Patent Publication Number: US-2011062349-A1

Title: Cassette and Device for Testing Objects

Description:
RELATED APPLICATIONS 
     This application claims the filing benefit of International Patent Application No. PCT/EP2009/001945, filed Mar. 17, 2009, which claims the filing benefit of German Patent Application No. 10 2008 020 611.3 filed Apr. 24, 2008, the contents of both are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to a cassette and to a device for testing objects. 
     BACKGROUND OF THE INVENTION 
     Testing devices of such a type find application in the non-destructive testing of materials, for example in connection with the inspection of weld seams. Extensive test regions are examined with the known devices in such a way that the radiation-source and the detection instrument are placed in succession above workpiece regions to be tested and then in each instance a partial test image is recorded which is evaluated for flaws. Such a shifting of radiation-source and detection instrument is time-consuming and requires human intervention. 
     Such testing devices are also used for military purposes, for police tasks and in the security field. 
     Analogous examination devices also find application in medicine, paediatrics, dental medicine and veterinary medicine. There are also many applications in which human intervention is not possible or is only possible under very difficult conditions. 
     Examples from materials testing are, for example, workpieces located underwater, for example cables, pipelines and also the so-called risers which in offshore oil wells are used to transport oil from a point situated on the sea bed to a loading station situated on the surface. The risers have to be flexible, in order to be able to give way sometimes during the movements of the sea and in order to be able to accommodate buckling and rolling movements of a tanker or of a loading pontoon that is being used for transporting or temporarily storing the crude oil. 
     The tubular material from which the risers are manufactured is subjected to strong mechanical influences and must also withstand chemical attacks in the long term. In practice it is a question of multi-layer composite materials consisting of plastic, metal, textile and insulating layers. 
     Given the frequent alternating loads to which the risers are exposed, it happens that fatigue fractures arise in the course of time. These have to be detected in good time before a leakage of the riser occurs. Leakages of such a type would result in considerable pollutions of the environment and would give rise to production losses which may amount to some millions of Euros per day. 
     Hitherto there has, for example, been no practicable process for an in-situ examination of the flawlessness of risers, because the cassette with the radiation-sensitive layer has to be brought into a scanner for the purpose of reading out the latent image, necessitating bringing the cassette up out of the water. 
     Also in other applications it is often difficult to bring the cassette to a scanner after exposure. 
     The present invention is directed to resolving these and other matters. 
     SUMMARY OF THE INVENTION 
     By virtue of the present invention, therefore, a cassette for use in the radiographic testing of an object is to be specified that can be read out at the place of use. 
     In accordance with invention this object may be achieved by a cassette for testing an object, using radiation, that is impervious to ambient light and exhibits a wall that is transparent to radiation at least in one region, with a planar light-sensitive recording layer which is arranged in its interior, wherein inside the cassette at least one part of an image-read-out instrument co-operating with the recording layer is arranged which includes: a read-out head, which at a given time co-operates with a partial region of the recording layer, a guide mechanism for the read-out head, a driving mechanism for moving the read-out head on the guide mechanism, and a position-measuring instrument for measuring the position of the read-out head on the guide mechanism. 
     The cassette according to the invention contains a read-out head and the mechanism for traversing it over the radiation-sensitive recording layer—that is to say, precisely the first part of a scanner, viewed in the signal-processing direction, and just so much that signals reproducing the latent image are obtained. 
     Since with the cassette according to the invention the recording layer may remain permanently in the cassette for its operational life, the dangers that arise as a result of contact of contaminants with the mechanically sensitive recording layer and as a result of manual handling of the same are also eliminated. 
     When use is made of the cassette according to the invention, a radiation-source that is used with it for testing can be shifted together with it, region by region, over an extensive object, a procedure that can be undertaken reliably and accurately, since the cassette does not have to be moved completely away from the object between the recordings. Accordingly, an overall image of the object can be generated rapidly and reliably that consists of smoothly combined partial images. One needs only to ascertain, in each instance, via an instrument for measuring the position of the cassette, which partial region of the object is being scanned. 
     Advantageous further developments of the invention are specified in the dependent claims. 
     With a further development of the invention it is ensured that the read-out head is always situated opposite the object under comparable conditions. 
     A further development of the invention is advantageous with regard to simple compact structure and reliable operation of the driving mechanism operating on the read-out head. 
     In this connection, with the further development of the invention it is guaranteed that the drive means exhibit large stroke and are of compact construction in the direction perpendicular to the direction of motion. 
     A further development of the invention permits the measurement of the position the read-out head in mechanically simple and space-saving manner. 
     With a cassette according to a further development of the invention, the measurement of the position of the read-out head can be performed simply and precisely. In this way a plurality of contiguous slit-shaped or strip-shaped partial images are obtained which together reproduce the partial region of the object being monitored. 
     In a further development of the invention the position of the read-out head can be performed simply by monitoring the application of the control signal for the head-driving mechanism. 
     In a further development the cassette is advantageous with regard to a rapid gauging of the object. Also with regard to the processing and evaluation of the test image it is advantageous if a line is simultaneously recorded in each instance. 
     In this connection, a screening of the detection face into pixels can be realised in straightforward manner. 
     In yet a further development of the invention, a cassette can be realised using semiconductor structural components that are already commercially available for other purposes. 
     In still a further development of the invention, a cassette, by way of radiation-source use may be made of a radiation-source emitting high-energy photon radiation or corpuscular radiation, and the radiation that has penetrated the object can be converted in straightforward manner into electrical signals, whereby known optoelectronic components may find application. 
     A further development of the invention permits the read-out of latent images from storage foils that contain metastable colour centres which are capable of being populated by radiation and which then relax as a result of being irradiated with read-out light, accompanied by output of shorter-wave fluorescent light. 
     In another development of the invention, the cassette is advantageous with regard to the avoidance of radiation damage to the read-out head. 
     With a cassette according to another embodiment of the invention, on the one hand a rapid read-out of the read-out head is obtained, with good separation of adjacent pixels. 
     In this connection, with the further development of the invention it is then ensured that no crosstalk occurs between detection elements. 
     According to a further development of the invention, the fine setting of the testing head within a smaller range of adjustment is measured accurately, the coarse setting less accurately. The partial images that are obtained in the case of successive coarse settings can be placed side by side correctly at the joint by evaluation of overlapping image regions. 
     With a cassette according to yet another embodiment of the invention, the overall setting of the testing head is composed of an absolute coarse value and a relative fine value. 
     A cassette according to an embodiment of the invention is particularly well suited for examining pipes and other objects exhibiting circular cross-section. 
     The further development of the invention permits the read-out head to be constructed to be smaller than 360° in the angular direction and nevertheless permits a continuous test image of a complete ring segment of the object to be obtained by the partial images taken in successive angular increments being joined electronically at the places of overlap. 
     A cassette according to a further development of the invention is particularly well suited for examining a tubular object rapidly. 
     With a cassette according to yet another development of the invention, after the read-out of the latent image an erasing of a residual image possibly remaining is effected. 
     With a cassette according to an embodiment of the invention the erasing unit can also, if necessary, be used as read-out head or as second read-out head for reading out a residual image. The control of the cassette only needs to be reprogrammed; in particular, the direction of motion of the read-out head needs to be reversed where appropriate. This emergency function is a great advantage in particular when a repair of the cassette on the spot is not possible or is only possible with major difficulties and major expenditure of time. 
     In this connection, with the device of the invention it is guaranteed that the location of the cassette relative to the object in one direction is predetermined by force. 
     In many cases it is necessary to be able to examine also relatively large objects rapidly. This can be obtained particularly easily by the object being scanned with a plurality of testing heads which each include a radiation-source and a cassette, which together cover the object. 
     In a further development is advantageous with regard to defined constant irradiation conditions for the testing of the object. 
     With a further development of the invention it is ensured that the bearing part and the radiation-source borne by it fill out the cross-section of the tubular object only partly, preferentially only to a small extent. This makes it possible to leave the bearing part and the radiation-source permanently inside the tubular object if this is desired. Also, fluid can continue to be conveyed through the tubular object during the time in which the object is being examined. 
     A further development of the invention is advantageous with regard to a tilt-free guidance of the bearing part in the tubular object. 
     Furthermore, guide arms constitute, at the same time, means for homogenising the flow in the tubular object. 
     A further development of the invention is advantageous with regard to a uniform irradiation of a tubular object with test radiation. 
     A further development of the invention is advantageous with regard to a particularly rapid generation of a test image, since relatively large regions of the object are irradiated simultaneously. 
     With a device a tubular object can be tested without a radiation-source needing to be brought into the interior of the tubular object. 
     In this connection, with the further development of the invention it is ensured that flaws that are located in the wall portion of the object facing towards the radiation-source can be discerned electronically from flaws that are located in the wall portion of the object situated in front of the read-out head. 
     A device as specified in an embodiment of the invention can also be employed in deep water. 
     The further development of the invention also serves for applicability of the device under high external pressures without the risk of damage or contamination of the read-out head. 
     The further development facilitates a correct aligning of read-out head and radiation-source if the two are capable of being moved independently of one another. 
     The further development of the invention permits an entire region of the object to be irradiated simultaneously. 
     With a device according to an embodiment of the invention, the radiation is moved over the object in the same way as the read-out head. This is also advantageous with regard to a rapid and automatic generation of the test image. 
     With a device according to yet another embodiment of the invention, the test beam and the read-out head can be moved jointly, which is advantageous for a simple and inexpensive structure of the device. With the device according to still another embodiment of the invention, it is also guaranteed that the test radiation does not reach the read-out head directly. This makes it possible to use sensitive components in the read-out head which could be damaged in the event of exposure to a high dose of test radiation. 
     In this connection, the phase shift between the motion of the test beam and the motion of the cassette is chosen in such a way that precisely no direct light of the radiation-source reaches the read-out head. 
     A device according to an embodiment of the invention, use may be made of read-out heads that exhibit radiation-sensitive elements which can withstand the test radiation. 
     The measure is also ensured that in no case does the test radiation reach the read-out head. 
     A device according to the invention can fully gauge an object exhibiting very large dimensions by using a testing head exhibiting small dimensions. 
     The further development of the invention permits a gauging of the position of the testing head on the object independently of zero displacements or of slippage, added up in the course of time, between head-position indicator and object. 
     In this connection, a device can measure the movements of the testing head with high resolution. 
     It is to be understood that the aspects and objects of the present invention described above may be combinable and that other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a side view of a tanker which has docked at a loading pontoon which is connected via flexible risers to several sources of crude oil situated on the seabed; 
         FIG. 2  an axial section through a riser testing head as shown in  FIG. 1 ; 
         FIG. 3  a transverse section through the axial centre of the testing head shown in  FIG. 2 ; 
         FIG. 4  a top view of the inside of a transducer unit of the testing head according to  FIGS. 2 and 3 ; 
         FIG. 5  a representation similar to  FIG. 4 , in which a modified read-out head is shown; 
         FIG. 6  an axial view of a modified testing head; 
         FIG. 7  a view similar to  FIG. 6 , in which a further modified testing head is shown; 
         FIG. 8  a view similar to  FIG. 7 , in which, however, a testing head for flat objects is shown; 
         FIG. 9  an axial view of a multi-head test unit; 
         FIG. 10  a section through the test unit of  FIG. 9  along the section line X-X therein; 
         FIG. 11  an axial view of a testing head that is capable of being rotated in the angular direction: 
         FIG. 12  a schematic representation of a coarse-setting sensor; 
         FIG. 13  a circuit for combining a coarse-setting signal with a fine-setting signal; 
         FIG. 14  a testing head similar to that in  FIG. 6 , wherein precautionary measures have been taken in order to protect the read-out head from test radiation; 
         FIG. 15  a top view of a flat cassette such as can be used for medical purposes; and 
         FIG. 16  a top view of a modified flat cassette which resembles that according to  FIG. 15 . 
     
    
    
     Denoted schematically by  10  in  FIG. 1  is a tanker which has moored to a loading pontoon denoted overall by  12 . The loading pontoon has supply ports  14   a ,  14   b  and  14   c  which via risers  16   a ,  16   b , and  16   c  are connected to tapping ports  18   a ,  18   b ,  18   c  which previously pertain to perforated heads  20   a ,  20   b ,  20   c . From the latter, drill pipes  22   a ,  22   b ,  22   c  go into the rock as far as a deposit of crude oil. 
     Wherever hereinafter the distinction does not matter, reference symbols  14 ,  16 ,  18 ,  20  and  22  are used without appended letters. 
     The risers  16  are produced from a composite material that comprises plastic, metal, fabric and insulating layers. The overall composite structure is flexible and pressure-resistant. 
     In order to be able to test a riser in operation for freedom from flaws, a testing head  24  is provided on this riser  16 . 
     Said testing head has, as shown in more detail in  FIGS. 2 and 3 , a housing  26  which exhibits an inner cylindrical peripheral wall  27  which is capable of being moved with the housing  26 , with sliding clearance, over the external surface of the riser  16 , and also an outer cylindrical peripheral wall  25 . These form, together with disc-shaped end walls, the pressure-tight and light-tight housing  26 . The latter is filled with inert gas under pressure (e.g. nitrogen), the level of the pressure being determined with regard to the external pressure prevailing around the housing. The inner peripheral wall  27  is transparent to X-radiation. 
     In order to position the testing head  24  by force in the angular direction, the peripheral wall  27  of the housing  26  is provided with an axial positioning groove  28  which co-operates with an axial positioning rib  30  which is provided along a generating line of the riser  16 . 
     In order not to impair the flexibility of the riser  16 , the positioning rib  30  may take the form of a toothed rib, the spacing of the teeth  32  being smaller than the axial dimension of the positioning groove  28 . 
     Inside the riser  16  a star-shaped base part  33  of an X-ray head  34  is capable of being displaced with sliding clearance. Via cables which are not shown, or via a rack provided on the inside of the riser  16  and a drive co-operating with said rack on the base part  33 , said base part is adjusted in a manner similar to that described further below for the axial adjustment of the X-ray source. 
     The base part  33  has a sheath-shaped hub part  36  and three radial arms  37  regularly distributed in the peripheral direction. In this manner, interspaces remain between the arms  37 , through which, if desired, crude oil can continue to be conveyed from the wellhead  20  to the loading pontoon  12  while the test is ongoing. 
     Inside the hub part  36  there is located a cylindrical fluid-tight bearing housing  38 , in which via bearings  39  a tube housing  40  is supported which receives an X-ray tube  41 . 
     For the purposes of clarification, it is assumed that the X-ray tube  41  is designed in such a way that it generates a cylindrical-sector-shaped radial fan  42  of X-rays. 
     The tube housing  40  is capable of being rotated about the riser axis by a motor  43 , so that the fan  42  revolves. 
     The upper end of the bearing housing  38  bears a gearwheel  44  running around a transverse axis, which co-operates with a rack  46  which is moulded on the inside of the hub part  36 . The drive of the gearwheel  44  is effected by an electric motor  48 . A position-indicator  50  is coupled to the axle of the electric motor  48 . 
     Components  44  to  50  together form a head-driving mechanism  51  for the X-ray head  34 . 
     Via a cable  52 , which runs through the hub part  36  to the loading pontoon  12 , the requisite operating voltage is supplied to the X-ray tube  41 , the electric motor  48  is energised, and the signal of the position-indicator  50  is passed to a controller  54  which is likewise accommodated on the loading pontoon  12  and which controls the operations necessary for the examination of the riser  16 . 
     The X-ray head  34  just described forms, together with a cassette  56 , the testing head  24 . 
     The cassette  56  will be described in more detail below. 
     Its housing  26  takes the form of an annular hollow body which runs, with sliding clearance, on the outside of the riser  16 , as already stated above. 
     In the annular interior space of the housing  26  a glass cylinder (or acrylic glass cylinder)  60  is provided which is fixed by press fit on the inner peripheral wall  27  of the housing  26  or is adhesion-bonded or mechanically wedged to the housing  26 . 
     The glass cylinder  60  bears a phosphorus layer  62  on its outside and forms with said layer a storage cylinder  61 . The phosphorus layer  62  includes a matrix  64  in which individual finely ground phosphorus particles  66  are homogeneously distributed. 
     The phosphorus particles  66  are obtained by grinding a solid material that contains colour centres or storage centres. In this connection it is a question of defects which may have metastable excited electron states. If an X-ray quantum impinges on such a storage centre, an electron of the storage centre can be energised into such a metastable excited state in which it then remains for a relatively long time (typically up to 20 minutes and more). 
     In the course of revolving about the axis of the riser  16 , the X-radiation fan  40  consequently generates an image of a cylindrical portion of the riser  16  in the phosphorus layer  62 . 
     Since all the parts of the testing device (with the exception of the phosphorus layer  62 ), in particular the base part  33  and the housing  26 , is produced from a material that is transparent to X-rays (e.g. plastic material or metal with a low atomic number), the annular wall portion of the riser  16 , at which the X-ray head  34  stands, is irradiated uniformly with X-ray light of the rotating radiation fan. 
     The X-ray light that has passed through the wall of the riser penetrates the glass cylinder  60  and impinges on the phosphorus layer  62  where it excites storage centres. This excitation is uniform if the wall of the riser is flawless. 
     But if the wall of the riser contains flaws, the density of the excited storage centres is locally variable, and the differently excited storage centres represent a latent radiograph of the riser  16 . 
     In order to be able to read out the latent radiograph, an axially aligned slat-shaped read-out head  68  is arranged in the annular space of the housing  26 . Said read-out head is borne by a carriage case  69 . The latter, in turn, runs on two axially spaced guide rails  70 ,  72  and bears on its upper side a ring of teeth  74 . 
     The teeth  74  mesh with a pinion  76  which is driven by a stepping motor  78  with angle-indicator  80 , which is borne by the housing  26 . The angular position of the read-out head  68  can also be ascertained by counting the control pulses that are transmitted to the stepping motor  78 . 
     Components  74  to  80  together form a head-driving mechanism  71 . 
     The angle-indicator  80  is set to zero if a reference mark  82 , which is fitted to the lower inner edge of the carriage case  69 , is detected by a light barrier  83  operating in reflection, which is provided at the lower end of the housing  26 . 
     In this manner the absolute position of the read-out head  68  in the angular direction is known. 
     The read-out head  68  exhibits a plurality of detector elements densely arranged along an axially parallel line, which each include an LED  84  and one or two photodiodes  86  closely adjacent to said LED, as evident from  FIG. 4 . 
     The LEDs  84  emit in the red, and by the light emitted by an LED the point of the phosphorus layer  62  that is situated directly ahead of it is irradiated. 
     The LEDs  84  and the photodiodes  86  are embedded in a material that in volume is black and opaque—that is to say, read-out light generated by the LEDs and blue fluorescent light absorbed equally. By this means, only the fluorescent light is received by each photodiode that was released by the LED assigned to it. 
     If excited storage centres are located in this punctiform region, the electrons located there are raised to a higher level which relaxes, accompanied by emission of blue fluorescent light. This fluorescent light is registered by the adjacent photodiode  86 . 
     In order to accelerate the read-out of the latent test image from the phosphorus layer  62 , the LEDs  84  and the photodiodes  86  can each be activated simultaneously if these are very closely adjacent to one another and the directional characteristics of light-emitting diode and photodiode are very narrow lobes. 
     If it is established that an LED  84  is also still reaching spaced points of the phosphorus layer  62  with appreciable intensity, so that the read-out of the latent image at one image point results in the emptying of storage centres at other image points, the LEDs  84  are combined into groups of interleaved diode sets in such a manner that the LEDs adjacent to one LED and the photodiodes thereof, in which there would be a crosstalk of read-out light of light-emitting diodes not pertaining to them, are in each instance not activated. 
     In practice, all the diodes can, for example, be combined that are distant from one another by three separations of the detection elements. There are then altogether three sets of LEDs  84  and photodiodes  86 , of which the diodes of one set can be read out jointly without crosstalk while the various sets of detection elements are read out in succession. 
     The combining of the interleaved pixel signals obtained in succession in this way is effected by an evaluating circuit  88  which receives the entire output signals of the photodiodes  86 . 
     By the stepping motor  78  being excited, the various axial image lines are read out in succession that together yield a test image of the annular wall portion of the riser  16 , with which the testing head  24  is co-operating. 
     Viewed in the direction of rotation, downstream of the read-out head  68  an axially aligned slat-shaped erasing unit  67  is fitted to the carriage case  69 . Said erasing unit may in practice have the same structure as the read-out head  68 , but normally it is operated differently: all the LEDs  84  are operated permanently. By means of the red light generated by said LEDs, storage centres that have not relaxed upon read-out, for instance, are emptied. This is effected securely, since by virtue of the erasing unit  67  the exposure-time is greater by a factor than that the read-out head  68 , which corresponds to the number of LEDs. 
     After the erasing of the residual image, the testing head  24  can be placed axially over a new region of the object surface which overlaps somewhat with the region just measured. 
     The traversing of the cassette  56  is effected by a cassette-driving mechanism  90  which includes a gearwheel  87 , meshing with the teeth  32  and supported in the housing  26 , and a stepping motor  89  operating on said gearwheel, with angle-indicator  91 . As also for the other angle-indicators, it is to be assumed that a counter is integrated into the angle-indicator, so that an unambiguous position signal for the detection head is obtained over the entire length of the riser  16 . 
     The traversing of the cassette  56  is concluded if a predetermined output signal of the angle-indicator  91  is received. 
     The traversing of the X-ray head  34  by the same distance is effected analogously. 
     Alternatively, on account of its identical structure to the read-out head  68 , the erasing unit  67  can also be used to read out the latent residual image of the storage disc. In this manner a further, weaker test image is obtained, in which only major flaws in the riser wall are reflected. 
     If the read-out head  68  fails—for example, as a result of failure of photodiodes—the erasing unit  69  can be used as a transducer unit, and the read-out head  68  can be employed as an erasing unit, in which case only their LEDs are activated. For this purpose, only the direction of rotation of the stepping motor  78  has to be reversed and the programming of the drive of the read-out head  68  and of the erasing unit  67  has to be transposed. 
     Again alternatively, use may be made of an erasing unit  67  in which only LEDs are provided and the photodiodes have been omitted or replaced by further LEDs. 
     In modification, use may also be made of a read-out head  68  such as is shown in  FIG. 5 . 
     With this read-out head, the individually addressable LEDs  84  are embedded in closely spaced manner in a transparent slat which at its ends is provided with parabolic caps  92 . Except at the points situated in front of the LEDs  84 , the slat is provided continuously with a reflective layer  94 , for example by vapour deposition of Al, Ag, Au. The material of the slat may have been dyed in its volume in such a way that it allows fluorescent light to pass through in substantially lossless manner but on the other hand absorbs the light generated by the LEDs  84 . 
     With this read-out head the LEDs  84  are activated individually and the detection of the fluorescent light is effected by only two photodiodes  86  which are located at the focal points of the two slat caps. The position of the image pixel just read out results from the angular position of the read-out head  68 , from the number of the LED  84  just activated, and from the output signal of the head-position indicator  80 . 
     This variant enables a high resolution with high sensitivity. 
     If an annular wall portion of the riser  16  has been examined as described above, the cassette  56  and the X-ray head  34  are moved, by corresponding excitation of their driving motors  78 ,  89 , by a distance in the axial direction that is somewhat smaller than the axial dimension of the fan  42 , of the phosphorus layer  62  and of the read-out head  68 . Then the recording is effected of a further annular region of the riser  16 , as described above. 
     The various partial test images that are obtained in this way are passed to the controller  54 , together with the position signals for the X-ray head  34  and the cassette  56 , via the cable  52  and a cable  96  which leads from the cassette  56  to the controller  54  of the loading pontoon  12 . Said controller can then combine the axially somewhat overlapping partial test images of the riser  16  to form an overall test image. 
     This test image can then be represented on a monitor  98 , in order to enable a visual inspection of the riser  16 . But alternatively the overall test image may also be evaluated for flaws with image-evaluation software, the flaws can be categorised, and the position and type of the flaws can be output in the form of a list. 
     In the exemplary embodiment described above, only a relative movement of the testing head  24  in relation to the riser  16  is necessary, namely an axial relative movement. Even without image-splitting, a smooth image of the object in the peripheral direction is obtained. But the testing head may be removed only via one end of a riser (as a rule, the upper end), which for this purpose has to be released. 
     The further exemplary embodiments show testing heads that without releasing a riser can be fitted to the riser and can be dismantled from the latter. 
       FIG. 6  shows a modified exemplary embodiment in which the cassette  56  extends only over an angular range of 140°. Parts of the cassette  56  that correspond functionally to components elucidated above with reference to  FIGS. 1 to 5  are again provided with the same reference symbols, even if they differ in details. These components also do not need to be described again in detailed manner below. 
     In the exemplary embodiment according to  FIG. 6  a toothed belt  100  which runs over two deflecting rollers  102 ,  104  serves for moving the read-out head  68 . Of these deflecting rollers, deflecting roller  104  is driven by the stepping motor  78 . 
     The guide rails  70 ,  72  are formed as box sections which have a longitudinal slot on the side facing towards the toothed belt. Through this slot, T-shaped guide lugs  100  of the toothed belt  101  engage in the guide rails  70 ,  72 , so that the toothed belt runs along the circular-arc-shaped guide rails  70 ,  72 , positively guided in both strands. 
     The glass cylinder  60  bearing a phosphorus layer  62  is replaced in the embodiment according to  FIG. 6  by an image-storage disc which exhibits a flexible transparent substrate  60  which and bears a phosphorus layer  62 . The phosphorus layer  62  is again arranged on the outside of the substrate  60 . 
     The testing head  24  according to  FIG. 6  generates in each instance a partial test image which registers somewhat more than 120° of the peripheral extent, for example 126°. 
     If use is made of the testing head  24  shown in  FIG. 6 , then by placing said testing head upstream of the riser  16  three times in angular positions that are offset 120° in relation to one another a full image of an annular portion of the riser  16  can again be generated. 
     The shifting of the testing head  24  in the peripheral direction may be effected in such a way that the testing head  24  is rotatably supported on a frame such as will be elucidated later still more precisely with reference to  FIG. 11 . 
     The fixing of the cassette  56  on the riser  16  in the position that has been set in the given case is effected, for example, by jaws  108  which co-operate with the outer surface of the riser  16  and are actuated by working cylinders  114  via elbow levers  110  which are rotatable about bolts  112  borne by a frame  106 . 
     A testing head such as is shown in  FIG. 6  can be fitted particularly easily to a riser  16  or dismantled from it without one end (as a rule, the upper end) of the riser having to be set free. The jaws  108  only have to be swivelled away, and then the testing head  24  can be removed from the riser  16  or attached to it in the transverse direction. 
     For this attaching or dismantling, the housing  26  of the cassette  56  does not need to be opened, which would involve the risk of the penetration of contaminants. 
     The exemplary embodiment according to  FIG. 7  differs from the exemplary embodiment according to  FIG. 6  by virtue of the fact that the X-ray head  34  is arranged outside the riser  16 . It is seated on a bent bracket  118  which is connected to the frame  106  in articulated manner by a bolt  117 , and in the working position shown in the drawing is capable of being locked mechanically or hydraulically. 
     The irradiation of the riser  16  is consequently effected from the outside, specifically from a point that is located opposite the cassette  56 . In this connection the conditions are chosen in such a way that the spacing between the X-ray tube  41  and the adjacent generating line of the wall of the riser  16  is considerably smaller than the spacing between the X-ray tube  41  and the opposite generating line of the wall of the riser  16 . 
     Two test images are now made in two positions of the testing head  58  in relation to the riser  16  that are not situated far apart in terms of angular measurement (for example)10°. By reason of the differing projection conditions, the images of those flaws which stem from the wall portion of the riser  16  facing away from the X-ray tube will be changed less than the shadows of those wall flaws which are located in the portion of the wall of the riser  16  adjacent to the X-ray tube. If the two partial images are compared, the flaws in the front wall portion can be distinguished from the flaws in the rear wall portion. 
     For the purpose of recording the two test images, also only the X-ray head  34  may be relocated. For this purpose, one arm of the bracket  118  may be divided and may be capable of being changed in length by means of a hydraulic cylinder  119 . 
     This distinction of the flaws in the front and rear walls can be effected automatically by the two partial images being transmitted via a change-over switch  120  into two different memories or memory areas  122 ,  124  and by the contents of these memory areas being separated from one another with appropriate image-evaluation software in an editing circuit  126 . 
     The exemplary embodiment according to  FIG. 8  relates to a testing device for a flat object  16 . In principle, the structure is similar to that in the case of the exemplary embodiment according to  FIG. 7 , only the guide rails  70  and  72  are straight and the stepping motor  89  operates via a worm wheel  87  on a rack  32  in order to bring about the feed of the transducer unit  68  in the perpendicular direction in relation to the plane of the drawing. 
     In the exemplary embodiment according to  FIG. 6  there was provision that the same cassette  56  is attached to the riser  16  at three places that are offset in relation to one another by 120°. 
     Alternatively, three cassettes  56  as were shown in  FIG. 6 , can also be releasably connected to a multiple cassette head  56 *, offset by 120° in differing axial positions, as shown in  FIGS. 9 and 10 . It is then sufficient to move the multiple cassette head  56 * in a permanent angular orientation along the riser  16 , it being possible for this to be effected again with a positioning groove  28  on the cassette head  56 * and with a positioning rib  30  on the riser  16 . 
     Shown in  FIGS. 9 and 10  are the three cassettes  56 A,  56 B and  56 C; their fronts are denoted respectively by V 56 A etc.; their rears are denoted by S 56 A etc. Releasable connections between the individual cassettes are denoted by  128 AB and  128 BC, respectively. 
     Dashed radial rays R are exactly 120° distant from one another. It can be discerned that the fronts and rears of the cassettes  56  lay in each instance ahead of and behind, respectively, a radial ray by an angle w, so that consecutive cassettes overlap, in each instance, by an angle  2   w . In the exemplary embodiment shown, w amounts to 10°. 
     For each of the cassettes  56 A,  56 B and  56 C an X-ray source  34  located opposite said cassette is provided, which in the drawing has been omitted for the sake of clarity. 
     The controller  54  then combines the various partial images that the cassettes  56 A,  56 B and  56 C supply, in such a way that altogether an overall test image of the riser is obtained. For this purpose, in each instance a current partial image of the first testing head, the partial image of the second testing head in the preceding cycle and the partial image of the third testing head in the antepenultimate test cycle are assembled to form a continuous smooth annular image. 
     In all the exemplary embodiments, the housing  26  of the cassette  56  and the bearing housing  38  of the X-ray head  34  have internal pressure applied to them, preferably with inert gas under high pressure. In this connection the setting of the pressure can be effected as a function of the depth of the water, which in the case of substantially known course of the riser  16  can be derived from the position signal for the testing head  24 . 
       FIG. 11  shows a single cassette  56  with 140° extent, which is rotatably arranged on a frame  106 . The latter comprises two frame parts  106 A and  106 B connected by a joint  130 , which are releasably held by a screw coupling  132  in a working position in which the frame  106  surrounds a riser  16  with clearance and is releasably fixed to said riser by jaws (shown in  FIG. 11 ), similarly as described above with reference to  FIG. 6  or  7 . 
     By virtue of the fact that the cassette  56  is set three times in angular positions offset 120° in relation to one another with axial position unchanged, a full image of an annular portion of the riser  16  is again obtained. 
     The angular shifting of the cassette  56  in the peripheral direction is effected in such a way that it is arranged on a split ring  134  which is provided with an external toothed rim  136  and which co-operates with a driven pinion  138  of a driving motor  140  which is equipped with an angle-indicator  142 . By the output signal of the angle-indicator  142  being monitored, the detection head  24  can be automatically moved successively into the three working positions offset by 120°. 
     On the front of the frame  106  there are located four bearing rollers  141  at equal distance from the riser axis and remote from one another at 90° angular distance bearing rollers  138 , which support the ring  134  on the periphery. 
     If it is desired to remove the testing head  24  shown in  FIG. 11  from the riser  16 , the screw coupling  132  is loosened; similarly, screw couplings  144  holding the ring halves together. After frame part  106 A has been swivelled away, the testing head  24  can then be removed in the transverse direction. 
     According to  FIG. 12 , combined marks  79 ,  82  and combined sensors  81 ,  83  can also be used for the purpose of position measurement. 
     The combined marks include—in addition to an optical mark  146  exhibiting small dimensions, which, for example, may be a reflecting mark—a transponder mark  148  from which information that reflects the absolute position of the mark  146  can be retrieved in wireless manner. 
     The combined sensors  81 ,  83  include an optical sensor  150 —for example, a light barrier operating in reflection—and a transponder sensor  152  which reads out in wireless manner the information stored in the transponder mark  148 . Components  148  and  152  may be, for example, Temic(R) components. 
     The output signals of a combined sensor consequently permit accurately and absolutely determined coarse positions of the testing head  24  on the riser  16  or of the transducer element  68  on the guide rails  70 ,  72  to be detected. Movements beyond these predetermined locations are measured by the fine-position indicators  80  and  91 . The absolute overall position results as the sum of coarse position and fine position. An appropriate circuit containing an adder  154  is shown in  FIG. 13 . 
     In the case of the testing head  24  shown in  FIG. 14 , which are very similar to that according to  FIG. 6 , measures have been taken in order to avoid an irradiation of the read-out head  68  with X-ray light. Hence the read-out head  68  may also include sensitive electronic components. 
     One possibility shown in the left half of  FIG. 14  consists in leading out the guide rails  70 ,  72  in a direction beyond the angular range denoted by U that is swept by the X-ray light. 
     One possibility shown in the right half of  FIG. 14  consists in providing in a protective position for the read-out head  68 —which here has been chosen, for example, at the right end of the stroke—a movable shield  160  which can optionally be placed in front of the transducer unit  68  or can be moved into a parked position releasing said transducer unit. 
     Where cassette and radiation head are capable of being moved independently of one another, between the two a further sensor device may have been provided, in order to align both heads with one another axially and, where appropriate, also in the peripheral direction. Such a sensor device may include, according to  FIG. 2 , for example a weakly radioactive sample  156  on the tube housing  40 , which emits gamma rays via a pinhole diaphragm, and a small gamma-ray detector  158  which is arranged in the radially inner peripheral wall of the housing  26 . The precise juxtaposition of the two heads is obtained when the output signal of the gamma-ray detector  158  has attained a maximum. The juxtaposition obtained in such a way will, as a rule, be more precise than the setting of identical absolute positions for X-ray head and cassette. 
       FIG. 15  shows a cassette  56  that can be used for medical purposes. Components of the cassette that correspond in terms of function to cassette parts already described are again provided with the same reference symbols, even if they differ in particulars. 
     Onto a shoulder  162  of a peripheral frame  164  which is rectangular in top view a window  27  that is transparent to X-radiation but opaque to ambient light is mounted in flush and light-tight manner. The rear of the cassette constitutes a wall  25  that does let light through. 
     The read-out head  68  is driven by a threaded spindle  166  which extends in parallel manner over the upper longitudinal edge of a storage foil  61 . Said spindle is accordingly unable to cast a shadow onto the storage foil. 
     An end portion  168  of the threaded spindle  166  provided with a squared end is guided outwards through the frame  164 . 
     The read-out head  68  is connected by a hinged cable  170  to a plug-connector part  172  borne by the frame  164 . 
     By attaching its instrument with a mechanical coupling part fitting the end portion  168  and with an electrical coupling part fitting the plug-connector part  172 , the prerequisites can consequently be completed that are necessary in order to read out the latent image of the storage foil  61 . For this purpose this instrument contains a driving motor and electronic image-recording and image-processing hardware. 
     The cassette  56  of  FIG. 16  corresponds largely to that of  FIG. 15 , only the end of the threaded spindle is directly connected to a driving motor  174 . Also arranged inside the cassette  56  is an image-recording and image-evaluating unit  176  which on the output side is connected to the plug-connector part  172 . With this cassette, recorded images can be transmitted as a whole to a PC or memory stick linked to the plug-connector part  172 . 
     In the exemplary embodiments described above, the illumination of the object was effected by X-radiation. Instead of this, use may also be made of radioactive emitters which emit electromagnetic radiation or particles that can generate, directly or indirectly, a latent radiograph in a phosphorus layer. 
     Sound, in particular ultrasound, may also be radiation in the sense of the claims and the foregoing description. 
     Instead of using a phosphorus layer, the radiograph can also be registered by a scintillation layer in combination with a photoelectric detector (e.g. a CCD). 
     For the purpose of better representation, various parts of the device have been represented as integral parts. It will be understood that a person skilled in the art can assemble these, where appropriate, from several separately produced parts. 
     Parts made of plastic material have been reproduced in alternating cross-hatching (single stroke and double strokes alternate); for light-transparent parts, in dashed cross-hatching. 
     All the parts of the testing head and of the receptacle for the X-ray source (the latter with the exception of shields that are necessary for reasons of radiation shielding) are produced from material that is highly transparent to X-ray light. 
     The movement of the testing head along the riser and, where appropriate, in the peripheral direction of the riser may also be effected by cables or even by propeller propulsion or jet propulsion. 
     The base part  33  may also have been moulded onto the inside of the riser  16 . The movement of the X-ray head  34  is then effected completely via the rack  46 . 
     Various relative movements were mentioned implicitly above, for example the movement the read-out head  68  with respect to the phosphorus layer  62 . It will be understood that here, in each instance, the concepts ‘moving part’ and ‘stationary part’ may be interchanged. 
     Furthermore, various position-indicators and drives were mentioned above which exhibit a moving part and a stationary part, for example marks and sensors co-operating with said sensors. It will be understood that these indicator parts and drive parts may be interchanged. 
     By way of radiation-source, an X-ray source was assumed above that makes a fan-shaped beam available which then had to be moved over the object. Alternatively, particularly in the case of the transirradiation of pipes and other elongated objects, use may be made of a radiation-source with cylindrical characteristic (rotating emitter). Sources of X-rays of such a type have, for example, an anode with conically rotationally symmetrical tip. Radioactive emitters are already naturally omnidirectional. With radiation-sources of such a type, more often than not a rotation of the source can be dispensed with, unless by the rotation it is desired to compensate for irregularities in the radiation characteristic. 
     It is to be understood that additional embodiments of the present invention described herein may be contemplated by one of ordinary skill in the art and that the scope of the present invention is not limited to the embodiments disclosed. While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.