993 Inspection Level S-1, 1.0 AQL.
As disclosed in this specification, a single batch of image intensifiers, built from a batch of tube modules and power supply modules, may be produced for both domestic use and export. After production is complete, some of the image intensifiers will be selected for export, and the FOM will be adjusted and fixed as described herein. A FOM may also be selected for image intensifiers identified for domestic use by providing the maximum possible photocathode current. To make the image intensifiers resistant to tampering, for example by an enemy wanting to increase the photocathode voltage to increase FOM, by disabling the adjustment mechanism. In one embodiment, the method of disabling includes backfilling the adjustment port with epoxy or other rigid material that will fix the adjustment mechanism in place. This renders the FOM fixed for that image intensifier. And because the entire image intensifier is encapsulated in epoxy, a new power supply cannot be substituted to increase the FOM. In a non-adjustable power supply, there are sometimes two holes in this epoxy that provide access to two adjustment screw-heads that are used to adjust two operating parameters (not FOM) of the night vision tube. After adjustment, these two cavities may be backfilled with soft RTV so that these adjustment screw heads are not accessible. But in some embodiments, epoxy or other more rigid material may be preferred for backfilling the adjustment mechanism for the photocathode voltage. For further security, after setting the adjustment mechanism in epoxy, part of the epoxy may be drilled out to provide access to the adjustment interface (such as a screw head on a potentiometer), which may also be disabled, for example by drilling out the screw head. The drilled-out adjustment port may then again be backfilled with epoxy. This provides redundant security for the adjustment mechanism.
Persons having skill in the art will recognize that there are numerous other methods for providing an adjustable voltage that can be disabled. For example, the mechanical adjustment mechanism, such as a screw, may receive a light coat of slow-acting adhesive (such as epoxy) before insertion, so that its position will be fixed after a time. In other embodiments, inserts may be used to physically block access to the port. In yet other embodiments, an electronic adjustment may be provided, and lead wires may be clipped or otherwise disabled before final encapsulation in epoxy, making the final product tamper resistant.
While the subject of this specification has been described in connection with one or more exemplary embodiments, it is not intended to limit the claims to the particular forms set forth. On the contrary, the appended claims are intended to cover such alternatives, modifications and equivalents as may be included within their spirit and scope.
This application claims the benefit of U.S. provisional application 60/967,475, filed Sep. 5, 2007 and entitled “Image Intensifier with Adjustable Signal-to-Noise Ratio and Imaging Resolution and Method for its Operation and Production,” which is hereby incorporated by reference.
Number | Date | Country | |
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60967475 | Sep 2007 | US |