GB2111716A - Improvements in or relating to optical scanning and optical image transmission - Google Patents

Abstract

An optical image transmission apparatus comprises a coherent bundle (1) of optical fibres having a plane input end face (2) and a plane output end face, a first optical scanning device (5) for scanning the image to be transmitted over the input face of the bundle, and a second optical scanning device at the output end of the bundle for scanning the resulting image at the output end face of the bundle in synchronism with the scanning of the image over the input end of the bundle. Each optical scanning device comprises an optical element (4) having an optical axis (9), a cage (7) in which the optical element is retained and within which the element is tiltable about axis normal to the optical axis, and a solenoid structure (11-14) for tilting the optical element about a tilt axis which is scanned around the optical axis. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical scanning and optical image transmission This invention relates to an optical scanning device and method for scanning a light signal through a range of positions, and to an apparatus and method of optical image transmission employing the scanning device and method.

According to one aspect, the present invention provides an optical scanning device, comprisng an optical element having an optical xis, means mounting the optical element for tilting about axes normal to the optical axis, and means for tilting the optical element about an axis of tilt which is scanned around the optical axis.

In a scanning device embodying the invention, a light signal received by the optical element from a given point in its object field is thus scanned through a range of positions.

In another aspect, the present invention provides a method of optical scanning, comprising providing an optical element having an optical axis, mounting the optical element for tilting about axes normal to the optical axis, tilting the optical element about a said axis of tilt, and scanning the axis of tilt around the optical axis.

In a preferred embodiment of the invention, the optical element of the scanning device is a light-transmitting plate having parallel faces.

Advantageously, the optical element is in the form of a disc and the mounting means comprises a circumferential cage which confines the disc so that it is freely, but to a limited extent, tiltable about any diameter thereof.

Desirably, the means for tilting the optical element comprises means for generating a rotating magnetic field which interacts with a magnetized rim of the optical element to cause the optical element to tilt about an axis of tilt which revolves around the optical axis.

The magnetized rim of the optical element may comprise a permanently magnetized ring attached to the circumference of the light transmitting plate, or individual local magnets distributed around the circumference of the light transmitting plate.

Preferably, the magnetized rim of the optical element is magnetized in a direction normal to the surface of the light transmitting plate, the means for producing the rotating magnetic field comprising a multiple pole solenoid having poles which are magnetizable in a direction parallel to the optical axis of the optical element, the poles having respective windings supplied with respective electrical signals of respective phases such as to produce a magnetic field which rotates around the optical axis of the optical device.

The solenoid producng the rotating magnetic field may have any convenient number of poles and, in one simple embodiment of the invention, has three poles spaced apart at 120 intervals around the optical axis of the optical element.

An optical scanning device and method according to the invention find particular application in improving the quality of images transmitted through fibre-optic bundles. Such image transmission is used extensively for medical purposes, as in endoscopes, and for diverse scientific and engineering applications.

Typically, a known optical image transmission device comprises an objective lens unit which throws an image to be transmitted onto a plane input end face of a coherent bundle of optical fibres. The image received at the input end of the bundle is transmitted through the optical fibres to a plane output end face of the bundle, where the image is available, for example, to be viewed through an ocular unit.

Each optical fibre in an image-transmitting bundle comprises a core and a surrounding light-isolating cladding, either in the form of a sheath around each fibre or in the form of a matrix material in which the individual cores are embedded. As a result, the image information is conveyed from the input to the output end of the bundle as an array of individual signals each corresponding to the average light intensity of an image element corresponding to the core area of an individual fibre. Since these image elements are spaced apart by the thickness of the cladding and any packing interstices in the bundle, the resulting image exhibits a mosaic pattern which is an unwelcome extraction to an observer. Moreover, damage to a fibre of the bundle causes loss of image information through that fibre, with the result that dark spots appear in the image.Further, the finite size and number of cores which can in practice be incorporated in a bundle place a severe restraint on the optical resolution obtainable.

Attempts have been made to improve the quality of images transmitted through such optical fibre bundles by decreasing the size of the fibre cores and the thickness of the cladding. However, as the core is made progressively smaller, it assumes the characteristics of a wave guide with resulting increased losses of transmitted light. Moreover, the smaller fibres naturally have increased fragility, both during manufacture and during the working life of the bundle. This increased fragility is especially pertinent where the handle is required to flex during use, as in the case of endoscopes. It is believed that the limit of image improvement by decreasing the size of the fibres of the bundle has been reached without providing an image quality which is acceptable, especially in the medical applications of such optical image transmission apparatus in endoscopes.

Various schemes have been proposed for weakening and removing the mosaic structure exhibited by images transmitted through bundles of optical fibres, one such suggestion being to scan the image formed on the input end of the fibre bundle rapidly over the end face of the bundle, so that information from each and every picture element is conveyed to the output end of the bundle through many different fibres. A synchronous similar scanning of the resulting image at the output end of the bundle provides a static image for viewing or recording purposes. One proposed scanning device for providing the required image movement over the input end face of the optical fibre bundle incorporates a wedgeshaped optical element which is rotated about the optical axis of the image transmission apparatus by means of a miniature electric motor.The potential size, complexity, relatively high production costs and low reliability of such a scanning device have limited the practical applications of such an approach and certainly render the proposal unsuitable for use in medical applications such as endoscopes.

One object of the present invention is to provide an optical scanning device for improving the quality of images transmitted through bundles of optical fibres and which, in particular, is sufficiently simple and compact in construction to be incorporated in an endoscope.

Accordingly, in a further aspect, the present invention provides an optical image transmission apparatus, comprising a coherent bundle of optical fibres having a plane input end face and a plane output end face, a first optical scanning device for scanning the image to be transmitted over the input face of the bundle, and a second optical scanning device at the output end of the bundle for scanning the resulting image at the output end face of the bundle in synchronism with the scanning of the image over the imput end of the bundle, in which each optical scanning device comprises an optical element having an optical axis, means mounting the optical element for tilting about axes normal to the optical axis, and means for tilting the optical element about a tilt axis which is scanned around the optical axis.

Apparatus embodying the invention may include an objective lens unit for focussing onto the input face of the bundle an image to be transmitted through the bundle.

The invention also provides a method of transmitting an image through a coherent bundle of optical fibres having a plane input end face and a plane output end face, including scanning the image to be transmitted through the bundle over the imput end face of the bundle using a scanning device in accordance with the invention, and scanning the resulting image at the output end face of the bundle using a second scanning device in accordance with the invention and operating synchronously with the first scanning device.

The invention is particularly applicable to endoscopes and, in accordance with this aspect of the invention, there is provided an endoscope including an image transmission apparatus according to the invention.

In order that the invention may be readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view of a distal end of the optical image transmission apparatus of an endoscope embodying the invention; Figure 2 is an exploded perspective view of the optical scanning device included in the apparatus of Fig. 1; Figure 3 is a view of the optical element; and Figure 4 diagrammatically illustrates a three-phase electrical power supply for driving the scanning device of the apparatus.

Referring to Fig. 1, an image transmission apparatus incorporated in an endoscope embodying the invention comprises a flexible coherent bundle 1 of optical fibres having a plane distal end face 2 and a plane proximal end face (not shown), the apparatus having an optical axis 9. An objective lens unit 3 is positioned forwardly of the distal end face 2 of the bundle 1 and serves to form on the face 2 the image of an object field, which image is to be transmitted through the bundle 1 to the proximal end face of the bundle, where it may be viewed or recorded through an ocular lens unit (not shown).

An optical scanning device 5 is provided at the distal end of the bundle 1 to scan the image to be transmitted over the distal end face 2 of the bundle during use of the endoscope and a similar synchronously operated scanning device (not shown) is provided at the proximal end of the bundle for compensating scanning of the resulting image at the proximal end face.

A tiltable optical element 4 of the scanning device 5 is interposed in the optical path between the objective lens unit 3 and the distal end face 2 of the bundle and takes the form of a disc 5 of optical glass mounted in a permanently magnetized ring 6 which is magnetized in a direction normal to the surface of the disc. The optical element 4 is captively received in a cage 7 having a cylindrical wall 8 concentric with the optical axis 9 of the apparatus and annular end walls 10 and 11.

The optical element 4 is tiltable within the cage 7 about axis normal to the optical axis 9, i.e. about any diameter of the element, the degree of tilting being limited by contact of the element 4 with the end walls of the cage.

The scanning device 5 includes a solenoid structure for producing in the vicinity of the optical element a magnetic field which extends in a direction parallel to the optical axis and which rotates about the optical axis. The solenoid structure has three poles and is received on the distal end of the optical fibre bundle. The solenoid comprises two end rings, one of which constitutes the end wall II of the cage 7, the other ring 1 2 being coaxially received on the bundle. Each pole of the solenoid structure comprises a magnetizable core 1 3 extending between the rings 11 and 1 2 and a respective winding 14. The solenoid structure may be encapsulated in a thin walled hollow cylinder having an internal diameter which is just sufficient to clear the optical path of the apparatus and the bundle 1.Fig. 2 shows the two part construction of the cage 7, the two paths being secured together in the assembled condition in any convenient manner.

Fig. 4 shows how the windings of the solenoid structures at the distal (windings 14) and proximal (windings 14') ends of the bundle 1 may be connected by star or delta wiring to a 3-phase sinusoidal electrical supply 1 5 generated at low voltage from either a small rotary alternator or a microcircuit, so as to operate synchronously.

In operation of the apparatus illustrated in Fig. 1, the windings of the solenoid cores are supplied by the electrical supply 1 5 with respective current phases which are mutually 1 2041 out of phase. Magnetic attraction or replusion between the permanently magnetized circumferential ring of the optical element and the solenoid cores causes the optical element 4 to tilt within the cage 7 about an axis of tilting X-X (see Fig. 3) coinciding with a diameter of the element. Rotation of the magnetic field as a result of the phase relationships between the cores of the solenoid causes the axis of the tilting to rotate about the optical axis of the apparatus, without necessarily including any rotary movement of the optical element itself.Thus, whilst it is envisaged that the optical element 4 may itself rotate slowly about the optical axis of the apparatus due to frictional forces at the points of contact of the optical element 4 with the cage 7, this motion does not influence the performance of the scanning device.

Thus, if S is the instantaneous phase angle from this supply across one connecting linepair, O + 2/3or and 0 - 2/3qr being the corresponding values for the other line-pairs, then the magnetic fields produce just the moment required to tilt the disc about the transverse X-X and this axis rotates to maintain the same angle aq from datum. The magnitude of this tilting moment remains constant for all phase angles and, consequently, the motion of the disc is very smooth.

As a result of the rotating axis of tilting of the optical element, each element of the image passing through the optical element is scanned in a local circular path over the distal end face 2 of the bundle 1. As a result, the information from each inage element is conveyed through many different fibres and reconstituted at the proximal end of the apparatus by the proximal scanning device, whose optical element is synchronously tilted so as to remain parallel to the optical element of the distal scanning device.

The frequency and magnitude of the scanninf performed by the scanning device can be chosen by selecting the degree of tilting and the scanning frequency to suit particular circumstances, but frequencies that are in excess of 50Hz are desirable for viewing and photographic recording of the image at the proximal end of the apparatus. The thickness of the glass disc of the optical element 4 and the degree of freedom to tilt within the cage 7 should be large enough to ensure that information from any point on the object is transmitted through sufficient fibres to smooth out the effects of broken fibres or other faults in the bundle of optical fibres.

Whilst the above embodiment employs a solenoid structure could have a different numder of poles. Similarly, although a solenoid structure is used on only one axial side of the cage 7 in the described embodiment, it is envisaged that the structure could be provided on both axial sides.

Claims (12)

1. An optical scanning device, comprising an optical element having an optical axis, means mounting the optical element for tilting about axes normal to the optical axis, and means for tilting the optical element about an axis of tilt which is scanned around the optical axis.

2. A device according to claim 1, wherein the optical element of the scanning device is a light-transmitting plate having parallel faces.

3. A device according to claim 1 or 2, wherein the optical element is in the form of a disc and the mounting means comprises a circumferential cage which confines the disc so that it is freely, but to a limited extent, tiltable about any diameter thereof.

4. A device according to claim 1, 2 or 3, wherein the means for tilting the optical element comprises means for generating a rotating magnetic field which interacts with a magnetized rim of the optical element to cause the optical element to tilt about an axis of tilt which revolves around the optical axis.

5. A device according to claim 4, wherein the magnetized rim of the optical element comprises a permanently magnetized ring attached to the circumference of the light transmitting plate.

6. A device according to claim 4, wherein the magnetised rim comprises individual local magnets distributed around the circumference of the light transmitting plate.

7. A device according to claim 5 or 6, wherein the magnetized rim of the optical element is magnetized in a direction normal to the surface of the light transmitting plate, the means for producing the rotating magnetic field comprising a multiple pole solenoid having poles which are magnetizable in a direction parallel to the optical axis of the optical element, the poles having respective windings supplied with respective electrical signals of respective phases such as to produce a magnetic field which rotates around the optical axis of the optical device.

8. A device accordng to claim 7, wherein the solenoid for producing the rotating magnetic field has three poles spaced apart at 120 intervals around the axis of the optical element.

9. A method of optical scanning, comprising providing an optical element having an optical axis, mounting the optical element for tilting about axes normal to the optical axis, tilting the optical element about a said axis of tilt, and scanning the axis of tilt around the optical axis.

1 0. An optical image transmission apparatus, comprising a coherent bundle of optical fibres having a plane input end face and a plane output end face, a first optical scanning device for scanning the image to be transmitted over the input face of the bundle, and a second optical scanning device at the output end of the bundle for scanning the resulting image at the output end face of the bundle in synchronism with the scanning of the image over the input end of the bundle, in which each optical scanning device comprises an optical element having an optical axis, means mounting the optical element for tilting about axes normal to the optical axis, and means for tilting the optical element about a tilt axis which is scanned around the optical axis.

11. Apparatus according to claim 10, including an objective lens unit for focussing onto the input face of the bundle an image to be transmitted through the bundle.

12. An endoscope including an optical image transmission apparatus according to claim 10 or 11.

1 3. A method of transmitting an image through a coherent bundle of optical fibres having a plane input end face and a plane output end face, including scanning the image to be transmitted through the bundle over the input end face of the bundle using a scanning device in accordance with the invention, and scanning the resulting image at the output end face of the bundle using a second scanning device in accordance with the invention and operating synchronously with the first scanning device.

1 4. An optical scanning device substantially as hereinbefore decribed with reference to the accompanying drawings.

1 5. A method of optical scanning substantially as hereinbefore described with reference to the accompanying drawings.

1 6. Any novel feature or combination features described herein.