GB2301679A - Endoscope optical system - Google Patents

Abstract

An endoscope comprises an optical system for forming an image of e.g. organs inside a patient's body, and a relay system e.g. optical fibres, for relaying the image. The optical system 1 produces peripheral pixels of the relayed image which correspond to a greater field of view of the object than the central pixels of the relayed image correspond to. This may be achieved by a lens 8 which has a greater magnifying power at its optical axis than at its edges or by arranging that peripheral optical fibres of a relay system accept a greater field of view than central fibres (Fig. 3).

Description

AN ENDOSCOPE This invention relates to endoscopes.

Endoscopes are devices which can be used in medicine for inspecting the internal organs of a patient e.g. for diagnosis or during surgery. Typically, such endoscopes comprise an optical system, usually a lens or lenses, for forming an image of an object and a relay system for relaying this image e.g. to a television monitor for viewing by a surgeon.

Endoscopes which are rigid have been used in neurosurgery, and these usually relay the image by a series of lenses. Endoscopes can also be flexible for ease of manipulation inside the body, and in which case the image is normally relayed by optical fibres.

One of the problems which would be encountered in such endoscopes is that a compromise has to be made between achieving the best image resolution for the instrument and attaining the maximum possible field of view. A wide field of view is desirable as it enables observation of the body surrounding the area of interest. This is particularly useful as insertion of the endoscope itself may cause a haemorrhage which can be detected quickly if the surgeon is able to see a wide field of view. The number of optical fibres or size of the lens which can be accommodated in an endoscope is limited by the maximum allowable diameter for the instrument, hence if a high resolution image is required the total field of view will be small and if a large field of view is required the image resolution will be poor.

Proposals have been made to alleviate this problem. For example. it has been proposed to use a zoom lens in the optical system in order to adjust the extent of the field of view without loss of image resolution. It has also been proposed to make use of a scanning arrangement whereby a moveable mirror scans a wide field of view and directs light from it into the optical system. However, both proposals involve the use of mechanical parts. The endoscope must undergo rigorous sterilisation procedures before use and such mechanical parts may not be sufficiently robust to survive such procedures.

The invention provides an endoscope comprising an optical system for forming an image of an object and a relay system for relaying the image wherein peripheral pixels of the relayed image correspond to a greater field of view of the object than central pixels of the relayed image correspond to.

This arrangement permits increased angular resolution in the central portion of the image to be achieved without restriction to the overall field of view.

One possible optical system which achieves greater field of view of peripheral pixels comprises an image-forming lens the magnification of which decreases in a direction from the optical axis of the lens to its edges (an anamorphic lens). Another possible optical system which achieves greater field of view of peripheral pixels, in the case in which the relay system comprises optical fibres, has the fibres of the coherent fibre bundle flared at their terminations, the diameters of the fibres thus increasing in the direction from the centre to the peripheral region of the bundle.

An endoscope constructed in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the endoscope, with suitable image-processing and illumination means; Figure 2 is a ray diagram of an optical system for forming an image for the endoscope of Figure 1; and Figure 3 is a ray diagram of an alternative optical system for forming an image for the endoscope of Figure 1.

Referring to Figure 1, the endoscope comprises an optical system, indicated generally by the reference numeral 1, which is able to form an image of objects in its field of view, and an optical relay system in the form of a fibre optic cable 2. The fibre optic cable 2 consists of a coherent fibre bundle, that is to say the optical fibres are carefully arranged so that their terminations occupy the same relative positions at both ends of the fibre optic cable. The fibre optic cable 2 relays the image as formed by the optical system 1 to an image processing arrangement which comprises a lens 3, a video camera 4 and a television monitor 5. The lens 3 is chosen to complement the optical system 1, so that relative image proportions are recovered at the external end of the endoscope.The image formed by the lens 3 is conveyed by video camera 4 to the television monitor 5 to enable e.g. a surgeon to see the field of view of the endoscope. There is also provided a second fibre optic cable 6, which has terminations close to the optical system 1 and conveys light from a source 7 in order that the field of view of the endoscope is illuminated when in use e.g. inside a human body.

Figure 2 shows an embodiment of the optical system 1. In this embodiment, light from the object plane in the field of view of the endoscope passes through an optically thick lens 8 which has a greater degree of magnification at its optical axis than at its edges (a so-called anamorphic lens). Thus, fibres 9 which are situated at the periphery of the coherent fibre bundle 10 will receive a greater field of view of the object than the fibre 11 which is at the centre of the coherent fibre bundle. This produces an effect known as negative or barrel distortion of the image, so that each point on the image is effectively displaced radially inwards towards the centre of the image, with the most distant points moving the greatest amount.This negatively distorted image is relayed by the coherent fibre bundle 9 to the lens 3 which complements the lens 7 so that the image is decompressed at its edges before it is transmitted via the camera 4 to the television monitor 5 for viewing. This displayed image presents to the observer a large field of view with highly resolved pixels in the central viewing area. Thus, a surgeon will be able to see a well-resolved image of the area he wishes to examine or operate on and will also be aware of the condition of the surrounding region.

Another embodiment of the optical system is shown in Figure 3. This embodiment comprises a convex lens 12 which forms an image of the object in the field of view. This image is received by the coherent fibre bundle which is manufactured such that the fibres 13 at the periphery of the coherent fibre bundle possess larger diameters than the fibre 14 at the centre of the bundle. The diameter of the apertures is varied continuously across the image plane from a minimum at the optical axis of the lens to a maximum at the periphery of the lens. Thus the fibres 13 receive a greater field of view than the fibre 14. This has the equivalent effect of the lens system of Figure 2, namely that the image undergoes negative distortion.This image is relayed by coherent fibre bundles, processed by lens and camera and displayed as before, presenting to the observer a large field of view with highly resolved pixels in the central viewing area.

Variations may be made without departing from the scope of the invention. For example, the lens of Figure 2 may be used in conjunction with the flared fibres of Figure 3, or it may be replaced by a compound lens arrangement having the same optical properties. Alternatively, concerning the embodiment in Figure 3, the terminations of the optical fibres near the periphery need not be flared. An equivalent effect may be achieved by positioning auxiliary microlenses in front of the terminations of the fibres, the microlenses in front of the ends of the fibres at the centre of the coherent bundle having smaller diameter than those microlenses at the periphery of the bundle. In another modification, in the case where a rigid endoscope is used, the relay system for relaying the image may comprise a series of lenses, rather than the coherent fibre bundle.

With regard to the image processing arrangement of Figure 1, the lens 3 may be omitted and the video signal from the camera 4 may be electronically processed in order to decompress the image for viewing. The endoscopes of the invention may be adapted for use in the abdominal areas (these being sometimes referred to as laparoscopes). Further, the endoscopes of the invention are not restricted to medical applications, for instance, in industry such endoscopes could be used for inspection and photography of remotely situated parts of machinery and equipment.

Our co-pending patent application number P/6061 1/MRC describes an endoscope which includes means for ascertaining the distance from the endoscope of objects in its field of view.

Claims (10)

1. An endoscope comprising an optical system for forming an image of an object and a relay system for relaying the image wherein peripheral pixels of the relayed image correspond to a greater field of view of the object than central pixels of the relayed image correspond to.

2. An endoscope as claimed in claim 1, wherein the optical system for forming an image is a lens, the magnification of which decreases with increasing distance from its optical axis.

3. An endoscope as claimed in claim 1, wherein the relay system comprises optical fibres and the peripheral optical fibres relay a greater field of view of the image produced by the optical system than the central optical fibres relay.

4. An endoscope as claimed in claim 3, wherein the terminations of the optical fibres are flared so that the peripheral optical fibres have a greater diameter than the central optical fibres have.

5. An endoscope as claimed in claim 3, wherein microlenses are positioned in front of the terminations of the optical fibres.

6. An endoscope as claimed in claims 1 to 5, wherein there is also provided means to compensate for the increased field of view of peripheral pixels.

7. An endoscope as claimed in claim 6, wherein the compensation means comprises electronic signal processing means.

8. An endoscope as claimed in claim 6, wherein the compensation means comprises optical means.

9. An endoscope substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.

10. Apparatus including an endoscope as claimed in any one of claims 1 to 9.