AN ILLUMINATION SOURCE

Abstract: An illumination source for illuminating a scene to be captured by a camera (1) comprises a laser diode (4) mounted on the camera housing (2). The output of the laser diode (4) is coupled, via a first single mode optical fibre (5) and connectors (6, 7), to a second stepped mode optical fibre (8) whose free end (10) forms the radiation source. The fibre (8) is formed into a coil (9), and is of sufficient length that the coherence of the laser radiation is destroyed by internal reflections within the fibre (8). This reduces laser "speckle" in the signal recieved by the camera (1).

Description: An Illumination Source The invention relates to an illumination source for illuminating a scene to be captured by a camera, and to a camera having associated therewith such an illumination source. The invention is particularly, but not exclusively, applicable to surveillance cameras which may be used for viewing vehicle number plates in covert surveillance operations at night. The covert reading of a vehicle registration or number plate at night has long presented problems in providing sufficient illumination for the camera to capture the detail of the number plate while remaining unobtrusive. Street lighting may be unavailable; and, even if available, may be insufficient to enable reliable capture of pictures that are sufficiently distinct to enable the number plate to be identified. Whilst it is acceptable to use high power flashes of visible light to capture images of vehicles for speeding offence purposes, the use of additional visible radiation is undesirable when attempting covert surveillance. It is known to use infrared illumination, but the traditional infrared source, which is produced by filtering an incandescent source, can be seen by some people, and has a significant power requirement. UK-B-2248994 discloses an apparatus for viewing a vehicle number plate in covert night-surveillance operations, the apparatus comprising an illuminator for emitting a series of high peak power pulses of infrared light, outside the range visible to the human eye, the illuminator being powered by a pulsed current of at least ten times the continuous rating, and a shuttered camera whose shutter open period is synchronised with the illuminator to receive the pulses of infrared light after reflection from the number plate. This document discloses, in particular, the use of an array of light emitting diodes (LEDs) as the illuminator, but also mentions that a source of laser light could be used. A problem that is not addressed in this document, but which is encountered when attempting to use laser radiation as the illumination source, is laser "speckle" which may prevent reliable identification of detail in a scene. A practical device currently manufactured by one of the proprietors of UK-B-2248994. uses an array of LEDs which takes a considerable amount of power, and still has a relatively limited range. It is an aim of the invention to provide an illumination source and a surveillance camera in which the effects of one or more of the disadvantages set forth above are mitigated. The invention provides an illumination source for illuminating a scene for capture by a camera, the illumination source comprising a laser diode coupled to one end of a first length of optical fibre for transmitting radiation emitted by the laser diode, the other end of the first length of optical fibre being coupled to one end of a second, mismatched optical fibre formed into a coil, the length of the second optical fibre and the diameter of the coil being such that the radiation emitted from the other end of the second optical fibre is substantially incoherent. It has been found that the use of laser radiation for automatic number plate recognition gives increased range for a given power requirement; as the source, which is effectively the end of the long length of fibre, can be arranged very close to the optical axis of the camera lens. The retro-reflective property of vehicle registration plates means that a very high proportion of the emitted radiation is reflected back into the camera lens. The use of laser radiation does, however, cause a problem, because its coherent nature allows interference patterns to form. These interference patterns give rise to the effect known as laser "speckle", which may prevent identification of detail in a scene. In many cases, this will prevent the reliable identification of the alphanumerics on a number plate. It has now been discovered that, when using a comparatively long length of coiled mixed mode optical fibre, a large number of internal reflections are produced. This produces effectively incoherent radiation at the end of the optical fibre. Thus, by connecting the output of the laser diode through a coiled length of mixed mode optical fibre, an intense beam of substantially incoherent radiation can be produced. Consequently, the problem of laser "speckle" can be overcome, or at least reduced to a sufficiently low level, so that the desired detail in the scene can be resolved. The first optical fibre may be a single mode fibre, and the second optical fibre a stepped mode fibre. It has been found that, with a radiation wavelength of 880 nm, if the first optical fibre is a short length of 100pm single mode fibre, a ten metre length of 200/230 pm stepped index fibre wound in a coil of about 100mm diameter is sufficient for effective destruction of the effect of coherence in this application. Of course, different lengths of fibre would be appropriate with different radiation wavelengths, different fibre types and sizes, and different coil diameters. Trial and error methods can be used to determine appropriate lengths. Thus, starting with given fibres, the tightness of coiling can be varied until sufficient incoherence is obtained for a given length of fibre. Alternatively, the length of fibre may be altered for a given coil diameter. The laser diode may emit radiation in the infrared range. This has the advantage that it is not visible, and consequently will not alert anyone to the fact that the taking of a photograph has occurred, thereby allowing covert surveillance. An additional advantage is that, because the arrangement of the present invention allows the use of low power radiation, the risk of anyone detecting the presence of this radiation is reduced as compared with the prior art. This is possible because of the high efficiency of use of the illumination. The invention further provides a surveillance camera including such an illumination source. The surveillance camera may comprise a camera body and a lens housing, said other end of the second optical fibre being located on the lens housing such that the radiation is transmitted parallel to the optical axis of the lens, and said other end of the second optical fibre being located close to the optical axis. In this manner, the illumination source may be combined with an existing surveillance camera with a minimum of changes to the basic camera. Consequently, the advantages of the invention can be realised with minimal changes to existing cameras. The invention will now be described in greater detail, by way of example, with reference to the accompanying drawing, the single figure of which shows, in elevation, a surveillance camera constructed according to the invention. Referring to the drawing, a television camera 1 has a camera body 2 and a lens housing 3. An illumination source comprising a laser diode 4 is mounted on the side of the camera by any convenient means. A short length of optical fibre 5 is coupled to the laser diode, and is provided with a connector 6. The connector 6 mates with a further connector 7 that is attached to a length of optical fibre 8. The optical fibres 5 and 8 are mismatched, so that optical interference is produced in the fibre 8. The fibre 5 may, for example, be a 100pm single mode fibre, while the fibre 8 may be a 200/230pm stepped index fibre ten metres long and formed into a coil 9 of approximately 100mm diameter. The fibre 8 is fixed to the lens housing 3 with its free, or remote, end 10 arranged parallel to the optical axis of the lens. The laser diode 4 preferably emits radiation in the infrared wavelengths when the camera and illumination source are intended for use in covert surveillance. This is, of course, not necessary if the surveillance is not intended to be covert, when radiation in the visible wavelengths could be used. A suitable infra red wavelength laser diode is that sold by Opto Power Corporation under the type number OPC-AOOl-mmm-FC/1 00. As can be seen from the drawing, the fibre 8 is attached to the outer surface of the lens housing 3 at a point close to the end of the housing, and the remote portion of that fibre is arranged substantially parallel to the optical axis of the lens. Consequently, the radiation emitted by the laser diode 4 will travel along the optical axis and, when it encounters a retro-reflective number plate, will be reflected back into the lens, and hence enter the camera through the shutter to provide maximum illumination to the camera detector (for example film or a CCD detector in the case of a CCTN camera). When used for surveillance purposes, for example in reading a vehicle registration plate, the arrangement shown in the drawing has the advantage of being much more compact than that described in UK-B 2248994 and the practical devices currently available. It also has the further advantage that it requires much less electrical power to provide sufficient illumination of the object to enable the camera to capture successfully a resolvable picture of the object. In particular, when used for number plate reading, the power requirements are very low, since the radiating end of the fibre can be placed very close to the optical axis of the lens, and, together with the retro-reflective property of the number plate, provides a very efficient use of the illumination. It is easy to construct the laser drive electronics so that the laser is turned on only when required, thus prolonging the laser life. As the cameras used in vehicle surveillance operations, which are typically mounted on gantries above motorways or other major roads or on roadside pylons, are usually operated at fast shutter speeds (1/1000th of a second is normal), it is useful to pulse the laser at 1/100th of a second synchronised to the camera shutter. This both maximises the laser life and minimises the total power requirements. This may be of importance when the camera is situated at a remote location, where easy access to a main power supply is not available. It will be clear that the particular form of the camera is not part of the present invention, which may be applied to many different types of camera. The invention is particularly useful for roadside CCTV type cameras, where the possibility of synchronising the switching of the laser diode with the shutter opening can be exploited for minimising the electrical power requirements. Such cameras mounted at roadsides already include an electrical signal for operating the shutter, often involving the use of a proximity detector for detecting the presence of an object (vehicle) to be photographed, or a vehicle speed sensor, and that signal or one derived from it can be used to pulse the laser. The embodiment shown in the drawing is particularly convenient for fitting to existing camera designs, since it merely needs to be fixed mechanically to the exterior of the camera with some access to signals within the camera, and thus requires a minimum of modification to the camera. A completely redesigned camera could allow the fibre to be carried within the camera housing. In that case, the fibre could be located even closer to the optical axis with the radiation shining through the lens rather than from alongside it, thereby increasing the efficiency of the illumination source still further. From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design and use of illumination sources and surveillance cameras and component parts thereof, and which may be used instead of, or in addition to, features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation of one or more of those features which would be obvious to persons skilled in the art, whether or not it relates to the same invention as presently claimed in any claim, and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims: Claims 1. An illumination source for illuminating a scene for capture by a camera, the illumination source comprising a laser diode coupled to one end of a first length of optical fibre for transmitting radiation emitted by the laser diode, the other end of the first length of optical fibre being coupled to one end of a second, mismatched optical fibre formed into a coil, the length of the second, optical fibre and the diameter of the coil being such that the radiation emitted from the other end of the second optical fibre is substantially incoherent. 2. An illumination source as claimed in claim 1, in which the laser diode emits radiation in the infrared range. 3. An illumination source as claimed in claim 1 or claim 2, in which the first optical fibre is a single mode fibre, and the second optical fibre is a stepped mode fibre. 4. A surveillance camera including an illumination source as claimed in any preceding claim. 5. A surveillance camera as claimed in claim 4, further comprising a camera body and a lens housing, said other end of the second optical fibre being located on the lens housing such that the radiation is transmitted parallel to the optical axis of the lens, and said other end of the second optical fibre being located close to the optical axis. 6. A surveillance camera as claimed in claim 5, further comprising means for pulsing the laser diode so that it emits radiation when the camera shutter is open. 7. A surveillance camera as claimed in any of claims 4 to 6, in which the camera is a television camera.

Inventors: SMITH DAVID RAYMOND, SMITH, DAVID, RAYMOND
Applicants: ADT SERVICES AG, SMITH DAVID RAYMOND, SMITH, DAVID, RAYMOND
Application year: 2000
Main classification: G08G1
IPC codes: G02B6, G08G1