Laser scanning imaging apparatus and method of ranging

We claim:

1. A method of range measurement comprising the steps of passing a laser beam through an optical system so as to be projected into an object field containing an object to be ranged, detecting light reflected from the object, varying the focus distance between an output of the optical system and the position at which the beam is focused by operation of a focus distance varying means, measuring a parameter of the detected light which is characteristic of laser speckle, determining a setting of the focus distance varying means at which the value of the speckle parameter is consistent with the beam being focused onto the object, and determining from calibration of the focus distance varying means the corresponding value of focus distance as a measurement of range, wherein the beam is directed onto a selected area of the object and scanned across the selected area, the reflected light being detected by means of a photodetector producing an electrical output signal, the speckle parameter being measured as the output of circuit means responsive to noise in the output signal and the presence of maximum noise in the output signal being taken as being consistent with the beam being focused onto the object.

2. A method as claimed in claim 1 wherein the focus distance varying means comprises a mirror system which is movable to vary the path length travelled by a converging or diverging portion of the beam.

3. A method as claimed in claim 2 wherein the focus distance varying means comprises at least one axially movable lens in the path of the beam.

4. A method as claimed in claim 1 in which range is measured by means of apparatus comprising the optical system, the apparatus being selectively operable to generate a television image of the object field by scanning the beam in raster fashion, the apparatus being switchable between ranging mode and imaging mode as required, and wherein the apparatus includes means operable to vary the depth of focus of the beam such that in the ranging mode the depth of focus is less than in the imaging mode.

5. A method as claimed in claim 4 wherein the depth of focus is varied by actuation of a zoom lens of the optical system which provides a field angle at the output of the optical system which is reduced in the ranging mode relative to the field angle provided in the imaging mode.

6. A method as claimed in claim 4 wherein the depth of focus is varied by passing the beam through different lenses in the ranging mode and imaging mode respectively.

7. A method as claimed in claim 4 wherein the apparatus further comprises a scanning head including at least one pivoting mirror, the scanning head being operable in the imaging mode to deflect the laser beam into an input of the optical system such that the angle between the beam and optical axis is scanned in raster fashion, a corresponding raster scan being produced in the beam projected from the output of the optical system to scan the object, and the scanning head being operable in the ranging mode to provide scanning of the beam only over a selected area of the object.

8. A method as claimed in claim 4 wherein the beam is deflected towards the selected area by light deflecting means operable to vary the direction of the optical axis at the output of the optical system when the selected area would otherwise be located outside of the reduced field angle of the optical system in the ranging mode.

9. A method of range measurement comprising the steps of passing a laser beam through an optical system so as to be projected into an object field containing an object to be ranged, detecting light reflected from the object, varying the focus distance between an output of the optical system and the position at which the beam is focused by operation of a focus distance varying means, measuring a parameter of the detected light which is characteristic of laser speckle, determining a setting of the focus distance varying means at which the value of the speckle parameter is consistent with the beam being focused onto the object, and determining from calibration of the focus distance varying means the corresponding value of focus distance as a measurement of range, wherein the optical system further comprises an optical relay, the method including the step of passing the laser beam through the optical relay, which optical relay defines an optical axis extending longitudinally through an elongate tube, the output of the optical system being located at a distal end of the tube which is insertable into confined spaces for range measurement of inaccessible objects.

10. Imaging apparatus comprising an optical system defining an optical axis, a scanning head connected to an input of the optical system, a laser light source connected to an input of the scanning head, the scanning head being operable to project a laser beam into the input of the optical system such that the angle between the beam and the axis is scanned in raster fashion, the optical system having an output comprising a light transmitting window from which the laser beam is projected towards an object field, a light receiving window located adjacent the transmitting window for collecting light reflected from the object field, detection means producing an electrical output signal responsive to light collected by the light receiving window, and electronic apparatus operable to produce a television signal from the detector means output signal whereby a television image of an object in the object field may be obtained, wherein the optical system comprises an elongate tube, the transmitting window and the receiving window being mounted in a distal end of the tube, the optical system including an optical relay through which the optical axis extends longitudinally within the tube, and wherein the tube is of small cross-section relative to the scanning head so as to be insertable into confined spaces for imaging inaccessible objects.

11. Imaging apparatus as claimed in claim 10 further comprising ranging means operable to measure the range of an object in the object field under inspection.

12. Apparatus as claimed in claim 11 wherein the ranging means comprises means varying the focus distance between the light transmitting window and the position at which the beam is focused and circuit means responsive to noise characteristic of laser speckle in the output signal of the detection means, the electronic apparatus being operable to determine a setting of the focus distance varying means at which maximum noise is present in the output signal and to determine from calibration of the focus distance varying means the corresponding value of focus distance as a measure of range.

13. Apparatus as claimed in claim 12 wherein the electronic apparatus includes a frame store containing image information of each pixel of the television image, the frame store information being refreshed at the scanning rate of the scanning head and the television signal being derived from the information in the frame store at a scanning rate which is independent of the scanning head.

14. Apparatus as claimed in claim 13 wherein the electronic apparatus provides for an image generated by a previous scan in image mode operation to be stored in the frame store during operation of the apparatus in ranging mode such that a television image may continue to be displayed.

15. Apparatus as claimed in claim 10 wherein the detection means comprises a fibre optic link extending between the light receiving window and a photodetector at the proximal end of the tube.

16. Apparatus as claimed in claim 11 wherein the scanning head is removably connected to the endoscope tube, the tube being connectable to an eyepiece for use in direct viewing through the optical system.

17. Apparatus as claimed in claim 11 wherein the optical system includes a zoom lens located in the distal end portion of the tube and means for remotely actuating the zoom lens.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to optical apparatus and in particular but not exclusively to electronic apparatus adapted for insertion into confined spaces to obtain a television image of an inaccessible object.

Such apparatus are well-known and are used as endoscopes to examine internal surfaces of, for example, human bodies and as borescopes to examine mechanical components such as engines, aero engines, and aircraft.

Whilst generally satisfactory there are a number of difficulties in utilizing such known apparatus. In particular, it can be difficult to obtain an image of sufficient contrast and resolution and furthermore, particularly when being used in an unfamiliar environment, it is difficult to obtain from the image produced an indication of the size of the objects being viewed.

It is known from EP0084435 to improve the contrast and resolution of an optical apparatus by providing means for focusing the optical system of the apparatus in which an intense spot of light is projected onto an object and the focus is adjusted until a clear image of the spot is formed. A disadvantage of such apparatus is that in some applications a high intensity source cannot be used and also the technique is not readily adaptable for use in an electronic imaging apparatus.

It is also known to provide a laser scanning camera for remote inspection purposes in which a laser beam is projected onto an object field and scanned in raster fashion, the reflected beam being detected and used to form a television image. A disadvantage of such cameras is that their bulk inhibits use in confined spaces.

It is also known to use an electronic imaging apparatus to provide a television image in which a solid state detector such as a charge coupled device is mounted at the tip of an apparatus tube. A disadvantage of such apparatus is that they cannot be used in certain hazardous environments where the tip is to be exposed to high levels of radiation such as in the inspection of nuclear reactors.

Throughout the specification we will refer to "light", "optical" and like expressions. It will be understood, however, that the present invention is not restricted to electromagnetic radiation of visible wavelengths, but may apply to other wavelengths such as infra-red and ultraviolet. The term "lens" used in the specification should also be understood to encompass groups of lens elements where appropriate.

SUMMARY OF THE INVENTION

According to the present invention there is disclosed a method of range measurement comprising the steps of passing a laser beam through an optical system so as to be projected into an object field containing an object to be ranged, detecting light reflected from the object, varying the focus distance between an output of the optical system and the position at which the beam is focused by operation of a focus distance varying means, measuring a parameter of the detected light which is characteristic of laser speckle, determining a setting of the focus distance varying means at which the value of the speckle parameter is consistent with the beam being focused onto the object, and determining from calibration of the focus distance varying means the corresponding value of focus distance as a measurement of range, wherein the optical system further comprises an optical relay, the method including the step of passing the laser beam through the optical relay, which optical relay defines an optical axis extending longitudinally through an elongate tube, the output of the optical system being located at a distal end of the tube which is insertable into confined spaces for range measurement of inaccessible objects.

According to a further aspect of the present invention there is disclosed imaging apparatus comprising an optical system defining an optical axis, a scanning head connected to an input of the optical system, a laser light source connected to an input of the scanning head, the scanning head being operable to project a laser beam into the input of the optical system such that the angle between the beam and the axis is scanned in raster fashion, the optical system having an output comprising a light transmitting window from which the laser beam is projected towards an object field, a light receiving window located adjacent the transmitting window for collecting light reflected from the object field, detection means producing an electrical output signal responsive to light collected by the light receiving window, and electronic apparatus operable to produce a television signal from the detector means output signal whereby a television image of an object in the object field may be obtained, wherein the optical system comprises an elongate tube, the transmitting window and the receiving window being mounted in a distal end of the tube, the optical system including an optical relay through which the optical axis extends longitudinally within the tube, and wherein the tube is of small cross-section relative to the scanning head so as to be insertable into confined spaces for imaging inaccessible objects.

According to a further aspect of the present invention there is disclosed an optical assembly comprising an objective lens and a field lens which is movable relative to the objective lens between a first position adjacent the objective lens and a second position spaced from the objective lens, the field lens and objective lens in combination defining a focal plane which is at the same location relative to the objective lens for both first and second positions of the field lens and wherein the field lens passes through the focal plane in moving between the first and second positions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a folded side view of apparatus including a forward viewing endoscope;

FIG. 2 is an end view of part of the apparatus of FIG. 1;

FIG. 3 is a schematic view of an alternative apparatus including a side viewing endoscope;

FIG. 4 is a detail of the tip of the endoscope of FIG. 3;

FIG. 5 is a perspective view of the internal components of a scanning head of the apparatus of FIG. 3;

FIG. 6 is a schematic diagram of a mirror system of the scanning head of FIG. 5;

FIG. 7 is a diagram showing a zoom lens of the endoscope of FIG. 3;

FIG. 8 is a diagram of the zoom lens of FIG. 7 in its ranging configuration;

FIG. 9 is a schematic diagram of a control apparatus of the apparatus of FIG. 3;

FIG. 10 is a perspective view of the proximal end of a forward viewing endoscope in a modified apparatus;

FIG. 11 is a schematic plan of the proximal end of the apparatus of FIG. 10;

FIG. 12 is a schematic view of a conventional zoom lens incorporated in a modification to the apparatus of FIG. 3;

FIG. 13 is a schematic view of the zoom lens of FIG. 12 in its ranging configuration;

FIG. 14 is a schematic view of a further zoom lens with field curvature correction;

FIG. 15 is a schematic view of the zoom lens of FIG. 14 in its ranging configuration;

FIG. 16 is a schematic plan of a proximal end of a modification to the apparatus of FIG. 3 in which the zoom lens is replaced by a fixed lens system and optical switching means;

FIG. 17 is a schematic view of an otherwise conventional endoscope modified to include a zoom lens;

FIG. 18 is a schematic diagram of a laser scanning camera; and

FIG. 19 is a perspective view of the scanning unit of the camera of FIG. 18 .

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An apparatus including optical apparatus for use as an endoscope or borescope illustrating a preferred embodiment of several aspects of the invention will now be described by way of example only and with reference to the accompanying drawings. Reference to the term "endoscope" should be understood to encompass the same apparatus used in non-medical uses as a borescope.

Referring to FIGS. 1 and 2 there is shown in diagrammatic sketch form an apparatus comprising an endoscope 10.

In FIG. 1 the length of endoscope 10 is chosen for the particular purpose required. The endoscope 10 comprises a rigid hollow tube 11 of 10 mm diameter containing an optical relay comprising a series of relay lenses 12 spaced along the tube so that the image plane of one coincides with the object plane of the next lens in the series.

Surrounding the optical relay is a longitudinally extending fibre optic bundle 13 of annular cross-section.

The endoscope 10 has a distal end 36 presented to an object 28 to be viewed and a proximal end 35 at which there is provided a laser scanning head 39 including a laser 14. A laser beam 15 passes from the laser 14 to a first scanning mirror 16 which is pivoted for scanning through a small angle about an axis X passing through the point at which the laser beam 15 strikes the mirror 16 such that the beam 15 is at 45.degree. to the mean position of the mirror. The beam reflected from the scanning mirror 16 passes to a second scanning mirror 18 via a lens 17, which mirror 18 is pivoted for scanning through a small angle about an axis Y at right angles to the axis X such that the beam is at 45.degree. to the mean position of the mirror.

The beam 15 reflected from the second scanning mirror 18 passes to a collecting lens group 19 comprising two spaced apart lenses directing the beam into the proximal end 35 of the endoscope 10 where it enters an input lens 21 of the optical relay 12.

The lens 17 is arranged so as to bring the laser beam 15 into focus in an image plane 22 located between the two lenses of the lens group 19 so that any dust on the lens surfaces does not interfere with the image. This image plane 22 is arranged to be the object plane of the first lens 21 of the optical relay lenses 12.

An output lens assembly 23 is provided at the distal end 36 of the endoscope 10 and has a projected image plane 34 which is illustrated in FIG. 1 as being coincident with the object 28.

A light collecting input surface 26 of the fibre optic bundle 1 surrounds the output lens assembly 23 and faces the object 28 so that light reflected from the object is collected at the surface and passes into the fibre optic bundle. The collected light is conveyed along the fibre optic bundle 13 to a photomultiplier 27 connected by a fibre optic link 37 to the proximal end 35 of the endoscope 10.

An interference filter 93 transmitting light only at the laser frequency is placed in front of the photomultiplier 27 to remove effects of ambient light.

The output signal from the photomultiplier 27 is passed to an electronic apparatus 29 which is synchronised with the scanning mirrors 16,18 to form a television image viewed on a television monitor screen 33.

In addition to the described components there is provided in the scanning head 39 a pair of lenses 31,32 which may selectively be moved into and out of the path of laser beam 15 between the first scanning mirror 16 and lens 17, this movement and their exact positions along the optical axis being controlled by a motor system 30. In FIG. 1 the lenses 31,32 are shown in broken lines at their positions when moved into the path of the laser beam 15 and are shown in unbroken lines at their normally retracted positions in which they are clear of the path of the laser beam 15.

The lenses 31,32 consist of a diverging lens 31 followed by a converging lens 32 which when introduced into the optical path have the effect of broadening the beam such that on emerging from the distal end 36 a projected laser beam 38 fills the aperture defined by the output lens 23. When retracted from the optical path the effective aperture of the apparatus 1 is reduced or in other words the beam 38 remains relatively narrow and is projected from the output lens 23 with a reduced convergence angle. Axial movement of the lenses 31,32 provides movement of the focal plane 34 at which the projected beam 38 is focused.

In use of the apparatus 1 the laser beam 15 is scanned horizontally and vertically across the image plane 22 by means of the first and second scanning mirrors 16,18 respectively in a raster form. Thus, a spot image of the beam 15 formed in the image plane 22 moves horizontally line by line and slowly vertically downwards as in a television picture. The limits of the range of movement of the beam 15 are generally illustrated in FIG. 1, but it will be understood that at any one time the beam 15 is deflected to a single point in the image plane 22. The terms vertical and horizontal used with reference to the raster scan are relative and do not necessarily comply with those particular directions in space.

The raster scan of the beam 15 is transmitted from the image plane 22, which forms the object plane of the first lens 21 of the optical relay 12, and hence is passed into the endoscope 10 and is transmitted through the endoscope to the object plane 24 of the output lens assembly 23. The projected laser beam 38 is then projected from the endoscope through the output lens assembly 23 so as to scan across the object 28 to be viewed in raster fashion (the beam at this point in FIG. 1 being shown in solid lines).

Thus, as the laser beam 38 scans across the object 28 reflected and scattered light is collected by the input surface 26 of the fibre optic bundle 13. The collected light passes down the fibre optic bundle 13 and is detected by the photomultiplier 27.

The photomultiplier 27 at any one time produces a single signal dependent upon the total amount of light entering the input surface 26.

The output signal from the photomultiplier 27 is passed to the electronic apparatus 29 and a corresponding image point is generated on the screen 33 which scans at the same rate and in the same way as the beam 38 is scanned across the object 28. Thus a composite image of that object 28 will be produced on the television screen 33 in each completed frame of the scan.

There are a number of advantages of producing a composite image in this manner. By the use of a laser beam, the light flux illuminating each element of the object under examination can be high. All of the light is concentrated on the spot, rather than generally illuminating the object under examination.

Because of the small convergence angle of the projected beam during imaging it is possible to have great depth of focus which reduces the aberration effects.

Similarly, the use of a laser enables the detected signal to have an inherently high signal to noise ratio. A monochromatic laser source can be matched to the coatings on the lenses so as to reduce to a very small level the reflection of the beam from the surface of the lenses and, indeed, the lenses themselves can be matched to the particular wavelength of the laser beam.

Furthermore, a clear high contrast high resolution image can be provided with relatively low input of total energy to the object under examination which is a considerable advantage in some applications, for example medical applications. Other applications include imaging of hazardous environments where high energy illumination must be avoided because of the flammability of vapours for example.

The apparatus 1 may also be used to measure and display information as to the range and size of objects imaged on the screen 33. This is particularly useful when the user is unfamiliar with the object being viewed so that it is difficult to gauge an impression of size from the television image in which a small object at a short range could appear to have the same size as a large object at a greater range.

The apparatus 1 is used to measure range of a viewed object 28 in its range finding mode in which lenses 31 and 32 are introduced into the path of laser beam 15. The range finding procedure requires that the focus distance D between the focal plane 34 and the distal end 36 be varied until an analysis of the output of photomultiplier 27 indicates that the object 28 is in focus i.e. that the projected laser beam 38 is focused on to a surface of the object 28. Movement of the focal plane 34 is accomplished by axial movement of the lenses 31 and 32. Analysis of the output of the photomultiplier 27 to determine the in-focus condition relies upon the phenomenon of laser speckle which is the random intensity distribution exhibited in light observed in non-specular reflection of a laser beam and results from constructive and destructive interference of coherent waveforms reflected from surfaces which are rough on the scale of the wavelength used. It is observed that the speckle pattern which appears on the reflecting surface changes as the beam is brought in and out of focus. Both the spacial frequency and amplitude of intensity modulation are seen to vary, the amplitude of modulation being at a maximum when the beam is in focus.

Speckle is detected in the output of the photomultiplier 27 as a strong AC modulation during a horizontal scan which is readily distinguishable from the less noisy DC signal resulting from a scan of the same object when the laser beam is out of focus. The beam is taken to be focused on the object when the amplitude of modulation is at a maximum. The frequency of modulation depends upon the rate at which the projected beam traverses the object surface, the distance across the reflecting surface traversed in one cycle of modulation being of the same order of magnitude as the cross-section of the projected beam 38 measured at the output lens assembly 23.

Effectiveness of the range finding mode is enhanced by the increased effective aperture resulting from insertion of the lenses 31 and 32 since this results in a marked decrease in the depth of focus of the projected beam 38.

The projected beam 38 in ranging mode of the apparatus 1 is shown in broken lines in FIG. 1 in which it is focused on to the object 28.

The distance D corresponding to the position of lenses 31 and 32 is then computed by the electronic apparatus 29 and displayed on screen 33 as a measurement of range R between the object 28 and the distal end 36. It is then possible to measure the width of a particular object at that distance by measuring the width of the image of the object on the screen 33 and multiplying this width by a conversion factor based on calibration measurements.

During ranging the pivotal movement of the scanning mirror 16 is stopped. In addition, because it is necessary to range over only a small part of the object, (since otherwise a variable range will be provided) the scanning movement of the mirror 18 is restricted.

The beam used during the ranging operation is illustrated by broken lines in FIG. 1 and as can be seen it is considerably wider at the lens assembly 23 than the beam represented in unbroken lines used during normal imaging.

It has been found optimum to arrange that the overall cross-section of the input surface 26 of the optical fibres 12 is the same as the cross-sectional area of the projected beam 38 at the output lens assembly 23 during the ranging operation and under these circumstances it has been found that as the focal plane 34 is moved back and forth (by relative movement of lenses 31 and 32) the modulation due to speckle in the signal detected by the photomultiplier 27 is at a maximum when the plane 34 coincides with the surface of the object 28.

As a result, this provides an automatic method of ranging in which movement of the lenses 31,32 to move the plane 34 is controlled by the motor system 30, the operation of which is under control of the electronic apparatus 29. Thus, in use, the electronic apparatus 29 causes the lenses 31,32 to move the plane 34 until the modulation of the signal detected by the photodetector 27 is a maximum. At that point the electronic apparatus 29 is able to determine the range R, that is the distance between the distal end 36 of the endoscope 10 and the object 28, by reference to stored data of focus distance D as a function of the position of lenses 31 and 32 from previous calibration experiments. When switched back to viewing mode the screen 33 may then carry, in addition to an image of the object 28, a scale from which the size of the object 28 may be directly measured.

To prevent mismeasurement, the arrangement may be such that when the endoscope 10 is moved the scale on the screen 33 disappears, since at that point a new ranging will be required.

The invention is not restricted to the details of the foregoing example. In particular in relation to the latter inventive concept, it would clearly be useful to regularly range and view. Thus, it may be provided that the lenses 31,32 are regularly moved into the beam path. Alternatively the beam path itself may be split so as to pass, in one parallel beam path through the lenses 31,32 and in the other not through those lenses, optical switching means being provided to switch back and forth between these two paths whereby the ranging can be regularly carried out. Hence the scale can be shown on the screen 33 continuously although it will vary continuously as the endoscope 10 is moved with respect to the object 28.

The fibre optic bundle 13 is shown in FIG. 1 as being annular but in an alternative arrangement the bundle may be of generally circular cross-section.

The ranging and hence scaling of the image has been described with respect to an endoscope 10. Clearly, however, this aspect of the invention may be applicable to other optical systems such as microscopes, telescopes and other scanning systems.

A further embodiment of the invention is shown in FIG. 3 in which corresponding reference numerals are used to those of FIGS. 1 and 2 where appropriate for corresponding elements.

The alternative apparatus 50 of FIG. 3 comprises a rigid endoscope tube 11 having a distal end 36 which is insertable into inaccessible areas for imaging purposes and is shown being directed towards an object 28.

The endoscope tube 11 has a proximal end 35 connected to a laser scanning head 39. A flexible single mode fibre optic link 51 connects a laser 14 to the laser scanning head 39, the link 51 comprising a single optical fibre which retains the coherence characteristics of the light carried.

As shown in FIG. 4 the endoscope tube contains an optical relay 12 for conveying a raster scanned laser beam from the laser scanning head 39 to a zoom lens assembly 52. A right angle prism 81 is arranged to deflect light emerging from the zoom lens assembly 52 so as to exit through a window 56 provided in a side wall 55 of the tube 11 as a projected laser beam 38.

The prism 81 is provided with a prism rotating mechanism 87 capable of tilting the prism about a vertical axis to produce horizontal deflection of the field scanned by the projected beam 38. Tilting of the prism 81 is actuated by a control wire (not shown) connected to an actuator (not shown) in the scanning head 39. Vertical deflection of the field scanned by the projected beam 38 is provided by rotation of the tube 11 about its longitudinal axis by means of a tube rotating mechanism (not shown). As described later, the prism rotating mechanism 87 and tube rotating mechanism provide means for varying the direction of the optical axis at the window 56 when the projected beam 38 is to be directed onto a selected area of the object which would otherwise be located outside of the reduced field angle available in ranging mode.

A fibre optic bundle 53 extends longitudinally through the tube 11 and terminates in a circular light collecting surface 54 forming part of the side wall 55.

The light collecting surface 54 is positioned side-by-side relative to the window 56 so that the apparatus 50 constitutes a side viewing endoscope.

A control wire 57 extends through the tube 11 and connects the zoom lens assembly 52 with a linear drive mechanism 91 shown in FIG. 5 in the laser scanning head 39.

A fibre optic link 37 connects the scanning head 39 to a photomultiplier 27, the connection being such that substantially all of the light collected at the surface 54 is conducted to the photomultiplier. An interference filter 93 transmitting light only at the laser frequency is positioned in front of the photomultiplier 27 to remove the effects of ambient light. An output signal from the photomultiplier 27 is input to an electronic apparatus 29 which in turn produces a signal driving a television monitor 58 having a screen 33.

The construction of the laser scanning head 39 is illustrated in FIG. 5 and comprises a housing 59 to which is rigidly connected the endoscope tube 11. The fibre optic link 37 is shown projecting from the proximal end 35 of the tube 11.

The single mode fibre optic link 51 carrying the laser beam from laser 14 enters the housing 59 and has a terminal 60 which incorporates a converging lens (not shown). A laser beam 15 emerges from the terminal 60 and is represented in the drawing by a single line extending along the optical axis of the scanning head 39 whereas in practice the beam will have finite width and convergence.

The beam 15 is directed on to a plane mirror 61 which is provided to fold the light path and direct the beam into a mirror system 62 represented schematically as a cube in the drawing. The mirror system 62 is shown in FIG. 6 and consists of two orthogonal mirrors 63 and 64 which are fixed relative to each other and mounted at the end of a radius arm 65 providing rotation about a pivot 66. A motor drive and transducer responsive to angular position (not shown) are provided by a range scanning unit 114 (see FIG. 5) for controlling the angular position of the mirrors 63 and 64. The effect of shifting position of the mirrors by pivotal motion is to increase or decrease the path length travelled by the converging laser beam 15. Consequently the mirror system 62 provides a means of shifting along the optical axis an image position 67 at which laser beam 15 is focused by the lens of the terminal 60. This in turn enables the focus distance D at which the projected beam 38 is focused relative to the window 56 to be dynamically varied.

The laser beam 15 on leaving the mirror system 62 passes through a converging lens 68 to a scanning mirror 69 arranged to produce scanning motion in the horizontal direction in the projected beam 38 emerging from the end window 56. The scanning mirror 69 is driven by a galvanometer type scanning unit 70 capable of positioning the mirror in any required position in a randomly addressable mode. This facility is required during ranging operation as described later.

The beam is then directed by a lens pair 71 to a further scanning mirror 16 driven by a further scanning unit 72 of a resonant galvanometer type producing horizontal line scan in the projected beam 38 a horizontal scanning frequency of 8 kHz. A cable 76 (represented in FIG. 3) connects the scanning head 39 to the electronic apparatus 29 and carries control signals.

From the scanning mirror 16 the beam 15 is relayed by a lens pair 73 to a final scanning mirror 18 driven by a final scanning unit 75 of a galvanometer type mechanism providing accurate control of the mirror position in a randomly addressable manner. The final scanning mirror 18 is arranged to produce vertical scanning of the projected beam 38.

A further lens pair 19 directs the scanned beam 15 into the optical relay 12 so that the beam is directed through the endoscope tube 11 into the zoom lens assembly 52.

The zoom lens assembly 52 is illustrated in detail in FIGS. 7 and 8. The lens assembly 52 is referred to as a "zoom" lens assembly to convey the meaning that the lens assembly has a variable focal length. In the present context it is intended that the focus need not be continuously variable. The present requirement is that the lens assembly should provide two possible configurations giving different focal lengths and hence two possible values of angular magnification, for each of which configurations the optics are well corrected in terms of aberrations. The zoom lens assembly 52 in its normal configuration when the projected beam 38 is being scanned for imaging purposes provides a field angle of 50.degree. over which the beam is scanned both horizontally and vertically. This configuration will be referred to as the imaging configuration of the zoom lens assembly 52.

The zoom lens assembly 52 has a second configuration which will be referred to as the ranging configuration in which the field angle is reduced to 10.degree.. In the imaging configuration the convergence angle of the projected beam 38 is relatively small as shown in solid lines in FIG. 4 whereas in the ranging configuration the projected beam 38 fills the aperture defined by the end window 56 and has a greater convergence angle and consequently provides a reduced depth of field during ranging.

The zoom lens 52 is shown in its imaging configuration in FIG. 7 and in its ranging configuration in FIG. 8. The zoom lens 52 comprises a fixed lens 77 of positive power and comprising compound lens elements 78 and 79. An axially movable lens 80 is located between the fixed lens 77 and the optical relay 12 so that the laser beam 15 passes from the optical relay, through the movable lens and then through the fixed lens 77. A right angle prism 81 deflects the beam 15 emerging from the fixed lens 77 through 90.degree., the light rays drawn to the right of line A--A should therefore be imagined as travelling in a plane orthogonal to the page.

The movable lens 80 and the fixed lens 77 constitute a field lens and objective lens respectively of the zoom lens.

The movable lens 80 is formed of BK7 glass and comprises a convex single thick element of 9.56 mm thickness having the same magnitude of curvature (0.17357 mm-1) on both its front and rear optical faces 82 and 83 respectively.

A fixed lens 77 comprises (from left to right in FIGS. 7 and 8) an element of SK10 glass of 1.5 mm thickness and curvature 0.11717 and -0.10386; an air gap of 0.10: an element of SK10 glass of thickness 1.50 and curvature 0.16440 and -0.00530; a further air gap of 0.10; an element of SK10 glass with thickness 4.0 and curvature -0.03633 and 0.20974; and an air gap of 1.0 following by the prism 81 (all dimensions of separations being in mm and curvature in mm-1)

The beam 15 is represented by groups of pencil rays 84 and 85 which correspond respectively to a beam projected along the optical axis of the apparatus 50 and a beam which is deflected by the scanning head 39 so as to form a projected beam 38 which is vertically deflected to the maximum limit available.

The optical relay 12 projects a real image at an image plane 86 which in the imaging configuration of the zoom lens 52 coincides with the front face 82 of the movable lens 80 which is also arranged to coincide with the focal plane of the zoom lens 52 i.e. the combination of the movable lens 80 and fixed lens 77.

In this configuration a wide field angle is provided by the zoom lens 52 so that the vertical and horizontal scan in each case covers 50.degree..

In FIG. 8 the zoom lens 52 is shown in its ranging configuration in which the moving lens 80 has been moved away from the fixed lens 77 so as to pass through the image plane 86 projected by the optical relay 12 to a new position in which the real image plane 86a coincides with the rear face 83 of the movable lens 80.

The image plane 86 is translated by the presence of the movable lens 80 in a direction towards the optical relay 12 but in its new position 86a as shown in FIG. 8 is coincident with the focal plane of the fixed lens 77.

In FIG. 8 three pencil rays 84 represent the laser beam projected by the scanning head 39 along the optical axis of the apparatus 50 and p