GB2215170A - Stereoscopic inspection system - Google Patents

Abstract

A stereoscopic x-ray inspection system has a binocular radiographic source 9L, 9R in which the binocular radiographic dimensions affecting presentation of the stereoscopic image are variable. The separation distance between x-ray sources and the convergence angle of the x-ray beams may be altered to change the individual perspective views comprising the final image. The acquired views are stored in video frame stores 14L, 14R ready for display in a manner appropriate to stereoscopic presentation and the lateral disparity between the images may also be altered to control the position in depth relative to the display screen in the perceived stereo image. The object 1 may be a cargo container. The x-ray sources may comprise accelerating waveguides 2L, 2R in which microwaves from magnetrons or klystrons 4L, 4R and waveguides 5L, 5R accelerate pulses of electrons form an electron gun (not shown) onto an anode (not shown). <IMAGE>

Description

STEREOSCOPIC INSPECTION SYSTEM The invention concerns a non-invasive inspection system which produces a stereoscopic x-ray image of an object.

The invention is intended to permit thorough inspection, for example, of sealed metal containers of the type which nowadays are used for packing cargo for transport by road, rail and ship. There is in use already an inspection system capable of handling such containers but this produces only two-dimensional x-ray images. Whilst this system produces images of the contents these are collapsed onto each other making identification and full appreciation of the form or configuration of obscure objects more difficult. The known system uses a single fixed x-ray beam to produce the commonly familiar type of radiographic image.

An object of the present invention is to provide an improved container inspection system which produces stereoscopic images of the contents to ease identification difficulties and so reduce operator fatigue and concentration loss.

According to the present invention a non-invasive stereoscopic inspection system capable of producing three-dimensional transparent images of an object and its contents comprises a binocular radiographic source means located at one side of a region within which the object is located, large scale imaging means located on the opposite side of the region, the radiographic source means, in use, projecting two laterally displaced x-ray beams towards the imaging means to produce thereon left and right perspective images, display means for presenting to an observer the left and right images in a manner to impart to him a stereoscopic perception of the object and its contents and wherein the binocular parameters of the radiographic source means are variable to alter the observer's perception of depth in the stereoscopic image.

Preferably the inspection system employs two x-ray beams produced by separate x-ray generators in order to obtain left and right perspective views of the object and its contents.

The x-ray generators include an accelerating waveguide of either the standing wave or travelling wave type which accelerates a pulsed beam of electrons towards an x-ray emitting target, and the waveguides are mounted for relative movement both laterally and pivotally so that an operator can adjust stereo image separation to suit his individual preference and limits of stereo perception and also to adjust magnification in depth in the fused image.

To give a better understanding of the invention two embodiments which use alternative x-ray beam configurations will be described with reference to the accompanying drawings, in which: Figure 1 shows a stereoscopic x-ray inspection system incorporating large scale area imaging means and dual x-ray generators, Figure 2 shows a stereoscopic x-ray inspection system incorporating linescan imaging means, and Figure 3 shows another form of binocular radiographic source comprising a single accelerator in combination with an x-ray emitting target and a dual beam collimator.

In Figure 1 a container ready to be inspected non-invasively is shown at 1 located in the x-ray inspection region. In the plane of the drawing, the binocular radiographic source means is depicted towards the left, and the imaging and display means is shown towards the right.

Much of the binocular radiographic sources of Figures 1 and 2 are identical. The differences result from the choice of either an area imaging means or a linescan imaging means and, as will be described below, essentially affect only the collimators required to produce the appropriate beam shapes.

The basis of the binocular radiographic source as used in Figures 1 and 2 is two linear accelerating waveguides schematically illustrated at 2L and 2R, where suffixes L and R indicate left and right as in the handedness of the perspective view. The two accelerating waveguides which may be of either the standing wave or travelling wave type are driven from a common modulator 3 and each derives its accelerating radio frequency power from a microwave source 4L and 4R, such as a magnetron or klystron. The output of each of the microwave sources is coupled via a centre-feed RF input to its associated waveguide 2L, 2R by flexible waveguides 5L and 5R respectively.

The modulator 3 controls operation of the magnetrons 4L and 4R by connections through a pulse transformer 6 which provides outputs at 7L and 7R connected with the field magnets of the magnetrons 4L and 4R respectively.

The RF output of a single magnetron may be switched by controlling the field strength of its attendant magnet. For connection to an accelerating waveguide the modulator pulse output network must be impedance matched to the input impedance of the magnetron field winding for efficient power transfer and minimum reflection. The magnetron impedance is strongly dependent upon its magnetic field. If the magnetic field is increased the magnetron impedance also increases giving an impedance mismatch which results in a proportion of the modulator output power being reflected back to the modulator. Consequently, the magnetic field strength in the magnetron has to be controlled to provide the correct impedance match and, thus, provides a ready method of switching the magnetron output by varying the magnetic field strength.

In the embodiment of Figure 1, having two accelerating waveguides each powered by its own magnetron, power can be selectively transferred to either one of the two magnetrons by switching its magnetic field to the correct value to obtain an impedance match and maintaining a high field strength in the other magnetron. Similarly, the magnetrons can be made to produce their RF outputs alternately by switching their respective magnetic fields alternately.

The magnetrons 4L and 4R are, therefore, switched in this manner under the control of modulator 3 so as to produce two x-ray beams 7L and 7R alternately from targets at one end of each of the waveguides 2L and 2R. At the same time a synchronising pulse output 8 is provided to control operation of the imaging system as will be described below.

In each of the waveguides an electron gun is pulsed synchronously with the RF output of its associated magnetron. The pulses of radio frequency energy act as a carrier wave within the accelerating waveguide to accelerate the bursts of electrons towards the x-ray emitting anode target at the opposite end. The beam of electrons can be focused to a narrow cross-section impinging on the target, typically the beam is focussed to about a 2.5 mm spot. On striking the target the electron energy is converted into x-rays which are scattered generally in a forward direction.

The target is located behind a collimator having a frusto-conical beam aperture co-axial with the longitudinal ~ axis of the waveguide and producing a conical beam. The cone angle of the beam is determined by aperture size and position of the collimator.The two waveguides 2L and 2R are mounted side by side on a common base (not shown in Figure 1) and spaced apart in a horizontal direction. The waveguides are mounted for relative movement both laterally and pivotally so that the separation and convergence of the left and right beams may be adjusted as required.

By changing the convergence point of the two beams and the separation distance of the x-ray sources an object being inspected can be viewed with a range of stereoscopic depths of field and scanned over a range of object distances.Source separation is an important stereoscopic parameter and has a powerful influence on stereoscopic perception in the way in which it affects the fusion of images by a human observer. The separation distance directly determines the parallax error which controls the depth of the stereoscopic field within which an observer is able to fuse together the left and right images.

In order to view the object stereoscopically the different perspective images of the object have to be presented superimposed and within the fusion limits of the observer. The two views can be presented simultaneously to an observer so that he sees one with his left eye and the other with his right eye, but this requires two display means. Largely for cost reasons, it is preferred to present the two views alternately to the appropriate eyes with sufficient rapidity that persistence of vision makes the images appear to be superimposed.

The convergence angle and separation distance of the x-ray beams considerably affect the appearance of the final image and the perception of depth and position of the imaged objects. However, the relative separation of the two perspective images, providing the observer's tolerance limits for stereo perception is not exceeded, can have a very powerful effect on the final appearance. If the two images are merely superimposed, so that the disparity between the images is caused by the original parallax error due to the relative positioning of the binocular sources, then the image will appear to recede behind the image screen.

By positively increasing the disparity between the images, that is by shifting the right image further to the right relative to the left image, the stereo image will appear to recede further behind the screen.

However, by moving the images in the opposite direction, thus giving them a negative disparity, the stereo image will appear to stand out from the screen.

Not only does this enhance the three dimensional effect seen by the observer but doubles the size of the image field within which stereo images can be produced.

Shifting the images too much in either direction, so as to exceed the observer's fusion limits, cause the perceived images to break-up into separate images lacking in depth. Of course, this occurs naturally towards the edges of a scene anyway so that in any view stereo-vision is perceived only towards the centre of the field of view. Varying the lateral disparity between the perspective images has the effect of further restricting the stereo field, but this is not necessarily a disadvantage.

In addition the separation distance between the x-ray beam sources can be used to control the magnification in depth of the displayed stereo images. In order to maintain sufficient beam overlap the convergence angle is preferably altered to maintain registration on the x-ray imaging means. The separation distance between the x-ray targets and the convergence angles of the beams should be consistent with production of images within the stereoscopic fusion limits of the observer.

The shaded area of Figure 1, where the beams overlap, represents the zone within which stereoscopic images of an object will be obtained. As shown the nominal convergence of the two beams is adjusted so that both cover as much as possible of a large area x-ray sensitive screen 10. The container 1 is positioned a short distance in front of the screen.

These containers are approximately 3 metres high by 2.5 to 3 metres wide and about 12 metres long. The container is positioned with its centre line roughly 1.5 metres from the screen which is about 3 metres high, 4 metres long and is spaced about 15 metres from the radiographic source. The source may be enclosed within a room or building with a relatively thin wall (not shown) between the source and the container.

The image screen 10 is formed with an x-ray sensitive phosphor panel on its rear face upon which appear the images created by x-rays penetrating the container 1.

The level of light- emitted by the phosphor is dependent upon the intensity of x-rays reaching the panel. The panel is viewed, in turn, by a low light level television camera 11, via reflection in a mirror 12. The latter permits camera 11 to be located out of a direct line of sight to the x-ray sources. The visible light images formed in panel 10 are digitised and transferred via an electronic gating switch 13 to one or the other of video frame stores 14L and 14R, according to the handedness of the perspective view, under the control of synchronising signal 8.

The stored video signals are read-out from the frame stores and displayed on a video monitor 15 via a direct signal path indicated by the dashed lines in the drawing. Normally, however, some image processing will be necessary and an alternative signal path is indicated by the solid line through image signal processing module 16.

The video system loads the frame stores 14L and 14R respectively with signals representing the left and right views of the object under the control ' of synchronising signal 8 and also reads them out again for display by the video display means 15. In the embodiment being described the left and right images are displayed alternately on the screen. An observer views the screen through a suitable viewing means so that he sees only the left perspective view with his left eye and the right perspective view only with his right eye.

The suitable viewing means may comprise optically switched means such as spectacles in which the eye-pieces are formed of liquid crystal cells in which the optical transmission factor is switched between opaque and clear by an electrical signal derived from the synchronising signal 8.

Another form of spectacles has circularly polarised eyepieces of opposite handedness. For example, the left eyepiece is polarised in a clock wise direction while the right eyepiece is polarised in an anti-clockwise direction. The image display monitor has in front of the screen a selectively controlled polarising screen which switches the polarisation directions of the screen in step with the presentation of the left and right images from the frame stores. At the rate of read-out from the frame stores normally employed, the observer will not perceive any switching to be occurring, but his brain will fuse the two images into a single composite stereoscopic image.

Circular polarisers are preferred to crossed linear polarisers in order to eliminate mutual interference effects which would otherwise occur when the observer tilts his head.

Grey scale enhancement of the stored images may be necessary before display can take place if the contrast range of the basic images are too compressed.

To correct this the video signal range containing the image data can be electronically "stretched" to make full use of the available contrast range. Further image processing algorithms may be applied, such as edge enhancement algorithms and, if desired, a false colour imaging process can be performed.

The linescan arrangement of Figure 2 employs the same basic binocular radiographic source with the substitution of different collimators adapted to produce two fan-shaped beams or curtains of x-ray which are directed towards two vertical linescan arrays spaced apart horizontally towards the edges of the stereoscopic x-ray region. In the drawing of Fig 2 parts which are common to the arrangement described with reference to Figure 1 are given like references.

The accelerating waveguides 2L and 2R are operated in the manner as described above but have collimators which produce fan-shaped beams 21L and 21R oriented vertically. These beams are angled, with respect to the horizontal direction of pathway la, to impinge upon vertical linescan detector arrays 22L and 22R.

These arrays comprise a multiplicity of x-ray sensitive light emitting diodes arranged in a single linear array. In order to obtain the different perspectives required of the two images the x-ray beam directions must either converge, as shown, or diverge.

In this arrangement, each of the perspective views of the object has to be built up by repeatedly scanning the arrays as the object 1 passes through the thin curtains of x-rays. In effect, both beams sweep the object but there is a disparity in time between the images.

This is easily corrected by applying a time delay to one of the linescan array signals when it is read out from the frame store,in this case the left signal from frame store 14L must be delayed relatively to the signal from frame store 14R. The final images will then appear as if they were obtained simultaneously when they are finally displayed together on the display monitor of the video system As before, the accelerating waveguides 2L and 2R are energised alternately. Each thin curtain of x-rays passes through the object and impinges upon the light emitting diodes of the corresponding array. These diodes are scanned electronically and the illumination level of each one is digitised and stored in the appropriate frame store location to represent one pixel in a vertical slice in the final image.This process is repeated alternately, column by column, under the control of electronic gating networks 23 controlled by the synchronising signal 8, until complete images are acquired. As each two dimensional image is built up by adding vertical slices of the images the object 1 has to move progressively passed the beams and linescan arrays.

Although, in the linescan imaging arrangement, the images which are effectively superimposed in the stereoscopic display are acquired at slightly different times this temporal disparity is easily corrected by introducing an opposite time delay when the images are recovered from the video frame stores.

The images can be displayed to obtain a "real time stereo image display if the later obtained image is displayed virtually straightaway and the earlier image merely delayed by the time it takes a given point on the object 1 to travel between the beams. It may not be possible to display the later image immediately because of the time taken by the linescan array to build up the image in vertical slices, whereas a conventional video display operates according to a horizontal raster scan.

The video signals representing the two perspective images are preferably stored electronically in respective frame stores sufficiently "long" in terms of numbers of storage locations to accommodate the whole of the images. Basic frame store chips may be extended by adding on further stores or memories.

Essentially each memory is mapped in two dimensions, say horizontally and vertically, and the linescan outputs are stored with corresponding orientation.

For physically long objects such as cargo containers the frame stores can be extended by adding further memory locations in the horizontal direction.

When it is wished to display the images to obtain a stereoscopic display output devices are electronically scrolled in step through appropriate locations in each of the two memories. Although there is a disparity between the times when the two images were acquired it is possible to effectively alter one of the stereoscopic parameters, the convergence point of thebeams by altering the time delay or separation between the outputs as they are read-out from the frame stores.

This method may be used to relatively shift the acquired images for display purposes. In the linescan type of arrangement an initial shift of one image relative to the other is necessary in order to nominally superimpose the images.

If this shift is merely to correct the temporal disparity when the images were acquired initially then, as has been described already, the perceived stereo image appears to recede behind the image screen. By varying the relative shift of the image signals as they are read out of the frame stores the stereo image can be moved backwards and forwards within the stereo image field. This technique can be applied equally to images acquired by the arrangement illustrated in and described above with respect to Figure 1.

In the Figure 2 arrangement the separation distance of the binocular x-ray sources may be altered as previously described, but in addition the convergence angle must be varied in order to maintain registration between the linescan arrays 22L and 22R and the path of the curtains 21L and 21R of x-rays. However, if it is wished to alter the separation distance and convergence angle of the radiographic source the object will have to be passed through the beams again.

Since it takes one complete ~ pass of the object to acquire the basic two stereoscopic images it is not possible to change the parameters without repeating the pass.

Fig 3 shows an alternative x-ray source suitable for use in the embodiment of Fig 2. This uses only one accelerating waveguide 30 which is fitted with an x-ray producing target 31 designed to produce two output beams 32R and 32L. In this case the two x-ray beams diverge as shown in the drawing, and are emitted along the axes of two collimating apertures 33R and 33L. This type of arrangement-may be used where space is particularly restricted.

A further radiographic source arrangement, not shown in the drawings, has a single accelerating waveguide and two x-ray producing targets which are spaced apart transversely with respect to the waveguide and each of which produces an x-ray beam. The waveguide is bifurcated and has an arm leading to each of the x-ray targets. The electron beam is switched between the arms are required, for example, after every pulse or after a group of pulses, so that the targets generate x-rays alternately.

Whilst the invention has been described above with reference to a container inspection system, it will be appreciated that other applications can be envisaged.

Claims (19)

1. A non-invasive stereoscopic inspection system capable of producing three-dimensional transparent images of an object and its contents comprises a binocular radiographic source means located at one side of a region within which the object is located, large scale imaging means located on the opposite side of the region, the radiographic source means, in use, projecting two laterally displaced x-ray beams towards the imaging means to produce thereon left and right perspective images, display means for presenting to an observer the left and right images in a manner to impart to him a stereoscopic perception of the object and its contents and wherein the binocular parameters of the radiographic source means are variable to alter the observer's perception of depth in the stereoscopic image.

2. An inspection system according to claim 1 wherein the two x-ray beams are produced by separate x-ray generators.

3. An inspection system according to claim 1 or 2 wherein the or each x-ray generators comprises an accelerating waveguide impinging an electron beam onto an x-ray emitting target.

4. An inspection system according to claim 3 wherein the or each of the accelerating waveguides comprises a centre-feed standing wave or a travelling wave linear accelerator.

5. An inspection system according to claim 4 wherein the or each of the accelerating waveguides is controlled by the output of a magnetron.

6 An inspection system according to claim 5 wherein the or each magnetron is operated to produce selectively an output to control generation of one or the other of the x-ray beams.

7. An inspection system according to claim 6 wherein operation of the or each magnetron is controlled by selectively energising the field coils producing its applied magnetic field.

8. An inspection system according to claim 7 wherein the magnetron magnetic fields are selectively energised by the output of a common modulator.

9. An inspection system according to claim 8 wherein the modulator output is supplied through a pulse transformer to the magnetrons alternately such that the accelerating waveguides are likewise energised alternately.

10. An inspection system according to any of claims 2 to 9 wherein the separation distance of the x-ray generators is variable by relative lateral movement of the accelerating waveguides whereby to alter the relative magnification in depth in the stereo image.

11. An inspection system according to any of claims 2 -to 10 wherein the angle of convergence of the x-ray beams is variable whereby to alter the stereoscopic depth of field in a perceived image.

12. An inspection system according to claim 1 wherein at least one of the accelerating waveguides is mounted for pivotal movement about an axis perpendicular to a plane containing both x-ray beams whereby to vary the convergence of said beams.

13. An inspection system according to any preceding claim further comprising large area imaging means for capturing the left and right perspective x-ray images.

14. An inspection system according to claim 13 wherein the large area imaging means comprises an x-ray sensitive phosphor screen and a video system.

15. An inspection system according to any of claims 1 to 12 wherein the large area imaging means comprises two x-ray sensitive linear arrays spaced apart in the path direction and disposed substantially perpendicularly, and a video system whereby as the object moves through the x-ray beams the linear arrays effectively provide two-dimensional left and right perspective images.

16. An inspection system according to claims 14 or 15 wherein the video system includes image detection means connected with video frame stores for loading video signals representing the left and right perspective views and display means connected with said frame stores for displaying the left and right images.

17. An inspection system according to claim 16 wherein the left and right images are displayed alternately on the display means to an observer viewing through stereoscopic viewing means presenting a left image to one of the observer's eyes and a right image to the other of his eyes.

18. An inspection system according to either claim 16 or claim 17 in combination with claim 9 wherein the loading of the video frame stores and operation of the display means in conjunction with #optical switching means is synchronised with energisation of the waveguides.

19. An inspection system substantially as described with reference to the accompanying drawings.