Abstract:
An optical interference apparatus for carrying out Fourier domain optical coherence tomography including means to provide multiple beams whereby interferograms are recorded simultaneously for a plurality of different focal depths within a substance to be examined, each interferogram being provided by one of the multiple beams. Means are provided for combining images derived from said interferograms for a plurality of different focal depths, whereby a single image may be constructed with an increased depth of field. The axial spacing of the foci is calculated to take into account the Rayleigh range of the focal waist in the substance to be examined.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an interference apparatus and method, particularly an optical coherence tomography apparatus and method and a probe for use therein. We will describe an optical probe and associated methods for use with an imaging technique known as optical coherence tomography (OCT). 
         [0002]    In a preferred arrangement, the optical probe may be used in any location which can be reached by a rigid endoscope (or borescope). Potential applications include medical examinations such as colposcopy (cervical cancer screening) and laparoscopy (e.g. in diagnosis and treatment of endometriosis). In another preferred arrangement, the optical probe may be used in more accessible locations which do not require an endoscope. Potential applications include dermatology (e.g. in skin cancer diagnosis). 
         [0003]    Internal medical examinations are typically carried out by using an endoscope in which the eye or a CCD camera images the view relayed from the distal end of a shaft of the probe. In a flexible endoscope, the image may be relayed using a coherent fibre bundle containing thousands of individual fibres; in a rigid probe or borescope, the image may be relayed via a system of lenses or rods. Effectively this gives a view of the surface of the relevant medical target, but to see changes in the structure below the surface, it is desirable to be able to obtain a cross-sectional image from within the bulk of the tissue. This is the capability which OCT can provide. Variants of OCT have been described which can extract additional information, such as blood flow velocity (Doppler), or alignment of muscle fibre (polarization). 
         [0004]    OCT may be used in the visible part of the spectrum for retinal examination, but to obtain reasonable penetration depth in other, more strongly scattering, tissues it is necessary to move to infrared wavelengths. 
         [0005]    OCT is based on the use of interferometry, where light in the measurement arm of an interferometer is passed to the object to be examined and a portion is scattered back to the interferometer. Light in the reference arm is passed to a mirror at a known distance and a reference beam is reflected back. The scattered measurement beam and the reflected reference beam are combined, and the interference between these two beams is detected and used to provide data about the examined object. 
         [0006]    Thus optical coherence tomography uses interferometry and the coherence properties of light to obtain depth-resolved images within a scattering medium, providing penetration and resolution which cannot be achieved with confocal microscopy alone. Clinically useful cross-sectional images of the retina and epithelial tissues have been obtained to a depth of 2-3 mm. 
         [0007]    There are three main types of OCT which can be categorized as follows: 
         [0008]    Time domain OCT; this uses a low coherence source and scans axially (in depth) by altering the reference path length of the interferometer. 
         [0009]    Spectral domain OCT; this uses a wide spectrum (i.e. low coherence) source, a stationary interferometer and a spectrometer. The spectrum of the interferogram is examined by the spectrometer and the axial response is obtained as the Fourier transform of the spectrum of the light at the output of the interferometer. 
         [0010]    Frequency domain OCT; this uses a swept-frequency narrow spectrum source and a stationary interferometer. The axial response is obtained as the Fourier transform of the time-varying intensity of the light at the output of the interferometer. 
         [0011]    We shall use the expression “Fourier domain” to cover both spectral domain and frequency domain. 
         [0012]    Time domain OCT (the original, and currently the most prevalent, type) is limited in acquisition speed by the need for mechanical depth scanning, and has relatively poor signal-to-noise performance. 
         [0013]    Fourier domain OCT (spectral or frequency domain) enables more rapid capture of high-resolution images without sacrificing sensitivity. The time for each axial scan (“A-scan” in ultrasound scanning terminology) is critical in medical in-vivo applications because of the need for the patient to stay still for the time that it takes to build up successive A-scans into a cross-sectional image (“B-scan”). 
         [0014]    However, time domain OCT has one significant advantage: it is easy to combine dynamic focal adjustment in step with the mechanical time-delay scan, giving the optimum spot size at the depth which is being probed. In contrast, Fourier domain OCT acquires information from the whole depth at the same time, so it is not possible to dynamically adjust focus for best lateral resolution. 
         [0015]    There are three main difficulties in providing a practical arrangement of an OCT probe in which the conflicting optical and medical requirements are resolved. 
         [0016]    Firstly, there are difficulties in obtaining an image which is suitably in focus over the depth of the (A scan) image. 
         [0017]    Secondly, to provide a B-scan image it is necessary to scan laterally across the surface. Designs exist for endoscopic probes which incorporate a miniature scanning device in the probe shaft tip, for instance using electromagnetic coils to move the end of an optical fibre. This approach has the disadvantage of placing moving parts, and the power to drive them, inside the patient&#39;s body, and may increase the difficulty of sterilizing the equipment. 
         [0018]    Thirdly, it is desirable to be able to provide a normal, full field, endoscope viewing channel at the same time. 
         [0019]    Through this specification we will refer to “optical”, “light” and such terms. It will be understood, however that such terms refer to radiation of infra-red, visible or ultra-violet wavelengths as appropriate. 
       SUMMARY OF THE INVENTION 
       [0020]    In order to deal with the first problem, according to a first aspect, the present invention provides an optical interference apparatus and method, preferably, but not restricted to an optical coherence tomography apparatus and method in which interferograms are recorded simultaneously for a plurality of different focal depths within the substance to be examined. 
         [0021]    Thus, each interferogram provides an A-scan image which is only in sharp focus over a limited depth range (the depth of focus, also known as the Rayleigh range), but by combining these images for a plurality of different focal depths, a single A-scan image may be constructed with an increased depth of field. 
         [0022]    The interferometer passes a measurement beam to the substance to be examined and the apparatus may provide a relevant measurement beam for each different focal depth. If the light is provided by a common source (as is most convenient)—which common source may be a laser—then optical means (such as an amplitude beam-splitter) may be provided to generate a plurality of beams. Different optical components (e.g. refractive elements) are then required in the path of each beam to bring them to different foci. 
         [0023]    The depth of focus of each measurement beam is proportional to the square of the diameter of the measurement beam (i.e. proportional to the spot area). Therefore we can halve the spot size (double the lateral resolution) by providing four spots instead of one. 
         [0024]    The axial spacing of the foci is calculated to take into account the wavelength of light in the target (which is smaller than that in air by the factor of the refractive index for the relevant wavelength range). 
         [0025]    To perform a B scan, it is necessary to relatively scan the beams and the surface being examined, and thus a scan means is provided. Usually a scan means is provided for scanning the beams along a line across the surface of the substance being examined. For a convenient optical design, it is desirable for the plurality of beams to be spaced along the scan line to a small extent. This leads to the information for different depth ranges at a given location arriving at slightly different times during the lateral scan, rather than simultaneously, an effect which has to be compensated for in assembling the combined image. 
         [0026]    In order to deal with the second problem, according to a second aspect, the present invention provides an optical probe (which may be used with coherence tomography apparatus or other optical arrangements, for example, a viewing endoscope in which an image is transmitted by the probe to a remote viewing lens or to a camera) in which a scanner (which is preferably a small rotating or oscillating mirror scanner), is provided at a proximal end of a probe, and optical components are provided within the probe to optically relay the scan to and from a distal end of the probe. 
         [0027]    By this means, no moving parts are placed at the distal end of the probe shaft and hence, where it is used for internal medical examination, no moving parts are within the patient. 
         [0028]    The probe preferably comprises a probe shaft, and a handle is preferably provided at the proximal end of the probe shaft, and preferably the scanner is mounted within the handle. The probe shaft may be detachable from the handle for cleaning (the probe shaft would normally be used within a disposable sheath however). Note that it is preferable to constrain the shaft to a specific orientation, so that any internal baffles which may be fitted within the shaft, or lens tilts to eliminate reflections, will align correctly with the scan direction. Because the scanner is not within the probe shaft itself, different variants of probe shaft may conveniently be provided, mating to the common handle, allowing different lengths of probe shaft, and probe shafts with angled views. If the length of the optical measurement path through the probe shaft is altered, a corresponding compensation in the reference path will be required. 
         [0029]    In order to deal with the third problem, according to a third aspect, the present invention provides an interference apparatus and method such as an optical coherence tomography apparatus for examining a substance, said apparatus including 
         [0030]    a viewing apparatus, 
         [0031]    an interference apparatus, 
         [0032]    a probe shaft including relay optical components in which viewing (illumination and imaging) is provided through the same relay optical components as are used for the interferometry (e.g. OCT), 
         [0033]    means to pass an interferometer (e.g. OCT) beam along the probe shaft to the distal end thereof to the substance to be examined and to pass the scattered interferometer (e.g. OCT) beam back along the probe shaft to the interference apparatus, 
         [0034]    a visible light source (such as a white light source), 
         [0000]    means to pass the visible light from the visible light source along the probe shaft to the distal end thereof to illuminate the substance to be examined, preferably uniformly, and to pass an image thereof back along the probe shaft to an image detector of the viewing apparatus, 
         [0035]    means to separate the returning image from the outgoing visible light, 
         [0000]    and a beam-splitter positioned between the proximal end of the probe shaft, and the viewing apparatus and interference apparatus respectively, to separate the interferometer beams (in both directions) from the visible light beams (in both directions) whereby the same part of the substance may be viewed using the visible light and examined using the interference beams at the same time. 
         [0036]    The beam-splitter is preferably a spectral beam-splitter. 
         [0037]    A scanner is preferably provided to scan the OCT beam across the substance to be examined and in this case the beam-splitter is preferably provided between the scanner and the probe shaft, so that in this case, the scanner is considered to be part of the interference apparatus. 
         [0038]    The visible light source is preferably an LED source to provide white light illumination, and the imaging detector is preferably a colour CCD camera to receive the reflected image of the surface of the substance being examined. 
         [0039]    Such an arrangement allows the clinician to view the surface of tissue, both when the probe is close above it and when the probe is in contact with it. The clinician can use the viewing device to select a particular part of the surface for more detailed in-depth examination by the OCT apparatus, then press the distal end of the probe shaft into contact with that part of the surface while continuing to observe it. 
         [0040]    The probe shaft will generally be rigid as this simplifies the optics, but in some circumstances may be at least partly flexible or jointed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: 
           [0042]      FIG. 1  is a block diagram showing the main components of the optical coherence tomography apparatus, 
           [0043]      FIG. 2  shows a perspective of the probe with some internal detail, 
           [0044]      FIG. 3  is an optical diagram of an optical coherence tomography apparatus comprising a four-spot probe used for frequency domain OCT in accordance with the invention (for clarity, some folds of the light paths have been removed), 
           [0045]      FIG. 4  is a an enlarged axial section of method of multiple beam generation, 
           [0046]      FIG. 5  is an enlarged detail of the measurement laser beams at the distal end of the probe, showing both axial and lateral separation of focus, 
           [0047]      FIG. 6  is an axial section of the probe assembly incorporating viewing optics including a camera and optical components to provide a view of the surface under examination, the figure showing the path of the laser OCT beams, 
           [0048]      FIG. 7  is an axial section of the probe assembly of  FIG. 3  showing the illumination light optical path excluding the laser beams, 
           [0049]      FIG. 8  is an expanded view of the method of mixing the illumination light path with the viewing light path, 
           [0050]      FIG. 9  shows the imaging light path from the distal end of the probe to the camera 
           [0051]      FIG. 10  is an enlarged detail of the multi-facet reference mirror structure, 
           [0052]      FIG. 11  is an enlarged detail of the interfering laser beams and the balance beam forming image foci on the detector plane, 
           [0053]      FIG. 12  shows an enlarged detail of the sensitive areas on the detector plane, and 
           [0054]      FIG. 13  shows a perspective view of the OCT apparatus 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     General Description 
       [0055]      FIG. 1  shows a block diagram of the OCT apparatus indicating a laser  10 , provided usually remotely from the probe  1 , but in some circumstances within the probe  1 . A laser beam  11  from the laser  10  is passed to the probe, usually through a single-mode optical fibre  2 . The laser  10  provides a swept spectrum over a wavelength range of at least 50 nm, within a region of the infra-red where tissue absorption is minimised. A wider spectrum improves the depth resolution. The probe  1  comprises a multi-beam interferometer  41 , a scanner  5 , a probe shaft  6  and camera with illumination system  50 ,  52 ,  53 , and other components detailed below. The processing and display system  9  and tissue under examination  33  are external to the probe  1 . 
         [0056]      FIG. 2  shows more detail of the probe  1 . The probe  1  comprises a handle  3  containing an multi beam interferometer  41  and scanner  5 , and a probe shaft  6 . The probe  1  is constructed so that the shaft  6  can be detached from the handle  3 . The shaft  6  is constrained to a specific orientation, so that an output lens which outputs a multiple beam set and which is tilted by a small angle to eliminate reflections, aligns correctly with the scan direction. Other components described below have been omitted from this diagram for clarity. 
         [0057]    For the particular application of imaging the uterine cervix, suitable probe shaft dimensions are  16  mm diameter at the proximal end  7  tapering to 12 mm diameter at the distal end  8  if required, and in the region of 220 mm length. The length of the scan line is made as large as possible, within the constraint of the shaft diameter, and in the described arrangement is 6.4 mm. The cone angle within the tissue is approximately f/8, which gives a depth of focus of about 0.3 mm. One beam of multiple beams used is essentially in focus from 0 to 0.3 mm depth, the next beam from 0.3 mm to 0.6 mm and so on through to 1.2 mm: the worst-case beam diameter at the tissue under examination (i.e. the width of a spot produced by the beam) is about 10 μm FWHM. 
         [0058]    The distal end  8  of the probe shaft is convex to apply even pressure over the whole front face to the soft tissue under examination, irrespective of small angular departures from the normal onto the surface. Some other internal components including rattle plate  13 , lens  25 , fold mirror  26 , scan mirror  27  and spectral beamsplitter  28  are shown to facilitate orientation. 
       Optical Description 
       [0059]    Referring to  FIG. 3 , the laser provides an output beam  11 , via single mode fibre  2 , which is passed to a converging lens  12 . After passing through the converging lens, the beam enters the rattle-plate beamsplitter  13 . It may be desirable to interpose additional optical components in beam  11  (between the output from the fibre—which may already be collimated—and the rattle-plate) so that the beam diameter can be adjusted, and hence the desired convergence can be produced at the measurement point. The rattle plate  13  splits the beam  11  into a number of weaker beams that are transmitted onwards; the detailed operation of the rattle plate is explained with reference to  FIG. 4 . 
         [0060]      FIG. 4  is an optical diagram showing the operation of a partially and fully reflecting pair of surfaces in forming a plurality of parallel beams. This arrangement is known as the rattle plate  13 . The apparatus comprises a parallel-sided glass plate  42 , which on the entry face  44  has a high efficiency reflective coating to provide a reflective surface over area  43 , leaving a non-reflective area  45  which may be either uncoated, or anti-reflection (AR) coated for better performance. The transition between these two areas is sharp. The exit face  46  is coated over the entire surface with a partially reflecting coating to provide a partially reflecting surface  47  such that typically 8% to 25% of the incident light is transmitted, and the remainder reflected. 
         [0061]    The incoming laser beam  11  passes through the non-reflective area  45  of face  44  (close to the boundary between the reflecting surface  43  and the non-reflective surface  45 ). Consequently, only a small amount of energy is lost on entry to plate  42  (i.e. the Fresnel reflection if there is no AR coating in this part of the plate, or less if AR coated). 
         [0062]    The laser beam  11  propagates through the plate  42 , and in this example 13% is transmitted at the partially reflecting surface  47  to provide the first beam  14 , and the remainder is reflected back towards the reflecting surface  43 . 
         [0063]    The plate  42  is tilted from orthogonal to the input beam  11  such that the beam reflected from the partially reflecting surface  47  is directed towards the high-efficiency reflecting surface  43 . Consequently the beam is then reflected back (approaching 100% of the energy is reflected) to the partially reflecting surface  47 , where a further 13% of the remaining beam power is transmitted to provide the second beam  15 . In this way, a series of beams of declining power are emitted from the plate, parallel to each other. 
         [0064]    If the input beam  11  at the rattle plate is arranged to be convergent rather than collimated (for example by taking a collimated laser beam and passing it through converging lens  12 ), then the beams  14 ,  15  etc leaving the glass plate  42  will focus at different axial positions relative to each other, since each successive beam follows a longer path through the plate  42 . The distance between the focal positions will depend upon the thickness, tilt angle and refractive index of the plate  42 . Alternatively, the rattle plate assembly may comprise a fully reflecting and partially reflecting surface separated by air, as opposed to glass. Also, the input beam  11  may be divergent rather than convergent with suitable changes to the optical components. 
         [0065]    The strongest five beams,  14  to  18 , are allowed to propagate onwards, the remainder are blocked by an opaque plate  19 . 
         [0066]    Returning to  FIG. 3 , the beams  14  to  18  from the rattle plate  13  are passed to a beam-splitter  20  which divides the beams into measurement beams  14 M to  18 M and reference beams  14 R to  18 R. The reference beam  18 R is manipulated in the same way as the reference beams  14 R to  17 R, but it is not used to interfere with a measurement beam, rather it provides compensation for laser amplitude variation. 
         [0067]    The reference beams  14 R to  18 R are reflected by the beam-splitter  20 , pass through lenses  21  and  22 , reflect at a multifaceted mirror structure  23  then re-pass through lenses  22  and  21 , and re-pass through beamsplitter  20 . The multifaceted mirror structure  23  has a reflecting surface for each of the reference beams, the individual reflecting surfaces are set at the foci of the respective beams. It may be advantageous to set the angles of the reflecting surfaces one to the next to ensure that the reference beams  14 R to  18 R are accurately retro-reflected. Alternatively, the power and position of lenses  21  and  22  may be selected such that the axes of reference beams  14 R to  18 R are parallel to each other. Note that the reference optical path is shown in the diagram as substantially shorter than measurement optical path. In practice, these paths would be very similar in length, because in a frequency domain OCT system the fringe frequency due to a target reflection is proportional to the path difference. Even if the electronic system could operate with unlimited bandwidth, there would be a constraint on maintaining similar path lengths, since the difference of the path lengths must be less than the coherence length of the laser  10  for interference to occur. Another criterion for good interference between measurement and reference beams is that the convergence and focal positions of the reference beams should match those of the measurement beams at the detectors. To achieve this, it is preferable to introduce additional reflecting or refracting optical components (such as an Offner relay) in the reference path to relay the focal points at or near beamsplitter  20  to the multifaceted reflecting surface  23 . 
         [0068]    The measurement beams  14 M to  17 M leave beamsplitter  20 , and the weakest beam  18 M is blocked by an opaque plate  24 . They are nominally collimated by lens  25 , but there will be a slight difference between the convergence of the four beams since the path length between lenses  12  and  25  is different for each beam. The separation between the two lenses is set so that the average optical path length would result in a collimated beam. The axes of the four beams  14 M to  17 M now converge towards each other. The beams are reflected at 90° orthogonal to the plane of the diagram at mirror  26 , and propagate onwards, with the axes meeting at a scan mirror  27 . 
         [0069]    The scan mirror  27  is driven to rotate nominally about an axis parallel to the original axis of the beam  11 , parallel to the plane of the diagram, scanning the measurement beams  14 M to  17 M. A further beamsplitter  28  is provided to reflect measurement beams  14 M to  17 M along a new axis nominally parallel to the original beam axis of beam  11 . The beamsplitter plate has a coating to selectively reflect IR radiation such as would be used for beams  14 M to  17 M, and to transmit visible white light. 
         [0070]    A probe shaft  6  is provided. It comprises a metal tube mounting various passive optical components (relay optical components) as will be described hereafter. 
         [0071]    The first (entry) lens group  30  in the probe shaft  6  forms a focus at  31  of each of the scanning measurement beams  14 M to  17 M within the probe shaft; other lenses relay the foci to a focus point just beyond the last lens  32  in the probe shaft, that is, just outside the distal end of the probe shaft. Because the measurement beams  14 M to  17 M enter the probe shaft with a slightly different divergence from each other, their final focus  14 F to  17 F outside the probe shaft  6  for the respective beams  14 M to  17 M as shown in  FIG. 5 , will be displaced axially relative to each other, allowing optimal signals to be derived from a different tissue depth (the tissue is indicated at  33 ). 
         [0072]    It will be seen that the last lens  32  forms the distal end of the probe shaft. In use, the distal end of the probe shaft formed by the lens  32  will be brought into contact with the medical surface tissue  33  to be examined, optionally through a thin transparent disposable sheath. 
         [0073]    As is shown in  FIG. 5 , the foci  14 F to  17 F of the four measurement beams  14 M to  17 M will fall inside the tissue to be examined. This allows provision of four laser beams which are focussed at different depths, and though each beam rapidly comes out of focus as the depth varies, it is possible to cover all of the depths of tissue of interest within the focal range of one of the four beams. The axial spacing of the four foci is calculated to take into account the Rayleigh range of the focal waist in the tissue to be examined 
         [0074]    Furthermore, because the four beams  14 M to  17 M strike the scan mirror  27  at slightly different angles, the four foci  14 F to  17 F outside the probe shaft are also separated along the scan line by a distance indicated at A in  FIG. 5 . The distance A is small (of the order of 0.2 mm) and so the time between each of the beams scanning across a particular point in the tissue under examination is small (a few percent of the total scan time) and so the tissue under examination should not change between the passage of each beam. 
         [0075]    Clearly as indicated above, one may have more or less than four beams which have foci at a range of depths within the tissue. It will be noted that the foci of the four beams are displaced both laterally and axially from one to the next. 
         [0076]    After scattering from the target tissue, components  14 MR to  17 MR of the four beams are confocally collected back through the probe shaft. These return beams  14 MR to  17 MR are de-scanned by the scan mirror  27  and pass back through lens  25 . 
         [0077]    A part of the each of the beams  14 MR to  17 MR is reflected by the beam-splitter  20  and combined with the corresponding reference beam  14 R to  17 R. The combined beams  14 MR/ 14 R to  17 MR/ 17 R pass through a lens  34  which forms focal points of each of the combined beams at detector  35 . It will be seen that the detector plane is tilted to the orthogonal angle of the incident combined beams axes from the normal to accommodate the focal shift originating from the rattle plate  13 . Interference between corresponding beams occurs at the surface of the detector  35 . The detector  35  will consist of a number of discrete sensitive areas, one for each of the combined beams, and an additional area for the reference beam  18 R, which is used as a balance signal. 
         [0078]    The beam-splitter  20 , reference mirror structure  23 , and individual detector sensitive areas  36  to  39 , and optical components form a Michelson interferometer  41 . The interferometer arrangement allows the use of OCT and in particular the optical components are provided in this preferred embodiment to use frequency domain OCT. 
         [0079]    It will be seen that if beamsplitter  20  is a polarising beamsplitter, and quarter wave-plates are interspersed in both measurement and reference paths such that the measurement beams  14 M to  17 M, and reference beams  14 R to  18 R pass and re-pass through the wave-plates, and if an additional analysing component is added to the combined path so that a common polarising component of each of the beams is selected, then the assembly will have a modified sensitivity to any polarised properties of the tissue under examination. 
         [0080]    Additional details are shown in  FIG. 6 and 7  to provide a viewing channel. 
         [0081]    In  FIG. 6 , the path of the OCT laser beams  14 M to  17 M is shown. The laser beams  14 M to  17 M are traced from lens  25  (not shown), via mirror  26  onto the scan mirror  27 , and through to the tissue at the distal end of the probe shaft  6 . A camera chip  48 , lens system  49  and illumination beamsplitter plate  50  are also shown. 
         [0082]      FIG. 7  shows the same components as  FIG. 6  but the illumination beams  51  and white light source  52  are shown, and the OCT laser beams are omitted for clarity.  FIG. 8  shows an additional view of the illumination beamsplitter plate  50 , which is a reflecting surface with a central aperture. Light from white light source  52  is largely reflected by the illumination beamsplitter plate  50 , although those parts of the beam which pass through the central aperture  54  are lost. 
         [0083]    The apparatus of  FIGS. 6 and 7  includes a spectral beam-splitter  28  which separates OCT laser light from white light. The illumination beam-splitter plate  50  and illumination source  52  are positioned to direct visible light which is preferably white light from the illumination light source  52  through the beamsplitter plate  28 , and to pass a beam  51  of white light from the source  52  along the optical axis within the probe shaft  6 . A white light LED is a suitable illumination source  52  but others are envisaged. Since the tissue surface  33  will be optically scattering, a component part of the returned reflected white light beam  51  will pass through the spectral beam-splitter  28 . A smaller component of this returned beam will pass through the aperture  54  in the illumination beamsplitter plate  50  to a camera  53  which includes a CCD detector  48 . This is illustrated in  FIG. 9 . 
         [0084]    As is clear from  FIGS. 6 and 7 , the spectral beam-splitter  28  allows an illuminating beam  51  to be passed to the surface under examination, the illuminating beam being mixed into the viewing channel by beam-splitter  50 . 
         [0085]    For preference, the entrance pupil  54  of the camera will be at a conjugate point to the reflective surface of the scan mirror  27 , and also coincident with aperture of the illumination beamsplitter plate  50 . 
         [0086]    The camera  53  includes one or more lenses  49  to form an image of a surface to be examined. The camera may be used to examine the surface  33  when it is in contact with the distal end of the probe shaft. Further, if the depth of focus of the camera is sufficient, it may be used when the distal end is spaced from the surface allowing the user to carry out a survey of the surface before selecting a particular part to be examined by OCT. 
         [0087]    Referring to  FIG. 9 , the image is focussed on either the image sensor surface  48  of the camera  53 , or in an alternative arrangement, an end surface of a coherent fibre bundle  55  which leads to a remote CCD. 
         [0088]    It will be noted that both the viewing optics and the OCT apparatus use the same distal end lens  32  and so the part of the tissue viewed by the camera  53  and the OCT interferometer  41  will be the same. Means may be provided for indicating on the displayed image the position of the OCT B-scan line. 
         [0089]      FIG. 10  shows a magnified view of the reference mirror structure  23 .  FIG. 11  shows the combined beams  14 MR/ 14 R to  17 MR/ 17 R, and balance beam  18 R forming individual foci on the detector surface  35 .  FIG. 12  shows the arrangement of the sensitive areas on the detector plane, one for each combined beam, and one for the balance beam  18 R. 
         [0090]    The embodiment so far described uses a single balance beam, and a compensation signal derived from this beam is applied to each of the (four) interference signals electronically. An alternative embodiment is to provide a separate balance beam matched optically to each reference beam; the paired beams are then detected using a balanced detector configuration. 
       Processing Description 
       [0091]    The laser provides a trigger signal to the processing system at the start of each frequency sweep. The processing system digitizes the analogue detector signals and stores the data (typically 1024 points) for the sweep, which provides the information to reconstruct one A-scan. The processing system may capture raw data for many A-scans (covering the entire movement of the scan mirror) before processing into a B-scan image, or alternatively capture and processing of A-scans may be overlapped in time. 
         [0092]    An ideal laser source for frequency domain OCT would sweep at a constant rate of optical frequency with time, and provide a constant level of power during the sweep. In this case it would only be necessary to perform a discrete Fourier transform of the raw data (with an appropriate window function, eg Hanning) to obtain the A-scan profile. 
         [0093]    For practical laser sources, the sweep rate varies across the spectrum, and so does the power. If uncorrected these effects would result in blurred images. Accordingly the raw data is corrected by resampling at unequal intervals using a local cubic interpolation algorithm, and by rescaling by varying factors. The discrete Fourier transform is then performed as above. 
         [0094]    The calibration for the above corrections is obtained by using a plain glass block as a target, to generate a single reflection of about 4% of incident power (the scan mirror is stationary, set to the central position, during calibration). The path difference is adjusted to give a suitably large number of fringes (for instance 100 across the scan), and the raw waveform is captured. After removing any residual dc component, the computer accurately determines the position of the fringe zero crossings using a local cubic interpolation algorithm, and hence obtains the required array of resampling positions. It also determines the envelope of the fringes, and hence obtains the required array of rescaling values. When the system is correctly calibrated, the glass block gives a sharp single peak in the A-scan. 
         [0095]      FIG. 13  shows a perspective view of the apparatus comprising a housing  100  mounting a computer system to analyse the interferograms and display the results on a screen  101 . The housing  100  also mounts the laser, the output beam of which is passed to the probe  1  via the flexible single-mode optical fibre  2 . 
         [0096]    The invention is not restricted to the details of the described examples.