Television – Camera – system and detail – Low light level
Patent
1992-11-16
1994-08-23
Mancuso, Joseph
Television
Camera, system and detail
Low light level
348255, 348673, 348678, H04N 5225, H04N 557
Patent
active
053412192
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device and method for controlling the contrast in an image observation system comprising an image intensifier provided with a microchannel plate and an image converter which converts the output image of the image intensifier into a video signal or a visible image.
2. Brief Description of the Prior Art
Such systems are already known and an image intensifier intended for use in such a system and having a microchannel plate, hereinafter called an MCP for brevity, is disclosed, for example, in "Microchannel Plates" Mullard Technical publication M81-0151, Mullard, Redhill, Surrey, GB and in "Image Tubes" by Illes P. Csorba, Howard W. Sams and Co. Inc. (1985), Indianapolis, U.S.A. The converter may, for example, be a CCD (charge coupled device) with associated read-out electronics which delivers a video signal for display on a video screen and which, for example, observes the image of the image intensifier via an optical fibre plate or relay optics. Such a system, which therefore forms a television camera system having an image intensifier placed in front of the actual imaging device is in fact called a "low light level television camera" and will therefore hereinafter be called an LLLTV camera for the sake of brevity. The image converter may, however, also be, for example, an optical system (for example, a magnifying glass) or an additional image intensifier in a night viewer.
A drawback of the known systems is that the contrast control they contain is not adequate in those cases in which a dark scene is observed in which one or more bright points, so-called bright spots, are present. Existing contrast control systems are, in fact, based on the average brightness and in such a case they therefore attempt to keep the average brightness constant, as a result of which the dark scene portions, which often contain the information of interest to an observer, become still darker, and this is precisely what is undesirable. In addition, in the case of a video signal, the existing contrast control system is designed to adjust back the brightness in the event of bright peaks in the video signal, regardless of the average brightness, and this only increases the problem outlined.
SUMMARY OF THE INVENTION
The object of the invention is to offer a solution to this problem and for this purpose, it provides a device of the abovementioned type which is provided with means for determining a control signal which is representative of the contrast range in the output signal of the converter and with means of controlling the gain of the image intensifier and the gain of the image converter in a manner such that, if the control signal has a first value which is representative of a large contrast range in the output signal of the converter, the gain of the image intensifier is high and that of the image converter is low, and if the control signal has a second value which is representative of a low contrast range in the output signal of the converter, the gain of the image intensifier is low and that of the image converter is high.
The invention also provides a method in which the voltage applied across the microchannel plate is increased and the gain of the image converter is reduced if the image formed by the image converter has a large contrast range and in which the voltage applied across the microchannel plate is reduced and the gain of the image converter is increased if the image formed by the image converter has a low contrast range.
The measures according to the invention make it possible, in the manner to be explained in more detail below, to control the contrast of the image formed by the converter on the basis of a dark portion or the darkest portion in the observed scene, the brighter spots in the scene, which can already often be observed even without which image intensifier, not affecting the total gain and said brighter spots not adversely affecting the image quality for the others.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will
REFERENCES:
patent: 3691302 (1972-09-01), Gaebele et al.
patent: 3823263 (1974-07-01), Guppy
patent: 3848085 (1974-11-01), Van Atta
Schelen Johannes B. J.
van den Berg J.
B.V. Optische Industrie "De Oude Delft"
Greening Wendy R.
Mancuso Joseph
Marn Louis E.
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