Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-12-03
2003-08-19
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S239000
Reexamination Certificate
active
06608298
ABSTRACT:
FIELD OF INVENTION
The present invention relates to optical sights, in particular to telescopic optical sights equipped with a switchable image intensifier tube, which can be switched between the non-operative position for use of the telescopic optical sight alone or to an operative position in which the tube can be used in combination with the telescopic sight for enhancing operation of the latter.
BACKGROUND OF THE INVENTION
There exist a great variety of optical sights of different types, in particular for application on hunting, combat, or training weapons. In general a sight is utilized for aiming a weapon during daytime operation at a directly visible target and during nighttime operation at a target, which is only visible through the use of some vision-aiding devices.
Daytime sight devices range from simple front and rear sights like those on ordinary rifles to complex optical systems in combination with laser range finders and laser aiming devices. Certain sophisticated types such as telescopes are utilized to magnify a target and to enable the user during normal daytime operation to view a magnification of the target area at which the viewer is aiming.
There are also nighttime vision devices or sights, which employ image intensifiers or similar structures. The function of an image intensifier is to multiply the amount of incident light received by it to produce a signal that is bright enough for presentation to the eyes of a viewer. As such, these devices have been employed by the military and in commercial products as well. Sights vary in size, magnification, type of reticle, weapon application and level of performance.
An image intensifier tube (IIT) is a vacuum photoelectronic device intended either for transformation of an invisible IR, UV, or X-ray image of an object into a visible image or for intensification of a visible image. An IIT normally consists of a photocathode, an image intensification system, and a cathode-luminescent screen. The photocathode transforms the original optical image into a so-called electronic image. With the use of the image-intensifying system, the electronic image is transferred to the screen where this image, in turn, is converted into a visible original image. In the IIT, the light reflected from the object causes emission of electrons (photocurrent) from the surface of the photocathode. In this case, a magnitude of photocurrent generated by various areas of the photocathode depends on distribution of density of images projected onto these areas. Photoelectrons accelerated and focused by the IIT's field, bombard the screen, thus causing it to luminesce. Since brightness on individual areas of the screen depends on density of the photocurrent, the screen reproduces a visible image of the object.
In its simplest form an IIT consists of two parallel electrodes, i.e., a photocathode and a screen, between which a voltage is applied. In a uniform electrostatic field of such an IIT, electrons are practically not focused (the electrons move along parabolas having parameters dependent on initial velocities of the electrons). For focusing of electrons, the IIT with a uniform electrostatic field is placed into a uniform magnetic field having the same direction as the electric field. In this case, the electrons emitted from individual points of the cathode begin to move along periodically converging spiral paths rather than along the diverging parabolas. The use of immersion-type electrostatic lenses makes it possible to obtain a good electronic image, even without the use of a magnetic field.
In an IIT, intensification of the original image is achieved due to additional acceleration imparted to the electrons as well as due to compression of the electronic image. In this case, brightness is also increased with a factor of 1/B
2
, where B is an optolectronic magnification. Brightness is increased with the use of a multiple-stage IIT, which comprises several IITs connected in series. From the screen of the first IIT, a luminous flow is directed to a photocathode of the second IIT, etc. Normally, multiple-stage IITs are encapsulated into a common shell. In order to prevent significant loss in resolution capacity, a thickness of a transparent partition between the stages should not exceed 5 to 10 &mgr;m.
Application of optical fiber plates makes it possible to connect individual IITs via direct optical contact between the surfaces of the plates. Multiple-stage IITs provide the maximum possible amplification of brightness when the output cathode-luminescent screen reproduces elements separately emitted from the photocathode. An IIT with a microchannel plate provides intensification of brightness close to the maximum possible limit. A microchannel plate is a glass plate with several million channels (having diameters within the range of 5 &mgr;m to 15 &mgr;m) with a voltage of about 1 kV applied to the end faces of this plate. In such an IIT, the electronic image is aligned with an input surface of the microchannel plate and is divided by the channels into separate elements. On its way through the channels, the electron flow of each element is multiplied by 10
3
to 10
4
times due to secondary emission of electrons caused by collision of the electrons with the walls of the channels. The obtained electronic image of increased density is transferred to the screen.
A basic parameter of an IIT is an integral sensitivity, which is a ratio of the photocurrent to a value of a light flow incident on the photocathode. For example, in an IIT with an oxygen-silver-cesium cathode intended for conversion of images in infra-red rays with the wavelength of 1.3 &mgr;m, image sensitivity may reach 50 mkA/lumen. A multiple-alkaline photocathode which contains compounds of Sb with Cs, K, and Na and which is used in IIT for amplification of a visible image, provides integral sensitivity up to 400 mkA/lumen. Other basic parameters are a resolution capacity (which is determined by the amount of separately seen black-and-white lines or dots per unit of length and which is within the range of 25 to 60 mm
−1
, or higher); a coefficient of transformation (a ratio of the luminous flow emitted from the screen to the luminous flow incident on the photocathode and which reaches several hundred in single-stage IITs and 5×10
4
in multiple-stage IITs; and time resolution, which in latest IITs reaches 10
−12
sec. Among other applications, the parameters listed above make it possible to use IITs also in night-vision systems, such as optical arm sights, as well as in range finders utilizing back-light systems for pulse illumination of objects, where illumination pulse may have time resolution of up to 10
−12
sec.
In general, an IIT alone is unsuitable for use as an optical sight because search of remote targets requires optical magnification and superposition of the target image onto the photocathode of the IIT. The above function is fulfilled by an objective lens, which isolates the area of interest and magnifies the target found in this area. Matching of the image reproduced by IIT with the pupil of the viewer's eye requires the use of an eyepiece. Thus, the IIT used in the optical sight comprises a complex optoelectronic system, which consists of an IIT per se, an objective lens unit, an eyepiece lens unit, and electronics.
Since it is advantageous to use the same weapon with the daytime and nighttime vision devices, many contemporary weapons are provided with possibility of installing both a daytime or nighttime sights.
For example, U.S. Pat. No. 4,822,994 discloses a configuration in which the front end of a telescopic sight is separable from the rest of the sight. For nighttime use an image intensifier module is inserted between the sections. However, for daytime operation, the user must disassemble the sight and remove and store the image intensifier module.
U.S. Pat. No. 4,629,295 is directed to another type of night viewing device, which is an add-on accessory to day rifle sights. This device is attached directly to the day rifle scope. I
American Technologies Network Corporation, Inc.
Le Que T.
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