APPARATUS AND METHOD FOR STEREOSCOPIC ENDOSCOPY

Abstract: Apparatus and method for providing stereo-image pairs, for use in endoscopy. A birefringent optical component (11) creates two virtual pupils (o.E) to provide spaced view points (P1,P2) of an object field through a single real pupil (16), the light from each of the two view points having a respectively different polarization. The birefringent component (11) may be in the form of a calcite slab or comprise liquid crystal material. By suitable orientation of two such components with respect to each other, the path lengths for the two polarizations of light may be made equivalent and rotation of the planes of polarization of this light through 90 degrees by means disposed between the birefringent components can improve the performance of the system. The rotation device may comprise a half-wave plate (21) or a layer of liquid crystal material. By tilting the slab of calcite, the observer may be provided with a change in view point and, thereby, some motion parallax. In one embodiment of the invention, a polarising beam splitter (7) directs light corresponding to each of the two view points to respectively different CCD's (6, 10) following its passage through the single pupil.

Description: APPARATUS AND METHOD FOR STEREOSCOPIC ENDOSCOPV This invention relates to the field of stereoscopic imaging and in particular to the provision of a stereoscopic image from an endoscope or similar optical relay system BACKGROUND Both in the medical field and in certain industrial applications it is desirable to relay an image through a tube of substantial length, such as an endoscope. This is used to inspect and manipulate items of interest in locations which are not easily accessible by other means In the medical field, the opportunity to reduce patient trauma and after care, by employing so-called Minimally Invasive techniques, is of major interest One of the more important aids to manipulation, when doing so normally, is binocular vision Each eye sees a slightly different perspective of the subject, which allows the precise determination of distance and depth An endoscope typically comprises a series of imaging and relay lenses, which transfer an image, usually wide angle, from the distal end of the instrument to the eye piece at the other end Here, there is a pupil What is meant by pupil is an effective aperture which may be a relayed image of a physical stop at any position in the optical system Conveniently, the user may place his eye at a pupil of the system to observe the image Often the eye's function is assumed by a small video camera suitably coupled to the optical system of the endoscope The simplest way, conceptually, to provide two different perspective views of a subject trough an endoscope, is to duplicate the optical relay system, with both systems running side by side down the instrument This approach is often adopted, but requires careful alignment of one system's optical axis to that of the other, in order to provide the correct correspondence between different points in each image A second approach, which is also well known, is to subdivide the pupil of the system, either sequentially using one camera or by sending the two slightly different images, formed from light from the different portions of the pupil, to respectively different cameras, simultaneously The disadvantage of this approach is that, whereas the majority of alignment problems are avoided, the stereopsis (difference in perspective) achieved is limited to that occurring within the dimensions of the pupil This can have a diameter as small as 0 3 mm or less at the distal end, where the subject will typically be at a range of distances of up to 60 mm or more The effective separation of the two halves of the pupil is of the order of 0 15 mm in the above example For the normal level of stereopsis at 1 5 mm, a separation of at least 0 5 mm is desirable In Patent Application GB 9607089 1 (Street), an embodiment of endoscopic apparatus is disclosed which, for stereoscopic use, may conveniently include the use of a birefringent component for the provision of two points of view This description develops the invention further and overcomes some of the attendant difficulties associated with its practical implementation SUMMARY OF THE INVENTION It is an object of the invention to provide a degree of stereopsis through a small entrance pupil which would not be achievable using pupil segmentation techniques alone It is a further object of the invention, in certain embodiments, to provide an endoscope which provides a measure of motion parallax when suitably coupled to movement of the observer Motion parallax is experienced when moving the observer's viewpoint during observation, thus causing objects at different distances to move relative to each other within the retinal image STATEMENTS OF INVENTION Thus according to the present invention there is provided apparatus for stereo- imaging having a single pupil through which an object field is viewed and comprising displacement means between the pupil and the object field, the displacement means being effective to create two virtual pupils providing spaced view points of the object field and to provide the single pupil with different respectively polarised light from the two view points Preferably the displacement means comprises birefringent material Advantageously the birefringent material is in the form of a slab In certain embodiments the birefringent material is calcite In some embodiments the birefringent material comprises a liquid crystal The displacement means may comprise a composite of at least two birefringent components so orientated as to ensure that the path lengths through the composite for the two polarisations of light are the same Advantageously the composite includes means for rotating the planes of polarisation of the light through 90 degrees disposed between two of the birefringent components In some embodiments the rotation means comprises a half-wave plate Alternatively, the rotation means comprises liquid crystal material According to a further aspect of the invention the displacement means is changeable to provide motion parallax The change may be effected by tilting the slab of birefringent material According to another aspect of the invention the light corresponding to each of the two view points is directed into respectively different optical paths by a polarisation selective component following passage through the single pupil Advantageously the polarisation selective component is a polarising beam splitter DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be described with reference to Figures 1 to 3 in which - Figure 1 shows diagrammatically an endoscope constructed in accordance with the invention Figure 2 is a birefringent element used in a preferred embodiment of the invention Figure 3 illustrates how motion parallax is provided in accordance with the invention An endoscope constructed in accordance with the invention is illustrated in Figure 1 Points Pl and P2 in an object field are imaged by an objective lens assembly 1 to the first image plane 2 of a relay system 3 For convenience this relay system is shown as a short assembly, whereas in- reality it would be quite long This is assumed to reproduce the image formed at plane 2 at an exit plane 4 A lens 5 produces an image of the object field on a CCD array 6, through a polarising beam splitter 7 Light with its polarisation direction in the plane of the diagram is transmitted by splitter 7 Light with its polarisation direction orthogonal to the diagram is reflected from the internal face 8 of splitter 7 This light forms, via a reflecting prism 9, another image of the object field on a second CCD array 10 If it were not for the properties of a special component positioned in the optical path intermediate the object field and objective assembly 1 , these two images would be the same A birefringent slab of calcite 1 1. with its optic axis 12 tilted out of the transmitting face 13 of slab 1 1 by an angle A of approximately 38°, causes light from points P l and P2 to travel via two distinct optical paths 14 and 15 before reaching the pupil of the optical system at plane 16 One set of paths 15 comprise ordinary rays of light which pass through slab 1 1 in a conventional manner according to Snell's laws of refraction and are polarised in a direction orthogonal to the diagram The second set of paths 14 are subject to an elliptical indicatrix of refraction due to the birefringence of the calcite and comprise light rays polarised in the plane of the diagram The difference between the refractive index along optic axis 12 and that at right angles to it causes these so-called Extraordinary rays to be deflected upon entering slab 1 1 by about 6° towards axis 12 in addition to any deflection expected from Snell's laws When exiting slab 1 1 , the Extraordinary rays loose this additional deflection and become parallel to their Ordinary counterparts Consequently a lateral displacement has occurred which, when viewed from the object field, is equivalent to the endoscope presenting two pupils displaced from one another by a distance which depends on the birefringence of slab 1 1 and its thickness In Figure 1 the centres of these two virtual pupils are denoted as O and E respectively It will be apparent that the effective point of view seen through the endoscope is different for the Extraordinary rays 14 to that provided by the Ordinary rays 15 The perspective separation achieved is 0 105 mm for each mm of thickness of slab 1 1 and accordingly a 5 mm thickness is found to provide a 0 525 mm shift between the two effective pupil positions This is more than adequate for the generation of a pair of stereo images of subject matter at working distances of 5 to 20 mm from the endoscope Pupil size is typically about 0 2 mm for a small endoscope and this does not provide scope for more than 0 1 mm of perspective separation The stereo image pair is provided by the two CCD's 6 and 10, each receiving rays having a plane of polarisation orthogonal to those received by the other, this being effected by the polarising beam splitter 7 The birefringence of the calcite slab 1 1 of Figure 1 has a second consequence The equivalent thickness in air of slab 1 1 is different for the Extraordinary rays and the Ordinary rays When the distance of the subject matter is much greater than the slab's thickness, or, more specifically, when light from object points is collimated on passing through the material, either by these points being distant or collimated by an intermediate lens (which can only be true for one distance), this has little significance However, for near field objects at a variety of distances which is the very nature of the endoscopic application, this consequence has a draw back There will be different scale changes for equivalent points in the two observed images One of these images is intended for the observer's left eye, whereas the other is for the right eye There should only be lateral disparity and no vertical disparity, the inevitable consequence at the top and bottom of the image if two images are not the same size Whilst this can be corrected for one distance, the simple arrangement of Figure 1 does not achieve this for arbitrary distance This problem can be overcome by splitting the calcite into two slabs, and rotating one with respect to the other If this rotation is 90°, the Extraordinary ray in the first becomes the Ordinary ray in the second and all rays experience the same equivalent thickness in air The total displacement however is reduced to the vector sum of two displacements orthogonal to each other, each of half the original magnitude The resultant effect is V( l/2) of its original value The alternative embodiment of Figure 2 avoids this reduced effect The sub-assembly is oriented for the sake of clarity to split the light which passes through it in a vertical plane In practice, it would be used horizontally to provide two horizontally spaced points of view Ray 17 from the object field enters calcite slab 18 which has its optic axis 18A turned through 38° in a clockwise direction with respect to the vertical edge 18B Two rays result The first is the ordinary ray 19 which, for the normal incidence case continues in the same direction Its plane of polarisation is horizontal as indicated by arrow 19A The extraordinary ray 20 is bent downwards and is vertically polarised as indicated by arrow 20A Both rays enter a half wave plate 21 with its principle axis 22 at 45° to the horizontal and vertical As a result, the direction of polarisation for both rays is turned by 90° and on entering the second calcite slab 23, which has its optic axis 23 A in a vertical plane containing the optic axis 18A of slab 18, what was the extraordinary ray 20 becomes the ordinary ray for slab 23 Likewise ray 19 becomes the extraordinary ray for the second calcite slab Its optic axis 23 A is tilted in the opposite direction from the vertical to that of slab 18 and, as a result, the two rays are separated further, whilst the optical path length for both is the same through the composite assembly The polarisation directions of the rays exiting the optical assembly are orthogonal to those on entry, as indicated by arrows 19B and 20B An alternative to the use of the half wave plate 22 is to use a twisted liquid crystal to turn the plane of polarisation of the light through 90° Figure 3 shows how an optically thick component such as the sub-assembly of Figure 2 can be incorporated in an endoscope to provide, in addition to two view points, a change in these view points, without moving the whole instrument Sub-assembly 24 constructed in accordance with the principles of Figure 2, may be tilted as indicated by arrows 25 This has the effect of moving the original positions 26A and 26B of the two virtual pupil positions to positions 27A and 27B, thus providing two new view points of the object field Such tilting of assembly 24 can be driven by changes in the viewer's co-ordinates, which are conveniently provided by a head or eye tracking system. It will be apparent that the same principle can be applied to a system which only provides one point of view, but where motion parallax is desirable Instead of a birefringent component being used, a thick slab of glass or other clear material would be suitable Whilst components comprised of naturally occurring calcite are employed in the illustrated embodiments of this invention and the birefringence of this material produces the necessary separation between two points of view of an object field, liquid crystal materials are known to exhibit high levels of birefringence, when their molecules are suitably aligned Suitable arrangements of optical structures incorporating such materials can be used to provide separation between orthogonally polarised image components in accordance with this invention

Claims: CLAIMS 1 Apparatus for stereo-imaging having a single pupil through which an object field is viewed and comprising displacement means between the pupil and the object field, the displacement means being effective to create two virtual pupils providing spaced view points of the object field and to provide the single pupil with different respectively polarised light from the two view points 2 Apparatus as claimed in Claim 1 wherein the displacement means comprises birefringent material. 3 Apparatus as claimed in Claim 2 in which the birefringent material is in the form of a slab 4 Apparatus as claimed in Claim 3 wherein the birefringent material is calcite. 5. Apparatus as claimed in any one of Claims 2 to 4 wherein the displacement means comprises a composite of at least two birefringent components so orientated as to ensure that the path lengths through the composite for the two polarisations of light are the same. 6 Apparatus as claimed in Claim 5 in which the composite includes means for rotating the planes of polarisation of the light through 90 degrees disposed between two of the birefringent components 7 Apparatus as claimed in Claim 6 in which the rotation means comprises a half- wave plate. 8 Apparatus as claimed in Claim 6 in which the rotation means comprises liquid crystal material 9 Apparatus as claimed in Claim 2 in which the birefringent material comprises liquid crystal material 10 Apparatus as claimed in any one of Claims 2 to 9 in which the displacement means is changeable to provide motion parallax The apparatus of Claim 10 as dependent on Claim 3 in which the change is effected by tilting the slab Apparatus as claimed in any preceding claim in which light corresponding to each of the two view points is directed into respectively different optical paths by a polarisation selective component following passage through the single pupil Apparatus as claimed in claim 12 in which the polarisation selective component is a polarising beam splitter A method for viewing a stereoscopic image of an object field through a single pupil which comprises providing displacement means between the pupil and the object field, thereby creating two virtual pupils providing spaced view points of the object field and providing the single pupil with respectively different polarised light from each of said two view points A method for providing motion parallax in an endoscopic viewing system comprising providing an image relay system, positioning a slab of transparent material intermediate an object field and a real pupil of said image relay system, and tilting said slab in accordance with the motion of an observer, whereby motion of a virtual image of said pupil, having a respective view point of said object field, and corresponding motion parallax, within the relayed image thereof, is provided

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