WO2014087396A1

WO2014087396A1 - Observation system

Info

Publication number: WO2014087396A1
Application number: PCT/IL2013/050955
Authority: WO
Inventors: Dan Vadim REGELMAN; Yan ITOVICH
Original assignee: Rafael Advanced Defense Systems Ltd.
Priority date: 2012-12-06
Filing date: 2013-11-19
Publication date: 2014-06-12
Also published as: IL223489A

Abstract

The invention is an omni-directional observation system comprised of an inverted multi-faceted pyramidal prism oriented such that each facet reflects an image of a field of view facing it towards the apex of the pyramid. Two discs containing apertures located beneath the apex of the pyramid rotate at the same speed in opposite directions, and an optical arrangement under the disks is arranged to focus light passing through apertures in both disks onto a focal plane array detector. The apertures in the disks and the speed of the rotation are adapted such that apertures in both disks align to allow the image from only one facet at a time to fall on the detector. In one complete revolution of the disks images from all facets fall on the detector; thereby providing a plurality of images that can be combined to form a panoramic view of the surroundings of the system.

Description

OBSERVATION SYSTEM

Field of the Invention

The present invention relates to the field of optical observation systems. More particularly, the present invention relates to systems intended for large field of view (omni-directional) observations.

Background of the Invention

Large field of view optical observation devices are of importance in optical surveillance, threat detection and security applications. One of the key parameters of a system with large given field of view (FOV - the total angular area observable by an optical system), is instantaneous field of view (IFOV - the angle observable by a single pixel of optical system's detector). The optical observation/surveillance system performance is usually determined in terms of probability of detection (PD) / false alarm rate (FAR) for threat detection systems. These parameters are directly linked to signal to clutter ratio (SCR), which improves when the IFOV is reduced. Therefore, it is highly advantageous to decrease the IFOV of the optical system, while keeping the required FOV constant. However, for a given FOV, decreasing IFOV means an increase in N - the number of pixels of the focal plane array (FPA) detector (FOV= N * IFOV).

This task of decreasing IFOV is commonly achieved by utilizing a larger pixel count detector or splitting the required FOV into several sub-FOVs, each assigned to another observation device (as shown in Figs. 1A and 1C). Both solutions increase the system complexity by increasing the FPA detector pixels count or the number of FPAs.

Another family of solutions is based on scanning devices, where a single detector is equipped with a scanning device, providing a view of part of FOV for each detector frame (as shown in Fig. ID). Those scanning devices are mechanically and optically complicated, requiring high precision and sometimes high rotation speeds. One of the main disadvantages of the scanning method is image rotation on FPA. This image rotation blurs the image for longer integration times and/or faster rotation speeds. Therefore, the following possibilities arise:

- Reducing the integration time, causing a subsequent decrease in SCR.

Reducing the scanning speed, causing less images acquired in a give time period.

Incorporating a complicated optical de-rotation mechanism into the optical system.

Yet another solution to keep scanning with a short integration time is to use a "jumping" mirror which jumps between different locations. However this approach is impractical because of high jumping speed and acceleration, requiring complex and not reliable mechanical actuation elements.

For threat detection systems, based on thermal signatures, one of the most important merits of performance is False Alarm Rate (FAR). False alarms are caused by different harmless thermal objects causing signals on the FPA similar to those caused by real targets. One of the well known means to reduce FAR is dual band detection. In this method, the scene imaging is performed at two distinct spectral bands and the signals obtained in both of them are compared. The spectral bands are selected in such a way, that real threats have a particularly high radiance difference in both bands compared to non-threat objects. Typically this type of dual band detection is implemented by two imaging detectors each equipped with A band or B band filters or by a jumping mirror. The first method requires an additional FPA and increases the system complexity and costs, while the second method requires fast jumping mirror, which is hard to implement as was mentioned above. For example, rocket propellants have strong emission peak at around 4.5 microns. Typically 3.6-4.0 (A band) and 4.5-4.8 (B band) micron bands are used for detection since rocket motors exhibit high ratio of B band to A band signals compared to sun reflections, hot objects and flames.

Fig. 1A shows a prior art conceptual layout of a staring panoramic optical system (as disclosed in US 6,304,285) where no moving parts are used. An omni-directional mirror (1) converts 360 degrees horizontal field of view into a toroidal shaped image (as if it was originated from a single point of view), through a lens (2) into the image plane (3). Fig. IB schematically illustrates a prior art image (4) covering 360 degrees FOV on an imaging detector array (5), as the detector of Fig. 1A. Peripheral and central areas of the imaging detector array are left unused. Typically smaller amount of pixels available on the detector are utilized for imaging in this type of systems.

Fig. 1C schematically illustrates a prior art split field of view imaging system, where each sensor covers part of the 360 degrees field of view, while small overlap between them allows a complete coverage of the horizontal field. Fig. ID shows an example of an imaging system with a horizontal scanning mirror (as disclosed in EP 0816891). The horizontal scanning creates an image blur on the FPA and requires short integration time, making this type of scanning systems especially unsuitable for bolometric detectors (in which the optical signal integration is continuous).

It is therefore a purpose of the present invention to provide an omnidirectional observation system adapted for scanning a narrow field of view, with a single imaging detector and static optical components.

It is a further purpose of the present invention to provide a system to implement multi band detection by using a single FPA with mechanical elements rotating at a speed adapted to allow multi band scanning. Further purposes and advantages of this invention will appear as the description proceeds.

Summary of the Invention

In a first aspect, the invention is an omni-directional observation system, comprising:

a) a stationary inverted multi-faceted pyramidal prism, having N equally shaped and dimensioned facets, oriented such that each facet reflects an image of a field of view facing it towards the apex of the pyramid;

b) optical energy absorbing baffles which separate the facets of the prism; c) two coaxial disks containing apertures, located between the apex of the pyramid and the optical arrangement, rotating in opposite directions at the same speed;

d) an assembly of gears and a motor adapted to cause the disks to rotate; e) an axle going through the center of the system, and connected to the motor at one of its ends and to the disks at its other end;

f) a focal plane array detector; and

g) an optical arrangement, arranged to focus light that passes through the aperture in both of the disks onto the focal plane array detector;

wherein the shapes and dimensions of the apertures in the disks and their rotation speed are adapted such that apertures of the two disks align, N times per revolution, to allow an image from only one facet at a time to fall onto the focal plane array detector.

In one embodiment of the invention, one of the disks comprises a single aperture in the shape and dimension of the projection of a single facet onto the disk plane, and the second disk comprises multiple apertures, each in the shape and dimension of the projection of a single facet onto the disk plane, wherein the number of the equally spaced openings in the second disk is half the number of facets (N/2). In a further embodiment of the invention, in one complete revolution of the disks images from all facets fall on the detector, such that a plurality of images may be combined to form a panoramic view of the surroundings of the system.

In a further embodiment of the invention, the system further comprises at least one spectral optical filter disk coaxial with and aligned parallel to the two disks, adapted to pass light of different spectral bands through different areas of its surface.

In an additional embodiment of the invention, the filter disk has half of the surface area comprising a filter for one wavelength band and the other half for a second wavelength band.

In an additional embodiment of the invention, the filter disks are adapted to rotate at a half of the speed than the other disks, so that each field of view will be captured in each of the wavelengths of the filter.

In a further embodiment of the invention, the rotating elements are controlled by an electronic board which also supports the data readout from the focal plane array detector.

In a further embodiment of the invention, the focal plane array detector is cooled by a cooling module.

In a further embodiment of the invention, one of the disks is replaced by a rotating optical periscope with a single pupil, comprising multiple mirrors and a balancing weight. In a second aspect, the invention is a method for omni-directional observation, consisting of supplying a system containing:

f) a focal plane array detector; and

and further comprising the following steps:

a. activating said motor and gears assembly to cause the disks to rotate at the same speed in opposite directions; and

b. using image processing technique to manipulate the images from each facet and connect them until a panoramic view is achieved.

All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of embodiments thereof, with reference to the appended drawings.

Brief Description of the Drawings

— Fig. 1A shows prior art omni-directional optical system.

— Fig. IB shows an image on the detector of a staring omni-directional system.

— Fig. 1C shows a split FOV omni-directional optical system. — Fig ID shows an example of scanning mirror system.

— Figs. 2A-2B show a prior art scanned scene in a staring/scanning system.

— Fig. 3A schematically illustrates an embodiment for a multi-faceted pyramidal prism.

— Fig. 3B schematically illustrates constantly rotating mechanical elements in the optical system.

— Fig. 4 shows the focal plane array detection timing diagram and the respective position of the scanning disks.

— Fig. 5 shows the detailed layout of the optical head.

— Fig. 6 shows an embodiment of the invention with a multiband filter.

— Fig. 7 schematically illustrates an embodiment of the staring/scanning system.

— Fig. 8A schematically illustrates a ray diagram for an optical layout for low f-number FPA.

— Fig. 8B schematically illustrates an optical layout for high f-number FPA.

— Fig. 9 shows the spatial coverage of the rectangular FPA.

Detailed Description of Embodiments of the Invention

The invention is an omni-directional observation system. The system is comprised of an inverted multi-faceted pyramidal prism oriented such that each facet reflects an image of a field of view facing it towards the apex of the pyramid. Two discs containing apertures located beneath the apex of the pyramid rotate at the same speed in opposite directions, and an optical arrangement under the disks is arranged to focus light passing through apertures in both disks onto a focal plane array detector. The apertures in the disks and the speed of the rotation are adapted such that apertures in both disks align to allow the image from only one facet at a time to fall on the detector. In one complete revolution of the disks images from all facets fall on the detector; thereby providing a plurality of images that can be combined to form a panoramic view of the surroundings of the system. Figs. 2 A and 2B schematically illustrate a prior art scanned image on the FPA resulting from observation of scene (30) by the mirror (1). The resulting image is an overlap of rotated images from each FOV associated with the multifaceted mirror facet. Those overlapped images (31-33) require that during a given FPA integration time images originated from all FOVs except the desired one will be optically blocked.

An embodiment of the invention is schematically illustrated in Fig. 3A, showing an inverted multi faceted pyramidal prism (29). The mirror surfaces on the facets of the prism (29) could be flat or curved depending on the specific optical design in order to optimize the system performance. The stationary faceted mirror (29) projects all partial FOVs from each of the facets onto a plane containing the first disk (44), below the apex of the pyramid. The first disk (44) contains a single aperture in a shape and dimension adapted to allow only the FOV of a single facet to pass through the aperture. The rotation of the first disk (44) scans the projected FOVs, while most of the time two neighboring FOVs are transmitted (part of each FOV is transmitted through the aperture). This causes image mixture and blur on the FPA. To eliminate this effect, a second disk (46) is added. The second disk (46) contains multiple apertures, where each aperture is in a shape and dimension adapted to allow only the FOV of a single facet to pass through. The second disk (46) rotates at the same speed (e.g. ~8 Hz) in the opposite direction from the first disk (44). The apertures in the disks (44, 46) and the speed of the rotation are synchronized such that apertures in both disks (44, 46) align to allow the image from only one facet at a time to pass through. The result is an aperture that opens and closes each time under a different facet (as shown in Fig. 3B). Finally, images from all FOVs are collected from all of the different facets. This way, a "jumping" scanning effect is achieved with two elements rotating at constant speed. The timing diagram of the FPA exposure is shown in Fig. 4 (where the bottom curve is the integration time window, and the upper curve is the frame transfer dwell). The FOV 1 (47) starts exposure while both disks are in a position that begins to transmit the light reflected from the mirror facet above the opening slit (49). The integration time ends when the slit is almost fully open, and then the frame transfer occurs. While the slit starts closing again, the next frame integration time begins, exposing another image of the FOV 1. When the FOV 2 (48) starts being exposed, the third frame integration time begins. This process continues periodically, exposing two images on the FPA for each FOV. Both frames collect equal amount of light since the exposure is symmetric with respect to slit opening and closing. The effective time averaged f- number (the ratio of the focal length to the diameter of the lens) of this optical system is FPA nominal for the direction parallel to the slit and about half-nominal in the perpendicular direction.

Fig. 5 schematically illustrates a detailed layout for an embodiment of the optical head. The faceted prism (29) has a hole in its center allowing the mechanical rotation of the two disks (44) and (46) located below the prism, by an axle which is connected to a motor and gears (50), wherein the motor (50) is located above the prism. The optically absorbing baffles (52) are separating the facets of the prism (29) in order to prevent out-of-FOV rays from neighboring facets, arriving at a larger angle, to enter the FOV of the facet currently opened to the imaging lens assembly (23) by the disks (44) and (46).

Fig. 6 schematically illustrates the concept for multiband detection (two band example). In addition to the two rotating disks (44) and (46), enabling the "jumping" scan, a third disk (60) carrying spectral optical filters of the two bands (60A, 60B) is added. The disk (60) rotates at half the angular velocity of the disks (44) and (46). This way in one rotation (61), half of the prism facets are exposed through the first spectral band (60A) and the second half through the second spectral band (60B). At the next rotation (62) of the disks (44), (46) and (60), the spectral bands interchange. This way in two sequential turns, every sub-FOV is exposed in both spectral bands.

The complete layout of the optical system (10) of the embodiment of Fig.5 is schematically illustrated in Fig. 7. An optical head with optically transparent windows (20), containing a faceted prism, is followed by two rotating disks (22), a lens assembly (23) and a focal plane array (24) with the system optionally cooled by a cooling module (28), to increase the signal to noise ratio. An electronic board (26) supports the FPA data readout and the rotation control of the disks.

Fig. 8A shows a ray diagram for the optical system of the embodiments shown in Fig. 5 and Fig. 7. The resulting effective f-number of the system is lower than the required FPA f-number in order to be able to detect light from all facets within the FOV.

Fig. 8B shows an additional embodiment of the invention with high f- number detectors. For this purpose a rotating periscope (70) is added below the disk with multiple apertures (replacing the disk with a singular aperture), rotating with the same angular speed and in the direction of rotation of the disk. An additional filter disk (as in Fig. 6) may also be added to this embodiment. The periscope consists of two mirrors (72) and (74) and a balancing weight (76). The optical assembly is designed in such a way that it has a single entrance pupil at the mirror (72), which rotates with the periscope. This way the light from each mirror facet is transferred to the optical assembly, so that the system will have smaller diameter optics.

Fig. 9 shows the spatial coverage of the rectangular FPA for exemplary mirror with facets. Because of the image plane rotation on the FPA, while scanning the mirror facets, the optical assembly focal length is designed to cover 44 degrees. This way, the vertical FOV at any sub-FOV covers at least 10 degrees above the FOV center.

Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, without exceeding the scope of the claims.

Claims

1. An omni-directional observation system, comprising:

b) optical energy absorbing baffles which separate the facets of the prism;

c) two coaxial disks containing apertures, located between the apex of the pyramid and the optical arrangement, rotating in opposite directions at the same speed;

d) an assembly of gears and a motor adapted to cause the disks to rotate;

e) an axle going through the center of the system, and connected to the motor at one of its ends and to the disks at its other end;

f) a focal plane array detector; and

g) an optical arrangement, arranged to focus light that passes through the aperture in both of the disks onto the focal plane array detector; wherein the shapes and dimensions of the apertures in the disks and their rotation speed are adapted such that apertures of the two disks align, N times per revolution, to allow an image from only one facet at a time to fall onto the focal plane array detector.

2. The system according to Claim 1, wherein one of the disks comprises a single aperture in the shape and dimension of the projection of a single facet onto the disk plane, and the second disk comprises multiple apertures, each in the shape and dimension of the projection of a single facet onto the disk plane, wherein the number of the equally spaced openings in the second disk is half the number of facets (N/2).

3. The system according to Claim 1, wherein in one complete revolution of the disks images from all facets fall on the detector, such that a plurality of images may be combined to form a panoramic view of the surroundings of the system.

4. The system according to Claim 1, wherein the system further comprises at least one spectral optical filter disk coaxial with and aligned parallel to the two disks, adapted to pass light of different spectral bands through different areas of its surface.

5. The system according to Claim 4, wherein the filter disk has half of the surface area comprising a filter for one wavelength band and the other half for a second wavelength band.

6. The system according to Claim 5, wherein the filter disks are adapted to rotate at a half of the speed than the other disks, so that each field of view will be captured in each of the wavelengths of the filter.

7. The system according to Claim 1, wherein the rotating elements are controlled by an electronic board which also supports the data readout from the focal plane array detector.

8. The system according to Claim 1, wherein the focal plane array detector is cooled by a cooling module.

9. The system according to Claim 1, wherein one of the disks is replaced by a rotating optical periscope with a single pupil, comprising multiple mirrors and a balancing weight.

10. A method for omni-directional observation, consisting of supplying a system containing: a) a stationary inverted multi-faceted pyramidal prism, having N equally shaped and dimensioned facets, oriented such that each facet reflects an image of a field of view facing it towards the apex of the pyramid;

b) optical energy absorbing baffles which separate the facets of the prism;

d) an assembly of gears and a motor adapted to cause the disks to rotate;

f) a focal plane array detector; and

g) an optical arrangement, arranged to focus light that passes through the aperture in both of the disks onto the focal plane array detector; and further comprising the following steps: