IL167317A - System and method for wide angle optical surveillance - Google Patents

Description

ram rna ^DIN ni ^1? no-iym nu1^ SYSTEM AND METHOD FOR WIDE ANGLE OPTICAL SURVEILLANCE SYSTEM AND METHOD FOR WIDE ANGLE OPTICAL SURVEILLANCE FIELD AND BACKGROUND OF THE INVENTION The present invention relates to a method and system for optical surveillance of a wide angle or panoramic field of view.

In certain imaging applications such as full earth surveillance from low altitude space platforms, missile launch warning from an airborne platform, or air-borne threat detection from a ground base location, an extremely wide field of view (120° or more) optical system is required so that a very large two dimensional region of the object space may be monitored either continuously or repeatedly at short intervals.

One approach to monitoring such large fields of view is the use of a scanning linear detector array. Examples of this approach are described in patent publications US 5347391 and EP 1416312 Al. Although such systems offer a cost-efficient solution for scanning large regions, they suffer from a number of shortcomings. Most notably, a scanning linear detector array by its very nature actually views each given pixel of object space for a very small proportion of each scanning cycle. As a result, there is a significant risk of transient events, such as the brief flash accompanying the launch of a missile, being missed between scans of the sensor.

An alternative approach is to use staring imaging sensors to monitor the region of interest. Examples of systems employing staring imaging sensors include patent publications US 6410897 Bl, US 5534697 A and US 5300780 A. In most cases, in order to achieve acceptable resolution and avoid problems caused by optical distortion, the field of view of each imaging sensor should be limited to 40-60°. In order to cover a larger solid-angle field of view, a scanning pattern is typically used, resulting in similar problems as described in the context of linear detector arrays discussed above. For truly continuous non-scanned monitoring of a large field-of-view at an acceptable resolution, a large number of imaging sensors deployed with overlapping fields-of-view would be required, thereby rending the system very expensive.

In a number of prior art systems, it has been proposed to use a single imaging sensor with optical multiplexing to perform more than one imaging function. Examples include the aforementioned US 6410897 Bl where a movable mirror is used to switch the optical sensor between a wide field of view optical objective and a narrow field of view optical objective. A similar concept of switching between narrow and wide fields of view is also disclosed in US 5049740, US 4486662 and US 3804976. Another example disclosed in US 4574197 provides a scanning rotating polygon which offers two fields of view used for stereoscopic viewing or for two independently steerable optical telescopes for display on separate screens. None of these references discloses optical multiplexing to offer two similar fields of view with different optical axes in fixed spatial relation as a solution for a staring surveillance system.

A further limitation of the aforementioned existing systems with optical multiplexing is that the optical switching frequency is typically limited by the read cycle rate of the sensor, i.e., the period taken to expose the array to incoming illumination and then read the resulting information from an array of capacitors associated with each sensor element. In order to avoid mixing of the content of the two images, the sensor array is exposed for a first integration time to the first field-of-view, the associated capacitors are read (a first read cycle), and then the sensor array is exposed to the second field of view and the capacitors are again read (a second read cycle). This mode of operation is referred to as "Read Then Integrate (RTI). For surveillance applications in which it is desired to detect transient events of duration similar to or less than the read-cycle of the sensor, this arrangement is problematic since an event may occur while the other field of view is being viewed and may therefore be missed by the sensor.

Finally, in the field of staring sensors with a single field of view, there exists a technique known as "Read While Integrate" (RWI) which substantially avoids dead-time during the output reading process between integration periods of a sensor. This technique is particularly useful when monitoring for transient flash events since it helps to ensure that even a transient event is picked-up by the sensor. "Read While Integrate" also effectively doubles the rate at which frames can be acquired using an array of light-sensitive sensors that produce electrical charge when the light that they are sensitive to impinges on them. The principle of RWI will now be illustrated with reference to Figures 1 and 2.

Specifically, Figure 1 shows one such sensor 10, for example an InSb detector sensitive to infrared light, coupled alternately to two capacitors 12 and 14 by a switch 16. Capacitors 12 and 14 in turn are alternately coupled to a readout circuit 18 by a switch 20. When a capacitor 12 or 14 is coupled to sensor 10, the capacitor 12 or 14 receives and accumulates ("integrates") the electrical charge produced by sensor 10 as a consequence of the light impinging on sensor 10. When a capacitor 12 or 14 is coupled to readout circuit 18, readout circuit 18 reads the charge accumulated in the capacitor 12 or 14 and discharges the capacitor 12 or 14.

Figure 2 shows the schedule of integration and readout used in the prior art RWI method to acquire images using an array of sensors 10 coupled to respective capacitors 12 and 14 and respective readout circuits 18 as illustrated in Figure 1. Time increases from left to right in Figure 2. During odd-numbered time intervals, capacitors 12 accumulate electrical charges while readout circuits 18 read the electrical charges accumulated in capacitors 14 during the immediately preceding time intervals. During even-numbered time intervals, capacitors 14 accumulate electrical charges while readout circuits 18 read the electrical charges accumulated in capacitors 12 during the immediately preceding time intervals. The read cycle, i.e., the period between successive readings from the same capacitor, corresponds to a pair of time intervals. The diagonal arrows in Figure 2 show the timing of the flow of accumulated electrical charge from the capacitors 12 or 14 to readout circuits 18. Note that Figure 1 illustrates the settings of switches 16 and 20 during odd-numbered time intervals.

Although RWI provides an effective solution for substantially continuous monitoring of an imaging system field of view, it is of limited value where a single imaging sensor is used to switch between two or more fields of view since each field of view would still remain unmonitored for at least half the read cycle.

There is therefore a need for a wide field-of-view surveillance system based upon staring imaging sensors which would provide quasi-continuous monitoring of a wide-angle field or view while requiring fewer imaging sensors than would be required for full field of view coverage.

SUMMARY OF THE INVENTION The present invention is a method and system for optical surveillance of a wide angle or panoramic field of view.

According to the present invention there is provided an imaging system comprising: (a) a two-dimensional imaging sensor array; and (b) an optical system including: (i) at least one optical arrangement associated with the imaging sensor array and defining a field of view of given angular dimensions; and (ii) an optical switching mechanism for alternately switching an optical axis of the imaging system between a first direction and a second direction, the optical switching mechanism and the at least one optical arrangement being deployed such that the imaging sensor array generates images of at least two substantially non-overlapping fields of view of equal angular dimensions, the substantially non-overlapping fields of view having diverging optical axes in fixed spatial relation. ; (c) a first set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; (d) a second set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; and (e) a sensor switching arrangement associated with said two-dimensional imaging sensor array, said first and second sets of read capacitors and said optical switching mechanism, said sensor switching arrangement being configured to switch connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said first and second sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view such that said first set of read capacitors accumulate information corresponding to a first of said two fields of view and said second set of read capacitors accumulate information corresponding to a second of said two fields of view.

According to a further feature of the present invention, the optical switching mechanism includes an apparatus that is alternately substantially transparent and substantially reflective.

According to a further feature of the present invention, the optical switching mechanism includes a rotatable disk including at least one pair of alternating segments, a first segment of each the pair being transparent and a second segment of each the pair being reflective.

According to a further feature of the present invention, the at least one pair includes at least two pairs of segments, wherein the transparent segments are transparent to non-identical ranges of wavelengths and the reflective segments are reflective to non-identical ranges of wavelengths.

According to a further feature of the present invention, the optical switching mechanism includes a plurality of microelectromechanical shutters, the apparatus being substantially transparent when the shutters are open and substantially reflective when the shutters are closed.

According to a further feature of the present invention, the optical switching mechanism includes a pair of prisms and a prism displacement mechanism operative to displace at least one of the pair of prisms such that the pair of prisms are alternately adjacent and apart, the apparatus being substantially transparent when the prisms are adjacent and substantially reflective when the prisms are apart.

According to a further feature of the present invention, there is provided an imaging assembly comprising a plurality of the aforementioned imaging systems, wherein the imaging systems are deployed in fixed spatial relation such that the substantially non-overlapping fields of view of the plurality of imaging systems together form a substantially contiguous effective field of view spanning at least 120°, and preferably at least 180°, and most preferably substantially 360°.

According to a further feature of the present invention, the plurality of the imaging systems includes at least three of the imaging systems.

According to a further feature of the present invention, the optical switching mechanism switches between the fields of view at a field-of-view switching rate, the imaging system further comprising a read arrangement for reading accumulated information from the two-dimensional imaging sensor array at a read cycle rate, wherein the field-of-view switching rate is greater than the read cycle rate.

According to a further feature of the present invention, there are also provided: (a) a third set of read capacitors including a capacitor associated with each sensor element of the two-dimensional imaging sensor array; and (b) a fourth set of read capacitors including a capacitor associated with each sensor element of the two-dimensional imaging sensor array, wherein the sensor switching arrangement and the read arrangement are further associated with the third and fourth sets of read capacitors, the sensor switching arrangement and the read arrangement being configured such that: (i) during a first half of the read cycle, the sensor switching arrangement switches connections from each sensor element of the two-dimensional imaging sensor array between corresponding capacitors of the first and second sets of read capacitors synchronously with switching of the optical switching mechanism between the two fields of view while the read arrangement reads the third and fourth sets of read capacitors; and (ii) during a second half of the read cycle, the sensor switching arrangement switches connections from each sensor element of the two-dimensional imaging sensor array between corresponding capacitors of the third and fourth sets of read capacitors synchronously with switching of the optical switching mechanism between the two fields of view while the read arrangement reads the first and second sets of read capacitors.

According to a further feature of the present invention, there is also provided a processor configured for analyzing a sequence of images from the imaging system to determine whether they indicate the presence of a transient event. 10 167317/2 BRIEF DESCRIPTION OF THE DRAWINGS The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1 illustrates a read- while-integrate sensor of the prior art; FIG. 2 is a timing chart for the sensor of FIG. 1; FIG. 3 illustrates a first implementation of an imaging system constructed and operative according to the teachings of the present invention; FIG. 4 illustrates a variant implementation of the imaging system of Figure 3; FIGS. 5A and 5B are schematic representations of components of a preferred implementation of a sensor assembly for use in the imaging system of Figure 3 or 4, the components being shown in first and second states, respectively; FIG. 6 is a timing chart for the sensor assembly of Figures 5A and 5B; FIGS. 7 A and 7B illustrate two implementations of a rotating disk for use in an optical switching arrangement in the imaging system of Figure 3 or 4; FIGS. 8A and 8B illustrate schematically an alternative implementation of an optical switching arrangement based upon relative displacement of a pair of prisms, shown in a transmitting state and a reflecting state, respectively; FIGS. 9A and 9B illustrate schematically an alternative implementation of an optical switching arrangement employing micro-electromechanical shutters shown in an open and closed state, respectively; FIG. 10 shows three of the imaging systems of Figure 4 deployed for monitoring a full 360 degrees of azimuth; and FIG. 11 illustrates schematically a further variant of the imaging system of Figure 4 for monitoring three fields of view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is a method and system for optical surveillance of a wide angle or panoramic field of view.

The principles and operation of methods and systems according to the present invention will be better understood with reference to the drawings and the accompanying description.

Turning now to the drawings, Figures 3-11 illustrate various preferred embodiments of an imaging system, generally designated 30, 30', constructed and operative according to the teachings of the present invention. Referring first particularly to Figures 3 and 4, generally speaking, imaging system 30, 30' includes a two-dimensional imaging sensor array 32 and an optical system including: at least one optical arrangement 34, 36a, 36b defining a field of view 38a, 38ft of given angular dimensions; and an optical switching mechanism 40. Optical switching mechanism 40 is configured to alternately switch an optical axis of the imaging system between a first direction 42a and a second direction 42b. Optical switching mechanism 40 and the at least one optical arrangement 34, 36a, 366 are deployed such that imaging sensor array 32 generates images of at least two substantially non-overlapping fields of view 38α, 386 of equal angular dimensions with diverging optical axes in fixed spatial relation.

At this stage, it will already be appreciated that the present invention offers profound advantages of economy in cost and size by employing a single imaging sensor array with optical switching for monitoring double the non-switched field of view without any loss of spatial resolution. Thus, if an imaging system with its associated optics is configured to provide a basic field of view spanning 60°, the present invention may be employed to provide images spanning 120° using the same single optical sensor and without loss of spatial resolution. This principle can be extended by employing more than one optical switching mechanism 40 to monitor three or more fields of view using a single sensor, as illustrated schematically in Figure 11. Furthermore, a small group of imaging systems may be used together to span wide angle or even panoramic views, as illustrated schematically in Figure 10. These and other advantages of the present invention will become clearer from the following description.

Turning now to the features of imaging system 30 in more detail, optical switching mechanism 40 typically includes an apparatus that is alternately substantially transparent and substantially reflective. Preferably, a high speed switching arrangement is used, capable of switching between its two states with a transition of less than one millisecond. One or more additional reflective element (mirror) is typically used as shown to ensure the desired geometrical relation between the fields of view, as will be clearly understood.

In the preferred implementations of Figures 3 and 4, optical switching mechanism 40 is based upon a rotatable disk 44 including at least one pair of alternating segments, a first segment 46 of each pair being transparent and a second segment 48 of each pair being reflective. A two-segment implementation of disk 44 is illustrated in Figure 7 A. The disk is preferably driven by a synchronous electric motor.

Figure 7B shows a further preferred implementation in which at least one pair of segments 46a and 48a are selectively transparent and reflective, respectively, in a selected wavelength band. The possibility of adding selective filtering provides an additional option for reducing false alarm rates (FAR) for flash detection. Specifically, it is known that a flash event such as launch of a missile produces an intensity peak at wavelengths between 4.0 and 4.8 microns.

Thus, in the preferred case of an imaging sensor of InSb detectors sensitive to infrared radiation of wavelengths between 3 and 5 microns, segment 46a is implemented as a bandpass filter with a passband between 4.5 and 4.8 microns and 46b is implemented as an absorptive bandpass filter that passes only wavelengths below 4 microns and absorbs other wavelengths. Similarly, segment 48a is implemented as a dichroic mirror that reflects only wavelengths between 4.5 and 4.8 microns while segment 48b is implemented as a dichroic mirror that reflects only wavelengths below 4 microns and absorbs other wavelengths. In this way, each revolution of the disk generates two image exposures of each FOV in different wavelength bands. By comparing the resulting images, it is possible to distinguish reliably between genuine "flash" events which have a large differential between the two bands and other transient intensity peaks such as the glint of sunlight reflected from a moving surface which tend to be relatively evenly dispersed between the two wavelength bands.

Turning now to Figures 8A and 8B, there is shown schematically an alternative implementation of optical switching mechanism 40 including a pair of prisms 50, 52 and a prism displacement mechanism (represented here schematically by arrow 54) operative to displace at least one of prisms 50, 52 such that prisms 50, 52 are alternately adjacent (as in Figure 8A) to provide a substantially transparent state in which total internal reflection is frustrated and apart (as in Figure 8B) to provide a substantially reflective state by total internal reflection.

Turning now to Figures 9A and 9B, there is shown schematically an alternative implementation of optical switching mechanism 40 including a plurality of reflective micro-electromechanical shutters 56. In this case, the mechanism is substantially transparent when shutters 56 are open (Figure 9A) and substantially reflective when shutters 56 are closed (Figure 9B). Such a mechanism can be implemented using, for example, the technology described in PCT publication no. WO02/13168. Suitable micro-electromechanical shutter elements are commercially available from Flixel Ltd., (Tel Aviv, Israel).

Referring briefly specifically to Figure 4, it should be noted that this is similar to the implementation of Figure 3 except for the use of an optical arrangement to reduce the size of a footprint of the optical bundle passing through optical switching mechanism 40. This is typically achieved by designing the optics to provide an intermediate image plane at or near optical switching mechanism 40. This allows the optical switching mechanism to be more compact than would otherwise be possible. In all other respects, the implementation of Figure 4 is fully analogous in structure and operation to that of Figure 3.

Turning now to the remaining features of imaging system 30, these include a sensor read arrangement 60 for reading data (typically accumulated charge) from sensor elements of the two-dimensional imaging sensor array and a controller 62 for controlling synchronous operation of the optical switching mechanism and the read arrangement to ensure correct separation of data from the two fields of view. Data from read arrangement 60 is transferred to a processor 63 for subsequent image processing, preferably analyzing a sequence of images from the imaging system to determine whether they indicate the presence of a transient event.

As mentioned above, switching between different fields of view may present problems for detection of flash events (e.g. a missile launch) of duration similar to or shorter than the period of the sensor read cycle. Specifically, there is a risk that the sensor may be viewing a different field of view for the entire duration of the flash event. It is a particular feature of certain most preferred embodiments of the present invention that switching between the plural fields of view monitored by the imaging sensor array is performed at a frequency greater than, and preferably at least four times greater than, the read cycle rate. This requires provision of a specially configured sensor read arrangement 60 as will now be described with reference to Figures 5 and 6.

Specifically, with reference to Figure 5, sensor read arrangement 60 includes a first set of read capacitors including a capacitor 112 associated with each sensor element 64 of the two-dimensional imaging sensor array and a second set of read capacitors including a capacitor 114 associated with each sensor element 64 of the two-dimensional imaging sensor array. A sensor switching arrangement 66 is configured perform fast switching (i.e. at a rate greater than the read cycle rate) of connections from each sensor element 64 between corresponding capacitors 112, 114, synchronously with switching of the optical switching mechanism between the two fields of view, such that the first set of read capacitors 112 accumulate information corresponding to the first field of view 38a and the second set of read capacitors 114 accumulate information corresponding to the second field of view 386. Thus, the integration time for the image of each FOV is made up of a summation of short exposures interspersed during a single read cycle of the imaging array. In this manner, the revisit delay during which a transient event in one FOV could be missed can be reduced to very much shorter than the read cycle period. The result is pseudo-continuous observation in the two FOVs while maintaining the overall sensitivity of the sensor system.

Although a basic implementation of this fast-switching methodology requires only two read capacitors per sensor element, as described up to this point, most preferred implementations employ four read capacitors per sensor element of the array to combine the fast switching methodology with the read-while-integrate approach described above. Thus, in the case illustrated here, capacitors 112 and 114 are supplemented by third and fourth sets of read capacitors 113 and 115. During a first time period as illustrated in Figure 5 A, fast optical/electrical switching is performed as described above to accumulate image data for the two FOVs in (sets of) capacitors 112 and 114 while the previous images of both FOVs are read from capacitors 113 and 115, either simultaneously or sequentially. Then, during a second half of the read-cycle, switching arrangement 66 performs fast switching between capacitors 113 and 115 while capacitors 112 and 114 are read. In this way, the revisit delay for each field of view can be further reduced.

Parenthetically, it should be appreciated that the choice of four read capacitors per sensor element is sufficient for the application described, but may be increased as necessary for alternative implementations according to the present invention. Thus, for example, the band filter implementation of Figure 7B would optimally be implemented with eight read capacitors per sensor element in order to record all four images within a read-while-integrate framework. Similarly, an imaging sensor with two optical switching mechanisms used to generate three distinct FOVs as illustrated in Figure 11 would require six read capacitors per sensor element.

It will also be clear to one ordinarily skilled in the art that the specific topography of the switching arrangements described in this example can be rearranged without departing from the principles of the present invention.

It will be appreciated that the principles of the present invention may be applied to a wide range of applications wherever it is useful to monitor a plurality of fields of view using a reduced number of staring sensors. Although applications employing a single imaging system fall within the scope of the present invention, most preferred implementations provide an imaging assembly where two or more imaging systems according to the present invention are deployed in fixed spatial relation such that the substantially non-overlapping fields of view of the plurality of imaging systems together form a substantially contiguous effective field of view spanning at least 120°, and more preferably at least 180°. For various surveillance applications, three or more field-switching imaging systems are employed to provide panoramic (360° azimuth) coverage. In this context, it will be noted that the plural FOVs of each individual imaging system need not be contiguous if the FOVs of the different imaging systems are interspersed in a complementary manner so as to together offer substantially contiguous coverage.

As mentioned earlier, the scope of the present invention is not confined to FOV doubling, but includes, in general, -fold FOV expansion. Figure 11 illustrates FOV tripling according to the present invention. Figure 11 shows imaging system 30 including two optical switching mechanisms 144 and 244 that are similar to optical switching mechanism 40 described above. For illustrational simplicity, the associated mirrors for folding the reflected optical paths 142 and 242 of the imaging system are not shown. When both mechanisms 144 and 244 are in their transparent state, the optical path of system 30 is the straight-ahead optical path of the imaging system 30. When optical switching mechanism 144 is in its reflective state, the optical path of system 30 is reflected optical path 142 corresponding to a reflected FOV to the left of Figure 11. When optical switching mechanism 144 is in its transparent state and optical switching mechanism 244 is in its reflective state, the optical path of the system is reflected optical path 242 corresponding to a reflected FOV to the right of Figure 11. 20 167317/2 While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Material that exceeds the scope of the claims, does not constitute part of the cl aimed invention.

Claims (16)

21 167317/4 What is claimed is:

1. . An imaging system comprising: (a) a two-dimensional imaging sensor array; (b) an optical system including: (i) at least one optical arrangement associated with the imaging sensor array and defining a field of view of given angular dimensions; and (ii) an optical switching mechanism for alternately switching an optical axis of the imaging system between a first direction and a second direction, said optical switching mechanism and said at least one optical arrangement being deployed such that said imaging sensor array generates images of at least two substantially non-overlapping fields of view of equal angular dimensions, said substantially non-overlapping fields of view having diverging optical axes in fixed spatial relation (c) a first set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; (d) a second set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; and (e) a sensor switching arrangement associated with said two-dimensional imaging sensor array, said first and second sets of read capacitors and said optical switching mechanism, said sensor switching arrangement being configured to switch connections from each sensor element of said two- 22 167317/4 dimensional imaging sensor array between corresponding capacitors of said first and second sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view such that said first set of read capacitors accumulate information corresponding to a first of said two fields of view and said second set of read capacitors accumulate information corresponding to a second of said two fields of view.

2. The imaging system of claim 1 , wherein said optical switching mechanism includes an apparatus that is alternately substantially transparent and substantially reflective.

3. The imaging system of claim 1 , wherein said optical switching mechanism includes a rotatable disk including at least one pair of alternating segments, a first segment of each said pair being transparent and a second segment of each said pair being reflective.

4. The imaging system of claim 3, wherein said at least one pair includes at least two pairs of segments, wherein said transparent segments are transparent to non-identical ranges of wavelengths and said reflective segments are reflective to non-identical ranges of wavelengths.

5. The imaging system of claim 1 , wherein said optical switching mechanism includes a plurality of microelectromechanical shutters, said apparatus being substantially transparent when said shutters are open and substantially reflective when said shutters are closed. 23 167317/4

6. The imaging system of claim 1 , wherein said optical switching mechanism includes a pair of prisms and a prism displacement mechanism operative to displace at least one of said pair of prisms such that said pair of prisms are alternately adjacent and apart, said apparatus being substantially transparent when said prisms are adjacent and substantially reflective when said prisms are apart.

7. An imaging assembly comprising a plurality of imaging systems according to claim 1 , wherein said imaging systems are deployed in fixed spatial relation such that said substantially non-overlapping fields of view of said plurality of imaging systems together form a substantially contiguous effective field of view spanning at least 120. degrees.

8. The imaging assembly of claim 7, wherein said substantially contiguous effective field of view spans at least 180. degrees.

9. The imaging assembly of claim 7, wherein said substantially contiguous effective field of view spans 360.degrees.

10. The imaging assembly of claim 7, wherein said plurality of said imaging systems includes at least three of said imaging systems.

11. 1 1. The imaging system of claim 1, wherein said sensor switching arrangement switches said connections at a field-of-view switching rate, the imaging system further comprising a read arrangement for reading accumulated information from said first and second sets of read capacitors at a read cycle rate, wherein said field-of-view switching rate is greater 24 167317/4 than said read cycle rate.

12. The imaging system of claim 1 1 , further comprising: (a) a third set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array; and (b) a fourth set of read capacitors including a capacitor associated with each sensor element of said two-dimensional imaging sensor array, wherein said sensor switching arrangement and said read arrangement are further associated with said third and fourth sets of read capacitors, said sensor switching arrangement and said read arrangement being configured such that: (i) during a first half of said read cycle, said sensor switching arrangement switches connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said first and second sets of read capacitors synchronously with switching of said optical switching mechanism between said two fields of view while said read arrangement reads said third and fourth sets of read capacitors; and (ii) during a second half of said read cycle, said sensor switching arrangement switches connections from each sensor element of said two-dimensional imaging sensor array between corresponding capacitors of said third and fourth sets of read capacitors synchronously with switching of said 25 167317/4 optical switching mechanism between said two fields of view while said read arrangement reads said first and second sets of read capacitors.

13. The imaging system of claim 1 , further comprising a processor configured for analyzing a sequence of images from said imaging system to determine whether they indicate the presence of a transient event.

14. A method for acquiring images of two fields of view using a single two-dimensional imaging sensor array, the method comprising: (a) providing an optical arrangement including an optical switching mechanism for alternately directing an image of the two fields of view onto the imaging sensor array; (b) providing for each sensor of the imaging sensor array at least onepair of read capacitors and a sensor switching arrangement forswitching a connection from each of said sensors between the corresponding pair of read capacitors; (c) synchronously operating said optical switching mechanism and said sensor switching arrangement at a field-of-view switching rate such that a first of said pair of read capacitors accumulates information corresponding to a pixel of a first of the fields of view and a second of said pair of read capacitors accumulates information corresponding to a pixel of a second of the fields of view; and (d) reading accumulated information from said pairs of read capacitors at a read cycle rate so as to acquire a sequence of images of 26 167317/4 each of the two fields of view, wherein said field-of-view switching rate is greater than said read cycle rate.

15. The method of claim 14, wherein said field-of-view switching rate is at least four times said read cycle rate.

16. The method of claim 14, further comprising analyzing said sequence of images to determine whether they indicate the presence of a transient event 7 Jabotinsky St. Ramat Gan 52520