Abstract:
The invention provides a confocal microscope or endoscope, having a source of coherent light for illuminating a sample, and an imaging optical fibre bundle ( 82 ) for receiving return light, whereby the fibre bundle ( 82 ) provides a return channel for fluorescent return light ( 78 ). The optical fibre bundle ( 82 ) preferably preserves, between entry and exit ends of the bundle, the relative spatial coordinates of the cores of individual fibres constituting the bundle.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a laser scanning confocal microscope with fibre bundle return, for—in particular improving the optical performance of laser scanning confocal microscopes. The invention may also have application in the miniaturisation of confocal endomicroscopes, in devices such as well plate readers, DNA chip scanners, in remote spectroscopy and as an optical system for a laser scanning ophthalmoscope.  
       BACKGROUND OF THE INVENTION  
       [0002]     Existing two fibre confocal microscopes and endoscopes typically require either precise fibre positioning and alignment in the probe head to provide separation of the light path. Existing systems also commonly require an extended beam path or further beam compressor optics in the microscope or endoscope head to give variable pinhole capability.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention provides a confocal microscope or endoscope, having: 
        a source of coherent light for illuminating a sample; and     an imaging optical fibre bundle for receiving return light;     whereby said fibre bundle provides a return channel for fluorescent return light.        
 
         [0007]     Thus, a return channel is provided that can be made optically independent and isolated from the laser delivery fibre fluorescence (and therefore decrease optical noise). The optical fibre bundle, being intended for imaging purposes, preserves—in a comparison of the entry and relative spatial coordinates of the cores of said individual fibres.  
         [0008]     Thus, a return channel is provided that can be made optically independent and isolated from the laser delivery fibre fluorescence (and therefore decrease optical noise). The optical fibre bundle, being intended for imaging purposes, preserves—in a comparison of the entry and exit ends of the bundle—the relative spatial coordinates of the cores of the individual fibres. Within this constraint, however, it is acceptable to transform these coordinates between the ends provided an image can still be formed. Thus, for example, the coordinates could be reversed so that a mirror image is formed. Other transformations, as will be apparent to those skilled in this art, are also possible. This constraint (i.e. that an image can be formed) means that the bundle might be termed ‘coherent’ in the sense that the fibre bundle maintains image orientation; this should not be confused with the coherence of the light from the light source, which refers to the maintenance of light propagation properties within the illuminating fibre.  
         [0009]     Further, this condition that the fibre bundle maintains image orientation only relates to the two ends of the fibre bundle. Thus, while a fused fibre bundle may be employed in some applications, in other applications the fibres may not be fused between the ends of the bundle, as long as the ends preserve a sufficient ‘coherence’ (i.e. image orientation maintenance) for the bundle to function as an imaging bundle. This latter type of bundle also has the advantage of greater flexibility over its length.  
         [0010]     It will also be understood that the term “confocal” is employed—as is understood in the art—to include conjugate focal point geometries that may not be purely confocal owing to the finite size of apertures, spatial filters, etc., and the occasional desirability of increasing the amount of detected light even if this means a minor loss of resolution. Such arrangements are nevertheless referred to as confocal (and are embraced by that term herein), as the collected light includes what might be termed ‘pure’ confocal return light.  
         [0011]     Preferably the microscope or endoscope further comprises a single mode fibre for transmitting said coherent light from said source and having an exit end mounted in a fixed spatial relationship to said entry end of said fibre bundle.  
         [0012]     The microscope or endoscope may be embodied as an ophthalmoscope, colonoscope or other optical instrument.  
         [0013]     Preferably the beam-splitter comprises a simple or compound prism. Alternatively, the beam-splitter could comprise a transmission or reflection diffraction grating.  
         [0014]     Preferably said microscope or endoscope includes a further beam-splitter, optically reversed relative to said beam-splitter and located optically after said fibre bundle, to improve focal plane isolation.  
         [0015]     Thus, in one embodiment, said beam-splitter comprises a simple or compound prism, and said microscope or endoscope includes a further prism, optically reversed relative to said prism and located optically after said fibre bundle, to improve focal plane isolation.  
         [0016]     Preferably said microscope or endoscope includes a spatial filter optically after said fibre bundle. More preferably said spatial filter comprises a variable aperture (such as a pinhole).  
         [0017]     Preferably said microscope or endoscope includes a scanner for providing scanning of said illumination volume relative to said sample.  
         [0018]     In one embodiment said scanner comprises a mirror, in another said scanner comprises a tuning fork. In one embodiment, said scanner comprises a pivotably mounted member provided with collimating optics for collimating said coherent light.  
         [0019]     Preferably said collimating optics comprises a simple or compound lens.  
         [0020]     Preferably said pivotably mounted member is mounted by means of, and is pivotable about, an axle. Alternatively, said pivotably mounted member is mounted by means of a pair of flexible supports that differ (preferably in length) so that said pivotably mounted member can be pivotted by being oscillated.  
         [0021]     Preferably said microscope or endoscope includes one or more shallow angle prisms located in an image plane to separate out different spectral bands, and a plurality of fibre bundles, each for receiving a respective spectral band, for producing multiple colour images. Preferably said microscope or endoscope includes a plurality of separate photo-detectors, each for detecting a respective spectral band transmitted by a respective fibre bundle. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0022]     In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0023]      FIG. 1  is a schematic view of an optical arrangement in which a prism is used as a beam-splitter (though not embodying the present invention);  
         [0024]      FIG. 2  is a schematic view of a laser scanning confocal microscope with fibre bundle return according to one embodiment of the present invention;  
         [0025]      FIG. 3A  is a schematic view of the exit end of the beam delivery fibre and entry end of the fibre bundle of the confocal microscope of  FIG. 2 ;  
         [0026]      FIG. 3B  is a schematic view of the exit end of the fibre bundle of the confocal microscope of  FIG. 2 ;  
         [0027]      FIG. 3C  is a schematic view of the exit end of the beam delivery fibre imaged between the jaws of the slit of the confocal microscope of  FIG. 2 ;  
         [0028]      FIG. 3D  is a schematic view of a variation of the arrangement shown in  FIG. 3A ;  
         [0029]      FIG. 4A  is a schematic view of a laser scanning confocal microscope with fibre bundle return according to a further embodiment of the present invention;  
         [0030]      FIG. 4B  is an enlargement of the final light path region (i.e. the Detector Unit) of the laser scanning confocal microscope of  FIG. 4A ;  
         [0031]      FIG. 4C  is a simplified view of a modification of the embodiment of  FIG. 4A , in which scanning is performed by means of a tuning fork and the fibre bundle is positioned to be scanned synchronously with the laser delivery fibre;  
         [0032]      FIG. 4D  is a simplified view of a modification of the embodiment of  FIG. 4A , in which shallow angle prisms are located in the image plane to separate out different spectral bands and direct them to separate photo-detectors to produce multiple colour images;  
         [0033]      FIG. 4E  is a simplified view of a modification of the embodiment of  FIG. 4A , in which the sample is in the form of a translatable gene chip;  
         [0034]      FIG. 4F  is a simplified view of a modification of the embodiment of  FIG. 4A , in which the sample is in the form of a translatable gene chip and the microscope includes a line of fibre bundles;  
         [0035]      FIG. 5  is a schematic view of a laser scanning confocal microscope with fibre bundle return according to a still further embodiment of the present invention, in which scanning is performed at the proximal end of the fibre bundle;  
         [0036]      FIG. 6  is a view of a variation of the microscope of  FIG. 5 ;  
         [0037]      FIG. 7  is a schematic view of a laser scanning confocal microscope with fibre bundle return according to a still further embodiment of the present invention, in which a laser diode chip is mounted beside the tip of the fibre bundle;  
         [0038]      FIG. 8  is a schematic view of a scanning system for use with various embodiments of the present invention;  
         [0039]      FIG. 9  is a schematic view of a further scanning system for use with various embodiments of the present invention; and  
         [0040]      FIG. 10  is a photograph of the exit end of a fibre optic bundle for use with the laser scanning confocal microscope of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0041]      FIG. 1  illustrates how a prism can be used as a beam-splitter in an optical system suitable for a laser scanning confocal microscope in fluorescent imaging mode, though without—in this figure—embodying the present invention. A laser source  2  provides a laser beam  4 , which is focussed by a lens  6  to a Gaussian waist  8  adjacent to the edge of a plane mirror  10 . The light then diverges from this focus  8  until it meets a collimating lens  12 , which collimates the beam  14  and projects it onto the face  16  of a glass prism  18 .  
         [0042]     The laser beam is TEMoo and monochromatic, and hence emerges from the prism  18  as a beam  20  with an unchanged parallel set of wave fronts. The beam passes through an XY scanner  22  and is focussed by a focussing lens  24  as diffraction limited spot  26  within the tissue sample  28 . Fluorescence generated by the laser light in the tissue is Stokes shifted to longer wavelength than the wavelength of the excitation light from the laser. Fluorescent light from the focussed spot region returning through the focussing lens  24  retraces the same general set of ray paths  20  through the XY scanner  22  until it reaches the prism  18 .  
         [0043]     Longer wavelengths are less refrangible in a normal dispersion regime than is laser light, so the longer wavelengths of the return light emerge from the prism  18  as a beam  30  at an angle with respect to the incoming laser beam  14 . The beam  30  is converged by collimating lens  12  and the rays indicated by  32  and  34  are reflected by the mirror  10  to a focus  36 . Combined with fluorescence of longer wavelengths, the light is deflected by mirror  10  and focuses to form a linear spectrum  38   a  to  38   b . All these beams of the full range of fluorescent wavelengths pass to a photomultiplier tube  40 , which generates an electrical signal.  
         [0044]     A pair of knife edges (not shown) is located at the focus just above and just below the plane of the figure at the focus  36 . These define a slit through which the spectral line passes. Light from out of focus planes will largely be blocked by the jaws of the slit allowing for the isolation of the focal plane. The width of the split can be adjusted to control the degree of focal plane isolation.  
         [0045]     The optical configuration shown in  FIG. 1 , however, does not offer very much of an advantage in the miniaturisation process. The slit mechanism, and the photomultiplier tube are large and both need to be located in the head to implement this design.  
         [0000]     Prism Beam-Splitter with Fibre Return:  
         [0046]     A major advantage is obtained by transferring the image of the slit plane out of the head and into the detector unit. This could be done using a flexible optic transfer element of small cross sectional area. A suitable component for doing this would be a “coherent” (i.e. an image orientation maintaining fibre bundle) optic fibre bundle or imaging bundle.  
         [0047]      FIG. 2  is a schematic view of a laser scanning confocal microscope with fibre bundle return according to one embodiment of the present invention, using a simple equilateral prism as a beam-splitter and a “coherent” (i.e. image orientation maintaining) optic fibre bundle. The Stokes shifted fluorescence returning from the focussed laser spot is moved by the prism laterally to fall as a line on the polished tip of a fibre bundle. This fibre bundle carries the fluorescent light back to the detector unit. This arrangement would facilitate factory adjustment and on site alignments would be considerably easier.  
         [0048]     The fluorescence, being broadband, is spread out over a large number of return fibres, which eliminates the need for XY positioning of the optical return transfer element in the head to enable the line to impinge on the photomultiplier tube. It may be seen from  FIG. 2  that any final adjustment of the system could be carried out in the detector unit. It is also clear that slit positioning precision and adjustment tolerances in the detector unit can be relaxed because the light selection is being carried out on a highly magnified image of the bundle tip in the detector unit.  
         [0049]     In  FIG. 2 a  laser  42  provides a beam of TEMoo light  44 , which is focussed by focussing lens  46  to the tip  48  of a single mode optic fibre  50 . The light travels along the fibre core and emerges from the fibre tip  52  as a divergent beam  54 . The light is collimated by collimating lens  56  to form a beam  58  that passes though a glass prism  60  from which it emerges as beam  62 . Passing through the prism does not change beam, as the light is monochromatic. The beam  62  is then deflected as an acquisition raster by the XY scanner  64 . The scanned beam  66  is then focussed by focussing lens  68  through tissue sample  70  to a focussed spot  72 . The reflected and fluorescent light from this spot retraces the same general set of ray paths back through focussing lens  68 , is descanned by the XY scanner  64  and travels along beam path  62  to prism  60 .  
         [0050]     Light that has been reflected or scattered from the focal spot  72  (being of unchanged wavelength) travels back along ray path  58  through collimating lens  56  and focuses back into the optic fibre  50 . This light is split off by a fused biconical taper coupler  74  to a photomultiplier tube  76  to allow the formation of a reflection image. Light generated by fluorescence at the focal volume also retraces the same general set of ray paths as the reflected light until it reaches the prism. Fluorescence is always of a longer wavelength than the excitation wavelength. Hence the fluorescent light is deflected by a smaller angle on passing through the prism  60  than the reflected excitation light and forms a beam  78 .  
         [0051]     The beam  78  passes through collimating lens  56  and is focussed onto the polished end  80  of a ‘coherent’ (i.e. an image orientation maintaining) fibre optic bundle  82 . The fluorescence is broad banded, that is, it consists of a range of wavelengths. Hence it is actually focussed as a spectral line on the end  80  of the bundle  82 , which is transmitted by the ‘coherent’ array of fibres constituting the bundle  82  to an image  84  on the exit end  86  of the bundle  82 . A lens  88  projects a magnified image  90  in space of the line  84  and the fibre bundle  82 . The light forming this image  90  continues on to impinge on the photomultiplier tube  92  and generates an electrical signal, which forms the image. A pair of adjustable jaws  94  and  96  are provided form a slit in width to allow a selected fraction of the near confocal fluorescent light to pass to the photomultiplier tube  92 , so that the depth of field isolation can be controlled.  
         [0052]     The last stages of the optics (carrying the light from fibre tip  86  to the photomultiplier tube  92 ) are enclosed in a light tight box  98 , thereby forming a module comprising the light tight box  98  and its contents. This enclosed module of the system is referred to below as the Detector Unit.  
         [0053]      FIG. 3A  shows a view of the tip  52  of the laser delivery fibre  50  and the tips  100  of the fibre bundle  82  in the microscope head. The laser delivery fibre  50  has a core  102 . The fibre bundle  82  has cores packed together each one of which can accept fluorescent light returning from the specimen. The confocal fluorescent return light forms a line  103  on the bundle end. If blue excitation light is used, the green end of the fluorescent spectrum will fall at  104  while the red end of the fluorescent spectrum will fall at  106 .  FIG. 3B  shows the exit end  86  of the bundle  82 , with the light emerging from the spectral line  84 .  FIG. 3C  shows the projected image of the fibre bundle  82  with the spectral line  90  between the two jaws  94  and  96  of an adjustable slit.  
         [0054]      FIG. 3D  depicts a variation of the arrangement of  FIG. 3A , and shows the end of the fibres in the microscope head. The confocal collection fibres  100  of the bundle  82  again collect light along the spectral line  103 . However, in this variation the delivery fibre  50  has one side  108  ground flat towards the core  102 . This brings the point from which light is emitted (i.e. the core  102 ) closer to the cores of the fibre bundle  82 , which minimizes “spot wander” (defined below) with fibre tip scanning. It also allows imaging to be carried using fluorescence, which is closer in wavelength to the excitation wavelength. In this embodiment, the shape of the bundle could also be altered so that the core  102  of fibre  50  is still closer—or even within—the bundle  82 , reducing or eliminating the amount of divergence that is required to be provided by prism  60 .  
         [0055]     A weakness of this first embodiment is that the isolation of the focal plane is not as great as the confocal isolation obtained with fibre or bulk optics pinholes. It would in fact be quantitatively the same as that of a slit scanning confocal system (inverse d) rather than the higher degree of confocal isolation obtained with a pinhole confocal return (inverse d 2 ).  
         [0056]     This can be addressed as shown in the embodiment of  FIG. 4A  by means of a second (identical) prism placed in the beam path in the Detector Unit. Referring to  FIG. 4A , the second prism is orientated so that its effect is in opposition to the effect of the prism (cf. prism  60  in  FIG. 2 ) in the microscope head. The principle of reciprocity in optical ray diagrams will result in the light from the focussed spectral line on the fibre bundle tip being re-focussed to a spot which passes through a variable iris diaphragm placed in front of the photomultiplier tube. Light from the out of focus fluorescence incident on the fibre bundle and returning to the detector unit will be focussed by the detector unit lens and prism as a spot surrounded by near confocal return light. This will have a radially symmetrical light distribution, which is close to spatially identical with a normal confocal pinhole return.  
         [0057]     Thus, in  FIG. 4A  laser  110  provides a beam of TEMoo light  112 , which is focussed by focussing lens  114  to the tip  116  of a single mode optic fibre  118 . The light travels along the fibre core and emerges from exit end  120  of fibre  118  as a divergent beam  122 . It is collimated by collimating lens  124  to form a beam  126  that traverses through a glass prism  128  as a beam  130 . As mentioned above, passing through the prism does not change the beam as the light is monochromatic. The beam  130  is then deflected as an acquisition raster by the XY scanner  132 . The scanned beam  134  is then focussed by focussing lens  136  through tissue sample  138  to a focussed spot  140 . The reflected and fluorescent light from this spot retraces the same general set of ray paths back through focussing lens  136  and is descanned by the XY scanner  132  and travels along beam path  130  to prism  128 . Light which has been reflected or scattered from the focal spot  140 , being of unchanged wavelength, travels back along ray path  126  through collimating lens  124  and is focussed back into the optic fibre  118 . This light can be split off by a fused biconical taper coupler  142  to a photomultiplier tube  144  to form a reflection image.  
         [0058]     Light generated by fluorescence at the focal volume also retraces the same general set of ray paths as the reflected light until it reaches the prism  128 . Fluorescence is always of a longer wavelength than the excitation wavelength, so the fluorescent light is deflected by a smaller angle on passing through the prism than the reflected excitation light and it forms a beam  146 . This beam  146  passes through collimating lens  124  and is focussed onto the polished end  148  of a ‘coherent’ (i.e. image orientation maintaining) fibre optic bundle  150 . The fluorescence is broad banded, that is, it consists of a range of wavelengths. Hence it is actually focussed as a spectral line  152  on the end of the bundle. This is transmitted by the ‘coherent’ array of fibres to form an image of the line  154  at the exit end  156  of the bundle  150 . A lens  158  is situated optically after the exit end  156  of the bundle  150 , the distance  160  between the exit end  156  and the lens  158  being the focal length of the lens  158 . Light from the green end  162  of the spectral line  154  (i.e. green fluorescence from blue light excitation) emerges from the fibre bundle and is collimated as  166 . Light from the red end  164  of the fluorescence spectral line is also collimated as a beam  168 . The green collimated beam and the red collimated beam are at an angle to one another but, when they pass through a second prism  170 , the green beam is refracted by a greater angle and both beams end up travelling parallel to one another. Thus when they enter an achromatic lens  172  they all are focussed to a single diffraction limited spot  174 , which passes through the central aperture of an iris diaphragm  176  and impinges on a photomultiplier tube  178  to produce the electrical signal.  
         [0059]      FIG. 4B  is an enlargement of the final light path region (i.e. the Detector Unit) of the system of  FIG. 4A . Out of focus return light emitted from other parts of the fibre bundle tip will also undergo deflection by the prism to a degree dependent on its colour. Thus confocal and near confocal return light will be reassigned back to a normally radially symmetrical disposition and thus the operation of the iris diaphragm can be used to vary the depth of field of the imaged plane within the specimen.  
         [0000]     Fibre Tip Scanning (Tuning Fork Design):  
         [0060]     In previous embodiments the scanning mechanism has been located so as to act after the excitation light has left the prism. This implies that the scanning mechanism would generally be an XY mirror pair or would use a specimen scanning or lens scanning arrangement. It is desirable to be able to use fibre tip scanning mechanisms, such as those in which the exit tip of the fibre is vibrated.  
         [0061]      FIG. 4C  shows how it would be possible to position the fibre bundle  150  so that it is scanned synchronously on the same vibrating assembly  180  (comprising a tuning fork) as the laser delivery fibre  118  in front of the collimating lens. This could be done with a tuning fork fast scan motion as shown in the figure, which replaces—in effect—the scanning mirror assembly  132  and permits further miniaturisation.  
         [0062]     The prism beam-splitter principle also addresses the problem of finding a way in which the confocal return fibre pinhole can be scanned in exact synchrony with the scan of the laser excitation fibre.  
         [0000]     Return ‘Spot Wander’ resulting from Prisms Aberrations:  
         [0063]     Fibre tip scanning introduces a difficulty with prism-based beam-splitter systems. Fluorescence of any particular Stokes shift, say 488 nm→520 nm, causes the confocal return spot of the 520 nm light to focus at a position which is displaced laterally from the emitting core of the laser light delivery fibre. For a two-fibre tuning fork system to work successfully it is important that the displacement of the confocal return spot is maintained at a fixed and constant displacement distance from the emitting core at the tuning fork tine. However prism (and polarisation based) compact design beam-splitter systems give varying distance of displacement if the scanning is done before the laser light enters the prism. If uncorrected this would result in a substantial vignetting of the image.  
         [0064]     There are two mechanisms responsible for this, one being exhibited for scanning motions of the emitting fibre tip in planes, which are parallel to the plane of symmetry of the prism, the other which is exhibited by scanning in the orthogonal directions.  
         [0065]     Fortunately there are a number of ways of minimising or eliminating these problems. Listed below are a number of methods that can do can this however it is possible that there may be further ways that could also work just as well.  
         [0066]     The first method is to use the fact that both mechanisms produce spot wander in the same direction i.e. in the direction of the spectral line. Hence an optical system in which the slit mechanism was used to isolate the focal plane would eliminate the effects of both aberrations.  
         [0067]     The effects can be more easily dealt with by the use of a compound prism designed so that the fluorescence emission wavelengths were all bunched up close together in the spectrum but were well separated in the spectrum from the excitation wavelength.  
         [0068]     In general both effects can be minimised (and possibly made insignificant) by designing the scanning optics so that the laser emission fibre tip is brought as close as possible to the tip of the fibre bundle, (still being coplanar with the flat polished tip). This can be achieved by polished down one side of the cladding of the laser emitting fibre (as in  FIG. 3D ) or by etching down the cladding using an ammonium bifluoride solution.  
         [0069]     The use of an Amici (direct vision spectroscope) prism will eliminate one form of spot wander.  
         [0070]     Another way of compensating would be to carry out synchronous mechanical scanning of the light beam via optical components in the Detector Unit. This, however, may introduce extra mechanical complexity in some embodiments.  
         [0071]     A number of other optical configurations specified under the systems heading “Hybrid Systems” would also reduce or eliminate these aberrations.  
         [0072]     Systems in which the scan mechanisms operate between the prism and the specimen or patient can be employed to reduce or eliminate these aberrations.  
         [0000]     Optical Design Producing Spectral Separation of Two or More Channels of Fluorescence  
         [0073]     The prism in the head produces spectral separation, which is maintained in the fibre bundle and projected as a real image of the spectrum (and the transfer fibres) in the Detector Unit. This spectral separation is desirable for dual channel (stain/counterstain) labelling. Mirrors or shallow angle prisms can be located in this image plane to separate out different spectral bands and direct them to separate photo-detectors to produce two or three colour images or even more channels if desired. A convenient way to achieve this using shallow angle prisms  182  is shown in  FIG. 4D ; this embodiment is shown with direct vision prisms but this is not essential for the operation of the device. This embodiment is more compact and has some other advantages over a sliding mirror system.  
         [0000]     Scanning in Remote Fluorescence Spectroscopy and Gene Chip Readers  
         [0074]     For applications in remote spectroscopy, plate readers or gene chip readers these considerations of prism aberration correction methods are probably immaterial as it is likely that the scanning would be carried out by motion of the entire head assembly or the specimen (see  FIG. 4E , in which a gene chip  184  is translated in x- and y-axes). Also, for these applications it may advantageous to use a specialised system using a line of fibre bundles  186  as shown in  FIG. 4F . This may have the advantage of simplicity and optical efficiency.  
         [0000]     Other Applications including Plate Scanners and Gene Chip Readers:  
         [0075]     It may also be desirable to include systems which use a focussed line of laser light which then is scanned over the area of interest and returns through the prism as a 2D pattern containing the spectral information of all the areas of interest covered by the excitation line. This could be conveyed via the fibre bundle and projected onto a 2D CCD chip to gain the information of the image line by line.  
         [0076]     The following points should be noted regarding this optical arrangement. 
    1) In most cases the area to be imaged is a flat plane and therefore does not need confocal isolation.     2) It is likely that a line scan using the prism arrangement, imaging directly onto a CCD is already known.     3) Some fast scan system point scanners may not be suitable for quantum dots and other narrow band emission marker fluorophores which have long excited state lifetimes. A slow line scan has advantages for these markers.     4) A system, which puts the CCD chip in the head itself, is likely to encounter stray excitation light. Blocking this by a long pass filter may result in filter fluorescence difficulties.     5) The long path in the Detector Unit allows a narrow band holographic blocking filter to operate and be used. This filter does not fluoresce, has a very high cut off for the excitation wavelength and also transmits fluorescence, which is very close to the excitation wavelength. 
 
 Ultra Miniaturisation: 
 
 Preferred Optical Arrangement for Proximal Scan Embodiment: 
   
 
         [0082]     The principle of using a prism as a beam-splitter can be applied to a system in which the scanning is all done at the proximal end of the fibre bundle. This would eliminate the necessity of having scan mechanisms at the endoscope tip and would result in a very compact system.  
         [0083]     This embodiment will have an optical resolving power that is reduced by a factor of 2.5 to 3 compared with the previous embodiments. This is the result of under sampling; the Nyquist resolution criterion is not fulfilled for images transferred by fibre bundle.  
         [0084]      FIG. 5  is a schematic view of a laser scanning confocal microscope system in which light  202  from a laser  204  is focussed by a lens  206  to diffraction limited spot  208  that grazes the edge of a mirror  210 . The light diverges from this focus until it reaches collimating lens  212 , which collimates the beam  214  and passes it through an XY scanning mechanism  216 . The scanned beam  218  passes to a focussing lens  220  which focuses the beam as a spot  222  scanned as a raster or other pattern across the polished end  224  of a ‘coherent’ (i.e. image orientation maintaining) optical fibre bundle  226 ; the light then passes along fibre  228 . Light emitted from fibre  228  at the other end  230  of the fibre bundle  226  diverges out to lens  232  and is converted to a collimated beam  234 . This light passes through a direct vision spectroscope prism  236  and is focussed by lens  238  into a tissue sample  240  within which it forms a diffraction limited volume spot  242 . Fluorescence generated at this focal volume travels back through the lens  238  to the prism  236 . The fluorescence light, being of a larger wavelength is refracted less than the blue laser excitation light and it emerges as a beam which is focussed by lens  232  to a line  244  at the polished optic fibre bundle end  230  and this is transferred along the bundle and emerges at the other end  224 . The light is descanned by the XY scanner  216 , focussed by lens  212  to a line, intercepted by the mirror  210  and reflected through further lens  246  and second prism  248 , which is reversed with respect to the initial prism  236 . The second prism  248  renders the beams of all the spectral colours parallel and thus when they pass through subsequent lens  250  they are all brought to a diffraction limited focus at  252 . This focus is at the centre of the pupil of a variable iris diaphragm  254  which can be adjusted to include a selected fraction of the near confocal light to pass with the fully confocal light and to impinge on the photomultiplier tube  256  and thus to produce the signal.  
         [0085]     Referring to  FIG. 6 , the knife edge beam-splitter arrangement provided by locating mirror  210  adjacent to the beam path could be replaced by a conventional dichroic or polarisation beam-splitter  260 . Also shown in this figure are laser  262 , tissue sample  264 , fibre bundle  266  and detector unit  268 .  
         [0000]     Mirror Scan Vibration and Damping:  
         [0086]     It might appear that, with a mirror scan at 850 Hz, unbalanced reaction forces could lead to damping problems due to transfer of vibration through the case. The mass distribution in the various embodiments, however, results in a very high moment of rotational inertia. Vibrational counter balancing should not be necessary if the fast scan mirror frame is rigidly attached to the main structural assembly. A micro machined silicon torsion mirror would be ideal from the point of view of size, but an Electro-Optical Products Corporation resonant scan mirror is also suitable.  
         [0000]     Hybrid Systems:  
         [0087]     There are a number of variations in configuration of the preceding optical systems, which can offer some advantages.  
         [0088]     For example, in one embodiment the slow scanning mechanism operates by the vibration of the delivery fibre and bundle. The fast scan mechanism could operate by the motion of a lens or a mirror on the other side of the prism. This configuration would eliminate one source of return “spot wander”.  
         [0089]     In another embodiment, the laser delivery fibre is scanned while the fibre bundle remains stationary. The descanning is then achieved by synchronized motion of a suitable optical component in the head. This approach may, however, involve an undesirable degree of complexity in arranging the synchronous scanner.  
         [0090]     An alternative possibility is for the fibre bundle in the head moved by the slow scanning mechanism but not to be moved by the fast scan mechanism.  
         [0091]     These designs would make it possible to use the existing tuning fork mechanism unchanged, as there is no extra mass to be scanned. The second design described above can also be used without descanning being required in the Detector Unit. This is achieved by using the slit as the confocal light isolation mechanism. These embodiments would benefit from the use of a prism combination in which the fluorescence spectrum is “bunched up” and is well separated from the excitation line. In general the slow scan mechanism is capable of moving bulky or heavy components while the fast scan mechanism cannot.  
         [0092]     It would also be possible for the slow scan mechanism to carry the collimating lens and the prism as well as the tuning fork (with the fibre bundle) in the slow scan of the raster. This configuration would have the advantage that it would eliminate one of the sources of “spot wander”.  
         [0000]     Fibre Characteristics:  
         [0000]     Optical Transfer Efficiency, Cladding Thickness and Cross Talk between Fibres:  
         [0093]     The theoretical area fraction for identical size circles packed in a square array is 71% and for hexagonal packing 86%, but in practice the actual area fraction obtained for a bundle of fibres is generally considerably lower. It is possible that, if a plane wave is incident on a large area of fibre bundle tips and in the absence of cladding, evanescent wave involvement can produce some improvement in the proportion of light that will be guided by the bundle. Generally, however, each fibre will require cladding, however, to prevent the leakage of light from one fibre to the next at the point of contact (viz. cross talk), and the cladding will further reduce the fraction of the light transmitted by the bundle. In some applications of this type of imaging, however, cross talk may be a lesser problem than it is for conventional exidoscopy, in which case thinner cladding can be employed.  
         [0000]     Optical Transfer Efficiency of Bundles:  
         [0094]     Measurements by Optiscan staff have put the optical transfer efficiency for fused bundles over 50%. This figure includes end reflection.  
         [0000]     Size, N.A and Modal Characteristics of Cores of the Return Fibres in the Bundles:  
         [0095]     It is desirable that the diameter of the return fibre cores be considerably less than the diameter of the excitation laser delivery fibre. This implies a corresponding increase in the numerical aperture or N.A. of the fibres. (The term N.A. for a single moded fibre is used herein to refer to the angle of the cone of light emitted from a fibre that includes a certain specified fraction of the optical energy being emitted. Unlike a multimode fibre or an imaging lens, this definition of N.A does not involve a sharp cut off of light at an edge.)  
         [0096]     With small diameter (high numerical aperture) return fibres the confocal spectral line are spread over a width covering 3 or more lines of fibres if they are single moded. This allows the full resolving power and focal plane resolution. Most fibres in test bundles were found to have high N.A., and manufacturers quote values of 0.3 to 0.4. However, most fibre bundle cores are not single moded at visible wavelengths but, rather, support a few modes. This is probably a trade off against light loss in the cladding necessary to prevent cross talk between fibres in the bundles.  
         [0097]     Another factor to be considered is that the optical configuration of these proposed systems however is such that the specimen (objective) lens is considerably over filled. Consequently the Airey disc of the return spot is larger than the core or modal field diameter of the laser emission fibre. This works advantageously in this parameter trade off.  
         [0098]     As is apparent from the above discussion, the N.A. and the mode acceptance characteristics of the cores, in the bundles that have been tested, produce focal plane isolation that is not far away from that of an optimised spot scan conventional confocal system.  
         [0000]     Fibre Bundle Stiffness:  
         [0099]     Two types of coherent (in the sense of image orientation maintaining) light guide arrays are available: 1) all glass fused fibre bundles, and 2) bundles made by winding fibre onto a rotating mandrel. The latter, wound bundles, are very flexible because the individual fibres can each bend independently like a clump of hair. Fused bundles are much stiffer. Commercially available fused imaging bundles containing 8,000-10,000 cores have been found to be too stiff to be used in the flexible tip endoscope, but such bundles with 2,000 cores have been found to transfer the spectral line to the detector unit satisfactorily and to have the required degree of flexibility. Indeed, the applicant&#39;s experience suggests that a bundle of 1,000 cores or even somewhat fewer would also transfer the spectral line to the detector unit satisfactorily.  
         [0000]     Fibre Fluorescence:  
         [0100]     Fused bundles, being all glass, show very little fluorescence. The wound bundles however are quite fluorescent, probably due to the glues, which are used to infiltrate the region of the hank, which is to be cut and polished. An alternative source of fluorescence is the organic coating applied to the fibre as it is drawn prior to winding on the spool. This is to prevent micro scratches and brittleness of the fibre in use. If the flexible wound bundles are to be used it may be necessary to design the head to minimize stray excitation wavelength light being reflected back into the bundle. Also low fluorescence glues and coating materials may be available.  
         [0000]     Delivery of Excitation Light by Means of Multimode Fibres (or Multimode Laser):  
         [0101]     Delivery of the laser light by means of a multimoded fibres could be used for microplate array or gene chip readers or for other remote spectroscopy purposes. It would offer the advantage of a less intense reading spot with reduced possibility of saturation of the fluorophore. It is less likely that multimode fibre would be of advantage for imaging purposes.  
         [0102]     It should be noted, however, that mode scrambling can be obtained by rapid movement of the fibre of illumination of the excitation spot, although the experience of the applicant suggests that it is unlikely that this would be of significant benefit.  
         [0000]     Fibre Tip Pattern and Orientation:  
         [0103]     The natural packing pattern in the fabrication of bundles is hexagonal. Square packing bundles are very difficult to make. If the return confocal light line was of the same width as the diameter of the return fibre core (i.e. similar core characteristics for delivery and return cores), highest resolution would be obtained by positioning the bundle so that the line fell squarely along a row of fibre tips, preferably of square packed fibre. To achieve this would involve a difficult adjustment process in the head.  
         [0104]     It may be more desirable to use return cores, the diameter of which was considerably smaller than the width of the confocal return line. This would make the orientation and packing pattern unimportant. The possibility of using a linear array of large core return fibres for the microplate readers and the like has already been mentioned.  
         [0000]     Minimization of Stray Excitation Light:  
         [0105]     The design aim and geometry of these systems means that the emission and collection fibres are close together. Consequently some reflected laser excitation light from scattering in the head will fall on the fibre bundle and will be conveyed to the photo detector. To minimize the spurious signal from this source it would be desirable to use a narrow band laser light exclusion filter. Narrow band holographic filters are available with extinction coefficients of 6-8 orders of magnitude for a 5-10 nm band centred on the laser wavelength. For these to operate they must be located in the optical path where the beam is collimated (or within a few degrees of collimation). A suitable position is shown in the detector unit. Kaiser Optical supplies such filters.  
         [0000]     Operation of System with Laser in the Head:  
         [0106]     Blue and near ultraviolet lasers based on Gallium Nitride have been used in confocal microscopes, but they are still connected using optic fibre delivery. Compact lasers such as laser diodes and frequency doubled YAG lasers are now sufficiently small that they could be located in the head itself.  
         [0107]     Referring to  FIG. 7 , it is possible to eliminate the delivery fibre and instead to mount a laser diode chip  270  directly beside the tip  272  of the fibre bundle  274 , and to utilize the prism beam-splitter  276  and fibre bundle return as described above.  
         [0000]     Hybrid System which Maintain Telecentricity and Pinhole Confocal Isolation:  
         [0108]     A compact hybrid system, which maintains scan telecentricity, is shown in  FIG. 8 . The fast scan resonant mirror  300  is very close to the back focal plane of objective lens  302  and thus is effectively telecentric. The prism  304 , lens  306 , fibre  308  and fibre bundle  310  are all rigidly mounted on a bracket  312 . This bracket  312  can be rotated about a pivot  314 , which means that the reflected beam  316  can be scanned about the telecentric plane of the lens.  
         [0109]     A slightly more compact version uses two flexure strips  318  and  320  of unequal length is shown in  FIG. 9 .  
         [0110]     In  FIG. 9  the fibre  322 , fibre bundle  324 , collimating lens  326  and the prism  328  are held on a rigid mount  330 , which is attached to the frame  332  of the endomicroscope head by two flexible strips of metal (flexure strips  318  and  320 ). Slow scanning motion is provided by actuator  334 . The unequal lengths of the two metal strips  318  and  320  cause the mount  330  to rotate as it moves away from the frame of the endomicroscope assembly. This rotation is centred on the back focal plane of the image-producing lens  336  or, more precisely, on the reflected position of the back focal plane as imaged in a mirror  338  which is used in resonant fast scan mode.  
         [0111]     This fast scan mirror  338  is located very close to the lens  336  and hence it is likewise operating in a close to telecentric condition.  
         [0112]     The simultaneous operation with two or more laser lines would introduce difficulties, as the two different laser colours would each be refracted to a differing degree by the prism, and two separate spots would be focussed in the specimen. Ways of adapting hardware and software according to the present invention to deal with this would be straightforward, and within the scope of the present invention, but are likely to be of limited value.  
         [0113]     A positive feature of the prism system, however, is that the excitation wavelength can be changed (such as from 488 nm to 514.5 nm) without the need to alter any other component (unlike dichroic beam-splitter arrangements).  
         [0114]      FIG. 10  is a photograph of the exit end  86  of a fibre optic bundle  82  for use with the scanning confocal microscope with fibre bundle return of  FIG. 2 . The photograph was taken with an Olympus™ brand SZ microscope; the diameter of the outer casing (1.5 mm) and of individual fibres (7 μm) are indicated. The illuminated line is the image of dispersed fluorescence through a direct vision prism; the insert is an enlarged view of a partly illuminated portion of the individual fibre tips.  
         [0115]     In the view of  FIG. 10 , the red end of the illuminated line is towards the upper left, and the blue end is towards the lower right. The boxed, enlarged portion of the illuminated line falls in the yellow region of the spectrum.  
         [0000]     Combined Slit and Iris Diaphragm Aperture:  
         [0116]     In some optical hybrid configurations it is imaginable that an advantage might be obtained by the use of both a slit aperture and a pinhole aperture in the detector unit. The slit aperture would be positioned at the first image plane while the iris pinhole would be located in front of the photomultiplier tube.  
         [0117]     In some embodiments, transmission or reflection diffraction gratings are used instead of prisms. The optical system could also be suitable for reflection imaging if used with polarization beam-splitter separation or an optical arrangement with a beam-splitter in the laser delivery path.  
         [0118]     Although a number of alternatives have been mentioned above, a mirror scanning system is likely to be the most accessible scanning design for implementing the present invention, and would be readily provided in compact, handheld form.  
         [0119]     The bundle system, when producing an image with the line fluorescence spectrum, should not introduce the image artefacts experienced with earlier two fibre systems. Any modal field mismatches should be evened out by the spread of the light over multiple fibres.  
         [0120]     Thus, modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.  
         [0121]     For the purposes of this specification it should be understood that the word “comprising” means “including but not limited to”, and that the word “comprises” has a corresponding meaning.  
         [0122]     Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge.