The present invention relates to a lightbar. More particularly, the invention relates to an intelligent lightbar having increased functionality and which is suitable for mounting to a vehicle.

Emergency services vehicles are generally fitted with emergency lighting which, when activated, alerts the drivers of other vehicles, as well as cyclists, pedestrians and other road users to the presence of the vehicle. Often other road users are required to give way to the emergency vehicle, or to stop or pull over to allow the emergency vehicle to pass by, when the emergency lights are activated. Emergency vehicles are known in which the emergency lighting is mounted to the roof of the emergency vehicle. Mounting the emergency lighting to the roof raises the profile of the lighting and increases the chance of other road users noticing that the emergency lighting is activated. Therefore other road users are more effectively alerted to the presence of the emergency services vehicle.

Emergency services vehicles may also comprise a video camera or closed circuit television (CCTV) camera. The video camera may be used to record a driver's eye view from the dashboard of the emergency vehicle. In an example where the emergency services vehicle is a police car, such a video camera fitted to the police car may be used to record video evidence of potentially criminal activity happening in front of the police car.

Conventional emergency lighting and video cameras for use on emergency vehicles are not intelligently designed and do not form part of an intelligent, integrated system. Accordingly the functionality provided by these components is limited.

We have appreciated that it would be desirable to provide an intelligent lightbar that has improved functionality.

SUMMARY OF THE INVENTION

The invention is defined in the independent claims to which reference should now be made. Advantageous features are set out in the dependent claims.

In a first aspect of the invention, a lightbar for mounting to a vehicle is provided. The lightbar comprises: one or more emergency lights; one or more video cameras; a first compartment; and a second compartment; wherein the one or more video cameras are not contained within the same compartment of the lightbar as the one or more emergency lights.

The lightbar may further comprising a dividing wall between the first compartment and the second compartment.

The first compartment may be located below the second compartment, such that when the lightbar is mounted to a vehicle the first compartment is closer to the vehicle than the second compartment; and the first compartment may contain the one or more video cameras.

The lightbar may further comprise an upper cover and a lower cover respectively covering upper and lower surfaces of the lightbar.

The upper and lower covers may comprise aluminium.

The lightbar may further comprise at least one connecting pillar between the upper and lower covers.

The lightbar may further comprise a heatsink.

At least one of the upper cover and the lower cover may be thermally connected to the heatsink.

The upper cover may comprise a UV reflection coating.

The first compartment may comprise a side wall extending between the lower cover and the dividing wall to enclose the one or more video cameras.

The second compartment may comprise a side wall comprising coloured glass and extending between the upper cover and the dividing wall to enclose the one or more emergency lights.

The lightbar may be dust tight and water tight.

The lightbar may comply with standard IP68.

The lightbar according to any preceding claim, further comprising a single circuit board comprising circuitry for controlling the at least one emergency light and the at least one video camera.

In a second aspect of the invention, corresponding methods for manufacturing the above lightbars are provided.

In a third aspect of the invention, a heatsink for dissipating heat from at least one infra-red illuminator is provided. The heatsink comprises: a first thermally conductive plate comprising a first aperture for receiving the infra-red illuminator, such that the infrared illuminator, when received in the first aperture, is in contact with the first thermally conductive plate; a second thermally conductive plate thermally connected to the first thermally conductive plate, the second thermally conductive plate extending along a plane substantially perpendicular to that of the first thermally conductive plate; and a plurality of thermally conductive blades, comprising: at least one blade connected to the first thermally conductive plate; and at least one blade connected to the second thermally conductive plate.

The first and second thermally conductive plates may be substantially rectangular, and substantially a whole edge of the first thermally conductive plate may be connected to substantially a whole edge of the second thermally conductive plate. The heatsink may further comprise a third thermally conductive plate connected to the second thermally conductive plate, wherein the third thermally conductive plate comprises a second aperture for receiving a video camera.

The plurality of blades may be substantially parallel to one another.

Each blade of the plurality may be spaced apart from one or more adjacent blades by substantially the same distance.

The blades may be substantially rectangular plates, and the angle between the surface normal of the blades and the surface normal of the first thermally conductive plate may be a predetermined non-zero angle.

The predetermined angle may be between about 30° and about 60°, preferably substantially 45°.

In a fourth aspect of the invention, a lightbar for mounting to a vehicle is provided.

The lightbar comprises a compartment, the compartment comprising: an infra-red illuminator; a heatsink according to the third aspect of the invention; and a video camera; wherein the infra-red illuminator is received in the first aperture of the heatsink, and is in contact with the first thermally conductive plate of the heatsink.

The heatsink may comprise a third thermally conductive plate connected to the second thermally conductive plate; and the third thermally conductive plate may comprise a second aperture through which the video camera is received.

The compartment may be enclosed by: a lower wall extending substantially along a plane; an upper wall extending substantially along a plane, the upper wall being

substantially parallel to the lower wall; and shielding extending between the upper and lower walls.

The heatsink may be thermally connected to the lower wall.

The heatsink may be thermally connected to the upper wall.

The upper wall and lower wall may comprise a thermally conductive material.

The thermally conductive material may be aluminium.

The lightbar may further comprise at least one pillar connecting the upper wall to the lower wall.

The at least one pillar may thermally connects the upper wall to the lower wall.

The first and second thermally conductive plates may each be connected to the upper and lower walls.

The at least one blade may be connected to each of the upper and lower walls. In a fifth aspect of the invention, corresponding methods for manufacturing the above lightbars are provided.

In a sixth aspect of the invention, a system is provided. The system comprises: a plurality of video cameras for a vehicle; and processing equipment for processing video signals obtained by the plurality of video cameras; wherein the plurality of video cameras are arranged in distinct first, second, and third groups, the plurality of video cameras comprising: at least one camera for carrying out number plate recognition, and at least one camera for carrying out facial recognition; wherein: in the first group, at least one video camera faces in a forward direction with respect to the vehicle; in the second group, at least one video camera faces in a backward direction, wherein the backward direction is substantially parallel and opposite to the forward direction; in the third group, at least one video camera faces in a sideways direction, wherein the sideways direction is substantially perpendicular to the forward direction; the at least one camera for carrying out number plate recognition is/are arranged to recognise number plates within an angular region extending substantially 360 degrees around the vehicle; and the at least one camera for carrying out facial recognition is/are arranged to recognise faces within an angular region extending substantially 360 degrees around the vehicle.

The plurality of video cameras may be mountable to a roof of the vehicle in an enclosed compartment.

The plurality of cameras may be mountable to a lightbar.

The system may further comprise at least one emergency light enclosed within a compartment separate to that in which the plurality of video cameras is enclosed.

The first group may comprise three or more cameras arranged in a row; the camera at a first end of the row may be rotated relative to the forward direction by a first angle between the forward direction and the sideways direction; and the camera at a second end of the row may be rotated relative to the forward direction by a second angle between the forward direction and the sideways direction.

The cameras rotated by the first and second angles may be rotated by substantially the same magnitude but in opposite senses relative to the forward direction.

The first group may comprise at least two cameras facing along the forward direction.

The second group may comprise two cameras arranged in a row.

The third group may comprise three or more cameras arranged in a row; the camera at a first end of the row may be rotated relative to the sideways direction by a third angle between the sideways direction and the forward direction; and the camera at a second end of the row may be rotated relative to the sideways direction by a fourth angle between the sideways direction and the backward direction.

The cameras rotated by the third and fourth angles may be rotated by substantially the same magnitude but in opposite senses relative to the sideways direction. The plurality of cameras may further comprise a fourth group of cameras in which at least one video camera faces in a direction opposite to the sideways direction.

The fourth group may comprise three or more cameras arranged in a row; the camera at a first end of the row may be rotated relative to the sideways direction by a fifth angle between the sideways direction and the forward direction; and the camera at a second end of the row may be rotated relative to the sideways direction by a sixth angle between the sideways direction and the backward direction.

The cameras rotated by the fifth and sixth angles may be rotated by substantially the same magnitude but in opposite senses relative to the sideways direction.

The first group may consist of four cameras, the second group may consist of two cameras, and the third and fourth groups may each consist of three cameras.

Each of the first, second, third, and fourth groups may comprise at least one camera for carrying out facial recognition and at least one camera for carrying out number plate recognition.

The plurality of cameras may comprise fewer facial recognition cameras than number plate recognition cameras.

At least one of the cameras may comprise a motorised lens for remotely adjusting the camera focus, and/or at least one of the cameras may comprise means for remotely adjusting the pan, tilt, and/or zoom of the camera .

In a seventh aspect of the invention, corresponding methods of providing a plurality of video cameras for a vehicle are provided.

In an eight aspect of the invention, a system is provided. The system comprises: an emergency light; a video camera; and a controller; wherein the controller is configured to: drive the emergency light with a signal modulated at a first frequency; and control the video camera to capture images at the first frequency.

The system may further comprise an illuminator for illuminating a subject of the video camera.

The illuminator may comprise an infrared light.

The infrared light may comprise an infra-red light emitting diode.

The controller may drive the illuminator at the first frequency.

The first frequency may be substantially equal to 25 Hz.

The system may further comprise a compartment in which the emergency light is enclosed, and a separate compartment in which the video camera is enclosed.

The signal driving the emergency light may be modulated with a further signal at a second frequency, wherein the second frequency is lower than the first frequency. The controller may be configured to provide a first driving signal to the emergency light such that the emergency light switches on at a rising edge of the first driving signal, and switches off at a falling edge of the first driving signal.

The controller may be configured to control the video camera by providing a second driving signal to the video camera such that the video camera captures a frame at a rising edge of the second driving signal.

The first and second driving signals may be out of phase with one another.

The first and second driving signals may be substantially π out of phase.

In a ninth aspect of the invention, corresponding methods of controlling an emergency light and a video camera are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 illustrates an upper perspective view of a lightbar according to the present invention;

Figure 2 illustrates a lower perspective view of a lightbar according to the present invention;

Figure 3 illustrates a front view of a lightbar according to the present invention; Figure 4 illustrates a back view of a lightbar according to the present invention;

Figure 4A illustrates a plan view of a video camera, infra-red illuminators, and heatsink according to the present invention;

Figure 4B illustrates a front elevation view of a heatsink according to the present invention;

Figure 4C is an exploded view illustrating the various components of a lightbar according to the present invention;

Figure 5 illustrates the video camera arrangement of a lightbar according to the present invention;

Figure 6 is a schematic diagram of a system including a lightbar according to the present invention;

Figure 7 illustrates signal waveforms used in driving various components of the lightbar according to the present invention; and

Figure 8 illustrates a lightbar according to the present invention mounted on an emergency vehicle. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS A lightbar according to an example of the present invention is illustrated in figures 1 to 4. Figures 1 and 2 respectively show upper and lower perspective views of the lightbar, and figures 3 and 4 respectively show front and back views of the lightbar.

Referring generally to figures 1 to 4, a lightbar 100 according to an example of the present invention comprises a first compartment 120 and a second compartment 140. In one embodiment, the lightbar 100 is mountable to the roof of a vehicle, such as an emergency services vehicle, including a police car or van, ambulance, or fire engine for example. The lightbar 100 may also be suitable for and mountable to any other vehicle, for example a non-emergency vehicle such as a school bus, lorry, rubbish truck, or a road- mapping and imaging vehicle. The first compartment 120 is arranged below the second compartment 140, such that the first compartment 120 is closer to the roof of the vehicle to which the lightbar 100 is mounted than the second compartment 140. The first

compartment 120 comprises one or more video cameras (for example, CCTV cameras) 122, 124, 126, 128, and optionally includes further devices such as an air analysis device, a thermometer, devices to capture the International Mobile Station Equipment Identity of any mobile devices in the vicinity of the lightbar, devices to capture information relating to WiFi or other computer networks broadcast within the vicinity of the lightbar, and so on. The second compartment 140 comprises one or more emergency lights 142, 144, 146, 148. As the second compartment 140 comprising the emergency lights is positioned above the first compartment 120 comprising the cameras, the emergency lights are mounted higher relative to the road upon which the vehicle may travel. This makes the emergency lights 142, 144, 146, 148 more visible during use, and means that the one or more video cameras 122, 124, 126, 128 are positioned at a more optimal height for capturing images of the faces of passers-by and people standing near the vehicle.

To assist in locating certain elements of the lightbar a forward direction 1 10 is defined relative to the direction a vehicle to which the lightbar 100 is mountable would normally drive. A backward direction 1 12 is defined as parallel and opposite to the forward direction 110. A first sideways direction (or driver's left direction) 1 14 is defined as perpendicular to the forward direction 100. A second sideways direction 1 16 (or driver's right direction) is defined as parallel and opposite to the first sideways direction 1 14.

The first compartment 120 comprises one or more video cameras 122, 124, 126, 128. The video cameras 122 face generally in the forward direction 1 10, video cameras 124 face generally in the backward direction 1 12, video cameras 126 face generally in the sideways direction 1 14, and video cameras 128 face generally in the sideways direction 1 16. The video cameras are therefore arranged in four groups: a first group comprising cameras 122, a second group comprising cameras 124, a third group comprising cameras 126, and a fourth group comprising cameras 128. Although the four groups respectively comprise cameras generally facing the forward 1 10, backward 1 12, driver's left 1 14, and driver's right 1 16 directions, the cameras within each specific group can be angularly repositioned azimuthally relative to one another to enable the cameras making up each group to cover a wider field of view along any particular side of the lightbar 100. In one embodiment, each of the first to the fourth groups of cameras 122, 124, 126, 128 respectively comprise at least one camera facing substantially along the forward 110, backward 1 12, driver's left 1 14, and driver's right 1 16 directions.

The second compartment 140 comprises one or more emergency lights 142, 144, 146, 148. The emergency lights may be arranged in sets or one or more rows of lights generally referred to herein as light banks. Light banks 142 face generally in the forward direction 1 10, light banks 144 face generally in the backward direction 1 12, light banks 146 face generally in the sideways direction 114, and light banks 148 face generally in the sideways direction 1 16. Each light bank may comprise, in one example, ten bright while LED lights arranged in two rows of five LEDs each, one row being positioned above the other. Other lighting means are also possible, and incandescent lights, fluorescent lights or tubes, lasers or any other type of light generating device or light bulb, or any combination thereof, can equally well be used. The lights banks 142 and 144 generally illuminate an area along the forward 110 and backward 1 12 directions respectively. In the example shown in the figures, eight forward-facing light banks 142 are provided, together with eight backward-facing light banks 144. The forward 142 and backward 144 facing light banks extend along a direction substantially parallel to one another. Light banks 146, 148 illuminate a side of the lightbar 100 along respective sideways directions 114, 116. In the example shown in the figures, three light banks 146 and three light banks 148 are provided, each set at an angle to one another and to the forward 142 and backward 144 facing light banks. Accordingly, the light banks 146, 148 generate a fan of light that extends radially outwards from the lightbar 100 to cover substantially all directions within the 180° between the forward 142 and backward 144 facing light banks on either side of the lightbar 100. Although three straight light banks 146 and three straight light banks 148 are illustrated, in other examples any number of light banks having any shape can be used, including one or more curved light banks extending between forward 142 and backward 144 facing light banks. For safety and for maximum visibility, arrangements are preferred in which at least one of the emergency lights 142, 144, 146, 148 is visible from any arbitrary azimuthal direction around the lightbar 100, and is visible from within a range of heights above and below the lightbar 100. In the case where LEDs are used in the light banks, each light bank can comprise an arbitrary number of LEDs according to the brightness and visibility desired. The use of a plurality of LEDs in each bank of lights is preferable because it provides a level of redundancy. Thus, if one LED fails there are other LEDs present to ensure that the emergency lights are still visible.

A lower cover 132 is provided to form the bottom of the first compartment 120, and an upper cover 152 is provided to form the top of the second compartment 140. A dividing wall 134 extends between the first 120 and second 140 compartments substantially parallel to the lower cover 132 and the upper cover 152, forming the top of the first compartment 120 and the bottom of the second compartment 140. The first 120 and second 140 compartments are therefore completely separate from one another and delineated by the exterior walls or covers of the light bar and the dividing wall 134. Camera shielding 121 extends between the dividing wall 134 and the lower cover 132 substantially perpendicular to the dividing wall 134 and the lower cover 132 to form a side wall of the first compartment 120 extending circumferentially around the lightbar 100. Light shielding 141 extends between the dividing wall 134 and the upper cover 152 substantially perpendicular to the dividing wall 134 and the upper cover 152 to form a side wall of second compartment 140 which also extends circumferentially around the lightbar 100. In one example at least one of the lower cover 132, the upper cover 152, and the dividing wall 134 is fabricated from aluminium. Optionally, a UV reflection coating may be applied to the top surface of the upper cover 152 to increase the amount UV light reflected by top of the lightbar. This assists in keeping the lightbar cool in bright sunlight. In some embodiments one or more photovoltaic cells or solar panels may be affixed to the top surface of the upper cover 152, for example to generate electricity to power the lightbar 100.

The light shielding 141 is formed of a material which keeps water, dust, and other material out of the second compartment 140 but allows the light generated by emergency lights 142, 144, 146, 148 to shine outwards from the upper compartment 140 away from the lightbar 100. In some embodiments the light shielding 141 comprises coloured glass or plastic and/or optical filters adapted to colour the light leaving the lightbar 100. The exact colouring applied can be chosen such that the emergency lights conform to those used in the jurisdiction in which the lightbar is to be operated. Different parts of the lightbar 100 can comprise differently-coloured filters, for example such that half of the lightbar emits blue light and half emits red light. In alternative embodiments the shielding is substantially clear (uncoloured) and the light colouring is set by the emergency lights themselves, for example by using coloured LEDs or lights with coloured bulbs. In all cases a high transparency of the light shielding 141 is desirable so as not to overly-attenuate the light leaving the lightbar 100. The light shielding 141 may also be arranged so as to optically modify the light leaving the lightbar 100, for example by focussing it towards one or more particular directions. In one example a Fresnel lens is used for this purpose. The internal surfaces of the second compartment 140 on the upper cover layer 152 and/or the dividing wall 134 may be covered with a high-reflection coating, such as a mirror coating. This reflects any light emitted by the emergency lights that illuminates the upper cover 152 or the dividing wall 134 back into the second compartment 140 such that it may subsequently exit the second compartment 140 via the light shielding 141.

The camera shielding 121 is also formed of a material which keeps water, dust, and other material out of the first compartment 120 but allows light to enter at least those regions of the shielding 121 that are in front of the lenses of the video cameras 122, 124, 126, 128. In some embodiments the shielding 121 may be tinted or comprise an anti- reflection coating which allows light to enter the first compartment 120 from the outside but prevents light from leaving the first compartment 120 from the inside. One-way glass or a similar material may be used for this purpose. This prevents onlookers from seeing the one or more video cameras 122, 124, 126, 128 whilst still allowing the video cameras to capture images from outside of the lightbar 100. This may be beneficial, for example, if it is desired to avoid alerting people around the lightbar to the presence of the video cameras, and it helps to keep the locations of the video cameras hidden from view. In other embodiments, the camera shielding 121 is formed of a UV coated shaded polycarbonate, with the material in front of the camera lenses being formed of a clear polycarbonate.

The video cameras can be used for automatic number plate recognition (ANPR), facial recognition, general surveillance around the lightbar 100, or any combination thereof. The lightbar 100 may also include one or more speed enforcement radar transmitters and receivers, thereby allowing for the simultaneous recording of both CCTV video and vehicle speed data in a 360 degree angular region around the lightbar 100. Alternatively, the lightbar 100 could communicate with an external speed enforcement radar device. In one embodiment, specific video cameras are used for number plate recognition and other video cameras are used for facial recognition. For example, one camera out of each of the four groups of video cameras 122, 124, 126, 128 may be used for facial recognition, with the remainder of the video cameras being used for automatic number plate recognition. The video cameras may also be configured to obtain or measure other characteristics of cars or other vehicles surrounding the lightbar 100 which may, for example, be of use to the authorities in tracking particular vehicles.

An automatic number plate recognition system analyses recorded images of vehicle number plates or registration plates obtained from the video camera(s), and performs optical character recognition (OCR) on the images to obtain the vehicle registration number. The vehicle registration number can then be compared against a vehicle registration database to obtain information such as the vehicle owner, whether the vehicle has been properly registered with the authorities, and so on. Optional infrared light emitting diodes (IR-LEDs) or other infrared illumination sources 123, 125, 127, 129 can be provided in addition to the video cameras. This enables the subject of the video cameras, whether a number plate on a vehicle or a face which is to be recognised by the facial recognition software, or any other subject, to be sufficiently illuminated such that images of a suitable quality can be obtained. The video cameras 122, 124, 126, 128 are sensitive to infrared light and therefore able to detect infrared light originating from the IR-LEDs 123, 125, 127, 129 and reflected by the subject. Using infrared radiation for this purpose brings several advantages. First, the video cameras can be used in any level of ambient lighting, and at any time of the day or night. Second, infrared light is invisible to the human eye, so onlookers are not aware of the infrared light emitted by the IR-LEDs. Third, using infrared light in preference to visible light also means that any "stray" visible light, for example that generated by car headlights located adjacent the car's number plate, will not saturate the video camera and prevent the number plate from being recognised. The video cameras may include optical filters which attenuate the visible light reaching the active area of the video camera but allow infrared light having the wavelength(s) emitted by the IR-LEDs to pass through into the video cameras to be recorded.

The IR-LEDs 123, 125, 127, 129 are positioned inside the first compartment 120. In one embodiment the IR-LEDs are located inside the first compartment 120, adjacent a particular video camera for which the IR-LED is designed to provide infrared illumination. Therefore the IR-LEDs point in substantially the same direction(s) as the video cameras. In one example, video cameras 122 are associated with IR-LEDs 123, which generally face along the forward direction 110. Similarly, video cameras 124, 126, 128 are respectively associated with IR-LEDs 125, 127, 129 which generally face along the backward, driver's left, and driver's right directions 1 12, 1 14, 116. If extra illumination is required it is possible to have more than one IR-LED per video camera. Some video cameras may have only one IR-LED associated with them, whilst others may have none, relying instead on the infrared light cast by the IR-LEDs associated with other video cameras or recording in the visible part of the spectrum. In one embodiment, a mixture of infrared light sensitive and visible light sensitive cameras are used to enable the lightbar to accurately image the vehicle surroundings regardless of the ambient light level, particularly at both night time and day time. In some embodiments the same video cameras are optimised for both visible light (day time recording) and infrared light (night time recording) modes of operation.

If the IR-LEDs 123, 125, 127, 129 are provided within the first compartment 120, the camera shielding 121 may be configured such that it is substantially transparent to infrared light. Alternatively, only those parts of the shielding 121 which are in front of an IR-LED and/or a video camera lens may comprise the infrared transparent material, with the remaining parts of the shielding made from glass or plastic which is not necessarily transparent and may, in some examples, be fully opaque. Such an arrangement avoids excess sunlight from entering the first compartment 120, such as sunlight which may enter the lightbar 100 between the video cameras such that it is not recorded by a particular video camera. This prevents the first compartment 120 from being heated up

unnecessarily.

Front and rear connecting pillars 136 are provided between the dividing wall 134 and the lower cover 132. Side connecting pillars 137 may also be provided. In one example the connecting pillars 136 and/or 137 are formed substantially of aluminium. The connecting pillars 136, 137 separate and maintain a constant distance between the dividing wall 134 and the lower cover 136. They also add structural rigidity to the lightbar, and may provide a surface onto which the camera shielding 121 can be fixed. In one example the front and rear connecting pillars 136 are wider than the side connecting pillars 137, as there is generally more space between the video cameras at the front and back of the lightbar 100 compared to at the sides. The connecting pillars 136, 137, the lower cover 132, and the dividing wall 134 form a number of openings in the aluminium case in which the shielding 121 is located.

The connecting pillars 136, 137 may also serve to thermally connect the lower cover 132 and the dividing wall 134, allowing heat generated by, for example, the infra-red illuminators 123, 125, 127, 129 to be dissipated.

The lightbar 100 may also comprise a dividing wall flange 135 fixed to or integral with the dividing wall 134. The flange 135 joins any gap between the outside surfaces of the first 120 and second 140 compartments, smooths out any stepped edge between the compartments 120, 140, strengthens the join between the two compartments, and/or improves the appearance of the lightbar 100. In some embodiments the front and rear connecting pillars 136, and the side connecting pillars 137, are provided between the dividing wall flange 135 and the lower cover 136, instead of or as well as between the dividing wall 134 and the lower cover 136.

The lower cover 132 may comprise one or more strengthening ribs 133, and the upper cover 152 may comprise one or more strengthening ribs 153. In one example, the strengthening ribs 133, 153 may be fixedly connected to or integral with the respective covers 132, 152, and may be made substantially of aluminium. The strengthening ribs 133, 153 allow the rigidity of the covers 132, 152 to be increased, without requiring an overall increase in the thickness of the covers which would add weight to the lightbar 100 and increase the cost of manufacture.

The lower cover 132 comprises a feedthrough or port 160 to enable wires, cables, and/or other connections to pass between the lightbar 100 and the vehicle to which it is mounted, for example via the roof of the vehicle. In one embodiment, this enables the lightbar 100 to communicate with an in-vehicle unit such at that described in relation to figure 6, below.

In one embodiment, the lightbar 100 is water tight and dust tight, and rated to at least one or more of the following standards: standard IP68; international standard EN 60529; British standard BS EN 60529: 1992; and European standard I EC 60509: 1989. Therefore the first 120 and second 140 compartments together as a whole are protected against dust and against complete, continuous submersion in water. Each of or both of the first 120 and second 140 compartments can also be individually sealed to one or more of the same standards. Therefore, in one embodiment, both the first 120 and second 140 compartments are individually sealed to meet the IP68 standard.

As the cameras play a role in security it is important to protect them from tampering or sabotage. Therefore, in another embodiment, the first compartment 120 containing the one or more video cameras may be permanently sealed such that it is not accessible to a maintenance engineer, whereas the second compartment 140 containing the emergency lights may be accessible to a maintenance engineer, for example by removing the upper cover 152 by unscrewing it, without needing to unseal the first compartment 120. This allows access to the second compartment 140 which, for example, may be needed to repair or replace one or more failed emergency lights. However, for security the

maintenance engineer will not be able to access the first compartment 120 containing the video cameras. Access can still be provided to an authorised operator by means of a special tool or key.

The dividing wall 134 serves as a barrier between the first 120 and second 140 compartments. The dividing wall 134 may therefore prevent air from circulating from the one compartment 120, 140 to the other, thereby preventing unnecessary exchange of heat between the compartments. As it is desirable to prevent the video cameras in the first compartment 120 from becoming too hot, the dividing wall may form a thermal barrier between the emergency lights in second compartment 140 which may heat up significantly during use, and the video cameras and associated electronics and circuitry in the first compartment 120 which should remain as cool as possible. For example, the dividing wall 134 may be arranged to dissipate the heat generated by the emergency lights in the second compartment 140, thereby preventing it from entering the first compartment 120. As the covers 132, 152 are made from a thermal conductor such as aluminium, heat is readily dissipated to the outside of the lightbar 100 during use.

One or more heatsinks 400 may be provided to dissipate any heat generated by the components in the first compartment 120, thereby keeping the first compartment 120 cool. In particular, the heatsink(s) dissipate heat from infra-red illuminators 123, 125, 127, 129, for example infra-red LEDs (IR-LEDs), which are used together with the video cameras to illuminate the video cameras' field of view. The heatsink(s) may be thermally connected to the lower cover 132, the upper cover 152, the dividing wall 134, and/or any combination thereof, to improve the efficiency with which it dissipates heat. The lower cover 132, the upper cover 152, and/or the dividing wall 134 may therefore act as part of the heatsink(s) to dissipate heat away from the assembly. Further details of the heatsinks 400 are provided below.

Figure 4A is a plan view of an assembly comprising a video camera 122, infra-red illuminators 123, and heatsink 400 according to an example of the present invention.

Although the video camera 122 and infra-red illuminators 123 are illustrated in figure 4A, a similar arrangement can also be used in respect of any of the video cameras 122, 124, 126, 128, and any of the illuminators 123, 125, 127, 129. The video camera 122 comprises a motor unit 130, which enables the zoom and/or focus of the video camera 122 to be changed as explained below in more detail. On a first side of the video camera 122, one or more infra-red illuminators 123-1 are provided. On a second side of the video camera 122, one or more infra-red illuminators 123-2 are provided. The illuminators 123 are responsible for generating a large amount of heat, and therefore the heatsink 400 is primarily positioned close to the illuminators 123.

Heatsink 400 may be formed of a material with high thermal conductivity. In one example aluminium is used. The heatsink 400 comprises first plates 402, 404 which are respectively in thermal contact with the infra-red illuminators 123-1 , 123-2. In one example the illuminators 123 are positioned within a hole or aperture 402a, 404a drilled into the first plates 402, 404, to ensure that the illuminators 123 are surrounded on all sides by the thermally conductive material of the heatsink. Connected to the first plates 402, 404 are second plates 406, 408, which extend in a direction substantially perpendicular to that of the first plates 402, 404. The end of the first plate 402 closest to the video camera 122 is connected to an end of the second plate 406. Similarly, the end of the first plate 404 closest to the video camera 122 is connected an end of the second plate 408. The other ends of the second plates 406, 408 are each connected to third plate 410 which extends in a direction substantially perpendicular to the second plates 406, 408, and connects the second plates 406, 408 together. The first 402, 404 and third 410 plates are substantially parallel to each other, and the second plates 406, 408 are substantially parallel to each other. The second 406, 408 and third 410 plates form a C-shaped recess into which the video camera 122 is mounted. The third plate 410 includes an aperture through which the video camera 122 passes.

A plurality of fins or blades 412, 422 are connected to the first, second, and/or third plates to increase the surface area of the heatsink 400. This improves the efficiency with which the heatsink 400 may dissipate heat away from the illuminators 123, for example by radiation. Some heat is also transferred to the air surrounding the blades 412, 422 which is then transferred elsewhere in the second compartment 120 by convection air currents. In the example shown in figure 4A, the blades 412 are arranged in a first group 41 1 , and the blades 422 are arranged in a second group 421. The blades 412 of the first group 411 are thermally connected to either the first plate 402 or the second plate 406, and extend at an angle with respect to the first plate 402. In one example the angle may be approximately 45°. The blades 412 are all substantially parallel to one another, and are spaced apart by a substantially constant distance 414. The blades 422 of the first group 421 are thermally connected to either the first plate 404 or the second plate 408, and extend at an angle with respect to the first plate 404. In one example the angle may be approximately 45°. The blades 422 are all substantially parallel to one another, and are spaced apart by a substantially constant distance 424. The distance 424 may be substantially equal to the distance 414. The blades 412 of the first group 411 may extend in a direction substantially perpendicular to the blades 422 of the second group 421. In other embodiments one or more of the blades 412, 422 are thermally connected to the third plate 410, in addition or as an alternative to being thermally connected to the first 402, 404 and/or second 406, 408 plates.

Figure 4B is a front elevation schematic view of the heatsink 400 according to the present invention. The video camera 122 and infra-red illuminators 123 have been omitted for clarity. First plate 402 comprises one or more apertures 402a through into which the infra-red illuminator(s) 123-1 are mounted as described above. In the example depicted in figure 4B, two apertures 402a are provided in the first plate 402, one above the other. Similarly, first plate 404 comprises one or more apertures 404a through into which the infra-red illuminator(s) 123-2 are mounted as described above. In the example depicted, two apertures 404a are provided in the first plate 404, one above the other. The third plate 410 is provided between the first plates 402, 404, and includes aperture 410a in which the camera 122 is located as described above.

As shown in figure 4B, the first 402, 404 and third 410 plates extend from the lower cover 132 to the dividing wall 134, and are thermally connected to both the lower cover 132 and the dividing wall 134. This enables the lower cover 132 and dividing wall 134 to effectively become part of the heatsink 400, thereby allowing heat to be dissipated by conduction through the plates 402, 404, 410 to the lower cover 132 and/or dividing wall 134. The lower cover 132 and dividing wall 134 have a large surface area, and therefore efficiently transfer the heat away from the second compartment 120, either by radiation of convection currents in the surrounding air. The blades 412, 422, and the second plates 406, 408, are not visible in figure 4B, but these elements may also be thermally connected to the lower cover 132 and/or the dividing wall 134.

Figure 4C is an exploded view showing the various components of the lightbar 100. The components have already been described in detail above and that description will not be repeated here. In the exploded view of figure 4C, the lower cover 132 and dividing wall 134 are shown separated from the camera shielding 121 , video cameras 122, 124, 126, 128, and infra-red illuminators 123, 125, 127, 129. The lower cover 132 comprises feedthrough or port 160 as described above. The first compartment 120 is enclosed by walls comprising the lower cover 132, the dividing wall 134, and the camera shielding 121. The heatsinks 400 are not shown in figure 4C to avoid complicating the figure. The exploded view of figure 4C also shows the dividing wall 134 and upper cover 152 separated from the light shielding 141 and the emergency lights 142, 144, 146, 148. The second compartment 140 is enclosed by walls comprising the dividing wall 134, the upper cover 152, and the light shielding 141.

The forward 1 10, backwards 1 12, and sideways 116, 118 directions are indicated in figure 4C. In a lightbar 100 according to an example of the present invention, the various layers shown separated in figure 4C are joined together to form the arrangement depicted in figures 1 to 4.

Figure 5 is a detailed illustration of the inside of the first compartment 120 of the lightbar 100, as would be seen underneath the dividing wall 134 and with the camera shielding 121 removed, and showing the arrangement of the video cameras 122, 124, 126, 128, IR-LEDs 123, 125, 127, 129, and heatsinks 400. Other components of the lightbar 100, such as the lower cover 132, strengthening ribs 133, and feedthrough or port 160, have already been described above and that description will not be repeated here. The forward 110, backward 112, and sideways 114, 116 directions are indicated.

At least one of the video cameras may be equipped with one or more motors to control the zoom and/or focus of the video camera. In the example of figure 5, each of the video cameras comprises motor unit 130 which enables the zoom and/or focus of each video camera to be changed in response to signals sent to the camera via controller 280 (see figure 6 and the corresponding description below).

A first group of cameras comprises video cameras 122a, 122b, 122c, and 122d located on the front of the lightbar 100. Cameras 122b and 122c are mounted to the lightbar so that they face substantially along the forward direction 1 10. Cameras 122a, 122c, and 122d are provided with IR-LEDs 123 on either side. Camera 122a and its associated IR-LEDs 127 are positioned at an angle away from the forward direction 110 towards the sideways direction 114. Similarly, camera 122d and its associated IR-LEDs 123 are positioned at an angle away from the forward direction 1 10 towards the sideways direction 116. In one example, the magnitude of the rotation of the cameras 122a and 122d and their associated IR-LEDs 123 away from the forward direction may be the same, although the sense of rotation is opposite for each of the cameras 122a, 122d.

A second group of cameras comprises video cameras 124a and 124b, located on the back of the lightbar 100. Cameras 124a and 124b are mounted to the lightbar so that they face substantially along the backward direction 1 12. Camera 124a is provided with IR- LEDs 125 on either side. In another example, the second group of cameras may comprise further video cameras 124 and IR-LEDs 125, which may be arranged similarly to those on the front of the lightbar 100 or in any other arrangement.

A third group of cameras comprises video cameras 126a, 126b, and 126c, on the driver's left side of the lightbar 100. Camera 126b is mounted to the lightbar so that it faces substantially along the sideways direction 114. Each of the cameras 126a, 126b, 126c is provided with IR-LEDs 127 on either side. Camera 126a and its associated IR-LEDs 127 are positioned at an angle away from the sideways direction 1 14 towards the forward direction 1 10. Similarly, camera 126c and its associated IR-LEDs 127 are positioned at an angle away from the sideways direction 114 towards the backward direction 112. In one example, the magnitude of the rotation of the cameras 126a and 126c and their associated IR-LEDs 127 away from the sideways direction 1 14 may be substantially the same, although the sense of rotation is opposite for each of the cameras 126a, 126c.

A fourth group of cameras comprises video cameras 128a, 128b, and 128c, on the driver's right side of the lightbar 100. Camera 128b is mounted to the lightbar so that it faces substantially along the sideways direction 116. Each of the cameras 128a, 128b, 128c is provided with IR-LEDs 129 on either side. Camera 128a and its associated IR- LEDs 129 are positioned at an angle away from the sideways direction 116 towards the forward direction 1 10. Similarly, camera 128c and its associated IR-LEDs 129 are positioned at an angle away from the sideways direction 116 towards the backward direction 1 12. In one example, the magnitude of the rotation of the cameras 128a and 128c and their associated IR-LEDs 129 away from the sideways direction 1 16 may be substantially the same, although the sense of rotation is opposite for each of the cameras 128a, 128c.

In one example, the cameras 122a, 122c, and 122d are positioned so that they can detect the numbers plates of vehicles driving within three lanes of traffic in front of the lightbar. Camera 122a is positioned to capture number plates in the lane to the left of the lightbar, camera 122d is positioned to capture number plates in the lane to the right of the lightbar, and camera 122c is positioned to capture number plates in the same lane as the vehicle is driving, directly in front of the lightbar. Similarly, the cameras 126c, 124a, and 128c are positioned so that they can detect the numbers plates of vehicles driving within the three lanes of traffic behind the lightbar. Camera 126a is positioned to capture number plates in the lane to the left of the lightbar, camera 128c is positioned to capture number plates in the lane to the right of the lightbar, and camera 124a is positioned to capture number plates in the same lane as the vehicle is driving, directly behind the lightbar.

Cameras 126a and 128a are positioned so that they can capture number plates of cars at the sides of the lightbar 100. This may be useful when the vehicle to which the lightbar is mounted is driving in a car park or parking lot, with a plurality of parked cars on one or both sides of the lightbar. By driving the vehicle along the rows of the parked cars, the lightbar can efficiently detect the number plate of every vehicle parked in the car park. The remaining cameras 122b, 124b, 126b, and 128b are used for general surveillance or facial recognition, and between them cover an area spanning substantially the entire periphery of the lightbar 100.

In an embodiment, the cameras are arranged such that the cameras for carrying out automatic number plate recognition are able to sense and recognise vehicle number plates in substantially the entire 360 degree azimuth around the vehicle to which the lightbar is mounted. Alternatively or additionally, the cameras for carrying out facial recognition are able to sense and recognise faces in substantially the entire 360 degree azimuth around the vehicle to which the lightbar is mounted. In a preferred embodiment, the lightbar cameras enable concurrently obtained full 360 degree views to be recorded and processed as described above.

Figure 6 is a schematic illustration of a system 200 in accordance with an embodiment of the present invention. The system 200 includes controller 280, transmitter and receiver unit 282, in-vehicle display unit 284, one or more video cameras 290, and one or more emergency lights 292. In one example the system 200 is implemented as a lightbar for a vehicle. The system 200 may be mounted to the vehicle.

Video cameras 290 comprise a first group of cameras 222, a second group of cameras 224, a third group of cameras 226, and a fourth group of cameras 228 in a similar arrangement to that described above. Thus, the cameras 222 of the first group may point in a generally forward direction, the cameras 224 of the second group point in a generally backward direction, and the cameras 226 and 228 of the third and fourth groups point generally in respectively opposite sideways directions. In one example the camera arrangement is similar to or the same as that described in relation to figure 5.

The video cameras 290 are connected to the controller 280 by means of connection 294, indicated schematically in figure 6 by means of a solid line. Connection 294 can be a wired or wireless connection, or a combination of wired and wireless connections. The wireless connection can, in some examples, be a radio frequency, IR, Bluetooth, or WiFi connection or any other type of wireless connection. In one example, connection 294 comprises a wired connection for transmitting power from the controller 280 to the video cameras 290, and a wireless connection for controlling the video cameras. Controlling can comprise adjusting the motors 130 to set the zoom, pan, tilt, and/or focus of at least one video camera, and/or setting the frame capture rate and phase as explained in more detail below. This control of the cameras may be carried out remotely.

In one example, one or more of the cameras is/are provided with embedded processors each running their own operating systems and software, and which are responsible for at least some of the functions of the controller 280. The embedded processors are part of the on board processing capability of the camera. Each of the embedded processors may communicate wirelessly with a centralised controller 280, and/or may communicate directly with the operations room or another vehicle, or an on-foot patrol for example as described in more detail below. In one embodiment the controller 280 is entirely distributed between the cameras in this way, therefore avoiding the need for a separate control module. The lightbar 100 may then solely communicate with the operations room, thereby avoiding any potential distraction for the vehicle driver.

In a further example, both the power and control signals are transmitted from the controller 280 to the video cameras 290 by wired connections. Alternatively, the video cameras 290 may obtain their power from a source independent of the controller 280, for example via a direct connection to a power supply unit (not shown in figure 6) which is connected to the power supply of the vehicle to which the system 200 is mounted. This power supply may be a car or vehicle battery. Other power supplies are possible, such as solar panels mounted to the vehicle or another part of the system 200. Solar panels may additionally or alternatively be mounted to the top surface of the lightbar 100 to charge batteries housed within the lightbar 100. This makes the lightbar 100 self-sufficient as far as its power requirements are concerned, and reduces or eliminates its dependence upon the vehicle for power. This also improves the efficiency with which the lightbar 100 can be installed on a vehicle, as no connection to the vehicle's power supply is required.

As explained in relation to previous embodiments, the video cameras 290 may be used both for automatic number plate recognition and for facial recognition. In one example, those video cameras 222, 224, 226, 228 marked with an "F" in figure 6 are used for facial recognition, with the remainder of the video cameras 222, 224, 226, 228 being used for number plate recognition.

The video cameras 290 optionally comprise infrared LEDs (IR-LEDs) associated with one or more or the video cameras 290 as described above. In the example illustrated in figure 6, video cameras 222 or the first group are associated with IR-LEDs 223, video cameras 224 of the second group are associated with IR-LEDs 225, video cameras 226 of the third group are associated with IR-LEDs 227, and video cameras 228 of the fourth group are associated with IR-LEDs 229. In some embodiments none, or only one, of the IR-LED(s) is provided. IR-LEDs 223, 225, 227, 229 are connected to the controller 280 via wired and/or wireless connection 296 indicated schematically in figure 6 by a long-dashed line. They may obtain their power and control signals in the same or a similar way to the examples described above with respect to the video cameras. The connections 294 and 296 may be combined into a single connection between the controller 280 and the video cameras and IR-LEDs 290.

The system 200 further comprises a transmitter and receiver (TX/RX) unit 282, connected by means of a wired and/or wireless connection 297 to controller 280. TX/RX unit 282 may be powered in the same way(s) as described above in relation to the powering of the video cameras 290. The TX/RX unit 282 allows the system 200, and in particular the controller 280, to communicate for example with an authorised operations room providing surveillance and control functions for the fleet of emergency vehicles. The system 200 can also be configured to communicate directly with another vehicle rather than communicating via the operations room, with an emergency services bike, to a foot- patrol officers, or any combination thereof. This enables, for example, a foot-patrol officer to receive and act upon live streaming video obtained from the lightbar 100. The TX/RX unit 282 may be configured for communication conforming to a wireless standard, such as the 3G or 4G (LTE) standards, or may conform to police radio communication standards. This enables images (still and moving) obtained from one or more of the video cameras 290 to be relayed to the operations room, and also allows the operations room to send instructions to a user of the system 200 to be displayed via the in-vehicle unit 284. In some examples, the TX/RX unit 282 is configured to allow remote control of the cameras 290, the in-vehicle unit 284, and/or the emergency lights 292, for example via an operator located in the operations room. In a similar way as explained above with respect to the controller 280, the TX/RX unit 282 can also be distributed between the on-board processors of the video cameras, thereby avoiding the need for a centralised TX-RX unit 280.

Optional in-vehicle unit 284 comprises a display 286, for example an LCD panel or other form of visual display, and an interface 288, for example a touch panel, mouse, keyboard or keypad, joystick, controller, voice command receiver, or any combination thereof. The in-vehicle unit 284 is connected to controller 280 via wired and/or wireless connection 299. The in-vehicle unit 284 may be powered in the same way(s) as described above in relation to the powering of the video cameras 290. In one example the display 286 may be combined with a touch panel interface 288 to form an interactive touch sensitive display. In one embodiment, the in-vehicle unit 284 comprises a portable touch panel linked to, for example, a police network via WiFi, 3G, or 4G, and which can be connected to a docking station mounted in the vehicle for recharging. This enables a police officer to temporarily remove the touch panel from the vehicle, for example if it is required at the scene or an accident. In-vehicle unit 284 manages communications with a driver or operative of a vehicle to which the lightbar system 200 is mounted. In some examples the display 286 shows a list of scanned vehicle registration numbers that are detected via the video cameras 290 operating in automatic number plate recognition mode. In-vehicle unit 284 also allows the driver or operative to interact with the system 200 via the interface 288. Such interaction may allow the driver or operative to control any aspect of system 200, for example controlling the behaviour of the video cameras 290 and/or the emergency lights 292.

The controller 280 is responsible for controlling the video cameras 290, the emergency lights 292, the transmitter and receiver unit 282, and the in-vehicle unit 284 as explained above. In one embodiment, a single controller unit 280 mounted on a single circuit board is provided to carry out each of these described functions. The controller unit 280 may be located inside the lightbar 100, or inside the vehicle to which the lightbar 100 is mounted. In one embodiment, a separate circuit board is provided for each of the first 120 and second 140 compartments of the lightbar 100.

In one example, one or more particular vehicle registration numbers are

communicated to the system 200 from an operations room via the TX/RX unit 282 and stored in memory at the controller 280. The controller 280 then controls the video cameras 290 to perform number plate recognition on vehicles surrounding the lightbar 100. If any of the video cameras 290 capture one of the particular vehicle registration numbers, the controller 280 instructs TX/RX unit 282 to send a message to the operations room. In one example, the message may be accompanied by one or more images obtained from the video cameras 290, such as an image of the number plate, an image of the vehicle as a whole, and/or an image of the driver of the vehicle. At the same time, an instruction may appear on the display 286 of the in-vehicle unit 284 informing a member of the emergency services that the vehicle is of interest and should be investigated.

The system 200 also includes emergency lights 292. The emergency lights 292 comprise four groups of lights 242, 244, 246, and 248 as described in relation to the previous embodiments. The emergency lights 292 are connected to the controller 280 by means of a wired and/or wireless connection 298, indicated schematically by a short- dashed line in figure 6, and may be powered in the same way(s) as described above in relation to the powering of the video cameras 290.

Although the above description refers to separate units, in other examples any or all of the units may be combined together. In one example, the transmitter receiver unit 282 may located within and form part of the controller unit 280. Other configurations are equally possible. Figure 7 is a graph showing several waveforms relevant to the operation of the video cameras 222, 224, 226, 228, the IR-LEDs 223, 225, 227, 229, and the emergency lights 242, 244, 246, 248. The x-axis 301 represents time, in seconds. The y-axis 302 represents the magnitude of the signals. The waveforms 310 to 318 shown on the graph have each been offset in the y-axis direction for clarity.

Waveform 310 is representative of a typical square wave used to power the emergency lights 292. The duty cycle is illustrated as approximately 50%, although other duty cycles can also be used. The base signal 310 oscillates between high and low magnitudes with a frequency of 25 Hz, so the emergency lights 292 switch on and off 25 times per second. In one example, when the base signal 310 is high the emergency lights are switched on, and when the base signal 310 is low the emergency lights are switched off. Wth such rapid switching on and off, the emergency lights powered by base signal 310 generally appear to an onlooker as if they are continuously lit. In one embodiment, the duty cycle of the waveform can altered to adjust the apparent brightness of the emergency lights.

The emergency lights may be capable of a strobing or flashing behaviour in order to make them more visible and eye-catching to an onlooker. To achieve this behaviour, the base signal 310 is modulated by a further signal 312, which in the present example adjusts the behaviour of the emergency lights such that they appear, to the onlooker, to emit five short flashes per second. The modulated signal is illustrated in waveform 314. More complex flashing or strobing patterns can be obtained, if desired, by adjusting the further signal 312.

Signal 316 illustrates one possible driving signal for the video cameras and

IR-LEDs. When signal 316 is high the IR-LEDs are switched on the video cameras capture an image. When the signal 316 is low the IR-LEDs are switched off and the video cameras do not capture an image. In one example, the video cameras may be timed to capture an image substantially simultaneously with the rising edge of signal 316, or to capture an image after the control electronics wait for a certain time after detecting the rising edge of the signal 316 whilst the signal 316 remains high. This causes the video cameras to capture images in phase with (that is, at the same time as) the illumination of the emergency lights. Such behaviour is useful if the emergency lights are operated in continuous illumination mode, where they are driven only with base signal 310 and no further modulation is present. In this case, the emergency lights enhance the illumination of the field of view of the one or more cameras 290, with the result that a more brightly illuminated image can be obtained.

Signal 318 illustrates another possible driving signal for the video cameras and IR-LEDs. When signal 318 is high the IR-LEDs are switched on the video cameras capture an image. When the signal 318 is low the IR-LEDs are switched off and the video cameras do not capture an image. In one example, the video cameras may be timed to capture an image substantially simultaneously with the rising edge of signal 318, or to capture an image after the control electronics wait for a certain time after detecting the rising edge of the signal 318 whilst the signal 318 remains high. This causes the video cameras to capture images out of phase with (that is, at a different time to) the illumination of the emergency lights, thereby ensuring that the cameras never capture images at the same time as the emergency lights are on. Such behaviour is useful in cases where the emergency lights detract from the image quality obtainable on the video cameras, for example if the emergency lights do not illuminate the field of view evenly, or the colour of the emergency lights detracts from the quality of the image.

In both cases, regardless of whether the video cameras and IR-LEDs are driven with a signal which is in phase or out of phase with the emergency lights, the frequency at which the cameras capture images (and the IR-LEDs illuminate the field of view of the cameras) as represented by the signals 316, 318 matches that of the base signal 310 used to drive the emergency lights. If a different frequency is used then a beat pattern may arise between the flashing of the emergency lights and the frames captured by the video camera, with the result that the moving image obtained on the camera(s) flickers as the emergency lights come in and out of phase with the frame rate of the video camera.

In a preferred embodiment, each of the video cameras and IR-LEDs in the lightbar

100 are driven with a signal which is out of phase with the emergency lights.

Figure 8 illustrates a vehicle 800 to which a lightbar 100 according to the present invention is mounted. The locations of the first 120 and second 140 compartments are indicated. The lightbar 100 is substantially rectangular in shape, such that it spans substantially the whole width of roof 801 of the vehicle 800. In one example, the lightbar 100 is mounted to the roof 801 by means of mounting pieces 802. One mounting piece 802 may be provided at each corner of the lightbar 100. The mounting pieces 802 are attached to the lightbar 100, for example via screws. The mounting pieces 802 are also screwed or fixed by any other suitable means to the roof of the 801 of the vehicle 800. The mounting pieces may be integral with the lower cover 132 of the lightbar 100.

Various modifications to the example embodiments described above are possible and will occur to those skilled in the art without departing from the scope of the invention which is defined by the following claims. In particular, it should be understood that features described in relation to a single embodiment can be present in other embodiments.