Patent Publication Number: US-2006001760-A1

Title: Apparatus and method for object shape detection

Description:
This invention relates to object shape detection using a backlight unit to create a silhouette of an object, with the shape of the object being determined from that silhouette. In particular, the invention relates object shape detection that is suitable for use with reflective objects.  
      It is known in the art to determine the shape of an object by using a backlight to generate a silhouette of the object and to determine the shape of the object using a threshold technique, as described below with reference to  FIG. 1 .  
       FIG. 1  shows light sources  2 ,  4 ,  6  and  8 , a diffusion panel  10 , an object being measured  12  and a camera  14  comprising a taking lens  16  and an imaging device  18 . The light sources  2 ,  4 ,  6  and  8  emit light in all directions and the diffusion panel  10  acts to scatter the light received from those light sources to provide a source of substantially isotropic lighting. The taking lens  16  is a converging (or positive) lens that takes light received from the light sources and focuses that light into the imaging device  18 .  
      Two exemplary beams of light are shown in  FIG. 1 . A first beam  20   a  just misses the top right-hand corner of the object  12  as shown in  FIG. 1  and passes through the centre point of the lens  16  before reaching the camera at point  22   a.  A second beam  20   b  just misses the bottom right-hand corner of the object  12  as shown in  FIG. 1  and passes through the centre point of the lens  16  before reaching the camera at point  22   b.  No light is received at the imaging device  18  between the points  22   a  and  22   b.  This is shown in  FIG. 2 , which shows the light profile for the system of  FIG. 1  in ideal circumstances, ignoring the possible presence of a penumbra and any effects of diffraction.  
       FIG. 2  shows the light received at the imaging device over a distance d. The point  22   a  is labelled a in  FIG. 2 : the point  22   b  is labelled b in  FIG. 2 . As shown in  FIG. 2 , the area between points  22   a  and  22   b  receives no light from the light sources  2 ,  4 ,  6  and  8  and so a silhouette is formed. The size of the object can be determined from that silhouette provided that the position of that object relative to the light source and the imaging device is known.  
      A problem with the method described with respect to  FIGS. 1 and 2  is that it does not work well when the object  12  is reflective.  
       FIG. 3  shows the system of  FIG. 1  when used to determine the size and shape of a reflective object. The system of  FIG. 3  consists of light sources  2 ′,  4 ′,  6 ′ and  8 ′, a diffusion panel  10 ′, and a camera  14 ′ comprising a taking lens  16 ′ and an imaging device  18 ′. An object being measured is shown positioned between the diffusion panel and the imaging device. In the example of  FIG. 3 , the object being measured is a metal cup, shown in cross-section in  FIG. 3 , the cup having a cylindrical main body  12 ′ and a handle  13 ′ with both the main body  12 ′ and the handle  13 ′ having reflective surfaces.  
      Two beams of light  20   a′  and  20   b′  are shown in  FIG. 3  and are similar to the beams of light  20   a  and  20   b  of  FIG. 1 , with the beam  20   a′  just missing the top of the main body  12 ′ of the object being measured and passing thorough the centre point of the lens  16 ′ before reaching the imaging device at point  22   a′  and the beam  20   b′  just missing the bottom of the main body  12 ′ before passing through the centre point of the lens  16 ′ before reaching the imaging device at point  22   b′.  In addition, a third beam of light  21 ′ is shown that strikes the handle  13 ′ and is reflected towards the imaging device  18 ′. As shown in  FIG. 3 , the beam  21 ′ strikes the imaging device outside the region between the points  22   a′  and  22   b′.  Reflections from both the main body  12 ′ and the handle  13 ′ of the cup are reflected in a number of different ways, with the result that the clear definitions of the edges of the silhouette shown in  FIG. 2  become blurred.  
       FIG. 4  shows an exemplary light profile for the system of  FIG. 3 .  FIG. 4  shows the light received at the imaging device over a distance d. The point  22   a′  is labelled a in  FIG. 4 : the point  22   b′  is labelled b in  FIG. 4 . The points a and b on  FIG. 4  mark the positions of the edge of the silhouette of the cylindrical main body  12 ′ of the cup, in an ideal system.  FIG. 4  also shows positions c and d that mark the positions of the edge of a silhouette of the handle  13 ′ in an ideal system.  
      The problem of reflections blurring the edges of silhouettes is clearly shown in the examples of  FIGS. 3 and 4 , but also exists in the example of  FIGS. 1 and 2  when the object  12  is reflective. In particular, incoming light at low angles can glance of the surface of the object and finds its way into the area that should form part of the silhouette.  
      One known technique for determining the position of the edge of an object is a system such as that of  FIGS. 1 and 3  is thresholding. The thresholding technique sets a light level, perhaps 50% of the maximum light level, that is taken to mark the edge of the silhouette (and hence the edge of the object being measured). Clearly, the thresholding technique would accurately define the edges of the object in the example of  FIGS. 1 and 2  but would be much less accurate in the example of  FIGS. 3 and 4 .  
      The problem of generating accurate silhouettes from reflective images is known in the art and one known solution is to use collimated lighting, as described below with reference to  FIGS. 5 and 6 .  
      The system of  FIG. 5  comprises a point light source  24  positioned at the focal point of a converging lens  26 . As is well known in the art, positioning a point light source at the focal point of a lens results in collimated lighting, as shown schematically in  FIG. 5 . An object being measured  28  is placed in the collimated light, between the lens  26  and the camera  30 . The camera  30  comprises a taking lens  32  and an imaging device  34  and is used to generate a silhouette image in a similar manner to the imaging devices described above with reference to  FIGS. 1 and 3 .  
       FIG. 6  shows a close-up of an object being measured using the system of  FIG. 5 . The object is a cup similar to that shown in  FIG. 3  and comprises a main body  28   b′  and a handle  29   b′.    FIG. 6  also shows a series of exemplary parallel light beams. As can be seen in  FIG. 6 , it is possible that a very small amount of light could be reflected from the main body  28   b′  onto the handle  29 ′ and from there into the camera  30 , but the amount of reflected light that will reach the imaging device  34  in the areas that should ideally be dark will be very small. Accordingly, the edges of the silhouette observed by the imaging device  34  will be much clearer than in the example of  FIGS. 3 and 4 . Furthermore, the well-known problem of penumbra does not arise when collimated lighting is used.  
      The effects of diffraction of light have been ignored in the analysis given above. This is because the effects of diffraction in the systems in which the present invention is intended to be used are much smaller than the effects of reflection and so the effects of diffraction can be overlooked.  
      There are a number of problems with the system of  FIG. 5 .  
      In order for the system of  FIG. 5  to take images of quite large objects, the lens  26  must have a large diameter. Further, the system of  FIG. 5  requires the light source to be placed at the focal length of the lens. The ratio of the diameter of a lens to the focal length of that lens is termed the F-stop or F-number of the lens. An F-stop value of the order of  1  generally leads to significant colour aberrations; values of  2 ,  3  or  4  are preferred. It follows that the system of  FIG. 5  will require a large, heavy lens having a long focal length (perhaps two or three times the diameter of the lens), resulting in a large and heavy optical system. In one exemplary use of the system of the present invention, the light unit (including the light source) is moved so that the silhouette of the object being measured can be taken from different positions or different orientations. Clearly, the use of a light source having a long focal length and incorporating large and heavy lens is a problem, especially if that light source is to be moved.  
      The use of collimated light is effective to reduce the problems caused by reflected light, but in order to capture enough of the light that passes the object to make an effective silhouette, the optical system of the image capturing device must be of a similar size to that of the light source (since the light from the light source is parallel). Thus, not only is the light source large and heavy, the image capturing device is also large and heavy.  
      The present invention seeks to overcome at least some of the problems identified above.  
      The present invention provides an apparatus for generating a silhouette of an object, the apparatus comprising a light projecting device, an image capturing device and an arrangement for mounting the said object between the light projecting device and the image capturing device, wherein said light projecting device includes a two-dimensional arrangement of light projecting elements, each light projecting element having a light source associated therewith, and wherein the light projecting elements are arranged to direct light towards said image capturing device, whereby a silhouette of said object is generated at said image capturing device.  
      The present invention also provides a light projecting device arranged, in use, to generate a silhouette of an object, the device including a two-dimensional arrangement of light projecting elements, each light projecting element having a light source associated therewith, wherein the light projecting elements are arranged, in use, to direct collimated or converging light towards said object.  
      The light projecting elements may be arranged to direct collimated light towards said image capturing device. Alternatively, the light projecting elements may be arranged to direct converging light towards said image capturing device.  
      Each light projecting element may comprise a converging lens arranged to direct light from the associated light source towards said image capturing device. Each light source may be positioned at the focal point of the lens with which it is associated. Each light source may be positioned relative to the lens with which it is associated such that converging light is directed towards said image capturing device. Each of said light sources may be a light emitting diode.  
      In one form of the invention, each of said converging lenses is a fresnel lens. This is advantageous due to the small size and low weight of fresnel lenses.  
      The converging lenses may be arranged in a honeycomb pattern, for example a honeycomb arrangement of hexagonal lenses. This is advantageous as it leads to fewer problems with colour aberrations compared to a rectangular arrangement of lenses of similar size.  
      The light projecting device may further comprise an additional converging lens positioned between said light projecting elements and said object. That additional converging lens may be a fresnel lens.  
      In one form of the invention, each of said light sources includes a mechanical adjustment mechanism for altering the position of that light source relative to the lens with which it is associated. Each light source may be moveable along an x-axis and a y-axis in order to align the light source with the centre of the lens with which it is associated. Alternatively, or in addition, each light source may be movable along a z-axis in order to position the light source either closer to, or further away from, the lens with which it is associated.  
      The image capturing device preferably includes a taking lens to form a camera.  
      In one form of the invention, one or more mechanical supports provide mechanical support to one or more of said the lenses of said light projecting elements. For example, a mechanical support may be provided between the lenses of said light projecting elements and said additional converging lens. Alternatively, or in addition, a mechanical support may be provided on the side of said additional converging lens facing said image capturing device.  
      In one form of the invention, the light projecting device is movable relative to said arrangement for mounting the said object in order to generate silhouettes of said object from different view points.  
      The present invention also provides a method of generating a silhouette of an object, the method comprising the steps of: 
          placing the said object between a light projecting device and an image capturing device;     directing light from a two-dimensional arrangement of light sources within the light projecting device towards the image capturing device; and     generating a silhouette of the object at the image capturing device.        

      The method may also include the step of converting the light from the light sources into either collimated or converging beams of light.  
    
    
      By way of example only, embodiments of the present invention will now be described with reference to the accompanying schematic drawings, of which:  
       FIG. 1  shows a known system for determining the shape of an object by measuring the silhouette generated by that object;  
       FIG. 2  shows an ideal light profile for the system of  FIG. 1 ;  
       FIG. 3  shows the use of the system of  FIG. 1  to measure the shape of a reflective object;  
       FIG. 4  shows an exemplary light profile for the system of  FIG. 3 ;  
       FIG. 5  shows an alternative system for determining the shape of an object using principles known in the art;  
       FIG. 6  shows a close-up of part of the system of  FIG. 5  when used to measure the shape of a reflective object;  
       FIG. 7  shows an imaging system in accordance with an embodiment of the present invention;  
       FIG. 8  shows a first arrangement of the array of lenses of the apparatus of  FIG. 7 ;  
       FIG. 9  shows the use of the imaging device of  FIG. 5  in the absence of an object being measured;  
       FIG. 10   a  shows schematically an image generated using the system of  FIG. 9 ;  
       FIG. 10   b  shows an exemplary image produced using the system of  FIG. 9 ;  
       FIG. 11  shows the use of the imaging device of  FIG. 7  in the absence of an object being measured;  
       FIG. 12  shows an imaging system in accordance with an embodiment of the present invention;  
       FIG. 13  shows an alternative arrangement of an array of lenses in accordance with an aspect of the present invention;  
       FIG. 14  demonstrates the positioning of a light source in accordance with an aspect of the present invention;  
       FIG. 15  is an exploded view of a light unit in accordance with an embodiment of the present invention;  
       FIG. 16  is a cross-sectional, schematic view of the light unit of  FIG. 15 ;  
       FIG. 17  is an isometric view of a photographic apparatus of in which a measuring system in accordance with the present invention can be used, viewed from a first direction;  
       FIG. 18  is an isometric view of the photographic apparatus of  FIG. 17 , viewed from a second direction;  
       FIG. 19  is a side view of the photographic apparatus of  FIG. 17  with the camera arm in a horizontal position;  
       FIG. 20  is a side view of the photographic apparatus of  FIG. 17  with the camera arm in a first position above the horizontal;  
       FIG. 21  is a side view of the photographic apparatus of  FIG. 17  with the camera arm in a second position above the horizontal;  
       FIG. 22  is a side view of the photographic apparatus of  FIG. 17  with the camera arm in a position below the horizontal;  
       FIG. 23  shows a calibration mat for use with the photographic apparatus of  FIG. 17 ; and  
       FIG. 24  shows a block diagram of a three-dimensional modelling system in which the measuring system of the present invention may be used. 
    
    
      A first embodiment of the present invention is described below with reference to  FIGS. 7 and 8 .  FIG. 7  shows a system having a 2-dimensional array of lenses  36 , a second lens  54 , an object being measured  56 , and a camera  58  comprising a taking lens  60  and an imaging device  62 . The 2-dimensional array of lenses  36  shown in  FIG. 7  is a cross-section in which lenses  38 ,  40 ,  42  and  44  are shown. As shown in  FIG. 8 , the lenses shown in the array  36  of  FIG. 7  are the lenses along the right-hand side of that array. Each convex lens, for example, lenses  38 ,  40 ,  42 , or  44 , of the array is arranged in a rectangular matrix, as shown in the  FIG. 8 .  
      Each of the lenses in the array  36  has a light source associated therewith, with that light source being positioned at the focal point of the lens (light sources  46 ,  48 ,  50  and  52 , associated with lenses  38 ,  40 ,  42  and  44  respectively, are shown in  FIG. 7 ). As discussed above, by placing the light source (such as a light emitting diode) at the focal point of the lens, that lens will convert the radial light from the light source into collimated light, as shown schematically in  FIG. 7 .  
      The lens  54  takes the collimated light from the array  36  and converges that light towards the camera  58 . The object being measured  56  blocks part of the converging light so that the imaging device  62  forms a silhouette, as described with reference to the prior art above.  
      The system described with reference to  FIGS. 7 and 8  has a number of advantageous features. As described above, the focal length of the lens might typically be at least twice the diameter of the lens, in order to keep colour aberrations down to an acceptable level. By using an array of small lenses (rather than one large lens), the focal length of each lens can be made much smaller without leading to unacceptable levels of colour aberration. Accordingly, using an array of lenses leads to a system in which the light sources can be placed closer to the lenses, thereby reducing the overall size of the light unit.  
      A further advantage of using a plurality of light sources, rather than a single light source, is that the amount of light being used can be increased, thereby improving the quality of the images generated by the imaging device  62 .  
      A third advantage with the system of  FIGS. 7 and 8  results from the provision of the second converging lens  54  and is explained below with reference to FIGS.  9  to  11 .  
       FIG. 9  shows the taking lens  32  and imaging device  34  of the system of  FIG. 5 , in which the object  28  has been removed so that no light is blocked. If the incoming light is parallel (i.e. collimated), the taking lens  32  focuses that light towards the focal point of the taking lens. It follows that an image taken by the imaging device  34  positioned close to the focal point will have a very bright central circle  33  surrounded by a dark area  35 , as shown schematically in  FIG. 10   a,  since all the incoming light is focused towards the central area of the imaging device.  FIG. 10   b  shows an image taken with an imaging device used in an arrangement similar to that of  FIG. 9 . The image of  FIG. 10   b  shows a bright central area  33   a  surrounded by a dark area  35   a  similar to the schematic representation shown in  FIG. 10   a.    
       FIG. 11  shows the taking lens  60  and imaging device  62  of the system of  FIG. 7 , in which the object  56  has been removed so that no light is blocked. The incoming light is converging. The taking lens  60  focuses the light, but now the light seen by the imaging device is spread over a wider area so that images taken by the imaging device have a more even and uniform light distribution, so that the problem of having bright central area and surrounding dark area, as shown in  FIGS. 10   a  and  10   b,  does not occur.  
      In one form of the invention, the lens  54  has a focal length similar to the distance between the lens  54  and the camera  58 .  
      A further problem associated with prior art systems, as discussed above, is the size and weight of the lenses required. The system of  FIG. 7  can be further improved by replacing the lens  54  and/or the array of lenses  36  with fresnel lenses.  
      As is well-known in the art, a fresnel lens has a surface of stepped concentric circles and is much flatter than a conventional lens having the same focal length. Accordingly, replacing one or more of the lenses of  FIG. 7  with one or more fresnel lenses reduces the size and weight of the lenses used.  FIG. 12  is a schematic view of a system in accordance with an embodiment of the present invention in which the lenses of the system of  FIG. 7  are replaced with fresnel lenses.  
      The system of  FIG. 12  comprises an array of fresnel lenses, indicated generally by the reference numeral  64 , a large fresnel lens  66 , and a camera  68  comprising a taking lens  70  and an imaging device  72 . The array of fresnel lenses  66  consists of a  2 D array of lenses, each having a light source associated therewith. An exemplary light source  74  is shown in  FIG. 12 . Some of the fresnel lenses in the array  66  are omitted for clarity and all light sources with the exception of light source  74  are omitted, again for clarity.  
      Each light source in the system of  FIG. 12  is positioned at the focal point of the fresnel lens with which it is associated. Accordingly, the light from the array  64  is collimated. It follows that the system of  FIG. 12  works in the same manner as the system of  FIG. 7 . The principal difference is the size and weight of the lenses used.  
      The lenses of the array  64  are arranged in a square pattern in a similar manner to that shown in  FIG. 8 . This arrangement can cause problems with colour aberrations in the resulting image.  
      The focal length of a lens is dependent on the wavelength of the light. Chromatic aberrations are caused by the different focal lengths of different colours of light and are made worse as the distance between the centre and the edges of a lens increases. Accordingly, using square lenses as shown in  FIG. 12  results in chromatic aberration caused by light arriving near the corners of those lenses. Clearly, the problem can be reduced by replacing the square lenses with circular lenses, but this does not fit well into an array. A better solution is to use an array arranged as a honeycomb of lenses, such as the array of hexagonal lenses shown in  FIG. 13 .  
       FIG. 14  shows one of the hexagonal fresnel lenses  73 ′ of the hexagonal array shown in  FIG. 13 . Also shown in  FIG. 14  is a light source  74 ′ for that lens. A light emitting diode may be used as the light source  74 ′. In practice, due in part to mechanical inaccuracies in the manufacture of light emitting diodes, it is difficult to ensure that each light source is correctly aligned in the lens array. Accordingly, in one embodiment of the invention, each light source is movable in three axial directions, as represented in  FIG. 14  by the x- y- and z-axes. Movements along the x- and y-axes align the light source with the centre of the lens. Movements along the z-axis ensure that the light source is positioned at the focal point of the lens.  
      Of course, the honeycomb arrangement of lenses shown in  FIG. 13  can also be applied to the array of lenses  36  of  FIG. 7 .  
       FIG. 15  shows an exploded view of a light unit, indicated generally by the reference numeral  76 , in accordance with an embodiment of the present invention. The light unit  76  comprises a main fresnel lens  78 , a hexagonal array of fresnel lenses indicated generally by the reference numeral  80 , and an array of light sources  82 , each light source being associated with one of the lenses of the array.  
       FIG. 16  is a cross-sectional view of the light unit  76 , including the main fresnel lens  78 , hexagonal array  80  of fresnel lens and the array of light sources  82 . A support  84 , for example, made of thin plain glass is provided on the exterior of the fresnel lens  78  to provide a mechanical support for that lens. A second support  86 , for example made of thin plain glass, is provided between the array of lenses  80  and the lens  78  to provide further mechanical support.  
      The supports  84  and  86  prevent the thin fresnel lenses from distorting caused by bending of the lenses. In one form of the invention, the supports  84  and  86  have a width of 2 mm. The supports  84  and  86  may be made of glass, which is advantageous because the optical properties of glass can be controlled; however other materials, such as acrylics, could be used.  
      One exemplary use of a light unit in accordance with the present invention, such as the light unit  76  described above, is in a system for generating a three-dimensional model of an object from a plurality of two-dimensional images taken from a plurality of positions. It is known that three-dimensional models of devices can be generated by determining the silhouettes of a number of photographed images of the device and using those silhouettes to generate a three-dimensional model of the object, the model consisting of a number of polygons. Photographed images are used to generate textures for application to each polygon of the three-dimensional images to generate the final model of the object.  
       FIGS. 17 and 18  shows two views of a photographic apparatus, indicated generally by the reference numeral  112 , for generating three-dimensional modules that makes use of a light unit in accordance with the present invention.  
      Photographic apparatus  112  includes a glass turntable  114  on which an object to be photographed can be placed. The glass turntable is rotatable about a central vertical axis  116  to enable an object placed on the turntable  114  to be photographed from many angles. A camera unit  118  is provided to take photographic images of an object on the turntable  114 . The camera unit  118  comprises a camera  120 , a zoom lens  122  and a mirror  123  with a tilting mechanical stage  123   a.  The zoom position of the zoom lens  122  is electrically controllable. Detailed descriptions of suitable controlling mechanisms for such a zoom lens are omitted from the present description since they do not relate directly to the present invention and suitable implementations are well known to persons skilled in the art.  
      A front fluorescent light unit  124  is provided on the camera unit  118  and a diffusion panel  125  is provided in front of the front fluorescent light unit to diffuse the light from front fluorescent light unit  124 , to reduce glare from the light unit, for example. The front fluorescent light unit  124  is used to provide appropriate lighting to enable the camera  120  to take photographs of an object placed on the turntable  114  for the generation of textural data for use by the three dimensional modelling software. The camera unit  118  is mounted on a central camera arm  126 . Central camera arm  126  extends from a left camera arm  128  to a right camera arm  130 .  
      A backlight unit  132 , such as the light unit  76  of  FIG. 15 , is positioned such that an object placed on the turntable  114  is located between the backlight unit  132  and the camera unit  118 . The backlight unit  132  is illuminated when the camera unit  118  is being used to capture a silhouette image of an object placed on the turntable  114 .  
      The backlight unit  132  is mounted between a right backlight arm  136  and a left backlight arm  138 . The right backlight arm  136  is connected to the right camera arm  130  by a right arm joint  140 . The left backlight arm  138  is connected to the left camera arm  128  by a left arm joint  142 .  
      A frame  144  is provided to support the elements that support the turntable  114  (described further below). Further, a right arm pillar  146  extends from the support frame  144  to the right camera arm  130  to support the right camera arm  130  and the right backlight arm  136 . In a similar manner, a left arm pillar  148  extends from the support frame  144  to the left camera arm  128  to support the left camera arm  128  and the left backlight arm  138 .  
      The turntable support frame  144  includes a drive wheel arrangement indicated generally by the reference numeral  150 , a first support wheel arrangement indicated generally by the reference numeral  152   a,  a second support wheel arrangement indicated generally by the reference numeral  152   b  and a third support wheel arrangement indicated generally by the reference numeral  152   c.  The support wheel arrangements  152   a,    152   b  and  152   c  are provided to support to the glass turntable  114 . The drive wheel arrangement  150  supports the turntable and is also provided to rotate the turntable as required.  
      As shown in  FIGS. 17 and 18 , each camera arm (camera arm left  128  and camera arm right  130 ) is attached to the corresponding backlight arm (backlight arm left  138  and backlight arm right  136  respectively) via an arm joint (left arm joint  142  and right arm joint  140  respectively). The camera arms and the backlight arms are held at a fixed position with respect to one another by the arm joints, but those arms can be rotated relative to the glass turntable about an axis of rotation  177 .  
      The left and right camera arms  128  and  130 , and the left and right backlight arms  138  and  136 , are connected together and can be rotated relative to the turntable  114  by arm drive  180 . FIGS.  19  to  22  show the camera and backlight arms in a number of different positions relative to the turntable.  
      In  FIG. 19 , the left camera arm  128  is horizontal, i.e. it extends along the axis of the turntable  114 . In  FIG. 20 , the left camera arm  128  is orientated  80  degrees above the axis of the turntable  114 . In  FIG. 21 , the left camera arm  128  is orientated  45  degrees above the axis of the turntable  114 . In  FIG. 22 , the left camera arm  128  is orientated 70 degrees below the axis of the turntable  114  (or at an angle of −70 degrees relative to the turntable). The camera arm is driven by arm drive  180  and the arm rotation position (or elevation angle) is controlled by driving the stepping motor  190  shown in the  FIG. 24 .  
      In the use of the photographic apparatus  112  to capture a plurality of images of an object, different images can be taken at different elevations. For example, views can be taken at raised positions relative to the turntable (as in  FIGS. 20 and 21 ) and below the object (as in  FIG. 22 ). Clearly, the arm drive  180  of the photographic apparatus  112  is able to position the camera arm in any position on the arc  182 ; the angles shown in FIGS.  19  to  22  are merely exemplary.  
      As shown in FIGS.  19  to  22 , the camera unit  118  and the backlight unit  132  rotate relative to turntable  114  on which an object to be modelled can be placed. Thus, the same backlight unit  132  is used for all positions of the camera  120 . This ensures uniformity in the distance from the camera  120  to the backlight unit  132  and also ensures uniformity in the brightness and hence in the image generated. The use of a single movable backlight unit is preferable to the use of multiple fixed backlight units for a number of reasons. For example, with fixed backlight units there is the potential for backlight units to be in the background of a captured image. Also, the use of multiple backlight units increases the size and cost of the photographic apparatus. The use of a single camera and backlight unit increases the flexibility of the system since the camera and backlight can be positioned at any angle relative to the turntable. This is simply not possible if fixed devices are used.  
       FIG. 23  shows a calibration mat  206  for use with the photographic apparatus  112  of the present invention. Calibration dots  208  are positioned on the calibration mat  206  to enable the detection of the position, orientation and focal length of the digital camera  120  with zoom lens  122  in each of its various positions of use. There are  32  calibration dots shown in the calibration mat  206 , four dots being located on each of eight different radii dividing the mat  206  into eight equal angles. The calibration dots may have different sizes, as shown, and preferably each set of four dots on a radius has a different pattern of dot sizes compared with the other sets. The calibration mat  206  has the same calibration dots located in exactly the same positions on the front and rear of the mat.  
      A number of images of the calibration mat are taken by the digital camera  120  during a calibration process. The images are processed to detect the calibration dots  208  on the calibration mat  206  in the captured image. The detected calibration dots are analysed to determine a central position of the calibration mat  206  for creating supposed three-dimensional coordinates. In accordance with the supposed three-dimensional coordinates, a position, an orientation and a focal length of the digital camera  120  can be obtained from the image of the calibration dots  208  by using perspective information. Further details of the calibration process, and how the calibration data obtained is used in the generation of three-dimensional objects of models are given below.  
       FIG. 24  is a block diagram of a three-dimensional modelling system incorporating the photographic apparatus  112  described above. The modelling system includes a computer system  210 . The computer system  210  may be any suitable personal computer and may be a PC platform conforming to the well-known PC/AT standard.  
      The computer system  210  includes a central processing unit (CPU)  212  that is used to execute an application program. Normally, the application program is stored in a ROM or a hard disk within the computer system  210  as object code. That program is read from storage and written into memory within the CPU  212  at system launch for execution by the computer system  210 . Detailed descriptions of data flow, control flow and memory construction are omitted from the present description since they do not relate directly to the present invention and suitable implementations are well known to persons skilled in the art.  
      A video monitor  214  is connected to the computer system  210 . A video signal to be displayed by the video monitor  214  is output from a video board  216  to which the monitor  214  is connected. The video board  216  is driven by a video driver  218 , the video driver  218  consisting of a set of software programs. A keyboard  220  and mouse  222  are provided to enable an operator of the system to manually input data. Such input data are interpreted by a keyboard and mouse interface  224  to which the keyboard  220  and mouse  222  are connected of course, other data input and output devices could be used in addition to, or instead of, the video monitor  214 , keyboard  220  and mouse  222  in order to enable the operator to communicate with the computer system  210 .  
      The digital camera  120  and zoom lens  122  are connected to the computer system  210  by a Universal Serial Bus (USB) port and HUB interface  226 . A USB device manager  228  manages USB port  226  (and any other USB ports under its control). The digital camera  120  and zoom lens  122  are controlled by a USB driver  230 . Control functions, including image capturing, exposure control, and zoom positioning are controlled by the computer system  210 .  
      An interface box  232 , external to the computer system  210 , controls communications between STM drivers  234 ,  236  and  238 , photodetector monitor  240 , lighting control unit  242  and the computer system  210 . STM driver  234  drives and controls a stepping motor  244  used to tilt the mechanical tilting stage  123   a  of a mirror  123 . STM driver  236  drives and controls the stepping motor  190  used to drive the arm drive  180 . STM driver  238  drives and controls the stepping motor  168  used to drive the drive wheel arrangement  150 . STM drivers  234 ,  236  and  238  control steeping motors  244 ,  190  and  168  respectively in accordance with outputs from digital-to-analogue converters (DACs)  246 ,  248  and  250  respectively. DACs  246 ,  248  and  250  each convert digital data received from the computer system  210  into analogue signals for use by the STM drivers  234 ,  236  and  238  respectively.  
      Photodetector monitor  240  detects an output from a photodetector device  176  indicating positions of one or more marks  252  composed of evaporated aluminium thin films or thin material located on a circumference of the turntable  114 . The analogue output of the photodetector monitor  240  is converted into digital data by analogue-to-digital converter (ADC)  254  for use by the computer system  210 .  
      The lighting control unit  242  has a register that controls front light unit  124  and backlight unit  132 . This register is a 2-bit register, the first bit controlling front light unit  124 , the second bit controlling backlight unit  132 . These control signals are created in accordance with the application program of computer system  210 .  
      The computer system  210  and interface box  232  communicate via serial interface  256  under the control of communication serial port driver (COM port driver)  258 . Digital data for use by STM drivers  234 ,  236  and  238  are sent from CPU  212  to those STM drivers via the serial interface  256  and the appropriate DACs  246 ,  248  and  250 . Data from photodetector monitor  240  is passed to the CPU via ADC  254  and serial interface  256 .  
      A hard disk unit  260  stores data  262  of texture images and silhouette images. A three-dimensional object model creating program is stored in a ROM or a hard disk within the computer system  210  as an object code and is represented in the block diagram by 3D Object Modelling Engine  264 . The program is read out from storage and written into a memory within the CPU  212  when the system is launched. The code is executed from the CPU  212 . The application program and the model creating program communicate through a communication (COM) interface. A program for displaying a graphical user interface (GUI) for the application is stored in the CPU  212  and is represented by the GUI block  266 .  
      The operation of the system of  FIG. 24  is described briefly below. The first step is to calibrate the system. First, the camera is calibrated using the calibration mat, such as that of  FIG. 23 . The appropriate mat is placed on the turntable by the user and an off-line calibration routine is activated in which images of the calibration mat are taken at different angles of the camera head (such as 80 degrees, 45 degrees, 10 degrees and −70 degrees). At each of the angles of the camera head, images are taken at a different rotational position of the glass turntable  114 . Once the calibration data has been obtained, the calibration mat is removed and an object to be modelled can be placed on the turntable  114 . Images of the object are taken at the same positions as images of the calibration mat were taken. Using the image data and the calibration data, a three dimensional model of the object can be generated by the 3D object modelling engine  264 .  
      Although the use of the light unit of the present invention in a photographic apparatus for generating three-dimensional models of objects has been described, the present invention is not limited to such a use.  
      The light system of the present invention has been described using converging lenses. Other arrangements are possible. For example, the array of converging lenses could be replaced with an array of parabolic reflectors to generate the collimated light source.