GB2188508A - Imaging system - Google Patents

Abstract

A moving object such as a container 10 on a conveyor belt 12 is imaged by an optical system 16 onto a CCD array 18 in which the lines of the array are arranged perpendicular to the direction of motion of the object. The speed of movement of the object is sensed to generate electrical signals which are processed to provide shift signals enabling the shifting of data row to row in the array in synchronism with the movement of the container. The electrical charge associated with a given point on the array is transferred from one line to the other until it appears at the last line of the array, from which it is read out in known manner in conjunction with all other electrical charges associated with the row of charge coupled devices in the last line of the array. Due to the integrating effect achieved, the aperture of the imaging system can be much smaller than otherwise would be required, and/or the level of light illumination can be reduced. The imaging system can be applied to X-ray inspection devices, aerial surveillance or scanning of moving documents is copying processes. <IMAGE>

Description

SPECIFICATION Improved imaging system Field of invention This invention concerns imaging systems and in particular systems incorporating charge coupled device arrays.

Background to the invention Charge coupled device (CCD) arrays of 512 + 512 bit resolution are now common and larger arrays are becoming available. Whilst the arrays have predominantly been developed to enable an optical image to be converted into an electrical charge pattern for serial readout, the mode of operation of these arrays during readout is such that the charge pattern along each line of the array is transfered by way of a parallel shift to an adjoining line and this parallel shift from one line to the next can be effected at very high speed.The serial removal of data from the last line can be achieved at normal clocking rates and because the time required to shift the information from the penultimate line into the last line ready for serial shift out of the array, is very small, the data relating to the whole of the array can be read out from the array as a single serial stream of data.

The present invention makes use of the fact that the time required to output the information from one line can be of the order of a millisecond so that information can be shifted in parallel accross the array in a direction perpendicular to the lines in the array at relatively high speeds thereby enabling the device to be used in a novel application.

Summary of the invention According to one aspect of the present invention, an imaging system for capturing an image of a moving object comprises: (1) a CCD array, (2) control means for shifting the electrical charge pattern from one line to the next (in known manner) and for serially reading out the data in the last line of the array (in known manner) as a video signal, (3) optical means for forming an image of an object on the array, the orientation of the array being such that the lines in the array are perpendicular to the direction of motion ofthe object relative to the array, (4) control means for controlling the rate at which the data is shifted from one line to the next in the array to link the shifting with the movement of the object relative to field of view of the imaging system, so that the shifting of the electrical charge pattern from line to line is synchronised to the movement of the object and therefore to the image of the object on the array, so that light from any part of the object incident on the array is accumulated for the whole of the time that that part of the object is imaged on the array.

The invention not only allows a moving object to be "frozen" for the purpose of deriving an electrical signal corresponding to the image thereof, but additionally, because of the accumulation of the light from each point during the whole of the exposure of that point to the array as the object moves relative to the array, the imaging system has a sensitivity which is increased by approximately the number of lines in the array relative to the sensitivity that the device would have if a single line of charge coupled devices had been used or if the array were simply exposed once using a flash technique and the entire electrical image read out serially before the next flash, in conventional manner.

The invention is equally applicable to situations in which the array is moving and the object is stationary or to arrangements in which both array and object are moving and a relative speed exists between the two. In all situations it is the relative movement of the optical image over the surface of the array which is the key factor and the shifting of the lines of data in the array must be synchronised to the relative movement to effect the freezing of the image, and obtain the increased sensitivity.

The invention can thus be used in the field of aerial surveillance of stationary or moving objects and can also be used in the field of document scanning to permit a document to be scanned "on the fly".

In combination with an X-ray source and fluorescent imaging device, the invention can also be used in combination with an X-ray inspection system. In this context the invention allows an X-ray inspection system to be operated at higher speed than has hitherto been the case.

According therefore to a further aspect of the invention, an X-ray inspection system for inspecting elongate objects comprises: (1) a colimated X-ray source adapted to irradiate a given area of the object when the latter is positioned in an inspection station; (2) means for moving the object through the inspection station; (3) a fluorescent screen beyond the object and adapted to receive X-rays which have penetrated the object; (4) an imaging system comprising a CCD array camera and optics associated therewith for focussing the fluorescent image from the screen onto the CCD array, the orientation of the array being such that the lines of charge coupled devices in the array are perpendicular to the direction of movement of the image of the object across the array due to movement of the object;; (5) means for controlling the shifting of the charge pattern from one line to the next in the array in synchronism with the movement of the object across the field of view; and (6) means responsive to the output from the camera for producing an image on a television screen or other display device.

The synchronisation of the shifting of the charge pattern from one line to the next in the CCD array is conveniently achieved by deriving position related signals from the the object and generating shift signals from the position related signals.

Where the object is on a conveyor belt, a signal related to the linear speed of the conveyor belt may be sufficiently accurate. Where this is not so, the actual movement of the object must be measured independently.

Where a signal relating to belt speed is sufficient then a convenient position related signal is obtained by marking or punching or otherwise forming the belt so as to interact with an encoder to generate electrical pulses, the repetition rate of which is indicative of the actual linear speed of the belt past the encoder. Thus for example, a light positioned above the belt and in line with a row of equally spaced apart apertures in the belt and a photocell or similar device located below the belt will produce a series of electrical pulses from the photocell, each of which will indicate a new position of the belt (and object thereon) and the frequency of which will depend on the speed of the belt.

The electrical pulses concerned can be shaped and used to generate shift signals for the array.

The output from the camera is is a varying amplitude electrical signal and if the signal is required in digital form, the imaging system preferably includes an analogue to digital converter (ADC) to produce a digital version of the analogue signal for storage in a digital store such as a frame store and/or for processing in a digital computer, for image enhancement or otherwise.

Switch means may be provided to advantage to enable the analogue video signal output from the camera or the digital version thereof to control the display of the image on a monitor screen or the like.

If required hard copy can be produced by means of a dot matrix or thermal printer or laser printer or the like to which the serial digital data can be supplied in real time or from a store or after processing for image enhancement purposes.

It will be seen that any length of object can be inspected in this manner since the electrical signal from the camera will comprise a data stream corresponding to the leading edge of the object through to the trailing edge of the object as the latter moves relative to the camera (or vice versa). However, there may be a limit to the height of-the object which can be inspected since using a standard array of 512 by 512 bits, the resolution required in the vertical plane may prevent the whole height of the object to be imaged onto the array.

Where this is the case either a larger array must be employed (ie more bits per line) or two or more cameras may be stacked one above the other each adapted to inspect a part of the height of the object. Thus for example if the height of the field of view of the camera is 1 metre, then two cameras mounted one above the other can accommodate an object which is 2 metres high.

The considerations as to height etc apply equally to the optical and X-ray inspection systems previously mentioned.

Whatever form of illumination is used the amount of radiation needed to obtain a particular light level on the CCD array will be significantly reduced by virtue of the fact that each vertical "slice" of the object which is reflecting of transmitting radiation (either light of Xrays) is presented to each of the lines in the array in turn so that whatever the radiation level, this is integrated for the whole of the time that it takes any point on the object to move across the field of view of the camera.

Where X-rays and fluorescent screen are employed and the latter is stationary, it is of course important that the decay time of the fluorescent screen is sufficiently small as to prevent smearing of the fluorescent image due to relative movement between the screen and the object. Typically the decay time of the fluorescent phospher should be of the order of one millisecond.

Where the phospher decat is too long, the size of the screen can be increased so that it occupies for example the whole of the length of the object and the screen is limited to the object so as to move therewith. In this way a stationary image will be formed on the screen (stationary that is relative to the material of the screen) although the optical image produced by the fluorecence moves relative to the camera and of course is still captured by the camera in the same way as previously described, provided the shifting of the information from line to line in the array is synchronised with the movement of the object as before mentioned.

In a preferred embodiment of the invention in which a CCD array camera forms part of an inspection system, the CCD array is orientated with its lines vertical so that the line shift is in a horizontal direction, the line shift speed is adjusted to match the speed of an object passing through the inspection station, speed sensing means converts the linear speed of the object into an electrical signal to control the line shift speed in the CCD array, a colimated X-ray source irradiates an area some 2.5 metres high by 1.25 metres wide at the rate of 3000 rad per minute, the object is conveyed through the inspection station at a uniform speed typically of the order of 0.6 metres per second and an optical system, which may include a mirror, views a fluorescent screen located on the opposite side of the inspection station from the X-ray source and forms an image of the fluorescent screen on the CCD array in the camera.

Where the camera output is to be digitised, the analogue signal is preferably converted to an 8 bit digital signal giving a 256 level grey scale and the resulting digital video signal can either be displayed directly on an appropriate monitor, or the information can be stored in a frame store which can then be addressed in any convenient manner so as to produce a signal which can be reproduced on any suitable display terminal.

Where a hard copy (ie a printout) of the image is required, a thermal printer or laser printer or the like may be employed.

The invention has particular application in the field of container and baggage inspection at docks and airports. Typically a container some 6 metres long and 2-1/2 metres high will require some 10 minutes of inspection time using conventional X-ray inspection equipment. The invention will allow the same container to be inspected in 10 seconds or even less depending on the speed of movement of the container past the inspection stage. Indeed the speed of image transfer within the charge couple device array is such that the limitation on the speed of inspection is not so much dictated by the speed of image capture as the speed at which the object can be physically moved through the inspection station.

In its optical mode, the invention is of particular application in the field of aerial surveillance, surveillance of moving objects and the scanning of documents whilst on the fly in high speed photocopying or printing processes.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates the principle on which the invention works; Figure 2 shows the essential ingredients of a camera system adapted to track a moving object; Figure 3 is a plan view of an overall inspection system utilising an X-ray source and fluorescent screen to provide the moving image to be tracked by the camera, and Figure 4 is an end view illustrating how a large object such as a container can be viewed by two cameras mounted one above the other.

Detailed description of the drawings Figure 1 illustrates the principle of the invention. The side of a container 10 carried by a conveyor belt 12 moving in the direction of the arrow 14 is imaged by an optical system denoted by a lens 16 onto a CCD array 18 in which the lines of the array are arranged vertically. The CCD array typically has 512 lines and 512 charge coupled devices along each line. In this context the word "line" has been used to describe what is sometimes alternatively referred to as a row.

The inverting function of the lens 16 means that the point 20 at the top right hand corner of the field of view appears as point 22 on the array 18, and with movement of the container 10 in the direction of the arrow 14, the point 22 will progress along the bottom of the array from one line to the next, as the point 20 moves across the field of view of the optical system. Finally, the point 22 will move off the right hand end of the array (as shown in Figure 1) as point 20 moves out of the field of view.

By shifting the data in each of the rows in the array in synchronism with the movement of the container, but in the opposite direction, so the electrical charge corresponding to the point 22 will be transferred from one to the other of each of the 512 lines in the array until it appears at the last line, at which point it will be available to be read out as a serial output signal in known manner together with all the other electrical charges associated with the row of charge coupled devices making up the last line in the array. These other charges may for example correspond to other points in the vertical "slice" containing the point 22, such as points 24 and 26.

It will be seen that because the charge pattern to which the light from point 20 (and points 24 and 26) contribute is itself shifted across the array as the points on the surface of the container move in the opposite direction across the field of view, so an integrating effect is obtained, and only a relatively small amount of light emanating from point 20 will nevertheless produce, after integration, a larger electrical charge which is much greater than the charge obtaied from the same amount of light if for example the array were exposed momentarily to the same area of the container shown shaded in the Figure in accordance with conventional high speed flash scanning technology.

The net result is that the aperture of the optical system can be much smaller than would otherwise have to be the case, and/or the light level can be reduced.

Where the intention is to inspect a surface of the object 10 it is merely necessary to illuminate the surface with light (or UV or IR) in known manner either by transmission or reflection, and move the object across the field of view as described.

If however, the intention is to examine the contents of a container using X-rays, then it is necessary to locate a fluorescent screen at the position shown by the shaded rectangle 28 in Figure 1 and cause the latter to fluoresce due to the impingement thereon of Xrays which have penetrated the container. The result will be a moving picture display on the fluorescent screen corresponding to the con tents of the container as outlined by the Xrays passing therethrough.

Figure 2 shows the essential parts of an image capture system which may be used for example to inspect objects illuminated by ordinary light moving along a conveyor belt. To this end an object 30 is shown carried by a conveyor belt 32 and the lens 16 of Figure 1 forms an image of the object 30 on a CCD camera containing a CCD array typically having 512 lines each containing 512 devices therealong (although the invention is not limited to this size of array). The camera is denoted by reference numeral 34.

The serial data stream obtained from the camera during normal operation is transferred via a control circuit 36 to a digitiser 38 which functions as a fast analogue to digital convertor. Thereafter the digital information be stored in a frame store such as 40 and may be proces for image enhancement or other purposes using circuitry such as 42 and a central processor 44. The video signal either in digital or converted back to analogue form can be displayed using a monitor screen such as 46 and if required hard copy can be made either using a laser printer or a thermal printer or the like.

The camera control circuit 36 is itself actuated by signals from a conveyor position sensor 50. The shifting of information from one line to the next in the camera is controlled in synchronism with the changing position of the object across the field of view.

Figure 3 shows how a system such as shown in Figure 2 can be adapted to image a picture obtained on a fluorescent screen 52 produced by X-rays from a colimated source 54 after they have passed through an object under inspection such as a container 56. The mirror 58 merely rotates the light from the fluorescent screen through a right angle to enable the inspection equipment to be located remote from the zone bombarded by X-rays.

Where the height of the object or container 56 is too great to be accommodated within the field of view of one camera, two cameras may be located, one above the other, as at 60 and 62 in Figure 4, so as to encompass within the overall field of view the total height of the container or other object. In this event the signals from the two cameras will need to be multiplexed for subsequent display on a single monitor screen. More than two cameras may be mounted in this way.

Where the fluorescent screen 52 has an insufficiently short decay time, a larger fluorescent screen may be employed attached to the entire side of the container so that there is no relative movement between the container and the screen.

It will be noted that if the object in the field of view moves in a series of steps (identical or otherwise) the imaging system of the invention will stil function if the line to line shift in the array is performed in response to the stepwise movement of the object.

If the object is distant and no link is possible, a clock pulse generator of adjustable frequency may be employed to generate the shift control signals for the array.

Thus the arrangement shown in Figure 2 may be used as a speed measuring device when viewing distant moving objects, a speed indicating signal being derived from the signal which must be supplied to the camera control to just arrest movement of the object in the display on the monitor 46.

Claims (16)

1. An imaging system for capturing an image of a moving object comprising: (1) a CCD array, (2) control means for shifting the electrical charge pattern from one line to the next (in known manner) and for serially reading out the data in the last line of the array (in known manner) as a video signal, (3) optical means for forming an image of the object on the array, the orientation of the array being such that the lines in the array are perpendicular to the direction of motion of the object relative to the array, and (4) control means for controlling the rate at which the data is shifted from one line to the next in the array to link the shifting with the movement of the object relative to the field of view of the imaging system, so that the shifting of the electrical charge pattern from line to line is synchronised with the movement of the object and therefore to the image of the object on the array, so that light from any part of the object incident on the array is accumulated for the whole of the time that that part of the object is imaged on the array.

2. An X-ray inspection system for inspecting elongate objects comprising: (1) a collimated X-ray source adapted to irradiate a given area of the object when the latter is postioned in an inspection station, (2) means for moving the object through the inspection station (3) a fluorescent screen beyond the object and adapted to receive X-rays which have penetrated the object, (4) an imaging system comprising a CCD array camera and optics associated therewith for focussing the fluorescent image from the screen onto the CCD array, the orientation of the array being such that the lines of charge coupled devices in the array are perpendicular to the direction of movement of the image of the object across the array due to movement of the object, (5) means for controlling the shifting of the charge pattern from one line to the next in the array in synchronisim with the movement of the object across the field of view, and (6) means responsive to the output from the camera for producing an image on a television screen or other display device.

3. A system according to claim 1 or claim 2, wherein synchronisation of the shifting of the charge pattern from one line to the next in the CCD array is achieved by deriving position related signals from the object and generating shift signals from the position related signals.

4. A system as claimed in claim 3, in which the object is carried on a conveyor belt, and signals related to the linear speed of the belt are used to generate the shift signals.

5. A system according to claim 4, wherein the signals related to belt speed are obtained by forming the belt with means which interact with an encoder to generate electrical pulses, the repetition rate of which is indicative of the actual linear speed of the belt past the encoder.

6. A system according to claim 5, wherein the belt is provided with spaced apart apertures and the encoder comprises a light source positioned on one side of the belt and a photo receptor located on the other side of the belt

7. A system according to claim 6, including pulse shaping means for acting on the electrical pulses derived from the encoder in order to generate the shift signals for the array.

8. A system according to any one of the preceding claims, wherein the output from the camera is a varying amplitude electrical signal and the imaging system includes an analogue to digital convertor to produce a digital equivalent of the analogue signal for storage in a digital store or for processing in a digital computer.

9. A system according to any one of the preceding claims, including switch means to enable the analogue or digital video signal to control the display of the image on a monitor screen or the like.

10. A system according to claim 8, including a printing means to which the digital data are supplied in real time from the store or after processing.

11. A system according to claim 2 or any claim appendant to claim 2, wherein the decay time of the fluorescent screen is sufficiently small substantially to prevent smearing of the fluorescent image.

12. A system according to claim 11, wherein the decay time of the fluorescent phospher on the screen is of the order of one millisecond.

13. A system according to claim 2, wherein the screen extends the entire length of the object and is coupled to the object so as to move therewith.

14. A system according to claim 2, in which a CCD array camera forms part of an inspection system, wherein the CCD array is orientated with its lines vertical so that the line shift is in a horizontal direction, the line shift speed is adjusted to match the speed of an object passing through the inspection station, speed sensing means convert the linear speed of the object into an electrical signal to control the line shift speed in the CCD array, a collimated X-ray source irradiates an area approximately 2.5 metres high by 1.25 metres wide at a rate of three thousand rad per minute, the object is conveyed through the inspection station at a uniform speed of the order of 0.6 metres per second and the optical system, which may include a mirror views a fluorescent screen located on the opposite side of the inspection station from the X-ray source and forms an image of the fluorescent screen on the CCD array in the camera.

15 A system according to claim 14, when incorporated in container and baggage inspection equipment.

16. An imaging system for capturing an image of an object substantially as hereinbefore described with reference to the accompanying drawings.