Stereoscopic radiographic inspection system
RELATED APPLICATIONS 
     This application is a continuation of Ser. No. 07/614,557 filed Nov. 13, 1990, and now abandoned which is a continuation of Ser. No. 07/137,642 filed Dec. 24, 1987, and now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention pertains generally to the field of radiographic inspection, and more particularly to a technique for acquiring and displaying stereoscopic radiographic images. 
     BACKGROUND OF THE INVENTION 
     Techniques developed in the prior art for stereoscopically displaying radiographic images have generally required the use of two radiation sources and two radiation detectors. According to the prior art, two radiation sources are arranged with respect to two radiation detectors so that a test object is irradiated by two radiation beams that are angularly displaced from each other, whereby two images of the test object are acquired and viewed stereoscopically. 
     Where steroscopic imaging was required to be performed in real time in the prior art, techniques were developed that used two radiation sources and a single radiation detector for acquiring a stereoscopic pair of radiographic images of the test object. The resulting stereoscopic pair of radiographic images could generally be displayed by conventional means. 
     No technique was developed in the prior art for stereoscopically acquiring and displaying radiographic images in real time using only a single radiation source and a single radiation detector. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a technique for stereoscopically acquiring and displaying radiographic images in substantially real time using only a single radiation source and a single radiation detector. 
     In accordance with the present invention, a test object that is to be radiographically inspected is positioned in a radiation beam so that a first electronic image can be formed, and the test object is then displaced relative to the source of the beam so that a second electronic image can be formed, whereby the first and second electronic images represent two views of the test object slightly displaced from each other. A feature of the present invention is that the first image is electronically delayed with respect to the second image so that the first and second images can be displayed as a stereoscopic pair. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of a stereoscopic radiographic inspection system according to the present invention. 
     FIG. 2 is a perspective view of the stereoscopic radiographic inspection system illustrated schematically in FIG. 1. 
     FIG. 3 is a schematic illustration of a first embodiment of the stereoscopic viewing technique shown in the stereoscopic radiographic inspection system of FIG. 1. 
     FIG. 4 is a schematic illustration of a second embodiment of the stereoscopic viewing technique shown in the stereoscopic radiographic inspection system of FIG. 1. 
     FIG. 5 is a schematic illustration of a third embodiment of the stereoscopic viewing technique shown in the stereoscopic radiographic inspection system of FIG. 1. 
     FIG. 6 is a schematic illustration of an alternative stereoscopic viewing technique for a stereoscopic radiographic inspection system according to the present invention. 
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
     As illustrated in FIG. 1, a test object 10 for which a stereoscopic radiographic image is to be displayed is supported by means of a positioning system 11 in a beam of penetrating radiation emanating from a radiation source 12. To facilitate illustration of the invention, the test object 10 is shown as having a cubic configuration, and is positioned symmetrically with respect to the beam during irradiation. However, in principle, the test object 10 need not be of regular configuration, and need not be symmetrically disposed with respect to the irradiating beam. The invention is described herein specifically in terms of an X-ray inspection system in which the radiation source 12 produces an X-ray beam. However, in principle, the radiation source 12 could be a gamma ray source, a neutron source, or any other penetrating radiation source. 
     The positioning system 11 can be operated (either manually or automatically) to cause relative motion of the test object 10 with respect to the X-ray beam produced by the X-ray source 12, so that the X-ray beam can irradiate the test object 10 in each one of a sequence of different positions. The motion system 11 could be a conventional conveyor system, which translates the test object 10 through the X-ray beam. In a typical embodiment of the invention, the test object 10 is translated through the X-ray beam in a direction perpendicular to a central axis of a cone of radiation emanating from the X-ray source 12 such that the central axis defines a direction of propagation of the X-ray beam. Alternatively, the positioning system 11 could be a device for rotating the test object 10 in the X-ray beam, in which case the test object 10 would typically be rotated about an axis perpendicular to the direction of propagation of the X-ray beam. 
     Motion of the test object 10 with respect to the X-ray source 12 could be produced during time intervals between sequential irradiations of the test object 10, although it is not a requirement of the invention that the test object 10 be fixed in position with respect to the X-ray beam during each irradiation. Regardless of the manner in which the test object 10 is irradiated, it is a requirement of the invention that a plurality of images of the test object 10 in a sequence of different positions be acquired. 
     Radiation that passes through the test object 10 during each sequential irradiation thereof is converted by an imaging system 13 into an electronic signal, which corresponds to the position of the test object 10 relative to the radiation source 12. When the radiation source 12 is an X-ray source, the imaging system 13 is an X-ray imaging system. Conventionally, the X-ray imaging system 13 described herein could comprise an image intensifier, or a phosphor screen, or a rare earth screen, which generates a visual image when activated by X-ray radiation impinging thereon. Conversion of the visual image into the electronic signal corresponding to the position of the test object 10 can be accomplished conventionally in the X-ray imaging system 13 by using, e.g., a video camera. 
     The electronic signal produced by the X-ray imaging system 13 during a particular irradiation of the test object 10 becomes an input to an image converter 14, which comprises means for producing a pair of electronic signals designated as &#34;Signal A&#34; and &#34;Signal B&#34;. One signal of the pair (i.e., either Signal A or Signal B) could be substantially undelayed with respect to the electronic input signal produced by the X-ray imaging system 13, or both Signal A and Signal B could be delayed (but by different amounts) with respect to the electronic input signal. Signal A and Signal B are substantially identical to each other, but must be delayed in time by different amounts with respect to the electronic input signal. 
     The image converter 14 comprises a pair of image storage and delay systems 15 and 16, which operate on the electronic input signal to produce the unequally delayed Signal A and Signal B, respectively. In a particular application, one of the image storage and delay systems 15 or 16 could be a direct feed-through, in which case the delay produced thereby with respect to the electronic input signal would be substantially zero. The means utilized by the other image storage and delay system of the pair (or by both of them, when both Signal A and Signal B are to be delayed with respect to the electronic input signal) could be a conventional delay device such as, e.g., a video tape recorder, a digital buffer, a video disc, or a digital disc drive. 
     Signal A and Signal B become inputs to a stereoscopic display system 17, which produces a corresponding pair of visual X-ray images of the test object 10. Because of the time delay between Signal A and Signal B, the two visual X-ray images produced by the stereoscopic display system 17 correspond to two different positions of the test object 10 relative to the X-ray source 12. Thus, as illustrated in FIG. 1, Signal A could correspond to a present position of the test object 10 in the X-ray beam, in which case Signal B would correspond to a previous position of the test object 10 in the X-ray beam. 
     In the particular embodiment of the invention shown in FIG. 1, the stereoscopic display system 17 comprises a pair of conventional video monitors 18 and 19. Signal A is input to the monitor 18 and Signal B is input to the monitor 19. Since Signals A and B correspond to different positions of the test object 10, the monitors 18 and 19 display visual X-ray images of the test object 10 in correspondingly different positions. An optical viewing system 20 (which is schematically indicated in FIG. 1 to represent a plurality of different viewing system embodiments) causes the visual image displayed on the monitor 18 to be visible only to one eye of a human observer, and causes the visual image displayed on the monitor 19 to be visible only to the other eye of the observer. Thus, the observer is enabled to perceive the two visual images of the test object 10 as a stereoscopic pair. 
     As the test object 10 is moved by the positioning system 11 sequentially through different positions in the X-ray beam, successive irradiations of the test object 10 in the different positions are performed. The X-ray imaging system 13 thereby acquires a succession of visual images (i.e., a &#34;moving picture&#34;) of the test object 10 in the different positions. The &#34;moving picture&#34; is then converted by the video camera (or by whatever other means is used) in the X-ray imaging system 13 into a series of electronic signals corresponding to the sequence of different positions of the test object 10. The series of electronic signals produced by the X-ray imaging system 13 becomes an input to the image converter 14, which then produces a pair of unequally delayed series of Signals A and B. 
     The pair of series of Signals A and B produced by the image converter 14 becomes an input to the stereoscopic display system 17, which produces a corresponding pair of series of visual X-ray images of the test object 10. The series of visual X-ray images corresponding to the series of Signal A electronic signals, and the series of visual images corresponding to the series of Signal B electronic signals, are presented by the optical viewing system 20 to different eyes of the observer during a time interval that is sufficiently short in terms of the physiology and phychology of human visual perception so that the two series of visual images are perceived by the observer as being substantially simultaneous. 
     The observer sees a series of visual X-ray images corresponding to the series of Signal A electronic signals with one eye, and a series of visual X-ray images corresponding to the series of Signal B electronic signals with the other eye, substantially simultaneously. Because of the timedelay between the Signal A series and the Signal B series of electronic signals, the series of visual X-ray images seen by the observer&#39;s right and left eyes, respectively, are attributable to successive irradiations of the test object at different positions in the X-ray beam. The time delay of the Signal A series relative to the Signal B series of electronic signals corresponds to the direction of displacement of the test object 10 relative to the X-ray source 12, so that the observer is enabled to perceive a &#34;moving picture&#34; of a series of visual X-ray images of the test object 10 stereoscopically. 
     A generalized illustration of equipment components for a stereoscopic radiographic apparatus as illustrated schematically in FIG. 1 is shown in perspective view in FIG. 2. 
     Several different embodiments are possible for the optical viewing system 20, which enables the human observer to perceive separate visual images (or separate series of visual images) on the monitors 18 and 19 in stereoscopic view. In an embodiment schematically illustrated in FIG. 3, the visual images (or series of visual images) displayed on the monitors 18 and 19, respectively, can be reflected to the right and left eyes, respectively, of the observer by corresponding optical systems 21 and 22, which could include mirrors and prisms; and the reflected images (or series of images) can be focussed onto the observer&#39;s right and left eyes, respectively, by corresponding lenses (or lens systems) 23 and 24. In an embodiment schematically illustrated in FIG. 4, the visual images (or series of visual images) displayed on the monitors 18 and 19, respectively, can be refracted by corresponding refracting systems 25 and 26 to the right and left eyes, respectively, of the observer. The refracting systems 25 and 26 could comprise, e.g., single prisms, or arrays of prisms and lenses. 
     In another embodiment of the optical viewing system 20, as shown in FIG. 5, the separate visual images (or separate series of visual images) displayed on the monitors 18 and 19, respectively, are cross-polarized by means of corresponding polarizing screens 27 and 28. The cross-polarized visual images are then combined by a beamsplitter 29, and the combined visual images are seen by the observer through polarizing glasses 30. The polarizing glasses 30 cause the visual image (or series of visual images) on the monitor 18 to be visible only to the observer&#39;s right eye, and cause the visual image (or series of visual images) on the monitor 19 to be visual only to the observer&#39;s left eye. 
     Different configurations are also possible for the stereoscopic display system 17. For example, in the embodiment shown in FIG. 6, only a single monitor (e.g., the monitor 18) is required, in which case the pair of unequally delayed series of Signals A and B produced by the image storage and delay systems 15 and 16, respectively, of the image converter 14 becomes a pair of inputs to a stereoscopic display synchronizer 31. The stereoscopic display synchronizer 31 causes the Signal A series of electronic signals and the Signal B series of electronic signals, in sequence, to be presented as alternating display fields on the monitor 18. Simultaneously, a polarizing screen 32 positioned in front of the monitor 18 is synchronously switched by the stereoscopic display synchronizer 31 from one polarization to another. The observer wears a pair of polarizing glasses 33 to produce cross-polarization with respect to the polarizing screen 32, whereby the series of visual X-ray images displayed on the monitor 18 by the Signal A series of electronic signals is visible only to one eye of the observer, and the series of visual X-ray images displayed on the monitor 18 by the Signal B series of electronic signals is visible only to other eye of the observer. 
     In a variation of the stereoscopic display system 17 illustrated in FIG. 6, instead of having a synchronously switched polarizing screen and a pair of polarizing glasses, the stereoscopic display synchronizer 31 synchronously actuates electro-optical shutters covering the eyes of the observer so that the series of visual X-ray images displayed on the monitor 18 by the Signal A series is visible only to one eye of the observer, and the Signal B series is visible only to the other eye of the observer. The observer is thereby enabled to perceive the two series of visual X-ray images as a stereoscopic pair. 
     In another variation, the stereoscopic display synchronizer 13 shown in FIG. 6 could comprise a conventional color monitor on which the unequally delayed Signal A series and Signal B series of electronic signals produce visual X-ray images of two different complementary colors (e.g., red and green). Color filter glasses of the same two complementary colors are worn by the observer, so that the series of visual X-ray images produced on the color monitor by the Signal A series is predominantly visible to one eye of the observer, and so that the Signal B series is predominantly visible to the other eye of the observer. The observer is thereby enabled to perceive the two series of visual X-ray images as a stereoscopic pair. 
     The present invention has been described above in terms of particular embodiments. However, other embodiments would be apparent to practitioners skilled in the art of radiographic inspection upon perusal of the above description and the accompanying drawing. Therefore, the foregoing specification of the invention is to be considered merely as illustrative of the invention, which is defined more generally by the following claims and their equivalents.