STEREO-ENDOSCOPE

Abstract: A stereo-endoscope, by which two perspective views of an objective field are viewed, is provided. A divergent optical arrangement, positioned between the object field and two objective lenses, each of which forms a respective image of the object field, reduces the disparity between corresponding points in the two image fields, where such disparity is caused by distortion of the image formed.

Description: STEREO-ENDOSCOPE This invention is concerned with the field of three-dimensional imaging and particularly with the provision of stereoscopic images through an endoscope by providing different perspective views of an object field to each eye of the observer. BACKGROUND In International Patent Application PCT/GB94/00405 (Street) apparatus is described in which a stereoscopic pair of two-dimensional images are combined with the aid of an optical system, so that each eye of an observer sees a different perspective of an object field, but in the same location This causes the brain to fuse these perspectives into one three-dimensional image For such fusion to be natural, it is important that the binocular disparity between corresponding points in each of these images is the same for points at a given distance, and that there should be little or no vertical disparity for such corresponding points A typical stereo-endoscope comprises two optical telescopes, each having an objective, an image relay system, and an eye piece or camera lens. When video cameras are employed to present the images on a stereo monitor, the eye piece becomes the final lens in the chain and forms a suitably scaled image on the CCD sensor of the video camera In most endoscopic designs, the objective lens has a very short focal length, providing a substantial field of view (say 60°). The camera lens is typically of longer focal length, and the relay system which may comprise a coherent fibre bundle, a series of discrete lens elements, or a graded index lens system, provides a 1 1 magnification of the image formed by the objective, at the object plane of the camera lens In many designs, the objective lens (typically comprising 2 or 3 elements) will image a flat square grid onto the CCD with some apparent distortion. Equally spaced points, further from the centre are seen as closer together than such points at the centre of the field. When two optical systems are combined side by side in a stereo-endoscope, any point at a finite distance from the distal end of the scope and at the centre of the field of view of one telescope, will be observed at some distance from the corresponding axis of the other telescope. It will be apparent that the spacing of any two points will typically be different for each optical system This difference can be quite severe towards the edges of the useful field of view It is found that, deliberate lateral displacement of each CCD, a natural consequence of defining a point at a finite distance of a few centimetres from the objectives as the central point of convergence for the stereo system, accentuates this asymmetry This invention overcomes the problem by utilising the asymmetric properties of an additional optical element when acting in combination with the two juxtaposed optical systems SUMMARY OF THE INVENTION It is an object of the current invention to minimise local scale differences between the images of pairs of points, corresponding to different perspectives thereof, where such points are substantially equidistant from the view point It is a further objective of the invention to minimise the vertical disparity between the two images of any given point Thus according to the current invention there is provided a stereo-endoscope for viewing an object field, the endoscope comprising two objective lenses, each of which forms an image of the object field from a different respective view point, and a divergent optical arrangement between the object field and the lenses Preferably the divergent optical arrangement is a divergent lens In certain embodiments of the invention, the divergent optical arrangement comprises prismatic components Advantageously the distance of the divergent optical arrangement from the corrected convergence point is substantially equal to the magnitude of the length of divergence of the divergent optical arrangement In general, a divergent optical arrangement has a length of divergence which is the distance of the apparent point of origin of oppositely deflected, originally parallel rays, following passage thereof through the arrangement For a divergent lens, the length of divergence is the negative focal length of the lens DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be described with reference to Figures 1 to 3 in which - Figure 1 shows the form of distortion which the invention seeks to control, Figure 2 shows diagrammatically an endoscope constructed in accordance with the invention, and Figure 3 shows the resultant effect of the invention on the distortions of Figure 1 In Figure 1 the left eye image 1 and right eye image 2 of a square grid, as would typically be viewed through two conventional endoscopic telescopes are illustrated The distortions are slightly exaggerated for the sake of clarity The region centred on the two images of the same point in the object field, depicted as points Cl and C2 in each respective case, is shown as a dashed line It will be clear that, when points Cl and C2 are made to be central to the observed image and coincident, by moving each respective CCD accordingly and thereby making C 1 and C2 a selected common convergence point, the distance to an off axis point shown as PI and P2 in the two respective views will be nearer the central point Cl in the left hand view than it is to C2 in the right hand view Figure 2 shows schematically an endoscope constructed in accordance with the invention Two optical relay systems 3 and 4 transfer a pair of images from the distal end of the endoscope to an image plane, where two corresponding CCD's 5 and 6 are positioned The objective lens groups 7 and 8 each form an image at a first image plane within each respective relay system, and this is typically magnified at the corresponding CCD It is this magnification which introduces the distortions observed The divergent lens 9 requires that the CCD's be moved further off axis than would have been the case without, in order to bring point C to the centre of each. By making this off-set about twice the amount required without lens 9, which means a negative focal length approximately equivalent to the distance D, mirror symmetric distortions are introduced into each image ofthe grid of Figure 1 and of a magnitude which makes the resulting total distortions the same with respect to the now corrected common convergence point in each view Figure 3 shows the result of applying the principles outlined to each image field The grid distortions are skewed, so that the corrected common convergence point C is at the point of symmetry By achieving this, corresponding points in each view now map correctly to each other without the unwanted disparity that would otherwise arise Whereas the divergent optical arrangement of Figure 2 is a negative lens 9, the desired image distortion compensation may be achieved by placing two optical wedges or prisms side by side with the two narrow ends juxtaposed, each in a respective optical path In this way, the two prisms combine to form the required divergent optical system and the desired compensation is achieved Whilst missing the demagnifying property of lens 9, which in itself has little effect on image distortions, such a prismatic divergent system is substantially equivalent to lens 9, in the context of the invention It provides, for each objective lens, the respective prismatic component of lens 9 that would be encountered by light passing through the optical system

Claims: CLAIMS 1 A stereo-endoscope for viewing an object field, the endoscope comprising two objective lenses, each of which forms an image of the object field from a different respective view point, and a divergent optical arrangement between the object field and the lenses 2 A stereo-endoscope as claimed in Claim 1 wherein the divergent optical arrangement is a divergent lens 3 A stereo-endoscope as claimed in Claim 1 wherein the divergent optical arrangement comprises prismatic components A stereo-endoscope as claimed in any preceding claim which has a corrected convergence point, the divergent optical arrangement has a length of divergence and the distance of the divergent optical arrangement from the corrected convergence point is substantially equal to the magnitude of said length of divergence A method for viewing a stereo-endoscopic image of an object field which comprises providing two objective lenses for forming respective images of said object field, each from a different respective view point, and positioning a divergent optical arrangement intermediate the object field and said objective lenses

Inventors: STREET GRAHAM STEWART BRANDON, STREET, GRAHAM, STEWART, BRANDON
Applicants: STREET GRAHAM S B, STREET, GRAHAM, STEWART, BRANDON
Application year: 1997
Main classification: G02B23
IPC codes: A61B1, G02B23