FIELD OF THE INVENTION This invention relates to cameras adapted to capture two fields of view simultaneously.

BACKGROUND OF THE INVENTION Cameras, such as surveillance cameras for military use, typically comprise several optical trains, each adapted for a different type of viewing. For example, a wide field of view (FOV) optical train may be used for target acquisition and for providing real-time situational awareness. Optical trains adapted for more narrow FOVs may be used for target detection, recognition, and identification. There may be a single sensor or separate sensors for the different FOVs. In addition, sensors to detect non-visible wavelengths, such as infrared, are typically provided to supplement the acquired viewable images. Since the narrow FOV typically involves the use of a telephoto lens providing a high degree of zoom, it is very sensitive to jitter. If there is any need to move any components of the narrow FOV optical train, it must be done with a high degree of precision. This may be needed for steering the narrow FOV to track an identified target acquired by the wide FOV.

SUMMARY OF THE INVENTION The present invention is directed for use in camera, such as a surveillance camera, having at least a narrow FOV optical train and a wide FOV optical train, so that steering of the narrow FOV within the wide FOV is accomplished without movement of any of the components of the narrow FOV optical train within the camera. According to one aspect of the present invention, the camera is adapted for fast switching between two fields of view. It comprises a first optical train associated with a first optical path and a first field of view, which may be a narrow FOV, a second optical train associated with a second optical path and a second field of view, which may be a wide FOV, and a compensating mechanism. Steering of the first field of view is accomplished by movement of the camera, while the second field of view is held steady by the compensating mechanism. The compensating mechanism may be a movable mirror constituting part of the second optical train. All components of the first optical train are immovably fixed within the camera. The camera further comprises a single sensor, which may be an infrared sensor, at which both optical paths terminate. The compensating mechanism is adapted to ensure that only one optical path terminates at the sensor concurrently. The camera is adapted to present both fields of view concurrently in real time using the principle of timesharing. The movable mirror is shiftable between a first position, in which it obstructs the first optical path and constitutes a reflex mirror being part of the second optical train, and a second position, in which it is removed from the second optical path, thereby disrupting it, and allows for completion of the first optical path without obstruction. By performing the shifting rapidly enough to be synchronized with the frame rate of the sensor, fast switching between the two fields of view is accomplished so that a user observes both FOVs normally. The movable mirror may be of a half-disk shape. The shifting is accomplished by rotation of the movable mirror about the center of its diameter and within a plane coincident with its reflecting surface. The rotation is preferably done at a constant angular velocity and is unidirectional. The compensation may be enabled by disposing the movable mirror within a double freedom gimbals arrangement. The camera may also be moved by being disposed within a double gimbals arrangement. By disposing both the camera and the compensating arrangement in similar movement mechanisms, synchronization of the two movements is more easily accomplished. The camera may further comprise a dichroic mirror disposed within at least one of the optical paths and a supplemental sensor associated with one light path of the dichroic mirror. The supplemental sensor may be any desired sensor, such as a visible wavelength or a near infrared sensor. According to another aspect of the present invention, there is provided a method of imaging by a camera having two fields of view. The method comprises steering the first field of view by moving the camera, and compensating the second field of view for said movement. The compensation is accomplished by the use of a compensating mechanism as described above. The present invention has the advantage that steering of the narrow FOV does not require any movement of parts of the narrow FOV optical train. This aspect obviates the need for the great care which would otherwise be necessary to control jitter which may lead to greatly magnified deviations. The camera according to the present invention has the additional advantage that the narrow FOV may be steered within the wide FOV while it is held steady. In addition, the camera according to the present invention utilizes a single sensor for each wavelength to capture multiple fields of view in real time by using timesharing. The number of required components is lowered, which leads to a more compact and inexpensive camera. This is especially important with regards to sensors, such as infrared sensors, which may be expensive. The constant rate and unidirectional manner of rotation of the moveable mirror provides a well-balanced rotation, and reduces, and may even eliminate, vibration thereof, resulting in a steadier image. - A -

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, an embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. IA is a perspective view of a camera according to the present invention with sides of the housing removed; Fig. IB is a perspective view of the camera illustrated in Fig. IA with all of the housing removed; Fig. 2 is a plan view of a wide field of view mirror assembly for use with the camera illustrated in Figs. IA and IB; Figs. 3A and 3B are perspective views of the wide field of view mirror assembly illustrated in Fig. 2 at different positions; Figs. 4A and 4B are side views of the camera, wherein the wide field of view mirror assembly is in the positions illustrated in Figs. 3A and 3B, schematically showing the resulting field of views; and Figs. 5A through 5J are schematic views of the wide field of view mirror assembly in different positions.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS As seen in Fig. IA, there is provided a camera 10 with a housing 11 (side portions of the housing are not shown). Fig. IB illustrates the camera 10 with the entire housing 11 removed. The camera comprises a narrow field of view (FOV) optical train, indicated by components labeled A, and a wide FOV optical train, indicated by components labeled B. A narrow FOV mirror 18 defines a leading portion of a narrow FOV optical path, and a side FOV mirror 20 defines a leading portion of a wide FOV optical path. Common reflex mirrors 14 and dichroic mirror 26 define a common trailing portion of the two optical paths. The camera 10 further comprises a primary sensor 12, common lenses 13, narrow FOV lens 15a and 15b, and a wide FOV mirror assembly, generally indicated at 22, in which the wide FOV mirror 20 is disposed. The camera also comprises a supplemental sensor 24. In the event that a supplemental sensor is not provided, a regular mirror may be substituted for the dichroic mirror 26. The primary sensor 12 is preferably a high frame rate video detector, and may be any known sensor appropriate for the intended use of the camera, such as an infrared sensor. The primary sensor 12 is situated at on end of the common optical path, which is described in more detail below. The supplemental sensor 24 is adapted to detect electromagnetic radiation having a wavelength different from that of the primary sensor 12. For example, it may be a visible wavelength or a near infrared sensor. The dichroic mirror 26 is selected so that the primary sensor 12 is adapted to detect light having its excitation wavelength and the supplemental sensor 24 is adapted to detect light having its emission wavelength. The dichroic mirror 26 is disposed within the common optical path, preferably at a 45° angle thereto. The primary sensor 12 is positioned below the dichroic mirror 26 so that the excitation light is reflected theretoward. The common reflex mirrors 14 are fixed within the camera and are mutually disposed so that they are adapted to transmit images from either the narrow FOV mirror 18 or the wide FOV mirror 20 to the sensors 12, 24 depending on the position of the wide FOV mirror, as detailed below. The narrow FOV mirror 18 is fixed within the camera. Together with the common reflex mirrors, it constitutes a portion of a narrow FOV optical train. The narrow FOV optical path overlaps the common optical path defined by the common reflex mirrors 14. Since all components of the narrow FOV optical train are fixed within the camera 10, steering of the narrow FOV is accomplished by movement of the entire camera. The wide FOV mirror 20 is flat and half-disk in shape. Alternatively, it may be a quarter or a third of a disk, or any other portion thereof. As seen more clearly in Fig. 2, it is disposed within a two-axis double freedom gimbals 28, and dynamically balanced about a rotation axis passing through the mirror at a point 30 which lies at the center of the diameter thereof. The gimbals permit rotations of the wide FOV mirror 20 about a pitch axis 34 and about an azimuth axis 36. Rotation about each axis is accomplished by a corresponding motor drive (not shown). These rotations enable compensation of the wide FOV for movement of the camera 10, allowing the wide FOV to be held steady, even when the camera is moved. The wide FOV mirror 20 and the gimbals 28 constitute main portions of the wide FOV mirror assembly 22. The mirror assembly is adapted to rotate the wide FOV mirror 20 about point 30. This may be accomplished by a rotor drive (not shown) disposed within the gimbals adjacent the wide FOV mirror 20. The common lenses 13 are situated within the common optical path, and are adapted to focus the incoming images to correspond to the sensors. The narrow FOV lenses 15a and 15b are long focal length lenses situated within the narrow FOV optical path and constituting the narrow FOV optics. In operation, the entire camera 10 is moved to steer the position of the narrow FOV. In order to maintain the position of the wide FOV, the gimbals 28 are adjusted to compensate the wide FOV mirror 20 for the movement of the camera. In this way, a target may be identified and tracked by the narrow FOV optical path, while situational awareness provided by the wide VOF optical path is not affected by movement of the device. Due to the azimuth angle change of the wide FOV mirror 20 relative to the primary and supplementary sensors 12, 24, a wide FOV image rotation effect may be caused. This slight shift may be corrected by applying any known video processing algorithm on the captured data. The wide FOV mirror 20 is rotated, as described above, at a predetermined rate so that it passes between a first position, shown in Fig. 3A, in which the wide FOV mirror 20 occupies an upper portion of the interior of the gimbals 28, and a second position, shown in Fig. 3B, in which the wide FOV mirror 20 occupies a lower portion of the interior of the gimbals 28. In the first position, the wide FOV mirror 20 obstructs the narrow FOV optical path, and completes the wide FOV optical path. In the second position, the narrow FOV optical path is unobstructed by the wide FOV mirror 20. The bearings of the two positions on the optical path are shown schematically in Figs. 4A and 4B, with Fig. 4A corresponding to the mirror position illustrated in Fig. 3A and Fig. 4B corresponding to the mirror position illustrated in Fig. 3B. An electric motor drive (not seen), adapted to rotate the wide FOV mirror 20, is situated on the gimbals. The motor imparts a continuous rotation to the mirror, synchronized with the frame rate of the sensor. The rotation is of a constant angular velocity and is unidirectional. The rate of rotation of the wide FOV mirror 20 is synchronized with the frame rate of the sensors 12, 24, which is preferably twice the rotation rate (i.e., for every rotation of the wide FOV mirror, each sensor captures two images). Alternatively, the sensor may capture several images while the mirror is in each position. This is possible while still maintaining a constant rotation rate since the mirror does not need to fully occupy either the upper or lower portion of the interior of the gimbals in order for the sensors to capture an image. As illustrated in Figs. 5 A through 5E, the region 32 which represents the intersection of the optical path with the plane of the wide FOV mirror 20 is intersected by the mirror at a range of angular positions thereof. An image of the wide FOV may be captured at any point within this range. Conversely, an image of the narrow FOV may be captured at any point within the range of angular positions of the wide VOF mirror 20 in which the region 32 does not intersect the mirror at all, as illustrated in Figs. 5F through 5J. This further allows for only one image of each FOV to be captured per rotation of the wide FOV mirror 20, but not necessarily when the mirror fully occupies either the upper or lower portion of the interior of the gimbals (i.e., the respective images may be captured when the mirror is in the positions illustrated, e.g., in Figs. 5B and 5G). Each sensor 12, 24 is connected to two output paths. When an image is captured, it is transmitted via the appropriate path, depending on whether it is from the narrow FOV or the wide FOV. In this way, the videos may be displayed separately from one another, or as a picture-in-picture. Alternatively, there may be one output path connected to a processor. The processor is adapted to determine which image is associated with which FOV, and display them separately. By providing switching which is synchronized with the frame rate of the sensor, both FOVs may be concurrently displayed so that a user observes each as being normal. The rotation of the mirror may be externally controlled, allowing other viewing possibilities. For example, the rotation may be stopped, permitting only with wide FOV or the narrow FOV to be captured by the camera at the discretion of the user. The frame rate of the sensors 12, 24 and the rotation of the wide FOV mirror 20 may be synchronized in advance. Alternatively, the synchronization may be performed automatically. This may be accomplished, for example, by providing one or more detector (not shown) around the wide FOV mirror 20, and one or more features on the mirror adapted to be detected by the detector. For each detection, a signal to capture an image is transmitted to the sensors. This arrangement has the advantage that the camera 10 does not require synchronization before use, and cumulative effects of slight deviations from expected behavior are mitigated. Those skilled in the art to which this invention pertains will readily appreciate that numerous changes, variations and modifications can be made without departing from the scope of the invention mutatis mutandis. For example, multiple elements similar to the wide VOF mirror assembly may be used to allow more than two fields of view to be switched as described herein. In addition, the gimbals may be replaced by actuators or another arrangement which permits the requires motion.