GB2354898A

GB2354898A - Vehicle licence plate imaging using two-part optical filter

Info

Publication number: GB2354898A
Application number: GB9915965A
Authority: GB
Inventors: Brian David Vincent Smith
Original assignee: Pearpoint Ltd
Current assignee: Pearpoint Ltd
Priority date: 1999-07-07
Filing date: 1999-07-07
Publication date: 2001-04-04
Anticipated expiration: 2019-07-07
Also published as: GB9915965D0; GB2354898B

Abstract

A vehicle licencing plate imaging apparatus comprises a camera 2 with a lens 3 selected to provide a field of view 4 within which a passing vehicle (5, Figure 3a) is imaged by the camera 2 via a filter 6. An artificial illumination source 7 has a wavelength distinct from that of normal ambient illumination, and the filter 6 is divided into regions (11,12, Figure 2) that respectively pass ambient illumination and illumination from the artificial light source 7. The camera 2 is arranged so that images of a passing vehicle are recorded repetitively, at a frequency such that images of the vehicle licence plate are recorded at least once in each of a plurality of sections of the field of view 4 that correspond to each of the sections (11,12) of the filter 6. Owing to the imaging of the vehicle licence plate respectively with light of different wavelengths, a reliable image of the vehicle licence plate can be obtained with licence plates of differing reflective characteristics.

Description

1 2354898 Vehicle licence plate imaginct The present invention relates to

a vehicle licence plate imaging apparatus particularly although not exclusively for forming an image of plates having different optical spectral and retroreflective characteristics.

There are at least two types of vehicle licence plate in use on a worldwide basis. Some have simple painted characters forming the vehicle registration number, often with the characters embossed in the metal plate. Others have either the characters or the plate background made from or painted with a retroreflecting material, such that they strongly reflect incident light back in the direction of the source of illumination. Some countries e.g. United Kingdom, exclusively use retroreflecting vehicle licence plates. Others, e.g. United States of America, will use both retro-reflecting and non-retro-reflecting vehicle licence plates within the country as a whole, and sometimes also within a state. In addition, the text and background colours of a licence plate may vary, particularly with the U.S.A. and Canada.

It is well known. that the intensity of illumination and optical filtering required to produce a high quality video image of a vehicle licence plate depends upon the optical spectral properties and retro- reflecting capabilities, of the materials and paints used in the manufacture of the plate. Conventionally, this has been achieved by utilising multiple cameras, each of which is configured to look for plates with a particular optical spectrum or retro-reflective characteristic.

It is an object of the present invention to provide simplified apparatus for imaging vehicle licence plates having differing 2 optical spectrum and/or retro-reflective characteristics that obviates the requirement for multiple cameras. It is a further object of the invention to provide apparatus for imaging vehicle licence plates having the minimum possible moving parts so as to simplify construction and improve reliability.

According to the invention, there is provided imaging apparatus for reading a vehicle licence plate, comprising a camera, an optical filter positioned between camera optics and an image sensor and an artificial illumination source, the filter being subdivided into at least two sections, one of which transmits light generated by said source such that a frame generated by the camera is subdivided into regions corresponding to a particular sector of said filter.

It is envisaged that the filter will be subdivided into two sections although the filter could be divided into further sections having different characteristics subject only to the need for the image of the vehicle licence plate to appear complete within each section of the filter and the corresponding portion of the field. one section of the filter will therefore transmit ambient light suitable for reading non-reflective licence plates. The other section will have the characteristics necessary to transmit light returned from the artificial illumination source by retroflective licence plates for example.

Indeed, the filter characteristics may be chosen to differentiate between not only retro and non retro, reflective plates but also between plates having other optical spectral properties. Thus the apparatus may be configured to identify more clearly vehicle licence plates having differing text and background colours such as, for example, the different plate styles common in the U.S.A. and Canada for example. Preferably, the artificial illumination will be provided by an infra-red 3 source of an appropriate wavelength. Furthermore, the artificial illumination source may be pulsed at a repetition rate sufficient to ensure that at least one frame contains an image of a licence plate which has passed through the appropriate section of the filter. Clearly, where the ambient light is insufficient to permit an image to be formed, the apparatus may further include an additional artificial illumination source or sources, which generate light at an appropriate set of wavelengths. These further illumination sources may also be pulsed as described above. Although it is envisaged that the camera could be a video camera having a CCD image sensor image could as easily be formed on conventional photographic f ilm or other known image sensor.

Preferably, the sections are positioned within the filter such that the light received from a licence plate passes through each filter in succession as a vehicle carrying the plate either advances towards or retreats from the camera.

In order to aid in understanding the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

Figure I is a diagrammatic sectional side view of apparatus for imaging a vehicle licence plate according to the invention; Figure 2 is a front elevation of an optical filter forming part of the apparatus of Figure 1; Figure 3a is a diagrammatic side view of the apparatus of Figure 1 in use, showing a vehicle entering a f ield of view of the apparatus; Figure 3b is a view of a f ield obtained by the apparatus of Figure 1, the f ield containing an image of the vehicle shown in Figure 3a; 4 Figure 4a is a diagrammatic side view of the apparatus of Figure 1 in use, showing a vehicle exiting a field of view of the apparatus; and

Figure 4b is a view of a field obtained by the apparatus of Figure 1, the field containing an image of the vehicle shown in Figure 4a.

With reference to Figure 1, there is shown a vehicle licence plate imaging apparatus 1 comprising a conventional video camera 2 with a lens 3 selected to provide a f ield of view 4 within which a passing vehicle 5 is imaged by the camera 2 via a f ilter 6 located between the lens or camera optics 3 and CCD imaging sensor 17. The apparatus further includes a source of infra-red illumination 7, which is arranged to illuminate the field of view of the camera 2 and will be most effectively reflected by retroreflective vehicle plate types. The artificial light source 7 most conveniently takes the form of a ring of Light Emitting Diodes (LEDs) arranged around the circumference of the camera optics or lens 3 so as to form a colinear source of retro-illumination.

The filter 6 characteristics are such that it differentiates the optical image in upper and lower portions 8,9 of the resultant field 10 generated by the camera 2. Thus, the filter 6 shown in Figure 2 is divided into two sections 11,12 corresponding to upper and lower portions 8,9 of the field generated by the camera. The filter 6 is orientated so that the motion of a vehicle 5 to be imaged tracks across each section 11,12 in turn. Thus, where a vehicle 5 is travelling towards a downwardly (or upwards) angled camera the filter sections 11,12 are stacked vertically as shown in Figure 2. One section 11 has a f ilter characteristic that responds to ambient illumination in order to detect non-retro-reflective vehicle licence plates. The second section 12 has different optical filter characteristic that responds to the infra-red illumination in order to detect retroreflective vehicle licence plates. The filter 6 is positioned immediately in front of the lens 3 so as to minimise the proportion of light incident on the lens 2 that has passed through the interface region of the filter 6 where the differing optical characteristics abut.

The camera 2 is deployed on a gantry, mast or similar support 13 at an depressed angle such that as the vehicle 5 passes into the f ield of view 4 its image 14 enters along a top edge 15 of a field 10 generated by the camera 2. As the vehicle 5 continues to travel through the field of view 4, i.e. towards the camera 2, the resultant image 14 of the vehicle 5 appears to move down the field 10 until it exits along the bottom edge thereof 16. Thus, Figure 3b shows a f ield 10 generated by the camera 2 in which the image 14 of the vehicle 5 is substantially within the upper portion 6 of the f ield 10. Figure 4b shows an image 14 the same vehicle 5 at a later time period when the image 14 occupies a lower portion 9 of a field 10 generated by the camera 2.

In order to reduce blurring caused by the movement of the vehicle 5, the camera 2 is shuttered so that, in effect, a succession of still fields are taken of the moving vehicle 5. If the vehicle 5 is moving at high speed, then with a conventional 50 or 60 fields per second (fps) video camera, often there will only be a small number of video fields 10 with the vehicle image 14 present therein. The lens 3 used and angle of deployment might even result in only a single image 14 of the vehicle 5 being captured. However, for slow moving traffic, e.g. approaching a parking barrier, a deployment can be arranged

6 such that the vehicle 5 appears in many successive video fields 10.

Thus, a vehicle licence plate 15, which is secured to the vehicle, will occur reliably in well-known portions 8,9 of a field 10. The result is that the camera 2 generates a sequence of video fields 10 in which the vehicle appears in sometimes in an upper portion 8 of a field 10 and sometimes in a lower portion 9 of a field 10. Thus, depending on the type of licence plate 15 carried on the vehicle 5, it will be more reliably imaged when it appears in a portion of a f ield 10 in which the f ilter 6 characteristic is optimised.

The portion 8, 9 in which the vehicle licence plate 15 is present is not known until the video field 10 has been generated. Where an intermittently operable infra-red source 7 is utilised then it is necessary to illuminate the field of view 4 at regular intervals sufficient to ensure that infra-red light is present at least once whilst the vehicle image 14 is within that portion of a field 10 on which the second filter section 12 characteristic operates. Furthermore, the camera 2 gain and camera shutter speed settings are sequenced to optimise the video image 14 of the vehicle in a particular portion of a f ield 10.

It will be further appreciated by those skilled in the art that different artificial light sources, filter characteristics and geometry may be used to ensure that a reliable image of a vehicle licence plate is obtained. For example, the filter may, instead of two semicircular sections, comprise other geometric arrangements of the sections, particularly to compensate for irregular vehicle motion. Furthermore, the filter 7 characteristics and light source(s) may be selected to identify particular text and background combinations of licence plate.

8

Claims

Claims:

1. An imaging apparatus for reading a vehicle licence plate, comprising a camera, an optical filter positioned between camera optics and an image sensor and an artificial illumination source, the filter being subdivided into at least two sections, one of which transmits light generated by said source, such that a frame generated by the camera is subdivided into regions each corresponding to a particular sector of said filter.

2. An imaging apparatus according to Claim 1, wherein said artificial illumination source limits illumination having a wavelength distinct from that of ambient illumination, and sections of the filter are respectively such as to transmit ambient illumination and illumination from the artificial illumination source, respectively.

3. An imaging apparatus according to Claim 1 or 2, wherein said camera is arranged to record images repetitively, at a rate such that, in use, a licence plate of a passing vehicle moving at a relevant speed is recorded at least once in each of respective sections of a field of view of said camera that correspond to the said sections of the filter.

4. An imaging apparatus according to any one of Claims 1 to 3 wherein said artificial illumination source is a source of infra red illumination.

5. An imaging apparatus according to any preceding Claim, wherein said artificial illumination source is arranged to transmit illumination in pulses at a repetition rate corresponding to the rate at which repeated images are to be recorded.

6. An imaging apparatus according to any preceding Claim, comprising an additional artificial illumination source arranged to emit illumination of a wavelength differing from said first artificial illumination source.

9 7. An imaging apparatus according to Claim 6 wherein said additional artificial illumination source is arranged to transmit repetitive pulses of illumination.

8. An imaging apparatus substantially as described herein with reference to accompanying drawings.

IC>.

AMENDMENTS TO TBE CLAIM[S, HAVE BEEN FILED AS FOLLOWS CLAIMS 1 An imaging apparatus for reading a vehicle licence plate, comprising a camera, an optical filter positioned between came ra optics and an image sensor and at least one artificial illumination source of wavelength or wavelengths distinct from ambient illumination, the filter being subdivided into at least two sections, respectively arranged to transmit light of different wavelengths which may be ambient light or light generated by said source or lo sources, such that an image generated by the camera is subdivided into regions each corresponding to a particular sector of said filter.

2. An imaging apparatus according to Claim I, wherein said camera is arranged to record images repetitively, at a rate such that, in use, a licence plate of a passing vehicle moving at a relevant speed is recorded at least once in each of respective sections of a field of view of said camera that correspond to the said sections of the filter.

3. An imaging apparatus according to Claim 2, wherein a said artificial illumination source is arranged to transmit illumination in pulses at a repetition rate corresponding to the rate at which repeated images are to be recorded.

4. An imaging apparatus according to any preceding Claim wherein a said artificial illumination source is a source of infra red illumination.

5. An imaging apparatus according to any preceding Claim, comprising an additional artificial illumination source arranged to emit illumination of a wavelength differing from a first artificial illumination source.

6. An imaging apparatus according to Claim 5 as appended to Claim 2 or 3 wherein said additional artificial illumination source is arranged to transmit repetitive pulses of illumination.

7. An imaging apparatus substantially as described herein with reference to accompanying drawings.