GB2578735A

GB2578735A - Camera system

Info

Publication number: GB2578735A
Application number: GB1818046.3A
Authority: GB
Inventors: Booker Robert
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2018-11-05
Filing date: 2018-11-05
Publication date: 2020-05-27
Also published as: GB201818046D0

Abstract

A camera system comprises a camera 101 centred on an optical axis 103, and an element 107 having a reflective face for reflecting light towards the camera 101, the element 107 being arranged such that a point 109 on the reflective face lies on the optical axis 103, forming an angle between the optical axis 103 and a normal 111 to the reflective face. The element 107 rotates about the optical axis 103, and the camera 101 captures successive image frames as the element 107 rotates. The element 107 may rotate through an angle φ, wherein the rate of change of φ is non-linear. The element 107 may rotate 360 degrees about the optical axis 103, and/or may rotate alternatively clockwise and anti-clockwise. The camera 101 may comprise an athermalised lens, and/or an element to control infrared radiation reaching the element 107 when the element 107 is in a particular rotational orientation, i.e. a dark plate to calibrate the camera 101. The camera system may be mounted on the underside of an aircraft to scan its field of view along a line that is substantially perpendicular to the direction of travel of the aircraft, as the element 107 rotates.

Description

Camera system

FIELD

Embodiments described herein relate to a camera system, and in particular, but not exclusively, to camera systems for carrying onboard an aircraft for overhead mapping of terrain and/or reconnaissance.

BACKGROUND

Camera systems for surveillance and/or reconnaissance are known in the art. In order to capture images over a wide area, these systems either make use of multiple image sensors and/or involve complicated scanning mechanisms. Such systems are often limited to imaging in certain wavebands or at certain distances and may require complex alterations in order to extend their range or wavelength sensitivity.

It is desirable, therefore, to provide a camera system capable of capturing images from a wide Field of Regard (FOR) having greater simplicity and/or ease for adapting to different imaging environments.

SUMMARY

According to a first aspect of the present invention, there is provided a camera system comprising: a camera centred on an optical axis of the system; and an element having a reflective face for reflecting light towards the camera, the element being arranged such that a point on the reflective face lies on the optical axis, the reflective face being inclined with respect to the optical axis such that an angle is formed between the optical axis and a normal to the reflective face that passes through said point; wherein the element having the reflective face is arranged to rotate about the optical axis; the camera being configured to capture successive image frames as the element having the reflective face rotates about the optical axis.

In some embodiments, the angle formed between the optical axis and the normal to the reflective face is 45 +/-20 degrees. In some embodiments, the angle formed between the optical axis and the normal to the reflective face is 45 +1-5 degrees.

In some embodiments, as the element rotates about the axis, the angle formed between the optical axis and the normal remains constant.

In some embodiments, the element having the reflective face is arranged to rotate through an angle p about the optical axis, wherein the rate of change of the angle cp is non-linear. The rate of change of the angle cp may be non-linear during a period of the rotation in which the camera captures the successive image frames.

In some embodiments, the element having the reflective face is arranged to rotate 360 degrees about the optical axis.

In some embodiments, the element having the reflective face is arranged to rotate alternatively clockwise and anticlockwise about the optical axis. In some embodiments, the element having the reflective face is arranged to rotate through an angle in either the clockwise or anticlockwise direction before switching to rotate in the opposite direction, wherein cp is less than 360 degrees. In some embodiments, the angle cp. is less than or equal to 180 degrees.

In some embodiments, the camera comprises an athermalised lens.

In some embodiments, the system further comprises a controller for triggering the capture of image frames by the camera, the controller being configured to trigger the camera in synchronization with the rotation of the element having the reflective face.

In some embodiments, the system comprises an element arranged to control IR radiation reaching the element having the reflective face when the element having the reflective face is in a particular rotational orientation.

According to a second aspect of the present invention, there is provided an aircraft comprising a camera system according to the first aspect of the present invention.

In some embodiments, the camera system is mounted on the aircraft such that light from beneath the aircraft is reflected by the element having the reflective face towards the camera.

In some embodiments, the element having the reflective face is arranged to scan the field of the view of the camera along a line that is substantially perpendicular to the direction of travel of the aircraft as the element having the reflective face rotates.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic of a camera system according to an embodiment; Figure 2 shows a schematic of a rotating reflective element as used in a camera system according to an embodiment; Figure 3 shows a schematic of a camera system according to a further embodiment; Figure 4 shows a schematic of a camera system according to a further embodiment; Figure 5 shows examples of different scanning profiles as may be employed in an embodiment; Figure 6 shows a schematic of a camera system according to a further embodiment; Figure 7 shows a schematic of a camera system according to a further embodiment; Figure 8 shows a schematic of a camera system according to a further embodiment; Figure 9 shows a schematic of a camera system according to a further embodiment; Figure 10 shows a schematic of a camera system according to a further embodiment; Figure 11 shows an example of a series of scans comprised of individual images as captured by a camera system according to an embodiment, Figure 12 shows a schematic of a camera system according to a further embodiment; and Figure 13 shows a schematic of a camera system according to a further embodiment.

DETAILED DESCRIPTION

Figure 1 shows a schematic of a camera system according to an embodiment. As shown in Figure 1, the system comprises a camera 101, which is aligned on the optical axis 103 of the system and a lens 105 arranged to form an image of a field of view on the camera. The camera itself may be a cooled CMT or COD, for example. The lens may be a simple fixed lens or a zoom lens. The lens may have a controlled focus to set or maintain a given depth of field. For example, the lens may be configured to change focus to compensate for changes in air pressure (e.g. at different altitudes) or changing height above ground. The lens may have an athermalised focus. The athermalised focus may be achieved by optical design, passive means such as controlled expansion tubes or active means such as a motor.

The system also includes a reflective element 107 having a reflective face arranged to direct light towards the lens. In this example, the reflective face is a planar mirror although it will be appreciated that other types of reflective element or surface may be used. For example, the reflective element may comprise a prism, and the reflective face may comprise a surface of the prism. The reflective element 107 is positioned such that a point 109 on the reflective face lies on the optical axis. The reflective face is inclined with respect to the optical axis 103, such that an angle B is formed between the optical axis and a normal 111 extending from the point 109 on the reflective face that lies on the optical axis 103.

The reflective element 107 is movable so as to alter the field of view that is imaged by the lens 103 onto the camera 105. Specifically, the reflective element is rotatable about that part of the optical axis 103 that extends from the reflective face towards the camera. As the reflective element 107 rotates about the optical axis by angle co, the point 109 on the reflective face that lies on the optical axis does not move and the angle formed between the optical axis and the normal 111 to that point remains as G. However, the actual direction in which the normal 111 extends changes as the element rotates (i.e. as the angle (p changes); accordingly, different fields of view at imaged onto the camera as the element rotates. The images captured of each individual field of view may be stitched together using appropriate image processing techniques, so as to build up a single image of the FoR and so provide a much larger view or panorama.

The rotation of the reflective element and the consequent shift in the instantaneous Field of View (FoV) can be further understood with reference to Figure 2, which shows a reflective element according to one embodiment. In the embodiment shown in Figure 2, the reflective element is shown to be prism-shaped with a planar reflecting surface on one side. However, it will further be understood that it is by no means essential for the element to be prism shaped and other shapes are possible. Figure 2 shows views of the reflective element at different time points t1-t8 during its rotation about the optical axis. At each time point, and associated rotation angle co, the optical axis is shown by a dashed line and the normal to the reflective face is shown by a continuous line. In each case, an arrow shows the direction in which a ray of light travelling towards the reflective face along the optical axis will be reflected by that face.

In the example shown in Figure 2, the angle 0 between the optical axis and the normal to the reflective face is chosen as being approximately 45 degrees. The prism-shaped element is depicted as rotating clockwise about the optical axis, but the same principles will apply in the event that the rotation is anticlockwise. It will be understood from Figure 2 that, having reached the orientation shown at time point ts, the reflective element will continue to rotate and in so doing arrive back at the orientation shown at time point ti, at which point the process will repeat.

As can be seen in Figure 2, the rotation of the reflective element causes the arrow to rotate. In this example, where the angle e is chosen as being approximately 45 degrees, the arrow's rotation describes a circle in a plane that is perpendicular to the part of the optical axis that extends from the reflective face towards the camera. This is perhaps most evident by comparing the views shown at time points t1 and t8. At time point t8, the reflective element has rotated half-way about the optical axis compared to its position at time point t1; at time point t1, the arrow is pointing straight downwards, but by the time the element has rotated half-way around the optical axis, the arrow is now pointing directly upwards, having rotated through an angle of 180 degrees from its position at time point t1.

It will be appreciated that the reflective element 107 need not be arranged directly in line with the camera; additional optics may be used to "fold" the optical path between the reflective scanning element and the camera, in the interests of saving space. Figure 3 shows an example embodiment in which a fixed folding mirror 113 is positioned between the camera 101 and the reflective scanning element 107. The folding mirror 113 can help maintain the optimum path length between the reflective element 107 and the camera 101, whilst ensuring the camera and reflective element can be housed together within a smaller volume than might otherwise be the case. In other embodiments, a prism may be used in place of the folding mirror 113. In other embodiments, folds may be included between the elements from which the lens is constructed i.e. the lens may be designed and built to include single or multiple folds in the optical path. One or more of the folds may be adjustable to allow for correct alignment of the camera with the optical axis.

Figure 4 shows an example of how the camera system may be used to obtain images across a Field of Regard (FoR) by successively sampling the camera's instantaneous field of view as the reflective element rotates about its axis. In this embodiment, the reflective element comprises a mirror positioned at one end of a motor 401 that is configured to rotate about the optical axis of the camera 403. As can be seen from Figure 4A, the rotation of the motor, and by extension the mirror, will cause the sight line of the camera to sweep across the ground, so shifting the field of view captured on the camera. In the embodiment shown in Figure 4A, the mirror is inclined such that a normal to the mirror forms a 45 degree angle with the optical axis. As a result, the sight line is swept across a plane that is perpendicular to that part of the optical axis that extends from the mirror towards the camera. Figure 4B provides an illustration of how the shape of the instantaneous field of view (more specifically, its projection onto a flat plane) varies as the reflective element rotates. The image orientation 'tumbles' as the reflective element rotates, producing the swath of images shown in Figure 4B. A complete image is built by mosaicking these overlapping and incrementally rotated images.

It will be understood that in some embodiments, the reflective element need not perform an entire 360 degree rotation. For example, the element may rotate through a smaller angle before rotating back again in the other direction, in a pendulum fashion.

This might be appropriate where, for example, it is desired to capture images from a plane that lies either beneath the camera system (as in Figure 4) or above the camera system, but not both.

In some embodiments, depending on the FoR scan angle, different types of scan, including raster scans, may be implemented. These scan types can be implemented by varying the speed at which the reflective element rotates about the camera optical axis as a function of time -in other words, by varying the rate of change of angle (p as a function of time. Examples of these different scan types are shown in Figures 5A and 5B. In Figure 5A, the graph 501 represents a continuous "helical" scan pattern, whilst graph 503 represents a "saw-tooth" scan, and the graph 505 shown in Figure 5B represents a "zig-zag" scan. In each case, the solid lines represent the active parts of the scan i.e. a time period during which images are captured, whilst the dashed lines represent inactive parts of the scan in which the reflective element rotates but no images are captured. In some cases, the active part of the scan may be non-linear (i.e. the rate of change of angle co may not be constant throughout the active part of the scan). Referring back to Figure 43, it can be seen that the ground footprint of the images captured during a single scan will vary across the swath; by varying the rate of change in the angle (p throughout the scan, it may be possible to compensate for differences in resolution between the various images, as well as helping to optimise the overall rate of image acquisition.

The graph 507 represents a fourth scanning arrangement, in which the reflective element rotates continuously, through an angle greater than 180 degrees, and in which the rotation of the element accelerates during passive (inactive) portions of the scan. The zig-zag scan 503 and saw-tooth scan 505 will typically be implemented when the total active scan is significantly less than 180 degrees i.e. the reflective element rotates through a smaller set of angles.

Figure 6 shows a more detailed schematic of the components of the camera system in an embodiment. In this embodiment, the camera 601, lens 603 and reflective element 605 are contained within a housing 607, the housing 607 being carried onboard, or suspended from, an airborne platform such as an aircraft, for example. An opening 609 is provided in one side of the housing to allow light from a region of interest to enter the housing 607. The light entering the housing may then be reflected by the reflective element 605 towards the camera 601. In addition to the camera 601, lens 603 and reflective element 605, the housing contains a motor drive 611 and power supply unit PSU 613 for controlling the rotation of the reflective element 605 about the optical axis. Also included within the housing is a controller 615 for controlling the operation of the camera and the motor drive 611, and an Attitude and Heading Reference System (AHARS) 617, which provides navigational information to the controller 615. The navigational information includes, for example, aircraft position data, attitude, and time. The rotation of the reflective element 605 is measured relative to the AHARS datum; when the desired scan position is reached, the controller 615 sends trigger signals to the camera to control the precise times at which images are captured and the integration times to be used for each respective image. The controller also sends signals to the motor drive 611 to control the rotation of the reflective element 605. By doing so, the controller 615 is able to synchronize the image capture with the reflective element's rotation.

The image frames captured on the camera are read out and stored in a memory for subsequent processing. Each image frame may be accompanied by meta-data that describes the frame characteristics e.g. time of capture, integration time, rotational orientation of the reflective element at the time of capture, etc In the present example, in which the camera is carried onboard or suspended from an aircraft, the meta-data may also include information concerning the aircraft position and altitude, etc. Once captured, the image frames may be forwarded to an image processing module (not shown) for stitching together the frames.

Figure 7 shows a schematic of the camera system in an embodiment in which the IR part of the spectrum is used to obtain the camera images. Here, the system includes a thermal reference such as a dark plate or baffle 619 that serves to control radiation reaching the camera when the reflective face is oriented towards the calibration device.

Figure 7A shows the case in which the reflective face is facing downwards, so as to reflect light entering the housing towards the camera, and Figure 7B shows the system at a later point in time at which the reflective element has rotated through 180 degrees such that the reflective face is now facing upwards towards the thermal reference.

With the reflective element in the orientation shown in Figure 7B, the thermal reference presents a uniform black body such that a controlled amount of radiation is provided to the optical system, to aid detector non-uniformity correction.

In some embodiments, the housing of Figures 6 or 7 may be mounted on the underside of an aircraft, such as an aeroplane, UAV, or helicopter. As the aircraft moves in a particular direction of travel, the rotation of the reflective element will allow the camera to record images of an extended field of view on either side of the aircraft. In this way, an ultra-wide field of view can be built up by successively sampling the camera instantaneous field of view whilst the reflective element moves the sightline across the ground under the aircraft.

In one embodiment, the housing may be mounted such that the part of the optical axis that extends from the reflective face towards the camera remains parallel with the direction of motion of the aircraft. For example, in the event that the aircraft is travelling along a line parallel with the ground, the part of the optical axis that extends from the mirror towards the camera may also be parallel to the ground, whilst in the event that the aircraft is climbing at an angle of 20 degrees to the ground, that part of the optical axis may also be oriented at 20 degrees to the ground. An example of this arrangement is shown in Figure 8. Figure 8A shows the housing 801 mounted on the underside of an aircraft 803, such that the part of the optical axis that extends from the reflective face towards the camera is oriented parallel to the aircraft's direction of motion. Figure 8B shows how the instantaneous field of view varies as the reflective element rotates about the optical axis. With the angle of incline of the mirror set at 45 degrees (specifically, the angle between the optical axis and the normal to the point on the surface of the reflective face that lies on the optical axis), the shift in field of view will be similar to that shown in Figure 4B, with the reflective element scanning the field of view along a line that is perpendicular to the aircraft's direction of travel.

In another embodiment, the housing may be mounted such that the part of the optical axis that extends from the reflective face towards the camera is oriented at an angle y to the direction of motion of the aircraft. For example, in the event that the aircraft is travelling along a line parallel with the ground, the part of the optical axis that extends from the reflective face towards the camera may be oriented at 30 degrees to the ground, whilst in the event that the aircraft is climbing at an angle of 20 degrees to the ground, that part of the optical axis may be oriented at 50 degrees to the ground. An example of this arrangement is shown in Figure 9. Figure 9A shows the housing mounted on the underside of an aircraft, such that the optical axis between the reflective element and the camera oriented is oriented at an angle y to the aircraft's direction of motion. Figure 98 shows how the instantaneous field of view varies as the reflective element rotates about the optical axis. In this case, the tilt of the optical axis means that the field of view (when projected onto a curved Earth surface, and when considering large distances) is scanned in an arc, rather than a straight line (as would be seen on a flat Earth projection).

In another embodiment, the housing may be mounted such that the part of the optical axis that extends from the reflective face towards the camera remains parallel with the direction of motion of the aircraft, but the angle between the optical axis and the normal to the point on the surface of the reflective face that lies on the optical axis is set to be smaller than 45 degrees; in other words, the reflective face is tilted to be closer to a vertical orientation than is the case in Figure 8. An example of this arrangement is shown in Figure 10A. Figure 10B shows how the instantaneous field of view varies as the reflective element rotates about the optical axis. With the angle of incline of the mirror set to be smaller than 45 degrees (specifically, the angle between the optical axis and the normal to the point on the surface of the reflective face that lies on the optical axis), the field of view is scanned in an arc (even when considering a flat Earth projection), rather than a straight line. Here, the curvature of the arc is in the opposite direction to that of the case shown in Figure 9; the field of view is projected further forwards towards the edges of the scan, and lies further back towards the midpoint of the scan. It will be appreciated that the opposite effect will occur in the event that the angle between the optical axis and the normal to the reflective face exceeds 45 degrees.

It will be understood that in each case above, the camera may be mounted facing either towards or away from the aircraft's direction of motion. Moreover, the shape of the ground swath (i.e. the region of the ground from which images are acquired as the reflective element rotates) may be adapted for different applications by utilising a combination of the arrangements shown in Figures 9 and 10. For example, the angle y at which the camera is oriented relative to the aircraft's direction of motion may be varied at the same time as varying the angle between the optical axis and the normal to the point on the surface of the reflective face that lies on the optical axis. In some embodiments, the components of the system may be arranged to allow dynamic variation of the parameters y and 0 (as opposed to these being fixed parameters of the system).

Figure 11 shows an example of how images covering an extended region of interest can be obtained as the aircraft travels above that region. In this example, it is assumed that the camera and reflective element are arranged as shown in Figure 8. As the aircraft moves forward, the field of view is repeatedly scanned across a line close to perpendicular to the line along which the aircraft is travelling, thereby building up a series of scans that cover the region. Each scan is similar to that shown in Figure 4B, with successive scans being shifted further forwards as a result of the aircraft's forwards motion.

It will be appreciated that the period of time that elapses between capturing successive scans may be varied, so as to increase or decrease the overlap between the successive scans. For example, with reference to Figure 11, as the aircraft travels in a forward direction, the extent L to which the successive frame capture swaths overlap one another in that direction can be adjusted by varying the rate at which the successive scans are carried out.

As noted above in relation to Figure 4B, the image orientation 'tumbles' as the reflective element rotates. Thus, appropriate image processing techniques may be required in order to build up a single complete image by mosaicking the overlapping and incrementally rotated images.

In order to stitch the captured images into a single composite image, it is helpful to consider how the instantaneous field of view changes as the reflective element rotates. To do so, we can consider how the path of a ray of light, as seen by the camera, is reflected off the reflective element (i.e. to consider the camera to be a projector, so that the correct incoming ray path can be determined).

As a first step, we can define a number of parameters (it will be appreciated that the values for each parameter as provided here are by way of example only): Look Forward Angle: v (Nu) = 20 degrees Nominal mirror fold angle: 13 = 45 degrees Along Track FoV: FoVAT = 8 degrees Across Track FoV: FoVx-r = 6 degrees Height of a/c datum above flat earth: height = 1000m We also define: Position in Longitudinal (X) axis {Forward / Back}; Position in Transverse (Y) axis {Left / Right}; Position in Normal (Z) axis {Up / Down}; Angles in Azimuth (w) around the Normal axis; Inclination (0) around the Transverse axis; and Bank (ro) around the Longitudinal axis.

In the present analysis, all angles are taken to be positive in the clockwise direction when looking along the positive direction of their relative axis of rotation.

Next, we can define 3D vector and point forms and matrix manipulations. Each point is defined in the form from the origin, and a vector is a unit vector in the same axis frame.

The platform original point PA is given by:

ION

The nadir vector v"adir, from a/c, is given by: nadir The Ground Ref PG (point below the aircraft datum) is given by: The Ground surface normal NG (assuming a flat Earth) is given by: We can create a function t(v) to calculate the interception of the ray with the plane: (PQPA) q1/4) Equation (1) 1/4-NG where t is the distance along the line.

We find the point Groundpoidv,t) in the plane by moving from point P a distance t along the vector: Cimuri1/41poulto, ,0 (PA +. (v) Equation (2) Next, we can use conventional rotation matrices for the angles (t), B and lit around the X, Y and Z axes, respectively (assuming a right handed coordinate set): /1 0 0 0 cos(4>) ---sin(4)) çØ sin (<10) cos(4) cos(e) 0 sin (0) 0 1 0 ty-sin (0) 0 cos( (3) cos(V) -sin( ',110 \1 R ( sin ( '1) cos( q() 0 0 0 1, Next, we can create a surface normal for the mirror, given a pre-set Look and Fold angle, for any rotation angle, by carrying out the following 3 steps: (1) Set up a normal vector pointing forward along the Longitudinal (X) axis.

v start t.) (2) Angle the vector towards the Scanner zero angle datum (upwards) by mirror angle 0 to form the starting mirror normal.

We can define: -Rot (0) scanner zero -= y 0.707 scanner it.o v--0.707 (3) Adjust the mirror normal for scanner "Scan Angle" yo (rotate about the X axis by the scan angle): We define the function: NScan 1'.1(4'):= Rotxt(tki'Nscanner Next, we consider the camera and a reflected ray of light: (4) First, set up a boresight ray from the camera pointing backwards along the X axis. 20 Rcim where Ream can be a matrix where each column vector represents the different camera rays (such as the corner rays when used to project the observed footprint onto the 25 ground).

(5) Next, we can reflect that ray off the mirror using the mirror normal from step (3) above. We define: Rmitror( :" Ream 2c(Re) Scan MOM 'NScan M() The ray "scanned" bundle now has to be rotated forward to allow the system to "look forward" by angle v: RaYexit(0) R°100.-Rmi rorM We can use this to find the intercept with the ground using Equations (1) and (2): Groandcoord) := Groundpoint( Ra Iray" co)) Converting from the aircraft Body_Axis to conventional ground co-ordinates X, Y, Z, yields: GrouuMM) (Ground,to coorc * Accordingly, if one considers the camera to be a projector, one can determine the point on the ground at which each light ray from the projector will intercept the ground, for a particular time point in the rotation of the reflective element. The reverse then also applies; namely, that one can determine, for each light ray arriving at the camera, the point on the ground from which that ray originates. In this way, one can establish the instantaneous camera footprint or field of view at different rotational angles of the reflective element.

Figure 12 shows an embodiment in which the camera 1201 and reflective element 1203 are mounted such that the part of the optical axis that extends from the reflective face towards the camera is oriented vertically. Here, the rotation of the reflective element scans the field of view in a circular path, with the images 1205 acquired from each successive 360 degree rotation forming a ring on the surface of a (virtual) hemisphere, 1207 as shown in Figure 12B. As shown in Figures 13A and 138, the relative position of the ring can be moved up the hemisphere by increasing the tilt of the reflective face closer to the vertical, such that the angle S formed between the optical axis and the normal to the reflective face is increased. More generally, the relative position of the ring can be changed by increasing or decreasing the tilt of the reflective face closer to or further away from the vertical, such that the angle 0 formed between the optical axis and the normal to the reflective face is increased or decreased. Thus, by adjusting the tilt on the reflective face, it is possible to obtain scans directed at different regions of space above or below the horizon.

It will be appreciated that, in addition to the features discussed above, embodiments may include further features such as motion compensation and/or the addition of a slot-less interferometer to provide high resolution spatial imaging and/or low resolution hyperspectral capability. A counter scan may be used to compensate for image motion during the camera shutter or stare period.

Embodiments provide a simplified means for achieving an ultra-wide field of view, with high resolution imagery at sub-FMV (full motion video) rates. By virtue of a simplified design, a camera systems according to the embodiments described herein can provide a high level of reliability. As the camera or other components in the system become obsolete, these can be easily replaced without having to necessarily adapt other components in the system. Similarly, the system provides flexibility in terms of wavebands and modalities, since cameras with different wavelength sensitivities (visible, UV, SWIR, IR, for example) can be readily exchanged and implemented in the system with minimal need for adjustment of the other core scanning optics. The use of a zoom lens and / or flexible scanner allows the system to be easily adapted for dynamic changes in platform height and/or speed when used in airborne applications, for example.

Embodiments can also provide a reduced stare time (shutter speed) and improved noise performance compared to conventional EO/IR detectors.

Embodiments described herein can be used in monitoring applications, such as in IR Ultra-Wide field of view reconnaissance sensing applications, for example.

Embodiments are applicable to Wide Field of Regard situational awareness systems such as ADAD, periscopes, IRST and military vehicle sights where repeated horizon scan is required. It will be appreciated that, whilst a number of the embodiments described herein pertain to aircraft based systems, embodiments may also be implemented in ships, ground vehicles and or used in static / fixed operation.

While certain embodiments have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel methods, devices and systems described herein may be embodied in a variety of forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.

Claims

CLAIMS1. A camera system comprising: a camera centred on an optical axis of the system; and an element having a reflective face for reflecting light towards the camera, the element being arranged such that a point on the reflective face lies on the optical axis, the reflective face being inclined with respect to the optical axis such that an angle is formed between the optical axis and a normal to the reflective face that passes through said point; wherein the element having the reflective face is arranged to rotate about the optical axis; the camera being configured to capture successive image frames as the element having the reflective face rotates about the optical axis.
2. A camera system according to claim 1, wherein the angle is 45 +/-20 degrees.
3. A camera system according to claim 1, wherein the angle is 45 +/-5 degrees.
4 A camera system according to any one of the preceding claims, wherein as the element rotates about the axis, the angle formed between the optical axis and the normal remains constant.
5. A camera system according to any one of the preceding claims, wherein the element having the reflective face is arranged to rotate through an angle c about the optical axis, wherein the rate of change of the angle c is non-linear.
6. A camera system according to claim 5, wherein the rate of change of the angle c is non-linear during a period of the rotation in which the camera captures the successive image frames
7. A camera system according to any one of the preceding claims, wherein the element having the reflective face is arranged to rotate 360 degrees about the optical axis.
8. A camera system according to any one of the preceding claims, wherein the element having the reflective face is arranged to rotate alternatively clockwise and anticlockwise about the optical axis.
9. A camera system according to claim 8, wherein the element having the reflective face is arranged to rotate through an angle y in either the clockwise or anticlockwise direction before switching to rotate in the opposite direction, wherein q is less than 360 degrees.
10. A camera system according to claim 9, wherein cp is less than or equal to 180 degrees.
11. A camera system according to any one of the preceding claims, wherein the camera comprises an athermalised lens.
12 A camera system according to any one of the preceding claims, further comprising a controller for triggering the capture of image frames by the camera, the controller being configured to trigger the camera in synchronization with the rotation of the element having the reflective face.
13. A camera system according to any one of the preceding claims, wherein the system comprises an element arranged to control IR radiation reaching the element having the reflective face when the element having the reflective face is in a particular rotational orientation.
14. An aircraft comprising a camera system according to any one of the preceding claims
15. An aircraft according to claim 14, wherein the camera system is mounted on the aircraft such that light from beneath the aircraft is reflected by the element having the reflective face towards the camera.
16. An aircraft according to claim 14 or 15, wherein the element having the reflective face is arranged to scan the field of the view of the camera along a line that is substantially perpendicular to the direction of travel of the aircraft as the element having the reflective face rotates.