Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1999-03-03
2000-10-31
Lee, John R.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250214VT, 250214LA, H01J 2998, H01J 3150
Patent
active
061406287
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a fast power supply for an image-intensifier tube such as those used in night vision devices.
BACKGROUND OF THE INVENTION
In a system for assisting with night-time vision, an image-intensifier tube makes it possible to deliver an image of the outside world similar to that obtained under daytime conditions. Such a system may consist of night vision sights or binoculars which are placed in front of the observer's eyes and are designed so that an intensified image of the outside world is formed on the retina.
These sights or binoculars may be independent or, for example, integrated in an aircraft pilot's helmet.
A system for assisting with night-time vision may also comprise a light-intensifier tube associated with a CCD camera and a display device.
In the known way, an image-intensifier tube generally comprises a photocathode placed behind an input window of the tube, a microchannel wafer and an output screen. It operates in the following way: window of the tube and enter the photocathode, and cause the latter to emit electrons which are accelerated between the photocathode and the input of the wafer. wafer by a phenomenon involving multiple collisions against the walls of the channels, highly positive potential of the output screen and strike the latter's photoemissive layer, which causes the emission of photons and the reproduction of a visible image at the output.
Ambient luminosity which is too weak to allow visibility to the naked eye can thus be sufficient to obtain a visible image at the output of the image-intensifier tube.
The accelerations and the multiplication of the electrons in the tube are due to the application of appropriate voltages to various elements of the image-intensifier tube, such as the photocathode, the microchannel wafer and the output screen. These voltages are delivered by the electrical power supply of the tube.
An advantageous power supply has to comply with a variety of constraints which are difficult to reconcile.
Firstly, when the ambient luminosity is very weak, the power supply must allow enough light intensification to present a visible image to the operator using the system for assisting with night-time vision.
Furthermore, when the ambient luminosity is too strong, the power supply must allow protection of the tube to prevent, for example, damage to the output screen or the microchannel wafer an excessive number of, or too highly energetic electrons. In this case, the power supply may fully cut off the light intensification or very greatly reduce the gain of the tube.
In the intermediate range of values of ambient luminosity, the power supply must also make it possible to control the gain of the tube in order to intensify the light received at the input of the tube while avoiding the presentation of too bright an image to the operator at the output of the tube.
An advantageous power supply will automatically ensure, on the output screen of the intensifier tube, an image with approximately constant average illumination irrespective of the value of the ambient luminosity above a minimum threshold. It should as far as possible react quickly enough to prevent dazzle in case of an abrupt increase in the outside luminosity, and to prevent "black holes" in case of an abrupt drop in luminosity. The human eye is very sensitive to excessively strong luminosity, even over a very short time (of the order of a few hundred microseconds); it furthermore reacts slowly to adapt to an abrupt decrease in luminosity.
With current devices, and under some working conditions, the reaction time of the device when confronted with abrupt variations in luminosity is too high, on the one hand in the case of increase but, on the other hand, above all in case of decrease. These various requirements impose stringent constraints on the voltage supplies of the elements of the image intensifier.
Furthermore, an additional constraint comes from the fact that a system for assisting with night-time vision may be used, for example, by an aircraft pilot or by a land vehic
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Fauvel Eric
Sontag Yves
Lee John R.
Sextant Avionique
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