Several techniques are in use for traffic speed measurement. These are based upon technologies using spaced-apart sensors (usually magnetic), ultrasonic detectors, infra red sensors and optical imaging devices. Usually, however, some attempt is made to monitor the movement, and thus (when timed) the speed of traffic by making measurements on individual vehicles. This can lead to difficulties and anomalies associated with the characterisation of particular vehicles and discrimination from other vehicles, especially where there is heavy traffic on a multi-lane highway.

An aim of the present invention is to evaluate average traffic speed simultaneously for a plurality of vehicles in a plurality of traffic lanes, without recourse to individual characterisation of said vehicles.

According to the present invention there is provided apparatus for detection and measurement of speed of moving items, the apparatus comprising: means for capturing first and second time-spaced images of the items, means for transforming said images and means for superimposing said transformed images so as to produce an interference pattern, means for conveying said interference pattern to an optical processing system, wherein an inverse transform is presented to an output device which device provides an

output signal indicative of the speed of the items during the interval between capturing the first and second images.

Preferably the items are vehicles moving along a highway.

Preferably the means for capturing the time-spaced images of the highway comprises a spatial light modulator (SLM). The SLM obtains an image of the traffic during a first time interval, then a second image is obtained during a subsequent time interval, said time intervals are separated by a short duration which is advantageously less than one second and preferably less than 0.1 second. A second SLM may be used on which images may be formed. Alternatively a liquid crystal device may be used. However, an optically addressable SLM is preferred.

Transform means for performing a transform of the image is interposed between the first and second spatial light modulators. The transform means may comprise a lens, which is preferably a cylindrical lens. A cylindrical lens is preferred because it has different properties along two perpendicular axes and this provides Fourier transformed information of the forward velocity component of the vehicles and disregards any sideways movement, for example occurring as a result of lane changing.

An embodiment of the invention, for use as a traffic speed indicator, will now be described with reference to the Figures in which:

Figure 1 shows, in block diagrammatic form, apparatus in accordance with one example of the invention; Figure 2a shows, schematically, items (vehicles) on a road; and Figure 2b shows an image derived from said items using the system in Figure 1.

Referring to the figures, first and second time-spaced images of a monitored highway 10 (shown diagrammatically in Figure 2a) are obtained using the apparatus shown in Figure 1. Vehicles are shown as item 9. The two time-spaced images are shown in Figure 2a and differ in that the second image (shaded) changes from the first (unshaded) by virtue only of the motion of vehicles 9 past a monitoring position. In practise, the two images are very similar as they are typically separated by a time duration of less than one second and preferably less than 0.1 second.

Figure 1 shows diagrammatically two spatial light modulator devices 2 and 5 (SLM's) of differing characteristics, as will be explained. The two SLM's 2 and 5 are incorporated in an optical system. The system exhibits significantly different imaging characteristics in directions along and transverse to the direction of traffic flow.

Images of vehicles 9 are obtained using infra red camera 1. Images are superimposed on a first, transmissive, spatial light modulator 2. SLM 2 can be of the kind known as a VGA spatial light modulator, for example of the type manufactured by the Applicant company Central Research Laboratory Limited and described in the brochure "A new

generation in Spatial Light Modulators". Substantially all the active area of the SLM 2 is illuminated by laser beam 3. Laser radiation 3 is reflected from SLM 2, and is modified by the interaction with the first and second superimposed images appearing on the SLM 2. Reflected modified radiation is incident on an optical element 4. Optical element 4 effectively performs a Fourier transform on the radiation 3 in one plane only and focuses the transformed image onto the second SLM 5. Element 4 comprises a cylindricalispherical lens combination and has different focal powers: one in the direction which corresponds to that of vehicle movement at the monitored position; and the other in the direction substantially at right angles thereto. The difference in the focal powers in each direction of optical element 4 is such that, in the first-mentioned direction (that corresponding to the direction of traffic flow at the monitored position), the image formed on the second SLM 5 is in the Fourier transform plane and such that in the second direction (transverse to the first), the image of SLM 2 is located in the image plane of the SLM 5. The production of such a lens combination is apparent to one skilled in the art. SLM 5 is a device which is optically addressable and is known as an OASLM.

If the traffic in all lanes is moving at about the same speed, Young's interference fringes are created in the OASLM device 5 as a result of interference between the first and second time-spaced images produced by the optical element 4. The spacing of the fringes is dependent upon the amount of vehicle movement which has occurred in the duration between obtaining each time-spaced image. If the spacing is too great compensation means (not shown) may be provided for adjusting the time interval so that it can be reduced. The compensation means is linked to a signal processor (not shown)

and is used as a scale factor for calculating the speed. The OASLM device 5 converts the distribution of the amplitudes of the Young's fringes into an intensity distribution.

This information is "read" by a collimated source of radiation such as a second laser beam 6, which is projected onto the back surface of OASLM device 5 and is reflected therefrom to an extent determined at each pixel, by the intensity distribution contained on the OASLM 5. Reflected laser radiation 6 is directed, via a polarisation-sensitive beam splitter 7, to a second combination lens 8. Lens 8 has similar optical characteristics to lens 4, and effectively performs an inverse Fourier transform, whereby the Fourier plane in the direction corresponding to traffic movement coincides with the image plane in the orthogonal direction of a CCD camera 14. CCD camera 14 (or similar device) placed in that plane, receives a transformed image (of the object shown in Figure 2a) such as that which is shown in Figure 2b.

It is seen that the output of the distance between each auto correlation spot 12 and a central spot 11, which appears as an image on the CCD 14, is proportional to the distance moved between each sampling event. Accordingly, the average speed is readily obtainable from the image or can be computed digitally.

When "samples" are referred to in this specification, it will be appreciated that what is referred to are two time-spaced images of moving objects. It will also be appreciated that new images are generated on a regular and rapid basis, and that the speed indication is thus updated and displayed substantially continuously.

Although the present invention has been described with reference to traffic moving in lanes along a highway, the invention finds other applications in the measurement of, for example, items moving across a shop floor, items moving along an endless belt, items moving across a fluid bed, edible items moving along a production line, livestock moving in fields or particles carried in suspension.

Variation may be made to the aforementioned embodiment without departing from the scope of the invention. For example, the invention may find application in a system for measuring speed of ships in sea lanes, or other forms of traffic. Such a system may be located on a satellite in geostationary orbit.