Abstract:
A method of detecting an object located within a dynamic scattering media, includes i) directing a continuous coherent light wave of predetermined wavelength into the media; ii) detecting dynamically scattered light emerging from the media; iii) correlating the detected light photons in the time or frequency domain; iv) determining the presence of an object from analysis of differences between the correlation and a correlation which would arise from photons scattered by the media only; and v) determining the approximate position of the object within the media from the analysis of the correlation and knowledge of the mean transport path of the light wave of predetermined wavelength within the media.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for detecting and/or imaging an object. 
     2. Related Art 
     Image through turbid media is often carried out using Time of Flight measurements. Various methods of time of flight measurement are known. The majority of these methods measure the time taken by pulses of light to travel a return path. Since the velocity of light is both constant and known for most materials, measurements of time of flight can be readily converted into images of the medium through which the light has passed. 
     Time of Flight imaging is particularly useful when imaging over large distances or in semi-turbid media. However, when dealing with short distances image resolution decreases rapidly, since the speed of light is so great that, for example, a spatial resolution of 1 m will require a detector with a temporal resolution of 5 ns. The shortest measurable distance and the image resolution obtainable using Time of Flight imaging is thus limited by the response time of the detectors used. A further disadvantage or Time of Flight imaging is that the reset time of the detectors used is considerably longer than the jitter time, so that the detectors are only capable of detection for a very short period of their total operating time. An alternative method of imagine through turbid media is it use acoustic waves (eg. ultrasound imaging). However, acoustic imagine suffers from lack of spatial resolution due to the large divergence of acoustic waves. 
     A further known method of imaging comprises forming an image of an object from interference of two beams of coherent light, one of which has been scattered from a target (ie. holographic imaging). Holograms have been used to analyse non-visible parameters of a target, for example, vibration of an engine block. Holographic imaging suffers from several disadvantages Firstly, holograms require two investigative beams that must interfere and be coherent over the distance to the target. Secondly, holograms are not suited to imaging through opaque media. An image cannot therefore be produced at a distance greater than a single photon transport path of the light used to obtain the hologram. Thirdly, holograms arc unsuited for measurement through media that exhibit dynamic scattering, and the scattering will reduce the quality of images obtained. 
     Several known imaging techniques exist where an investigative wave is perturbed as it passes, scatters, reflects or is absorbed by a target. These techniques require that the form of energy used for the investigative wave, and its frequency, must be chosen to interact with the target and cannot thus be fully optimised for detection (i.e. low absorption an/or high spatial resolution and/or high signal to noise). 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome or substantially mitigate the above disadvantages. 
     According to a first aspect of the present invention there is provided a method of detecting an object located within a dynamic scattering media, the method composing: 
     i) directing a continuous coherent light wave of predetermined wavelength into the media; 
     ii) detecting dynamically scattered light emerging from the media; 
     iii) correlating the detected light photons in the time or frequency domain; 
     iv) determining the presence of an object from analysis of differences between said correlation and the correlation which would arise from photons scattered by the media only; and 
     v) determining the approximate position of the object within the media from said analysis of the correlation and knowledge of the mean transport path of the light wave of predetermined wavelength within the media. 
     The term “light wave” is not limited to visible light but is to be interpreted as encompassing electromagnetic radiation of any suitable wavelength. 
     The media may be a turbid media of relatively high density and the object may be more or less dense and more or less viscous than the media. The object may be an object which absorbs and/or reflects the incident light waves. 
     The invention (from hereon referred to as Diffuse Wave Imaging) incorporates aspects of the known technique of Diffusing Wave Spectroscopy (DWS). DWS is used for sub-micron particle sizing and bulk rheology measurements in dense suspensions or emuisions. DWS is not applicable to the imaging of single objects within turbid media. Furthermore, DWS is not used to identify individual particles. The suppression of photons close to the axis of the auto-correlation traces is treated as a limitation of DWS. The inventors have realised that this suppression of photons which have undergone multiple scattering events can be used to determine the presence of an object, and by taking many measurements, to form an image. 
     The invention allows images to be obtained using photons which have travelled optical distances greater than 2 photon transport paths. This is opposite to conventional imaging of dense media, which generally removes or filters out light which has been scattered more than once. Since the invention does not require photons which have undergone ballistic and low order scattering, it is suitable for imaging very dense suspensions where ballistic scattering is limited. 
     The steps (i) to (v) may be iterated either sequentially or simultaneously using coherent light waves of different predetermined frequencies having different mean transport paths within the dynamic scattering media, to thereby obtain further information as to the approximate position of the detected object from the analysis of the respective correlations and knowledge of the respective mean transport paths. For instance, light of three different wavelengths may be used. 
     The or each iteration of steps (i) to (v) may be repeated, either sequentially or simultaneously, for additional locations within the media, the results then being combined to construct an image of the object within the media. Where only one wavelength of light is used the image will be two dimensional. However, using two or more different wavelengths as mentioned above (which effectively probe into different depths of the media and/or object, enables the construction of a, three dimensional image. 
     The light emerging from the media may be detected at one or more predetermined scattering angles, preferably a scattering angle of 180° and/or 0°. 
     The media may be modulated to induce, or enhance, dynamic motion within the media to provide or enhance the required dynamic scattering. Similarly, when the object is an object which is at least partially reflective of the or each light wave, the object may be modulated to enhance phase chances in light reflected therefrom. 
     The method may included the step of selecting for detection light which has a predetermined component of polarisation. The selection may be accomplished using polarising filters or fibre optic cables which preserve only one particular component of polarisation. 
     The or each light wave may be passed through a window prior to entering the media, the window being arranged to reflect light which is detected together with light emerging from the media, thereby producing a heterodyne signal. The window may be adjustably displaced relative to the origin of said light wave to allow control of the intensity of the reflected light which is detected. The window may be arranged to cause the reflected light to undergo multiple reflections before being detected, thereby enabling the path length travelled by the reflected light to be controlled. 
     The method may be performed on a human or animal body to detect the presence and approximate positions, or construct an image of a pathological entity within the body. 
     According to a second aspect of the present invention there is provided a method of detecting the presence of a pathological entity within the human or animal body, the method comprising: 
     i) directing a continuous coherent light wave of a first predetermined wavelength into the body; 
     ii) detecting dynamically scattered light emerging from the body; 
     iii) correlating the detected light photons in the time or frequency domain; and 
     iv) determining the presence of a pathological entity from analysis of differences between correlation and the correlation arising from photons scattered by the media surrounding the entity only. 
     According to a third aspect of the present invention there is provided apparatus for detecting an object located within a dynamic scattering media, the apparatus comprising means for directing a continuous coherent light wave of a predetermined wavelength into the media, means for detecting dynamically scattered light emerging from the media, means for correlating the detected light photons in the time or frequency domain, whereby the presence of an object can be determined from analysis of differences between said correlation and the correlation which would arise from photons scattered by the media only, the approximate position of the object within the media can be determined from said analysis of the correlation and knowledge of the mean transport path of the light wave of predetermined wavelength within the media. 
     A plurality of detectors may be arranged in an array to provide a series of measurements simultaneously. The array of detectors may be coupled to 3 CCD camera. 
     Preferably the or each detector is either located adjacent the emitter or is displaced from the emitter and is located on the axis of emission of the emitter 
     The coherent light producing means produces both visible light and infrared light. 
     Polarising filters may be located in front of the detection means to select either light with a polarisation perpendicular to the light emitted from the emitter, or to select light with the same polarisation as the light emitted from the emitter. 
     An object located within a medium may be caused to modulate to increase the contrast of the phase of scattered light, and thereby improve the resolution of the image. 
     The invention may be used in combination with Time of Flight apparatus to provide imaging over both short and long distances. 
     Where heterodyne detection is to be used, the coupling means may be provided with a window which will reflect a fraction of the light towards the detection means, thereby providing a heterodyne signal. 
     The window may be adjustably displaced from the coupling means to allow control of the intensity of reflected light incident at the detection means. 
     The window may be arranged to cause the reflected light to undergo multiple reflections before being incident at the detection means, thereby allowing the path length traveled by the reflected light to be controlled. 
     The coupling means and detection means may comprised polarisation preserving optical fibres. The fibres may be mounted so as to be rotatable through 90 degrees, thereby allowing modification of the effective numerical aperture of the fibres. 
     Preferably the means for producing coherent light comprises a laser. 
     Preferably four lasers operable at a different wavelength are used concurrently. 
     Preferably, two of the lasers are arranged to produce orthogonally polarised light, and a polarising beamsplitter cube is provided to couple the light into a polarisation preserving optical fibre. 
     The coupling means and detection means may comprise optical fibres with terminations located in a probe comprising a cylindrical head. 
     According to a fourth aspect of the present invention there is provided apparatus for detecting the presence of a pathological entity within the human or animal body, the apparatus comprising means for directing a continuous coherent light wave of a first predetermined wavelength into the body, means for detecting dynamically scattered light emerging form the body, and means for correlating the detected light photons in the time or frequency domain, whereby determining the presence of a pathological entity may be determined from analysis of differences between said correlation and the correlation arising from photons scattered by the media surrounding the entity only. 
     According to a fifth aspect of the present invention there is provided a method of detecting the presence of an object within a media, the method comprising 
     i) inducing vibration in the object at a predetermined frequency which does not propagate efficiently within the media; 
     ii) generating a continuous coherent light wave; 
     iii) modulating the generated light wave at a second predetermined frequency; 
     iv) directing the modulated coherent light wave into the media; 
     v) detecting scattered light emerging from the media; 
     vi) analysing the detected scattered light for the existence of a beat signal corresponding to the beat frequency between the first and second predetermined frequencies thereby indicating the presence of the object. 
     The frequency of the induced vibration is selected to correspond To the resonant frequency of the object, or regions within the object to be detected. Such regions may for instance be regions of stress, such as cracks in the object. 
     The vibrating frequency may be varied until the object, or parts of the object to be detected, resonates, and wherein the size of the resonating object, or region of the object, is determined as a function of the resonating frequency. 
     An image of the object, or parts of regions of the object to be detected, may be constructed by detecting light scattered from different parts of the object. 
     The method may be performed on a human or animal body to detect the presence, or construct an image, of a pathological entity within the body. 
     The invention also provides apparatus for detecting the presence of an object within a media, the apparatus comprising: 
     i) means for inducing vibration in the object at a predetermined frequency which does not propagate efficiently within the media; 
     ii) means for generating a continuous coherent light wave; 
     iii) modulating the generated light wave at a second predetermined frequency; 
     iv) means for directing thy modulated coherent light wave into the media; 
     v) means for,detecting scattered light emerging from the media; 
     vi) means for analysing the detected scattered light for the existence of a beat signal corresponding to the beat frequency between the first and second predetermined frequencies thereby indicating the presence of the object. 
     The method and apparatus according to the fifth aspect of the invention may be used to investigate properties that cannot be imaged directly such as scaling in an oil pipe or cracks in a metal structure. 
     The modulation frequency may be used to perform a secondary function on the object. For example, resonance may be used to cause descaling of a pipe whilst an image (by means of measurement of the change in the magnitude of resonance at a single frequency or preferably the change in the resonant frequency) is used to monitor the descaling operation in real time. Similarly, where the methods is used on a human or animal body, the same method may be used to remove or destroy pathological entities such as tumours (wherein the resonance may heat and kill the target), gall stone (wherein the resonance will physically break down the target), kidney stones, and other growths, foreign bodies and abnormalities. A similar method may be used to remove blockages etc. in non-medical applications, such as to remove blockages from target objects such as underwater pipe lines or cables. 
     Resonance may also be used to modify the target object means of chemical release or activation. 
     The invention may utilise a naturally occurring source as the source of the investigative wave. 
     The present invention may be combined with existing detection or imaging systems, such as time of flight systems, CAT, electron resonance etc. 
     Other possible features of the invention will become apparent from the description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of an imaging apparatus according to a first aspect of the present invention; 
     FIG. 2 is shows three auto-correlation traces which illustrate the operation of the apparatus of FIG. 1; 
     FIG. 3 is a schematic diagram of a first heterodyne window suitable for use with the present invention; 
     FIG. 4 is a schematic diagram of a second heterodyne window suitable for use with the invention; 
     FIG. 5 is a schematic diagram of a third heterodyne window suitable for use with to the invention; 
     FIG. 6 is a schematic diagram of a light generating apparatus; 
     FIG. 7 is a schematic diagram of a detection apparatus; 
     FIGS. 8 a  and  8   b  schematically diagram orthogonal cross-sections of a probe for use in the present invention; 
     FIG. 9 is a schematic diagram of a fibre block for use in the present invention; 
     FIG. 10 is a schematic illustration of an imaging apparatus according to a second aspect of the invention; 
     FIG. 11 is a schematic illustration of the operation of the apparatus of FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1 shows an imaging apparatus comprising three lasers  1  operating at different wavelengths. The three lasers produce light wavelengths 488 nm (argon ion), 633 nm (helium neon) and 1064 nm (Nd:YAG) respectively, the light from each laser  1  having a single transverse and longitudinal mode. Optics  2  (an example of which is described in more detail below with reference to FIG. 6) couple light from each laser  1  into a single mode polarisation maintaining optical fibre  3 . Light from each fibre  3  is launched into a turbid medium  4  within which is located an absorbing object  5 . 
     Light scattered by the turbid medium  4  is collected by three optical fibres  6  which may have the same construction as the launch fibres  3 . The collected light is collimated using farther optics  7  (an example of which is described in more detail below with reference to FIG. 7) and directed at polarising filters  8  and laser-line optical filters  9 . Light transmitted by the laser-line filters  9  is monitored by detectors  10 , a respective detector  10  being arranged to detect scattered light of each of the three laser  1  wavelengths. The detectors  10  are photon counting photon multiplier tubes or photon counting avalanche photodiodes, whichever is appropriate for the wavelength of light to be detected. An output it signal from each detector  10  passes to a digital correlator which produces a correlation trace (described below with reference to FIG.  2 ). The correlation trace may be displayed on a monitor  11 . 
     The illustrated apparatus is configured to detect light scattered through 180degrees (back scatter). The apparatus could be configured to detect light scattered through other angles including light which, although scattered, exits the turbid medium  4  in the same direction as the incident light (i.e. scattering through 0 degrees). Whilst operation of the apparatus at any angle is possible, the mathematical modelling required to obtain imaging becomes more complicated for scattering angles other than 180 degrees or 0 degrees and thus these two configurations are preferred. 
     FIG. 2, parts  2   a  to  2   c  show three correlation traces obtained from the scattered light detected at each of the detectors  10 . Each correlation is obtained by correlating detected light with itself (i.e. the known technique of auto-correlation). Each correlation trace shows the logarithm of the number of photons detected (vertical axis) versus the square root of the correlator delay time (i.e. the delay time of the auto-correlation of the detected signal). The region of the correlation trace near to the vertical axis represents photons which have undergone a great deal of scattering, lesser amounts of scattering are indicated further from the vertical axis (for light with purely random phase the correlation would be a flat line). It is important to note that some form of motion of the medium  4  is required so that scattering of the light by the medium will modify the phase of the light (Brownian Motion is often sufficient for this purpose). 
     The correlation trace of FIG. 2 a  is obtained from detection of the shortest wavelength light (i,e. 488 nm). The turbid medium  4  will scatter the relatively short wavelength light efficiently, and consequently in the illustrated example the average penetration of photons into the turbid medium  4  is less than the distance to the absorbing object  5 . Thus, the short wavelength light is largely unaffected by the absorbing object  5 , and the correlation trace obtained will have a shape characteristic of random scattering by the medium  4  (i.e. a straight line sloping downwardly away from a maximum value of 1). Thus the correlation trace indicates that the depth of the object  5  within the medium  4  is greater than the average scattering depth of the 488 nm light (the average scattering depth or 488 nm light in a given medium may be known from a reference text or from experimentation). 
     The correlation trace  2   b  is obtained from detection of the medium wavelength light (i.e. 633 nm), which has a greater average penetration depth than the relatively shorter wavelength light. The trace shows some attenuation close to the origin, which indicates that those photons which lave travelled the longest path through the medium  4  (approximating to diffuse photons) have been preferentially depleted by the absorbing object  5 . Further away from the origin of the trace those photons which have travelled a shorter path are unaffected by the object  5 , and the downward sloping straight line of the trace is unaffected by the shape of the correlation trace thus provides information pertaining to the depth and shape of the object  5 . 
     Correlation trace  2   c  represents long wavelength light (i.e. 1064 nm) which has a greater average scattering depth than the 633 nm light. The effect of the object  5  on the trace is more accentuated than the effect on correlation trace B, since more of the 1064 nm photons penetrate sufficiently far into the medium  4  that many of them are absorbed by the object  5 ). Thus the shape of the correlation trace  2   c  provides further information pertaining to the depth and shape of the object  5 . 
     The invention thus uses light of three different wavelengths to determine the position of the absorbing object  5 . The depth of the object is determined by comparison of the shapes of the three correlation traces  2   a-c.    
     A single measurement as described above will describe the position of a small area of the object  5 . To obtain an image of the object the position of the optics (not shown) which launch light from the fibres  3  into the medium, and/or the position of the optics (not shown) which couple light from the medium into the fibres  6 , are translated between measurements to provide a grid arrangement of measured areas. 
     The wavelengths used may be preselected to provide a desired range of penetration depths into a given medium. It is noted that a coherent light source capable of being tuned over a wire range of wavelengths (for example an Optical Parametric Oscillator) would provide the capability to pre-select wavelengths, and also to obtain many measurements at different wavelengths (although the three wavelengths used above should be sufficient to obtain accurate imaging). 
     The apparatus may comprise single emitters and multiple detectors, thereby allowing many point measurements. The fibres collecting scattered light could be coupled directly (or via an intensifier) to arrays of detectors such as charge-couple device (CCD) cameras. 
     Inclusion of the polarising filters  8  enables further resolution of the depth of the object  5 . When polarising filters  8  are set so as to transmit light which is polarised transverse to the light from the lasers  1 , only photons which have undergone many scattering events will be detected. This orientation of polarising filters thus suppresses detection of photons which have penetrated a relatively small distance into the medium  4 . When the polarising filters  8  are rotated to transmit light which has the same polarisation as the light from the lasers  1 , photons which have undergone many scattering events arc suppressed, and photons which have penetrated a relatively small distance into the medium  4  are detected. 
     When optical fibres  3 , 6  which support only a single eigen-mode of light at an appropriate wavelength are used, the polarising filters  8  are not needed. This is because the transverse polarisation of the light produced by the lasers  1  will be preserved by the fibres  3  which launch the light into the medium  4 . The orientation of the fibres  6  which couple scattered light from the medium can be set so as to couple light with the same polarisation as that emitted from the lasers  1 , or light with polarisation orthogonal to that emitted from the lasers. This will allow discrimination of penetration depth as described above. 
     Optical fibres  3 , 6  capable of supporting multiple wavelengths would reduce the number of fibres  3 , 6  required by the apparatus (it is noted that the use of polarisation maintaining beamsplitters would be advantageous in this arrangement to allow the combination of beams from the different lasers  1 ). 
     A fibre having multiple cores would allow different emitter/detector spacing within that fibre. 
     It is noted that the invention may be used to image reflecting objects, in addition to imagine absorbing objects as described above. Light reflected from the object will cause a perturbation at or close to the vertical axis of a trace. 
     The invention may be applied to imaging of the human body, for example to detect tumours. For body imaging, utilisation of both 180 and 0 degree measurements in a number of positions will produce the best results. The position of launch of light into the body or of collection of scattered light, may be varied by scanning the apparatus appropriately or by using multiple emitters and/or detectors. After operating the apparatus at a series of positions correlations obtained are normalised and compared. Differences in correlation functions close to the vertical axis will suggest a change in the effective ‘viscosity’ or ‘refractive index’ of the body at that point, which may indicate the presence of for example, a tumour. 
     Heterodyne signal processing may be used prior to auto-correlation. An apparatus which will provide a heterodyne signal is shown in FIG.  3 . Light is transmitted through emission optics  12  into a window  13 . At a lowermost surface of the window  13  the light passes into the turbid medium  4 , and a fraction of the light is reflected due to the difference in refractive index of the window  13  and the medium  4 . Detection optics  14  collect scattered light and reflected light which interferes to produce a beat signal, and the beat signal is processed to provide imaging in the same way as described in relation to FIG.  2 . The triangular area  15  located beneath the detection optics  14  represents the detection area of the optics  14  (i.e. corresponds to the numerical aperture of the optics  14 ). The emission optics  12  and detection optics  14  are index-matched to the window  13  to prevent waveguiding of light within the Window  13 . 
     The apparatus shown in FIG. 3 has two limitations. The first limitation is that the ratio of heterodyne (i.e. reflected) light relative to scattered (i.e. homodyne) tight cannot be controlled. The second limitation is that the heterodyne light and homodyne light travel different path lengths, as a consequence of which stringent coherence of the beam is required. These two limitations are linked, since when the homodyne light has a short path length in the medium  4  the intensity of scattered homodyne light detected will be high and a strong heterodyne signal will be required. 
     An apparatus which overcomes the first of the above limitations is shown in FIG.  4 . Emission optics  12  and detection optics  14  are located above optical window  13 . The optics  12 , 14  are not index-matched to the window  13 , and a proportion of light emitted from the optics  12  will be reflected by the window  13  and collected by the detection optics  14 . The window is index-matched to the turbid medium  4  so that there are no reflections from the lowermost surface of the window  13 . Light scattered by the medium  4  interferes with light reflected from the window  13  to provide a beat signal for professing as before. However, the separation of the window  13  from the optics  12 , 14  allows the optics to be moved relative to the window  13 , thereby controlling the proportion of reflected light which is collected by the detection optics  14  (i.e. moving the window  13  further from the optics  12 , 14  will increase the proportion of reflected light collected). 
     The apparatus illustrated in FIG. 4 does not allow control of the heterodyne path length (i.e. path length of the reflected signal). The mean path length of the heterodyne signal may be controlled using the window  13  as a guide for multiple reflections as shown in FIG.  5 . The emission optics  12  and detection optics  14  are not index-matched to the window  13 , and the window is not index-matched to the turbid medium  4 , so that a portions of the emitted light is reflected from the uppermost and that lowermost surfaces of the window  13 . Heterodyne light may enter the detection area ( 4 ) by multiple reflections from the surfaces of the window  13 . Moving the window  13  toward the emission optics  12  and the detection optics  14  will reduce the heterodyne signal, and increase the mean optical path length of the heterodyne signal. Reduction of the path length of the heterodyne component (by moving the window  13  further from the probe) will increase the heterodyne signal strength. Thus, using a windows  13  of selected material, optical coatings and thickness it is possible to provide a heterodyne signal of the desired path length and intensity. 
     It is noted that since light will be emitted by the emission optics with a finite divergence, there will be a spread in the path lengths travelled by the heterodyne light. 
     FIG. 6 illustrates a configuration for coupling light from several lasers into two optical fibres. A configuration based on that illustrated may be used to couple light from three lasers into three separate optical fibres, as is required by the apparatus shown schematically in FIG.  1  and described above. 
     The primary lasers are: a single longitudinal mode frequency doubled horizontally polarised Nd:YAG laser  16  (5 mw of output power at 532 nm), and a stabilised horizontally polarised 675 nm laser-diode unit  17 . Two further lasers may be used to provide light at wavelengths of 580-833 nm (eg. 780 nm stabilised diode laser  18 ) and 450-670 nm (eg. 488 nm single transverse mode argon ion laser  19 ). 
     Light from the Nd:YAG laser  16  and diode laser  17  is directed to a polarising beamsplitting cube  20  via baffles  21  and mirrors  22 . The dimensions of the baffles  21  and mirrors  22  are minimised to reduce the possibility of multiple reflections of light re-entering the lasers  16 , 17 . The mirrors  22  are capable of rotational and translational movement. Light from the lasers  16 , 17  may pass through laser line filters  23  (eg. birefringent filters) if required. 
     Since the light from the lasers  16 , 17  is horizontally polarised it will pass, without reflection, through the polarising beamsplitter cube  20 . Any component of the light which is not horizontally polarised will be directed down to a prism  24 . One short side of the prism  24  is painted black, to minimised reflection of the light back to the polarising beamsplitter cube  20 . 
     The polarising beamsplitter cube  20  is used to combine (vertically polarised) light from the 488 nm laser  19  With the light from the Nd:YAG laser  16 , and to combine (vertically polarised) light from the 780 nm stabilised diode laser  18  With light from the 675 nm laser-diode unit  17 . Light from the 488 nm single transverse mode argon ion laser  19  is coupled to the beamsplitter cube  20  using a suitable lens  25 . The lens  25  may for example be plano-convex, anti-reflection coated on a curved side and coupled to the polarising beamsplitter cube  20  to reduce reflections from the interface between the lens  25  and the polarising beamsplitter  20 . The 780 nm laser  18  may be collimated via a gradient index lens which is index-matched to the beamsplitter cube  20 . 
     The beams pass from the polarising beamsplitter cube  20  to two achromatic lenses  26  which focus the light into aligned fibres  27  held in two blocks  28 . 
     Light at 532 nm from the Nd:YAG laser  16  is launched onto The fast axis of a first of the optical fibres  27   a , and light from the 488 nm laser  19  is launched onto the slow axis of the first optical fibre  27   a . Similarly, light from the 675 nm laser-diode unit  17  is launched onto the fast axis of a second of the optical fibres  27   b , and light from the 780 nm stabilised diode laser  18  is launched onto the slow axis of the second optical fibre  27   b . Using the slow axis of the fibre  27  (which has a high numerical aperture) for the shorter wavelength 488 nm light accentuates the increased scattering suffered by the lower wavelength, once the 488 nm light will have a wider angle close field of view compared with the 532 nm light. The 675 nm light is directly comparable with the 532 nm light since both pass down a fast axis of a fibre  27 . However, the 675 nm light will suffer less scattering within a turbid medium and will thus analyse material weighted deeper in the sample. 
     The blocks  28  are capable of rotating the optical fibres  27  through 90 degrees to allow the eigenmodes of the fibres  27  to be changed. The part of the fibres  27  held in the blocks  23  is mode-stripped to prevent light being launched into the cladding of the fibres  27 . 
     An avalanche photodiode may be used to detect the 780 nm light, the other wavelengths being detected using photo-multiplier tubes. 
     Coupling light into a medium using low numerical aperture optics, and detecting light polarised at right angles to the coupled light will allow objects as deep as possible to be detected, and ensures that only high order multiple scattering is detected, which scattering is most suited to the known multiple light scattering models. By altering the wavelength of the coupled light, the depth monitored by the technique will be significantly affected, since scattering or light is wavelength dependant. Where both transmission (0 degree scattering) and back-scatter (180 degree scattering) measurements are possible, information pertaining to the depth of an object in the medium could be obtained from the difference between these two measurements. 
     FIG. 7 illustrates apparatus for detecting light scattered by a turbid medium, and may comprise part of the apparatus shown schematically in FIG.  1  and described above. 
     In the apparatus illustrated light is coupled by detection optics (not shown) into an optical fibre  29 . An emission end of the fibre  29  is held in a block  30  that is fixed in one of two positions to allow selection of polarisation modes. An achromatic lens  31 , of higher numerical aperture than the fibre to allow for misalignment, collimates light from the fibre  29 . The light may be attenuated by a simple moving beam block  32 ; this is more efficient than attenuating or reducing the laser intensity since both the signal and the background are attenuated. The light passes through a laser line (or other) filter  33  which removes unwanted laser frequencies and background noise. A polarising filter  34  selects the required polarisation state, and the light is then focused by an achromatic lens  35  onto a photo-multiplier tube  36  (or avalanche photodiode). The signal detected by the photo-multiplier tube  36  is amplified and filtered  37  prior to signal analysis. 
     FIGS. 8 a  and  8   b  shows a fibre probe which combines emission fibres  38 , 39  and detection fibres  39 , 40 . The fibre probe may be used as part of the apparatus shown schematically in FIG. 1 to couple light into and out of a turbid medium 
     Light of 488 nm and 532 nm is emitted down a first fibre of the probe  38  and collected by a second fibre  40 . Both fibres  38 , 40  are bow tie (polarisation preserving) fibres, their eigen modes being set at 90 degrees so that only light which has suffered a polarisation rotation through 90 degrees during scattering in the turbid medium will be detected. The 675 and 780 nm light is similarly emitted and detected by fibres  39 , 41 . The fibres  38 - 41  are glued at their tips  42  into a short capillary  43 , typically a few mm long. The tips  42  of the detection fibres  39 , 41 , and an adjacent portion thereof, are stripped of fibre cladding, and black paint is applied around the outside of the fibre to mode-strip the collected light (i.e. prevent light being coupled into the cladding). The short capillary  43  is filled with black paint. The emission  38 , 40  and detection  39 , 41  fibres are held in separate jackets (not shown), beyond the capillary  43 , to prevent cross talk of light between them. 
     A second capillary  44  surrounds the first capillary  43  and fibres  38 - 41  (typical diameter 3 mml, and further capillaries may be added. The second capillary  44  is encased to within 0.5 mm from the ends of the fibres  38 - 41  with a protective steel jacket  45 . The jacket  45  is a loose sliding fit on the capillary  44  and is affixed with silicone, to stop thermal stressing. The probe is fitted with a window holder  46  that forms a close sliding fit with the steel jacket  45 . A window  47  may be glued or fused to the holder  46 . A temperature sensor (not shown) may be attached to the window holder  46 . 
     A fibre block of the type which has been shown in FIGS. 6 and 7 is illustrated in detail in FIG.  9 . The fibre block  48  is cylindrical and is provided with two chamfered holes  49  spaced apart by 90 degrees about the block. A single ball bearing  50  provided in a block-mounting (not shown) is resiliantly biased to locate within either of the holes  49 , thereby allowing the block to be rotated accurately through 90 degrees about its axis. The block  48  has a recessed face  51  to allow polishing of a fibre held therein  52  and an inner capillary  53  without contamination. The inner capillary  53  is located in an outer capillary  54  allowing that part of the fibre  52  which is mode-stripped to be protected. The fibre  52  is glued to the inner capillary  53  and supported by silicone  55  as it exits the outer capillary  55 . 
     The area of a medium which is imaged by the apparatus described above will be influenced by the distance and angle between the detetor/s and emitter optics. 
     Auto-correlation of the detected signal, or heterodyne detection, may be substituted by any other processing which is sensitive to the dynamics of a system being imaged, for example pulse arrival distribution or a frequency scan from a etalon. 
     Where the object is reflective the object itself may be used to obtain a heterodyne signal. 
     The methods according to the present invention have been in terms of light of visible frequency, however any wave foam that may be produced with a coherence equivalent to the maximum path length difference of the quanta may be used. 
     The invention is applicable to any system where information is to be transmitted through turbid media, although it is particularly suited to body imaging and undersea surveillance. 
     For imaging of a reflective object, where insufficient dynamic information is present to obtain a useful correlation (due to in insufficient amount of Brownian Motion in the target), a modulator may be used to induce vibration of the object. Inducing resonance of an object will thus improve the contrast obtained via diffuse wave imaging (the resonance of the object will result in a heterodyne signal). 
     For imaging of an absorbing object, the vibration may be induced in the medium surrounding the object may be in the event that there is insufficient Brownian Motion to provide dynamic scattering of light in the medium. 
     A specific application of diffuse wave imaging in which resonant modulation of a target may be useful is imaging of the human body, for example where the body has a rumour growth, foreign body or other abnormality surrounded by normal tissue. A sonic wave may be used to set-up resonance of the target tissue without inducing significant modulation in the tissue surrounding the target. Tissue will give a very low frequency dynamic scattering signal (Brownian type motion) as particles in the tissue are constrained and scattering centres are located in a soft solid. 
     The resonance of the target tissue will depend upon its viscoelastic properties and size of the primary particles constituting the target. Diffuse Wave imaging apparatus may be used to view the target directly. Resonance of the target may also be used to treat the foreign body as it is being viewed, i.e. to break a gall stone, or to heat a tumour. Resonance may also be used to trigger a secondary (or more) chemical reaction such as a drug release whilst the target is imaged. 
     An alternative technique for obtaining an image of a modulating object which does not use Diffuse Wave Imaging will now be described with reference to FIGS. 10 and 11 (the technique will be referred to as Antenna Imaging). Antenna Imaging uses a modulation of an investigative wave (usually optical, and from hereon referred to as investigative beam) and modulation of a target by a modulation wave (usually acoustic). The frequency Of the modulation wave is chosen to be between 10 and 90% of the frequency applied to the investigative beam. Therefore, if the investigative beam has a frequency F. The modulation wave may have a frequency of 0.75F. A fraction of the investigative beam is diverted towards a detector, without first entering the medium, thereby providing a heterodyne beam. 
     The investigative beam will be scattered from the target, and a proportion of the investigative beam will also be scattered by the medium surrounding the target. Light from the investigative beam that has not been scattered from the target will have a modulations frequency F, although there will be a spread of modulation frequencies due to dynamic light scattering in the media. This light is combined with the heterodyne beam prior to detection to produce a beat signal, which in this case will have a frequency of 0Hz. Light that hits the target will have the modulation frequency of the target applied to it. When this light is combined with the heterodyne beam it will produce beats of frequency 0.25F (ie. F-0.75F), thus indicating the presence of the target. 
     The investigative beam and/or the modulation wave may be scanned (both spatially, or by frequency), thereby allowing an image to be built up. The wavelength and other physical properties (eg. polarisation) of the investigative wave and the detection means, may be used to alter the depth of penetration of the investigative wave into the media surrounding the target. 
     Where the media is absorbing of the investigative wave, and the absorption is a function of wavelength then a series of wavelengths may be used to analyse the depth of the target. Where the media is scattering and the scattering is a function of wavelength, this property may also be used to analyse depth of the target. 
     All returning light will have a slight frequency spread due to dynamic light scattering by the media. However, the detected signal is integrated between 0.1F and 0.4F before forming an image, thus lessening the effect of the frequency spread. Multiple scattering of the light (or other investigative wave) may lead to a slight loss of resolution, but since the invention uses the magnitude and position of the centre frequency of the detected light to form the image, dynamic scattering of the light will have only a limited effect. 
     Two forms of resonance modulation may be used to induce modulation of the target. The first form of modulation is modulation of the entire target, where the wavelength is a harmonic of the target. In this form the resonant frequency may be used to size the object very accurately. The second form of modulation is modulation of constituents of the target, where in primary particles within the target are made to modulate. The modulation could have a secondary function of heating or breaking down the target or objects attached to it. 
     The medium surrounding the object could be made to modulate whilst the object to be imaged is not modulated. For example, in imaging of the human or animal body it may be practical to choose an acoustic wavelength that passes through normal tissue but is highly attenuated by a tumour (in order to induce motion in the tissue, and thereby obtain dynamic light scattering). Correlation traces thus obtained will be of the same form as those shown FIG. 2, but with increased contrast. 
     In one specific example of an application of the invention, an undersea pipe may be modulated directly at an oil platform or a shore base, at a resonant frequency that does not propagate efficiently through water. An antenna camera may then be used to track the pipe and provide an image of the pipe. Alternatively, the pipe may be modulated by a transducer permanently located on the pipe. 
     The pipe may be imaged directly, for example to look for damage (ie. a source of light and detector may be located immediately adjacent the pipe). In this case a transducer may be included as part of a single piece of equipment which also contains the light source and detector. 
     Resonant modulation of the pipe may allow imaging of cracks, blockages and buildup within the pipe which would not otherwise be visible. This is done by choosing a modulation frequency which corresponds to a frequency of, for example, blockages located within the pipe. Sensitivity to a blockage may be improved by varying and measuring the resonant frequency at the blockage. 
     An apparatus which may be used to provide an image of a pipe located on the sea bed is shown in FIG. 10. A light source  56  emits coherent light which is coupled through a polarising filter  57  and an optical filter  53 . The light is then modulated at a frequency F by an optical modulator  59 , and passes via a variable mixer  60  into a medium in which the target  61  is located (in this example the medium is water, and the target  61  is a pipe). 
     A proportion of the coherent light is diverted towards a detector  62  without entering the water, and this light forms a reference. The remaining light is coupled into the water, and is scattered by the water and from the pipe  61 . Scattered light is collected and passes through the variable mixer  60 , via a polarising filter  63  and an optical filter  64  to the detector  62 . 
     A modulator  63  is located on the pipe  61  and is used to cause resonant modulation of the pipe  61 . This resonant modulation will be applied to the coherent light when it is scattered from the pipe  61 . When scattered light is detected it will interfere with the reference light to produce a beat signal, with a frequency determined by the difference between the frequency of modulation of the pipe  61  and the frequency of modulation of the coherent light by the modulator  59 . The generation of the beat signal is described above. A band-pass filter  66  is used to select the beat frequency from the signal produced by the detector  62 . 
     The polarising filters  57 ,  63  are set at orthogonal polarisations when the apparatus is used to detect light that has travelled more than one photon transport path, or when the target is not a specular reflector, since this minimises detection of photons which have been scattered by the water rather than the pipe  61 . When the apparatus is used in this way 50% of the light that has been scattered from the pipe  61  will be filtered out by the polarising filter  63 . However, the suppression of photons which have undergone low order scattering in the water offsets this loss of signal. The optical filter  58  may be used in conjunction with a spatial filter (not shown) to improve the coherence of the light prior to modulation. 
     FIG. 11 illustrates the operation of the apparatus of FIG.  10 . FIG. 11 a  shows the frequency F of the modulation applied to the coherent light by the modulator  4 . FIG. 11 b  shows the frequency applied to the pipe  61  by the acoustic modulator  10 . FIG. 11 c  shows the frequency spread of light incident at the detector  62 . Light which has been scattered from the pipe  61  will have a frequency component labelled ‘S’. There is significant broadening of the detected frequency about ‘S’ due to scattering of the light by the water. There will also be a significant noise band at OHz due to homodyne scattering of the light, and the line-width of this light will be broadened due to scattering. FIG. 11 c  shows a trace obtained by auto-correlation of the detected signal. The auto-correlation is used to provide imaging of the pipe. 
     Heterodyne imaging (i.e. where the investigative light is made to mix with light that has travelled a different path) does not necessarily require direct modulation of the investigative beam as described above. Heterodyning of the investigative beam could also occur between light modulated by the target and light scattered by the media, although this will limit control of the relative magnitude of the reference signal. Heterodyning of the investigative beam could also occur between light modulated by the target and a portion of light taken from the incident bean (conventional heterodyne). This gives a signal around 0Hz and is ideal for digital correlation of quanta but is noisy or signal analysers. 
     Apparatus for providing a heterodyne signal obtained directly from a source is illustrated in FIGS. 3 to  6  as described above.