DETAILED DESCRIPTION OF THE INVENTION

[0052] In FIG. 1 we show a generalized schematic volume probe array, 10 , whose function is to collect data from a plurality of points in a target sample. The system generally includes an appropriate light source 11 , whose light output is conditioned and may be multiplexed in block 12 to create a plurality of light sources to be relayed to an array 13 of light valves. These light valves can act as illuminating field stops or aperture field stops, and only one valve is open at a given time, thus providing for sequential illumination of volume elements in sample 19 . The light emanating from each light valve is then directed to a targeted volume element in the sample 19 with an appropriate illumination objective 14 . In some embodiments, a single objective lens is used, while in other embodiments, we incorporate an array of objective microlens having the same periodicity as that of the light valve array.

[0053] Responses from each targeted volume element in the form of light emanating from the volume elements, is collected through a collection optics objective 15 (which in some embodiments can be the same as the illumination objective and an array of microlens), and through an array 16 of light valves (which can also be the same array as the one used for illumination). The responses are then directed to one or more detectors 17 to determine their optical and spectral characteristics.

[0054] It should be emphasized that both the illumination optics and the collection optics each contain a field stop having dimensions that are relatively large in relation to the average wavelength of the illuminating radiation, and furthermore, these field stops are conjugated to each other through the volume element examined. As a result, a well defined volume element is illuminated at any time, and the optical response from the element collected through the field stop of the collection optics is essentially limited to responses emanating from the volume element.

[0055] A controller 18 is provided to control the sequencing of the volume elements scanned (in the x,y plane, the plane of the sample) and to control the depth of the volume elements examined (in the z direction.)

[0056] In FIG. 2, a simple example of an array volume microprobe system 20 is shown. The system includes a light source 21 . Light from the light source is condensed with a lens 22 onto an array of light shutters 24 , through a beam splitter 25 . In this embodiment, each element 28 in the light shutter array serves as a field stop which is being imaged through an objective lens 26 on a sample 27 . The dimensions and shape of the shutters determine the morphology of volume elements sampled in a manner discussed in detail in copending applications, Ser. Nos. 08/510,041 and 08/510,043. In essence, the mean dimension, d, of each shutter is selected to be larger than the wavelength divided by the numerical aperture, NA, of the objective or d>λ/NA. Thus the image of the field stop in the plane of the sample is larger than the diffraction limited resolution for the wavelength. As a result, a very large proportion of the light that traverses a given field stop and is imaged in the sample is within a well-defined volume element of the sample. Similarly, while the total response to the illumination is distributed over a very large spatial angle (essentially 4π steradians), only responses that are emanating from within the same volume element are imaged back onto the field stop and reach detector 29 , by being reflected on the beam splitter 25 onto a collector lens 23 which concentrates the response onto the detector 29 . This results from the fact that the respective field stops of the illumination and detection systems are conjugated to each other via the target volume element. In the embodiment shown in FIG. 2 , both field stops are embodied within the same aperture (an optical shutter or a light valve 28 within the optical shutter array).

[0057] In some embodiments, the beam splitter 25 can be a dichroic mirror, particularly when the light source is a short wavelength (UV) exciting light source and the responses are fluorescence responses. In other embodiments, the beam splitter 25 can be a half silvered mirror which separates the optical path of responses from the sample from the optical path of the exciting beam, for instance, when the exciting beam is provided with a broad spectrum light source, and the responses involve back scattering and reflections from the sample (and thus mostly the extent of absorption of the exciting beam in the targeted volume element is examined).

[0058] The array of light valves, or optical shutters, can be implemented in a number of different ways. One can use liquid crystal sandwiched between two electrode arrays (deposited, as is in the prior art on transparent glass or plastic sheets in the form of transparent electrodes made of Indium Tin Oxide (ITO) or Tin Oxide (TO), usually doped with fluorine to provide good areal conductivity). One can also use films of PDLC (polymer dispersed liquid crystals) that might be easier to handle and have lower production costs. Another embodiment contemplated, when the required scanning is particularly fast, is an array of ferroelectric elements, each acting as a light valve. Yet another embodiment of the light valves can involve an array of PVDF (Polyvinyl-difluoride) bimorphs, each coated to be reflective (or opaque on both sides) on the side facing the light source and designed to bend out of the light path so as to create a light valve. The typical dimensions of the light valve range from a low of about 20 microns to as much as 1000 microns. The size is determined primarily by the application, the nature of the sample analyzed and the particular design of the specific array volume microprobe utilized. When using the general design of FIG. 2 , with a single large objective lens serving as a common objective to all the field stops in the array, the space between adjacent light valves is usually kept as small as possible, so as to provide as closely spaced as possible scanned volume elements. It should be understood however that in some embodiments, the spacing is kept relatively large (as large as the field stop itself) when an image of the pathology consisting of well spaced discrete points is more appropriate.

[0059] In operation, the controller 18 keeps one of the light valves open and adjusts the position of the device so as to image the field stop at the desired volume element in the sample 27 . Once the general position of the device relative to the sample has been optimized, the controller causes scanning of the surface of the specimen in the xy (the plane of the specimen) direction by sequentially closing an open light valve and opening an adjacent light valve. The time interval of each light valve in the open position is a strong function of the intensity of the light source and the efficiency of collection of the response from each volume element. In some embodiments, this time interval can be shorter than a millisecond, while in other embodiments tens to hundreds milliseconds are required.

[0060] The controller 18 also controls the position of the volume elements within the sample in the z direction, generally an axis perpendicular to the plane of the samples. This can be achieved in a number of ways. For instance, the whole optical assembly can be moved back and forth in the z direction. In some embodiments, this translational movement of the image plane of the field stops in the sample can be achieved by moving the objective alone, or the array of light valves, or both of these elements simultaneously. The specific design depends on the particular embodiment of the device.

[0061] It should be appreciated that since the intensity of illumination is highest within the volume element probed by the excitation beam (relative to zones surrounding the volume element), and that the response detected by the sensor 29 is primarily from the same volume element (and contains very little illumination emanating from zones surrounding the volume element), that changing the position of the volume element within the sample in the z direction will provide responses from various depths of the sample. This, in essence allows for analysis, in vivo of tissues at various depths, as long as the overall absorption of the illuminating beam by the tissue and the responses to the beam are not excessive.

[0062] In FIG. 3, a slightly different embodiment of an array volume microprobe 30 of the present invention is shown. The system includes a light source 31 with appropriate optics (not shown) to project a common field stop 41 through a condenser lens 32 onto an addressable shutter array 34 . Each element in the addressable shutter array can be considered an aperture stop which serves to further limit the spatial distribution of the light impinging on a sample 37 . In lieu of using a large singular objective (as used in the embodiment described in FIG. 2 ) to image the field stop on each volume element in the sample 37 , a lens array 36 is interposed between the shutter array and the sample. The lens array 36 consists of a plurality of microlens 42 . The periodicity of the lens array is exactly the same as that of the shutter array, and each lens 42 within the lens array 36 corresponds to a light valve 38 within the light shutter array 34 . In most embodiments, the light impinging on the shutter array would be collimated, and the shutter array would be fixed in position relative to the lens array. The volume element probed would be at the focal point of the objective lens within the microlens array, and movement of the combination of the shutter and lens array in the z direction, can be used to probe different layers within the sample, as explained above when describing the array volume microprobe of FIG. 2 . One can, however, conceive of other arrangements where the lens array and the shutter array are capable of moving independently, and probing the sample in the z direction is achieved by translation in the z direction of the lens array alone.

[0063] Light responses to the exciting radiation from the light source 31 from each of the sampled volume elements are collected through the same objective elements through which illumination is effected. The light responses are separated from the illuminating beam by the beam splitter 35 . These responses are then imaged via a collecting lens 33 onto a collection field stop 43 that restricts the responses received by a sensor 39 to be essentially only from the probed volume element. In operation, the controller 18 opens a given shutter and allows the illumination of a single volume element. Furthermore, the same light shutter allows optical responses to the excitation to be recorded by the detector 39 . This is followed by closing the light valve and opening another light valve, so that sequentially discrete volume elements within the sample are scanned to obtain optical responses thereof. One can scan all the desired volume elements in the array in a given x,y plane and then rescan the array at a different depth (in the z axis), so as to obtain three dimensional information on the target sample. One can also choose to operate the array volume microprobe in such a way that for each pixel, the respective light valve 38 is kept open, while the controller causes the shutter array together with the lens array to move in the z direction, thus probing at the same x,y location volume elements at various depths of the specimen.

[0064] In yet another embodiment of the array volume microprobe, the illuminating and detecting optics are each provided with their own array of optical shutters. In FIG. 4 , such an embodiment is shown schematically. Specifically, the array volume microprobe 50 includes a light source 51 , a first field stop 52 , a collimating lens 53 , a first shutter array 54 , a first objective lens array 55 , beam splitting means 56 , a second objective lens array 58 , a second shutter array 59 , a second collimating lens 60 , a second field stop 61 and a detector 62 . Not shown in FIG. 4 are appropriate means to image the light source 51 and detector 62 onto their respective field stops 52 and 61 . In operation, the light source 51 is imaged onto the field stop 52 , having dimensions that are greater than the diffraction resolution limits of the exciting radiation. The light emanating from the field stop 52 is collimated into an essentially parallel beam that impinges on the back side of the shutter array 54 . At any given time, only one of the light valves in the light shutter array is opened and its corresponding light valve in the detector shutter array is open. The sequential illumination of an array of volume elements within the sample, in a manner similar to that described above, coupled with the synchronous opening of the appropriate light valve in the detector array, assures that at any given time, only responses from the probed volume element are detected. Similarly, scanning in the x,y direction is provided by controller 18 sequencing the opening and closing of the light valves in the two shutter arrays in synchronism. It should be understood that in this embodiment, the two field stops 52 and 61 are conjugated to each other via each of the volume elements 63 in the sample 57 .

[0065] In FIG. 4 A, an arrangement essentially identical to that described in FIG. 4 is shown, except that the array of microlens 58 is replaced with a single large lens 58 ′. Like elements in FIGS. 4 and 4 A have the same reference numbers.

[0066] Yet another embodiment of the invention is illustrated in FIG. 5 , which shows an array volume microprobe 70 . The system includes a light source 71 and a detector 72 having their optical axes orthogonal to each other and separated by a first beam splitter 73 . The light emanating from the source is condensed onto an array of field stops 74 with a condenser lens 75 and a second beam splitter 76 . The array of field stops 74 consists of an array of micromirrors 77 that can be tilted in and out of a plane generally parallel to the plane of the array. Light reflected from any one of these mirrors, while in the untilted position, is reflected back through the second beam splitter 76 and is imaged onto a sample 79 with an objective lens 77 . As shown in FIG. 5 , only one micromirror 78 at any given time is oriented to reflect light onto the sample. All other micromirrors in the array are tilted so that light impinging on them is reflected away from the sample. In FIG. 5 , rays 1 and 4 are limiting rays for the total field, and rays 2 and 3 are limiting rays for a single micromirror. In operation, the micromirrors 77 are sequentially brought to the untilted position by the controller 18 , and as a result of this sequential untilting of micromirrors, a sequence of responses from volume elements in the sample 79 is recorded in the detector 72 . An artificial image from the responses can then be recorded and displayed. As in prior embodiments, probing of the sample with the volume microprobe array in the z direction (depth) can be achieved by either moving the objective lens 77 or the array 74 in the z direction.

[0067] The control of the tilting mirror is performed by controller 18 , and the tilting mechanism can be implemented in a number of ways well known in the prior art. For instance, the mirrors can be micromachined in silicon, leaving a cantilever in the middle of the back of the mirrors. Two opposing electrodes cause the mirror to tilt about the cantilever due to charging one or the other electrode with a charge opposing the charge on the mirror itself. Another method of obtaining tilting mirrors is well known in the art of deformable mirrors, whereby each micromirror is mounted on a bipolar piezoelectric element.

[0068] Yet another embodiment of the invention, a variation of the embodiment shown in FIG. 5 , is illustrated in FIG. 6 . An array volume microprobe 80 includes a light source 81 from which light is conditioned to pass through a first field stop (not shown) and through a lens 82 . The light is collimated onto an array of micromirrors 83 . The micromirrors are tiltable as described above. However, each of the micromirrors is shaped to be an off axis segment of a paraboloid of revolution having its focal point tracing an arc of radius which is somewhat larger than the distance of the array from the sample. The geometry is such that when the mirrors are untilted (parallel to the plane of the array), the axis of the paraboloid of revolution (of which the specific mirror is an off axis segment) is perpendicular to the plane of the array. Thus a line between the focal point (of the paraboloid of revolution) and the micromirror is at a predetermined angle to the normal to the array. The micromirrors can be tilted through that angle so as to bring the focal point of the paraboloid onto the sample.

[0069] In one embodiment of the invention, the micromirrors are arranged in alternating right rows 89 and left rows 88 of off-axis segments of a paraboloid. The right mirrors can be termed the exciting mirrors and the left mirrors the detecting mirrors. The focal points of each segment of right rows 89 , upon tilting at the above mentioned angle, resides within the sample at the volume element 85 , and its respective paraboloid axis of revolution is parallel to the optics axis of the exciting beam, while the tilting in the opposing direction (at the same angle) of each micromirror in an adjacent left row causes the focal point of each micromirror to move to the same volume element ( 85 ) in the sample 84 , and its respective paraboloid axis of revolution is parallel with the optics axis of the detector.

[0070] Thus, all mirrors in right rows 89 are used to excite volume element 85 in the sample 84 and all left rows 88 are used to collect responses from volume element 85 . In operation, only one pair of mirrors is tilted at any given time and the axes of all other mirrors point down toward the sample. As a result, an exciting beam from the light source 81 is imaged onto the volume element 85 , or more accurately, the first field stop is so imaged, while light impinging on all other mirrors is scattered away from the sample in all directions. Similarly, only responses emanating from the volume element 85 are imaged back onto the second field stop in front of the detector. As a result, a very high degree of discrimination is obtained, since the intensity of the exciting beam decreases very rapidly outside of volume element 85 , and responses from outside volume element 85 are essentially blocked by the second field stop in front of the detector 87 . The controller 18 controls the sequence of tilting each pair of mirrors to obtain an array of responses from different volume elements in the sample. The depth of the volume element in the sample is also controlled by the controller 18 by moving the total array 83 along the z axis toward or away from sample 84 .

[0071] A slightly modified embodiment of the volume probe array shown in FIG. 6 is presented in FIG. 6A . This embodiment allows for using each off axis parabolic micromirror both as an excitation and detection mirror. The system is equivalent to that shown in FIG. 6 and described above, except that the micromirrors of array 83 ′ of the volume probe array 80 ′ are rotated by 90° to the right or the left. Thus, in the unrotated position, the axis of revolution (and thus the optical axis) of each mirror is at 90° to the optical axis of the exciting and detecting optics. However, when a mirror 89 ′ is rotated by 90° to the right, its axis of revolution becomes parallel to the axis of the excitation, and the focal point of the off-axis parabolic micromirror is in volume element 85 ′. If an adjacent mirror 88 ′ is simultaneously rotated 90° to the left, then its axis of revolution becomes parallel to the detection optics, and its focal point is in volume element 85 ′. The volume element 85 ′ is determined by the overlap of the images of the two field stops, as explained in detail in copending application Ser. Nos. 08/510,041 and 08/510,043. In this embodiment, as well as the one shown in FIG. 6 , sheared conjugation of the excitation and detection optics field stops is used to provide for spatial discrimination of the excitation beam to the target volume element as well as the spatial discrimination of the detected responses to be essentially from each volume element. In operation, the controller 18 causes two adjacent micromirrors ( 89 ′ and 88 ′) to be rotated simultaneously as described above and thus provides excitation of essentially only the desired volume element 85 ′ and responses which emanate essentially from the volume element 85 ′.

[0072] An advantage of the embodiment shown in FIG. 6A is that a higher resolution of volume elements is feasible for the same density of micromirrors, since each mirror can be used to either excite a volume element or to collect responses from an adjacent volume element. This differs from the embodiment shown in FIG. 6 , where all left mirrors can be used only to collect responses and all right mirrors can be used only to excite volume elements. In the embodiment of FIG. 6 , the tilting of the off-axis paraboloids of each segment is in a plane perpendicular to the plane of the array, while in the embodiment of FIG. 6 A, the plane of rotation is parallel to the plane of the array.

[0073] In FIG. 7 , yet another embodiment of the invention is shown. An array microprobe 90 includes an illumination optical assembly with a light source 91 and a first collimating lens 92 , and a response collection optical assembly having a detector 93 and a second collimating lens 94 . The respective optical axes of the light source assembly and the detector assembly are at 90° to each other. A beam splitter 95 is positioned at the intersection of the exciting and detected beam so as to separate the detected signal from the excitation signal. A lens 96 is used to focus the exciting beam into an optical fiber 102 which is interfaced with a fiber switching element 97 . The fiber switching element 97 is terminated on the opposing side with a plurality of fibers 98 , and the switching element is capable of connecting optically and sequentially (under the control of the controller 18 ) the proximal fiber 102 to any of the fibers 98 in the distal bundle. The ends of the individual fibers in the bundle are then arranged in an array 99 (this array may be either a linear array or a two dimensional array). An objective lens 100 then images the respective ends of the fibers in the fiber bundle onto the specimen. Each individual fiber end (within the fiber holder) defines a field stop which is imaged onto the sample. This field stop serves as a field stop to both the exciting beam and the detected responses from the sample. As is described in more detail in copending application Ser. Nos. 08/510,041 and 08/510,043, such an arrangement involves the conjugation of both the exciting and detected optics via the volume elements in which the field stop is imaged, and thus provides for spatial discrimination of both the excitation beam and the responses to be essentially from each volume element associated with each fiber in the array.

[0074] In operation, the fiber switching element 97 directs the exciting beam sequentially through all the fibers in the bundle 98 . As a result, a plurality of volume elements in the sample 101 (having a distribution corresponding to the array of fibers in the bundle 98 ) are excited sequentially. Responses are collected through the same field stop (the natural aperture of each fiber end) and are separated from the exciting beam by the beam splitter 95 to be detected in detector 93 . In this manner, one obtains responses from an array of volume elements that can then be displayed as an artificial image of the sample. This embodiment has the advantage that a higher intensity of excitation is feasible, since the light source is used sequentially by the different fibers in the bundle.

[0075] In FIG. 8 , yet another embodiment of a volume microprobe array 110 is shown. The device includes two optical assemblies, an excitation or source assembly 111 and a detector assembly 112 . Each of the assemblies is interfaced to its own individual fiber bundle 113 and 114 , respectively. The individual fibers are organized, in one embodiment, in two rows 116 and 117 , respectively, in a fiber holder 115 . When a linear array is desired, the fibers are organized in two opposing rows, one row consisting of the excitation fibers in bundle 113 and the other row of detection fibers in bundle 114 . When a two-dimensional array is desired, the fibers are organized in alternating rows of excitation fibers and detection fibers, with a small tilt of such rows relative to each other. The excitation optics 111 includes a light source 118 and focusing optics 119 that can focus the output of the light source on each fiber in the bundle 113 sequentially. A rotating mirror 120 is used to index the light source onto the opening apertures of the fibers in the bundle 113 . One should appreciate that the input aperture of the excitation fibers can be terminated in an appropriate way to improve collection of the light from the rotating mirror. Such termination may include, but is not limited to, flaring of the input end of the fiber, or termination of each fiber with a small compound parabolic concentrator, as is well known in the prior art.

[0076] In operation, the controller 18 causes the incremental rotation of the mirror 120 so as to direct the excitation beam to fibers in bundle 113 sequentially. This light then emanates through an excitation field stop at each distal end of the fiber, the field stop being essentially the aperture of each fiber. These field stops are imaged onto a sample 124 with objective microlens at the end of each fiber. The distal ends of both excitation and detection fibers are terminated into microlenses that serve as objectives. The excitation and detection fibers may be at a slight angle to each other, and sheared conjugation of their respective field stops (fiber apertures) defines the volume elements probed. The volume elements in the sample will be a mirror image of the fiber arrangement, namely a row or an array of points, depending on the organization of the fibers in the fiber holder 115 . The distance between the volume elements may be the same as that between the fibers in the fiber holder or may differ from the interspacing of the fibers in the fiber holder, and will depend on the magnification of a relay lens 125 . In some embodiments, one can allow for movement of the relay lens relative to the fiber holder so as to provide magnification (or demagnification). However, the size of each volume element will also be modified somewhat.

[0077] This configuration allows for the sequential illumination of an array of volume elements in sample 124 . An excited volume element will emit a response to the exciting beam. To ensure that responses that are essentially emanating only from the desired volume element are detected, the responses are collected with a dedicated fiber from bundle 114 . The optics are configured such that the respective field stops of the response fibers (their natural aperture) are each conjugated to the respective the field stops of the associated exciting fiber. As a result of this conjugation (or, more accurately sheared conjugation, since the exciting and detecting field stops are slightly spaced apart), the excitation beam has its highest intensity within the sample within the zone of sheared conjugation (the probed volume element), and the intensity declines very rapidly outside the volume element. Furthermore, responses collected by each detection fiber emanate essentially only from the volume element, and any response collected from adjacent tissues is very small relative to the response obtained from the zone of sheared conjugation of the excitation and detection field stops.

[0078] Since the excitation of the array of volume elements is carried out sequentially, response will be transmitted through the fiber bundle 114 sequentially to the detecting optics 112 . In a preferred embodiment of the invention, the response optics include a receiving rotating mirror 123 which directs (sequentially and in synchronism with the excitation mirror 120 ) the responses through a focusing lens 122 to a detector 121 . This assures that stray responses (namely, responses emanating outside the zone of sheared conjugation and thus outside the target volume element) and collected by adjacent fibers, do not reach the detector. In this manner, as before, spatial discrimination is obtained, and sequential detection of responses from specific volume elements is achieved.

[0079] In this embodiment, the use of a beam splitter is avoided, and only very simple optics are used at the distal end of the device. Such a device is particularly suitable when a distance between the sample and the optics (source and detector) is required, such as in laparoscopic and endoscopic devices. This embodiment has the additional advantage that higher excitation energies are feasible, since the light source resources are not distributed simultaneously over a full array as in some embodiments described above, and in this respect is similar to the embodiment shown in FIG. 7 and described above.

[0080] In FIGS. 9 and 10 , two additional embodiments of the invention, which differ from each other only in the position in the system of the addressable light shutters, are shown. In FIG. 9, a volume microprobe array 130 that includes a light source 131 , a detector 132 , and an optical fiber bundle 133 is shown. The proximal end of the optical fiber bundle interfaces an addressable array of optical shutters 134 . The optical shutters are under the control of the controller 18 . Each fiber in the bundle 113 is positioned in an array arrangement that corresponds to the addressable light shutter array. The light source 131 is coupled to the light shutter array via a condenser lens 135 and a shutter array coupling lens 136 . As a result, light from the light source is distributed over the light shutter array, and, when one of the shutters is in the open position, light is transmitted to the specific fiber coupled to that specific shutter. At the distal end of the fiber bundle, the device has objective optics 137 which essentially images each of the apertures 138 of the fibers on a sample 139 . The distal apertures of the fibers are, in essence, acting as the excitation and detection field stops for each of the volume microprobes in the volume microprobe array 130 of this embodiment. Responses to the exciting signals emitted from volume elements in the sample are collected by the fibers through the same objective optics 137 and the same fibers through which excitation was carried.

[0081] Since the field stops of both the exciting and detecting optics are conjugated within the volume element probed, the excitations and responses are limited to the individual volume elements probed by each fiber. In operation, the light shutter array is controlled by the controller 18 to sequentially open the light shutters in front of the fiber bundle sequentially in such as way that only one fiber is powered at any given time. Thus, by the synchronous detection of responses from fibers that are coupled to an open shutter, a full artificial image of pathologies in the targeted sample can be constructed. As in some of the embodiments described above, the response is separated from the excitation by a beam splitter 140 positioned at 45° to the optical axis of the excitation optics and detection optics.

[0082] In FIG. 10, a similar volume microprobe array 150 is presented. The essential difference is that a shutter array 154 is positioned at the distal end of the fiber bundle. This allows for selecting an array of field stops determined by each of the apertures within the shutter array rather than by the individual apertures of the optical fibers.

[0083] In some embodiments of the volume microprobe arrays described above, a plurality of detectors corresponding to adjacent full regions of the shutter array are employed. Each detector accepts responses from a subarray of the light shutter array, and thus from the sample. In these embodiments, data collection is accelerated by the simultaneous opening of a light valve in each of the subarrays in the light shutter array and detecting the response in their respective detectors. When using this approach, care is taken to assure that interferences (or noise) from responses outside each specific region are smaller than a preset value of the expected response from the sample in each region.

[0084] In several embodiments described above, an array of light shutters is employed to sequence the excitation of an array of volume elements in the sample as well as collect responses from the volume elements. In some embodiments, each shutter serves as an excitation and detection field stop, while in other embodiments other optical elements in the system perform the function of the field stop. Such light shutters are well known in the prior art and have been used in a number of display devices, whereby the sequence of opening and closing sets of optical shutters that are back illuminated provide either a fixed or a time variable image.

[0085] The actual embodiments of such shutters in the prior art can take many forms. The ost widely used light shutter array is an array of liquid crystal elements having two sheets of polarizer one each on the front and the back of the array. On each element in the array, a voltage can be applied. When the voltage is sufficiently high, the liquid crystal causes rotation of the plane of polarization of light passing through it. The two polarizers are oriented in such a way that no light passes through an element when no voltage is applied. Thus, the polarizers are cross polarized (their relative orientation is 90°, thus the first polarizer removes all light polarized in one direction, while the second polarizer blocks the light passing through the then inactive liquid crystal element). When a sufficiently high voltage is applied, the plane of polarization of the light passing through the liquid crystal cell is rotated, so that the second polarizer is essentially transparent to the light passing through the active liquid crystal cell. The addressing can be carried out as in the prior art, either as row and columns, so that only the sum of voltages applied to both a row and column is sufficient to cause the desired rotation of polarized light. Since the dimensions of our light shutters are relatively large and the number of shutters small relative to the current practice in liquid crystal display, such addressing is quite sufficient, and cross talk is minimal and insignificant in view of the strong spatial discrimination due to the conjugation of the excitation and detection field stops.

[0086] When very large arrays are desired, approaches such as used in active matrix liquid crystals display (namely the activation of a pixel through the direct switching of an individual transistor at each pixel) can be practiced as well.

[0087] In yet another embodiment, the shutter array consists of ferroelectric elements activated in a manner similar to that of liquid crystal light shutter arrays. These shutter arrays are useful when the switching rate desired, namely, the rate of opening and closing a given light shutter in the array, is faster.

[0088] In yet another embodiment, the light switching medium is a polymer dispersed liquid crystal (PDLC). In such films, a dispersion of droplets of liquid crystal is embedded in a polymer having an index of refraction equal to the field oriented index of refraction of the liquid crystal dispersion. When no electrical field is applied, the droplets are randomly oriented and light is scattered in all directions. Thus the shutter can be considered as closed. When a sufficiently large electric field is applied to a PDLC element, the liquid crystal droplets orient themselves with the field and thus, in the direction of the field, the index of refraction is essentially constant and light passes through uninterrupted. Thus the shutter is open.

[0089] In yet another embodiment, essentially electromechanical shutters are used. Such can be easily implemented with piezoelectric bimorphs, which when actuated bend out of the path of the light and when inactive, assume a straight geometry which blocks light transmission through a given shutter.

[0090] In FIG. 13 a, a top view of a light shutter array <