Optical: systems and elements – Lens – With light limiting or controlling means
Reexamination Certificate
1999-09-14
2001-01-30
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With light limiting or controlling means
C359S355000, C359S407000, C359S733000
Reexamination Certificate
active
06181486
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of optical architectures for an infrared vision system, which can be applied especially to a pair of binoculars or to a camera.
2. Dissussion of the Background
In a known manner, an optical architecture for an infrared vision system generally includes a front afocal, one of the functions of which is to collect the flux from a scene, an optical device forming an image of the scene and a cooled detector with its cold stop.
So-called first-generation cameras include scanning means which scan the scene two-dimensionally, that is to say horizontally and vertically. The detector is a quasi-point detector, that is to say a small matrix of elementary detectors having one or a few of these elementary detectors in each of two dimensions—horizontal and vertical. This detector therefore operates in AC mode, that is to say by eliminating the continuous component; it therefore takes into account only the variations in the signal during the scanning.
So-called second-generation cameras include a front afocal preferably of the Kepler type, scanning means which preferably include a mirror rotating about a vertical axis of the scene and allowing horizontal scanning of the scene, an optical device forming an image of the scene and a detector of the linear-array type, that is to say having a large number of elementary detectors in the vertical direction covering the entire scene in the vertical dimension, with a TDI effect (Time Delay Integration: that is to say the linear array is in fact a matrix having a few elementary detectors in the horizontal direction which “see” in succession the same point of the scene and the measured values of which, for the same point, are averaged). Throughout the rest of the text, the mirror of the scanning means will be referred to as the “scanner”. The detector operates in DC mode, that is to say by preserving the continuous component of the signal, so as not to lose contrast of the scene in the vertical direction. These cameras exhibit good photometric performance but require, on the one hand, the use of a re-imager which is composed of several optical elements, usually lenses, and the function of which consists in transporting the image of the aperture stop, which is at the cold stop, onto the scanner. On the other hand, these cameras require the use of a reference temperature which, for example may consist of a black body which will be “seen” by the detector by virtue of a slight overscanning of the scene by the scanning means, whereby allowing the elementary detectors of which the detector is composed to be homogenized. The above two components—re-imager and temperature reference—are expensive and bulky components.
U.S. Pat. No. 4,972,085 describes an example of an optical architecture according to the prior art, having a re-imager producing an image close to the cold screen.
Hence the idea of devising a simplified optical architecture while preserving the photometric properties of second-generation cameras.
This means using unconventional solutions to solve the usual problems, which will therefore make it possible to achieve very compact combinations for portable and high-performance equipment. The main characteristic resides in the omission of the very expensive and very bulky re-imager; this inevitably results in a certain degradation of the photometric performance. It is then possible to make the architecture photometrically sound by a few modifications of the original optical architecture, in particular by modifying the shape of the cold of the cooled detector.
SUMMARY OF THE INVENTION
A preferred form of the invention may be summarized in the following manner.
For cost and bulkiness reasons, the re-imager is omitted. Since the aperture stop is no longer imaged on the scanner, the size of the latter has a tendency to increase. Throughout the rest of the text, the term “stop” will be used to denote the physical embodiment of the aperture stop. In order to minimize the size of the scanner, it is necessary to move the aperture stop away from the cold stop and to move it closer to the scanner. Since the aperture stop is moved away from the cold stop, the total aperture of the latter becomes oversized with respect to the useful aperture of the camera and a large part of the structure flux reaches the detector; as may be seen below, this may become troublesome since this structure flux is by definition spurious flux, as opposed to the useful flux of the signal of the observed scene. A first-order photometric correction is possible by placing the conjugating mirror in such a way that, apart from the useful flux, the detector now sees only the cold flux.
However, the structure flux problems are not completely solved because of the residual spurious flux which will be explained in detail below. A second-order photometric correction is obtained by giving the cold stop a particular shape.
The invention therefore provides an optical architecture for an infrared vision system with an optical axis, having an aperture stop, and comprising, in sequence:
a front afocal,
scanning means which carry out horizontal scanning of the scene,
an optical device forming an image of the scene,
cooled detection means which include a linear-array-type detector located in the focal plane of the device and a cold stop,
characterized in that:
the optical device is a simple imager having one lens and being as close as possible to the scanning means,
the aperture stop is located at the lens of the simple imager and is physically formed by a conjugating mirror which delimits it and the reflecting face of which is turned towards the detector,
the cold stop has a shape such that the photometric inefficiency is smoothed out over the extent of the detector and/or is decreased.
The above considerations relate to a preferred embodiment of the invention which makes it possible to solve the more general problem, which is to smooth and/or decrease the photometric inefficiency over the extent of the detector when this inefficiency is due to the appearance of spurious structure flux, which appearance results from the fact that the aperture stop is separate from the cold stop.
This problem can be solved, according to another subject of the invention, by any optical architecture in which the cold stop is given a suitable particular shape.
Another aspect of the invention therefore provides an optical architecture for an infrared vision system with an optical axis, having an aperture stop, and comprising, in sequence:
means allowing the flux from a scene to be collected,
an optical device forming an image of the scene,
cooled detection means which include a detector located in the focal plane of the device and a cold stop,
characterized in that
the aperture stop is separate from the cold stop,
the cold stop has a shape such that the photometric inefficiency is smoothed out over the extent of the detector and/or is decreased.
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Forestier Bertrand
Ragot Dominique
Rollin Jo{umlaut over (e)}l
"Thomson-CSF"
Epps Georgia
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Schwartz Jordan M.
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