GB2249420A

GB2249420A - Intruder detection system

Info

Publication number: GB2249420A
Application number: GB9023818A
Authority: GB
Inventors: Christopher George Harris
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1990-10-31
Filing date: 1990-10-31
Publication date: 1992-05-06
Anticipated expiration: 2010-10-31
Also published as: GB9023818D0; GB2249420B

Abstract

An optical intruder detection system comprises a television camera 5 which produces video data comprising successive scan frames relating to the image of an area to be covered by the system, digitiser means 6 responsive to the video data for producing corresponding digital data, data storage means 7 comprising a plurality of store addresses for digital data appertaining to a matrix of pixels which in combination define a frame of the image, and data processor means 8-10 responsive to current store address data relating to each pixel and to a digital reference signal derived in dependence upon previously stored data relating to each pixel for providing an intruder detection signal when a predetermined relationship obtains for any pixel between the current data appertaining to that pixel and the said digital reference signal which appertains thereto. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO INTRUDER DETECTION SYSTEMS This invention relates to intruder detection systems and more especially it relates to optical intruder detection systems. Optical intruder detection systems are known comprising a photo sensitive detector forming part of a detector system which affords an alarm signal when light incident upon the detector changes from an ambient light condition by more than a predetermined amount. This kind of optical intruder detection system is not suitable for the detection of intruders when a large area is to be covered since a large number of individual detectors would be required. Nor is it suitable for use in an environment where sudden light changes could normally be expected as might be occasioned for example by the movement of a flag or the leaves of a tree with the wind.

According to the present invention an optical intruder detection system comprises, a television camera which produces video data comprising successive scan frames relating to the image of an area to be covered by the system, digitiser means responsive to the video data for producing corresponding digital data, data storage means comprising a plurality of store addresses for digital data appertaining to a matrix of pixels which in combination define a frame of the image, and data processor means responsive to current store address data relating to each pixel and to a digital reference signal derived in dependence upon previously stored data relating to each pixel for providing an intruder detection signal when a predetermined relationship obtains for any pixel between the current data appertaining to that pixel and the said digital reference signal which appertains thereto.

The digital reference signal may be derived in accordance with a predetermined algorithm from data received in a plurality of preceding frames whereby a normal pattern of light variations for each pixel is established and represented by means of its associated digital reference signal.

Thus any deviation in any pixel from what has been established as a normal range of light variations, will be detected and result in a corresponding positive intruder detection signal being given by the data processor means.

It will be appreciated that with a system according to the present invention a large area may be covered by means of a single TV camera and that false alarms may be substantially reduced since normal occasional sudden light changes in one or more pixels will be ignored for detection purposes if they form a part of a normal light variation pattern.

One embodiment of the invention will hereinafter be described by way of example with reference to the accompanying drawings in which; Figure 1 is a schematic block diagram of a software based implementation of an intruder detection system, Figure 2 is a schematic block diagram of a hardware based implementation of an intruder detection system and; Figure 3 is a schematic block diagram of a part of the system of Figure 2 shown in greater detail.

Referring now to Figure 1 which is a largely software realisation of the present invention, a video signal is produced by a TV camera 1 which is fed to a digitiser 2. A signal from the digitiser 2 is fed to an image memory board 3. The memory board 3 comprises a storage address for data appertaining to each of a plurality of pixels, one pixel to each storage address whereby data appertaining to each frame of an image produced by the TV camera is stored in digital form by the image memory board 3. The image memory board 3 is coupled to a microprocessor 4 arranged to be responsive to brightness variations in each pixel, which are updated from time to time and stored in the image memory board 3 for performing a variability estimation and for detecting brightness variations indicative of an intruder by means of signal processing techniques.

The theory on which such signal processing techniques are based will hereinafter be described. If I is the intensity of light detected at a pixel in a current image and I is the intensity of light detected at the same pixel in a preceding frame, movement event detection may be deemed to have occurred if II-I'I > a.S + b where S is the variability estimate of the pixel, and a and b are constants which depend on the acceptable false alarm rate and the quality of image digitisation. It will be appreciated that quality of image digitisation is dependant on the number of bits in an analogue to digital converter used and camera noise level.The variability estimate S adapts with time by being updated thus, S := S + (lI-Fl - S) /T where the decay time, T is mean time (in processed images) over which the variability average is calculated. This update does not have to be performed upon the acquisition of every new image, i.e.

from frame to frame, but may be performed infrequently. The decay time needs to be chosen to be sufficiently large to overcome lulls in the movement of wind-blown vegetation, and so a decay time in the order of minutes is appropriate.

Movement event D is deemed to occur when the modulus of image brightness differences exceeds the variability estimate, S, by some margin n, thus D = (II-I'I > n.S) As an alternative to the software implementation just before described with reference to Figure 1, a generally hardware based realisation of the invention will now be described with reference to Figure 2. Referring now to Figure 2, a video signal produced by a TV camera 5 is fed to a digitiser 6. The video signal corresponds to that normally produced in a scanned raster display having a frame rate of 25Hz. The digitiser serves to produce a corresponding digital signal wherein the image is represented by a matrix of pixels the brightness in each pixel being represented by a digital signal.A data stream I, relating to the pixel brightnesses is fed to a memory 7 wherein data relating to one frame of a complete image is stored.

The data stream I is fed also to a subtractor 8 which receives a data stream I' from the memory 7 relating to a previously received image. The subtractor thus produces an output signal for each pixel corresponding to the modulus of I-I'. It will be appreciated that as addresses in the memory 7 are updated by the data stream I, which contains data appertaining to a current image, data appertaining to a previous image and corresponding to the data stream I is outputted to the subtractor 8 which thus receives simultaneously signals which relate to the same pixel. Output signals from the subtractor 8 are fed to a look-up table 9 which is fed concurrently from a store 10 with signals corresponding to the variability estimate S hereinbefore referred to.

The look-up table 9 performs both an update of the variability estimate S calculated as hereinbefore described which is fed back via line 13 to the memory 10 and it provides an output signal on the line 14 indicative of the detection of a movement of the kind which might be associated with an intruder. One hardware implementation of the look-up table 9 is shown in Figure 3 wherein the modulus signal II-I'I is fed on a line 15 to a first subtractor 16 and a second subtractor 17.

The subtractor 16 is fed via a line 18 with a signal corresponding to the variable variability estimate S which is fed also to an adder 19 and a threshold detector 20. Output signals from the threshold detector 20 are fed via a line 21 to the subtractor 17 which provides on the line 14 output signals indicative of the detection of an intruder. Output signals from the subtractor 16 are fed to a delay device 22 which operates in accordance with the factor 1/T and provides an output signal on a line 23 for the adder 19 whereby an output signal on the line 13 is provided for the memory 10 as shown in Figure 2.

Output signals on the line 14 from the subtractor 17 of the look-up table 9 are fed also to a microprocessor 12 which operates for intruder tracking purposes whereby an intruder once detected can be tracked across a field of view which corresponds to the image as seen by the TV camera 5.

The hardware realisation just before described with reference to Figure 2 and Figure 3 enables all the pixels in an image (typically about a quarter of a million) to be processed for movement event detection, at speeds up to a normal TV frame scanning rate i.e. 25Hz.

It will be appreciated that the invention just before described is suitable for the automatic detection of intruders more especially in outdoor environments such as car-parks and at site parameter fences and gates for example.

Various modifications may be made to the embodiments just before described without departing from the scope of the invention and for example various alternative software implementations of the hardware system described may be conceived as will be apparent to those skilled in the art.

Claims

1. An optical intruder detection system comprising, a television camera which produces video data comprising successive scan frames relating to the image of an area to be covered by the system, digitiser means responsive to the video data for producing corresponding digital data, data storage means comprising a plurality of store addresses for digital data appertaining to a matrix of pixels which in combination define a frame of the image, and data processor means responsive to current store address data relating to each pixel and to a digital reference signal derived in dependence upon previously stored data relating to each pixel for providing an intruder detection signal when a predetermined relationship obtains for any pixel between the current data appertaining to that pixel and the said digital reference signal which appertains thereto.

2. A system a claim of 1 wherein the digital reference signal is derived in accordance with a predetermined algorithm from data received in a plurality of preceding frames whereby a normal pattern of light variations for each pixel is established and represented by means of its associated digital reference signal.

3. A system as claimed in of 1 or claim 2 and as hereinbefore desribed with reference to Figure 1 of the accompanying drawings.

4. A system as claimed in of 1 or claim 2 and as hereinbefore described with reference to Figure 2 of the accompanying drawings.