Abstract:
The embodiments of this invention use a multi-point scanning geometry. This design maintains the high frame rate of the slit-scan system and still allows both grayscale and multi-spectral imaging. In a confocal configuration, the multi-point scanning system&#39;s confocal performance is close to that of a single point scan system and is expected to yield improved depth imaging when compared to a slit-scan system, faster imaging than a point scan system, and the capability for multi-spectral imaging not readily achievable in a Nipkow disk based confocal system.

Description:
BACKGROUND 
       [0001]    This invention relates in general to an optical scanning instrument, and more particularly to a multi-point scan architecture. 
         [0002]    Confocal microendoscopy is an emerging technology that allows in situ confocal microscopic imaging of cells in live animals and humans. A number of approaches to confocal microendoscopy have been developed. The two most common methods are those that employ a coherent fiber optic bundle to relay the image plane of confocal microscope into the body, and those that build a miniaturized confocal microscope with the scanning mechanism into the distal tip of the flexible imaging device. 
         [0003]    For the miniaturized confocal microscope method, see Dickensheets, D. L., et al., “Micromachined scanning confocal optical microscope,”  Optics Letters  21, 764-766 (1996), Kiesslich R, et al., “Confocal laser endoscopy for diagnosing intraepithelial neoplasias and colorectal cancer in vivo,”  Gastroenterology  127, 706-713 (2004). For the coherent fiber optic bundle, see Gmitro A. F., et al., “Confocal Microscopy Through a Fiber-Optic Imaging Bundle,”  Optics Letters  18,565-567 (1993), Viellerobe, B., et al., “Mauna Kea technologies’ F400 prototype: a new tool for in vivo microscopic imaging during endoscopy,”  Proceedings of SP 1E. 6082 60820C (2006). 
         [0004]    For the development of a confocal microendoscope utilizing a slit-scan fluorescence confocal microscope coupled to a fiber bundle with a custom miniature objective lens, see Rouse A. R., et al., “Design and demonstration of a miniature catheter for a confocal microendoscope,”  Applied Optics  43, 5763-5771 (2004). This system operates at 30 frames per second and provides high quality fluorescent microscopic images of living tissue. This system has also been configured to allow multi-spectral imaging with essentially instantaneous switching between grayscale and multi-spectral modes of operation, see Rouse A. R., et al., “Design and demonstration of a miniature catheter for a confocal microendoscope,”  Applied Optics  43, 5763-5771 (2004). 
         [0005]    In the context of confocal microendoscopy, multi-spectral imaging provides a powerful capability allowing identification of multiple fluorophores and/or the sensing of subtle spectral shifts caused by tissue microenvironment. 
         [0006]    A slit-scan confocal microscope represents a compromise between speed of operation and confocal imaging performance. Theory predicts that lateral resolution is maintained but axial resolution is reduced for a slit scan versus a point scan system. These theoretical results are for a non-scattering medium. Recent Monte Carlo simulation results show that performance degrades with depth for both point-scan and slit-scan systems, but that in the slit-scanning geometry the effects are severe enough to limit the practical imaging depth for in vivo imaging applications. 
         [0007]    It is therefore desirable to provide an improved scan architecture in which the above disadvantages are not present. 
       SUMMARY 
       [0008]    In one embodiment, a method for scanning an object comprises providing a substantially one dimensional array of two or more illumination beams spread along a first dimension travelling in a forward direction. The array of two or more illumination beams are focused to an object plane in the object. Light from said first illumination beam is scanned along a second dimension transverse to the first dimension before such light reaches the object plane. The instrument providing the array of illumination beams or scanning along a second dimension also causes such light to scan along the first dimension, so that said array of two or more illumination beams scans across the object plane along said first and second dimensions. Light caused by interaction of the array of two or more illumination beams and the object is transmitted through a confocal aperture array in the reverse direction in a manner so that the transmitted light caused by the interaction matches a profile of the said array of two or more illumination beams. 
         [0009]    Another embodiment is directed to a confocal microscope comprising a light source providing a first illumination beam travelling in a forward direction, and a device that modulates the first illumination beam to provide a substantially one dimensional array of two or more illumination beams spread along a first dimension. Optics are employed that focus the array of two or more illumination beams to an object plane. A first mechanism is used to scan light from the first illumination beam along a second dimension transverse to the first dimension before such light reaches the object plane. The first mechanism or the device causes such light to scan along the first dimension, so that the array of two or more illumination beams scans across the object plane along said first and second dimensions. A confocal aperture array transmits light caused by interaction of the array of two or more illumination beams and the object in the reverse direction in a manner so that the transmitted light caused by the interaction matches a profile of the said array of two or more illumination beams. 
         [0010]    In yet another embodiment, a confocal microscope comprises a light source providing a first illumination beam travelling in a forward direction to an object, and a confocal aperture array. The confocal aperture array passes portions of the first illumination beam to form a substantially one dimensional array of two or more illumination beams spread along a first dimension travelling in the forward direction and transmitting light caused by interaction of the array of two or more illumination beams and the object in the reverse direction. The confocal aperture array is made to move so that different portions of the first illumination beam are passed at different times to form the array of two or more illumination beams in the forward direction, thereby causing the array of two or more illumination beams to scan the object along the first dimension. The microscope includes optics that focus the array of two or more illumination beams to an object plane and a mechanism to scan array of two or more illumination beams along a second dimension transverse to the first dimension, so that said array of two or more illumination beams scans across the object plane along said first and second dimensions. 
         [0011]    The above embodiments use a multi-point scanning geometry. This design maintains the high frame rate of the slit-scan system and still allows rapid switching between grayscale and multi-spectral imaging. In a confocal configuration, the multi-point scanning system&#39;s confocal performance is close to that of a single point scan system and is expected to yield improved depth imaging when compared to a slit-scan system. 
         [0012]    All patents, patent applications, articles, books, specifications, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of a term between any of the incorporated publications, documents or things and the text of the present document, the definition or use of the term in the present document shall prevail. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIGS. 1A ,  1 B, and  1 C are schematic views of the scan paths of respectively a single point scan system, a slit-scan system and a Nipkow disk scan system. 
           [0014]      FIGS. 1D and 1E  are schematic views of scan paths to illustrate the operation of different embodiments of this invention. 
           [0015]      FIG. 2A  is a schematic view of a multi-point scanning system employing a stationary confocal aperture array to illustrate a first embodiment of the invention. 
           [0016]      FIG. 2B  is a schematic view of the illumination points or beams in  FIG. 2A . 
           [0017]      FIG. 3A  is a schematic view of a multi-point scanning system employing a moving confocal aperture array to illustrate a second embodiment of the invention. 
           [0018]      FIG. 3B  is a schematic view of the illumination points or beams in  FIG. 3A . 
           [0019]      FIG. 4A  is a schematic view of a multi-point scanning system employing a moving confocal aperture array that causes multi-point illumination and scanning to illustrate a third embodiment of the invention. 
           [0020]      FIG. 4B  is a schematic view of the rotating star aperture array in  FIG. 4A . 
           [0021]      FIGS. 5A ,  5 B,  5 C and  5 D are schematic views of four different arrangements for creating multi-point illumination. 
           [0022]      FIG. 6A ,  6 B are graphical plots of the lateral and axial interquartile range performance in μm as a function of imaging depth for each of four types of apertures. 
       
    
    
       [0023]    For simplicity in description, identical components are labeled by the same numerals in this Application. 
       DETAILED DESCRIPTION 
       [0024]      FIGS. 1A ,  1 B and  1 C are schematic views of the scan paths (shown in broken lines) of respectively a single point scan system, a slit-scan system and a Nipkow disk scan system.  FIG. 1D  is a schematic view of the scan path (shown in broken lines) of a linear array of pinholes scan system.  FIG. 1E  is a schematic view of the scan paths (shown in broken lines) useful to illustrate some of the embodiments of this invention. All of the scan paths in the five figures cover the plane or surface scanned. 
         [0025]    As will be explained below, one advantage of the multi-point array scan architectures in  FIGS. 1D and 1E  over that of the Nipkow disk is that it enables multi-spectral imaging to be performed. The multi-point array scan architecture in  FIGS. 1D and 1E  yields improved depth imaging when compared to a slit-scan system. When used in a confocal microscope, the performance of this multi-point array is close to that of a single point scan system. 
         [0026]      FIG. 2A  is a schematic view of a multi-point scanning system employing a stationary confocal aperture array to illustrate a first embodiment of the invention. The system  10  utilizes a light source  12  and a beamsplitter  14 , to couple excitation light into the system. The light source  12  provides a substantially one dimensional array  20  of two or more illumination beams spread along a first dimension. In this application, the phrase “along a first dimension” means along a first line or direction, and the phrase “along a second dimension” means along a second line or direction. 
         [0027]    The array  20  of illumination beams is shown schematically in  FIG. 2B  as an array of illumination points, where the first dimension is along line or direction  20 ′. The light source  12  may include a laser or lamp (not shown), a cylindrical lens (not shown), and a grating or pinhole aperture array (not shown) for generating a substantially one dimensional array  20  of two or more illumination beams spread along a first dimension  20 ′. 
         [0028]    The lenses  22  and the microscope objective  24  combine to image the light source to the object plane  26  of the confocal microscope  10 . The array  20  of multiple points of light is imaged onto the object plane  26  in object  28 , where the light interacts with an object  28 . As illustrate in  FIG. 2A , the object plane  26  is inside the object  28 ; however, this need not be the case, and the object plane  26  can be located at the surface of the object  28 . Light caused by the interaction of the multiple points of light with the object returns through the system and is re-imaged onto the confocal aperture plane of a stationary confocal aperture array  40 . The returning light through the aperture array  40  is then imaged onto a detector array  42 . The interaction of the multiple points of light with the object may cause fluorescent light to return from the object  28 , or the returning light from the object can be reflection or scattering of array  20  of illumination beams. Where the interaction of the multiple points of light with the object causes fluorescent light to return from the object  28 , beamsplitter  14  is dichroic and passes only the fluorescence from the object and eliminates excitation light reflected from the object or scattered within the optical system. Where the interaction of the multiple points of light with the object causes reflection or scattering of array  20  of illumination beams to return from the object  28 , beamsplitter  14  is not dichroic and separates light directed to the object from light returning from the object. 
         [0029]    The array  20  of two or more illumination beams, also referred to herein as array of multiple points of light, has a substantially one dimensional profile, such as the one illustrated in  FIG. 2B . The stationary confocal aperture array  40  preferably has a profile that matches that of array  20 , to pass the light resulting from interaction of the multiple points of light with the object in a confocal configuration. 
         [0030]    The light passing back through the aperture array  40  would represent a line image of the object  28  at a selected depth and at a specific set of spatial positions on the object. To produce a 2D image scanning is performed by means of a 2D scanning mechanism  48  and the signals are digitized and processed by a computer  41  to generate the 2D image. 
         [0031]      FIG. 3A  is a schematic view of a multi-point scanning system  100  employing a moving confocal aperture array to illustrate a second embodiment of the invention. The system  100  differs from system  10  of  FIG. 2A  in that the light source  112  generates a substantially one dimensional array  120  of two or more illumination beams spread along a first dimension  120 ′ shown in  FIG. 3B , where the array  120  is scanned also substantially along the first dimension  120 ′. Hence instead of using a 2D scanning assembly as in system  10 , a scan mirror  152  scanning in a second direction or dimension transverse to the dimension  120 ′ (such as about an axis that is perpendicular to the page) is adequate to accomplish the 2D image scanning in the manner illustrated in  FIG. 1E . The generation of a scanning array of two or more illumination beams spread along a first dimension  120 ′ and also scanning along first dimension  120 ′ is described below in reference to  FIGS. 5A-5D . 
         [0032]    In  FIG. 3A , the light from array  120  is conveyed to the object (not shown) imaged by means of an optical imaging catheter or relay  102 . The array  120  of two or more illumination beams, also referred to herein as array of multiple points of light, has a substantially one dimensional profile, such as the one illustrated in  FIG. 3B . The confocal aperture array  140  preferably has a profile that matches that of array  120 . Since array  120  is scanned along the dimension  120 ′, the returning light from catheter or relay  102  will also move, along with the motion of array  120 . Hence, confocal aperture array  140  is also scanned in synchronism with array  120  by means of a motor  154  (or other means), so that the aperture array is in a position to pass the returning light from catheter or relay  102 , and block other stray light. Other techniques of creating aperture array motion, such as those described below in reference to  FIGS. 5A-5D , may also be used.? 
         [0033]    To build up a 2D image, a second synchronized scan mirror  156  is used to sweep the light returning through the confocal aperture array  140  across a 2D CCD detector  142 , such as that of a CCD camera. Both scan mirrors  152  and  156  are locked to the frame rate of the camera so that each frame output from the camera produces a full 2D confocal image. An eyepiece  44  observed by eye  46  may be used instead of CCD detector  142 . The interaction of the multiple points of light with the object may cause fluorescent light to return from the object, or the returning light from the object can be reflection or scattering of array  120  of illumination beams. Beam splitter  14  is dichroic only when the returning light from the object is fluorescent light. 
         [0034]    Multi-spectral imaging is accomplished by diverting the beam through a prism  162 . This is done by moving scan mirror  156  to a fixed angular offset position so that light returning from the sample goes through the prism. Light at different wavelengths is refracted at different angles, which map to different lateral positions on the CCD detector  142  in a CCD camera. For any given angular position of scan mirror  152 , the light distribution on the CCD is a 2D distribution where one direction (parallel to the dimension  120 ′) corresponds to a spatial coordinate, and the other direction (perpendicular to the dimension  120 ′ and in the dispersion direction) corresponds to the spectral coordinate. As scan mirror  152  rotates, multiple frames are read out of the CCD to build a 3D multi-spectral data set with a full spectrum at each spatial location in the 2D image, using computer  172  processing the outputs of CCD detector  142 . This requires slowing down the rotation of scan mirror  152 . 
         [0035]      FIG. 4A  is a schematic view of a multi-point scanning system  200  employing a moving confocal aperture array that causes multi-point illumination and scanning to illustrate a third embodiment of the invention.  FIG. 4B  is a schematic view of the rotating star aperture array in  FIG. 4A . The system  200  utilizes a fiber-coupled laser source  212  and anamorphic optics, consisting of a spherical lens  202 , a cylindrical lens  204 , and a beamsplitter  214 , to couple excitation light into the system. The anamorphic optics combined with the spherical lens  206  to the right of the beamsplitter  214  produce a line of light at the confocal aperture plane. In the third embodiment, the confocal aperture consists of a slit  208  and a rotating star-pattern aperture array  240 , whereas in a slit scanning system the confocal aperture is simply a slit. The star-pattern binary transmission aperture array  240  has a low duty cycle of open to opaque regions, effectively producing a set of multiple widely-spaced illumination points along the slit. As the star aperture array rotates, these points continuously move up the slit. The lens  224  shown to the right of the confocal aperture and the microscope objective  24  combine to image the aperture plane to the “object” plane of the confocal microscope. While using a cylindrical lens  204  makes efficient use of the light from the light source, it is not necessary where efficient use of the light is not a concern, and light from any source can simply be passed to the slit  208 . 
         [0036]    In a confocal microendoscope the proximal end of the flexible imaging catheter or relay  102  is placed at the “object” plane of the confocal microscope. The flexible fiber bundle in the imaging catheter relays this “object” plane to the distal end of the catheter, where a miniature objective lens images the “object” plane into the tissue at a selected depth. The depth of the imaging plane in tissue is controlled by a focus mechanism that moves the fiber relative to the miniature objective lens, which is held in contact with the tissue. The multiple points of light in the confocal aperture are imaged into the tissue and excite the fluorescence in the tissue. The fluorescent light returns through the system and is re-imaged onto the confocal aperture plane. As the star aperture array rotates, the points of illumination move along the slit direction. Where the light returning from the tissue is that of reflectance, reflected light is re-imaged onto the confocal aperture plane. 
         [0037]    Integration of the light passing back through the aperture would represent a line image of the tissue at a selected depth and at a specific lateral position. To produce a 2D image, scanning perpendicular to the slit direction is required. Scanning of the illumination is accomplished by scan mirror  152 . The fluorescent light returning from the tissue is de-scanned by this mirror to the fixed slit aperture location. To build up a 2D image, a second synchronized scan mirror  156  is used to sweep the image inside the slit across a CCD detector  142  of a CCD camera. Both scan mirrors are locked to the frame rate of the camera so that each frame output from the camera produces a full 2D confocal image. As in  FIG. 3A , an eyepiece (not shown) may be used instead of CCD detector  142 . 
         [0038]    The dichroic beamsplitter  214  passes only the fluorescence from the sample and eliminates excitation light reflected from the tissue or scattered within the optical system. The beam path shown in the diagram is for the direction where the cylindrical lens has no optical power. In the perpendicular direction the cylindrical lens comes to focus at the front focal point of the lens to the left of the slit. Therefore the chief ray for every field point is parallel to the optical axis as it passes through the slit aperture. It is also parallel to the optical axis as it hits the fiber bundle. The interaction of the multiple points of light with the object may cause fluorescent light to return from the object as described above, or the returning light from the object can be reflection or scattering of the illumination beams, in which case beamsplitter  214  is not dichroic. 
         [0039]    In  FIG. 4A , the beam path shown is for the direction where the cylindrical lens has no optical power. Multi-spectral imaging is accomplished by diverting the beam through a prism  162 . This is done by moving scan mirror  156  to a fixed angular offset position so that light returning from the sample and passed by the slit goes through the prism. Light at different wavelengths is refracted at different angles, which map to different lateral positions on the CCD camera. For any given angular position of scan mirror  152 , the light distribution on the CCD is a 2D distribution where one direction (parallel to the slit  208 ) corresponds to a spatial coordinate, and the other direction (perpendicular to the slit and in the dispersion direction) corresponds to the spectral coordinate. As scan mirror  152  rotates, multiple frames are read out of the CCD  142  to build a 3D multi-spectral data set with a full spectrum at each location in the 2D image, using computer  172  processing the outputs of CCD detector  142 . This requires slowing down the rotation of scan mirror  152 . For 128 data points per spectrum, the frame rate is 30/128 or approximately 4 seconds per complete data set. The scan mirrors can be under computer control so that one can switch rapidly (&lt;30 ms) between the mode where there is no dispersion by prism  162  of the light returning from the object and the multi-spectral mode of operation where there is such dispersion. 
         [0040]    The scan paths in the embodiments of  FIGS. 3A and 4A  are illustrated in  FIG. 1E . As shown in  FIG. 1E , the scan path is similar to that of  FIG. 1D , except that it is slanted upwards as the scanning proceeds. 
         [0041]    The multi-spectral capability described above is unique to this new approach and would not be easy to implement using a Nipkow disk. Moreover, the cylindrical lens makes the new approach more light efficient, which is important for a system utilizing a fiber bundle, which has significant transmission losses. 
         [0042]    While preferably systems  10 ,  100  and  200  are confocal, they can also be used in a non-confocal configuration, where no aperture array is needed. This configuration is not confocal and has no depth discrimination, but would work for imaging a planar surface. 
         [0043]      FIGS. 5A ,  5 B,  5 C and  5 D are schematic views of four different arrangements for creating multi-point illumination that may be used for the systems in  FIGS. 3A and 4A . 
         [0044]    The light source  112  of  FIG. 3A  may be implemented as shown in  FIG. 5A , in which light source  112  includes a laser or lamp (not shown), a slit  302  aligned along the first dimension  120 ′, and an array  304  of radial slits on a disk  300 . Slit  302  and array  304  overlap and pass light from the laser or lamp illustrated as dark areas in  FIG. 5A . Array  304  is rotated by a motor or other means (not shown) along arrow  306 , so that the light passing through slit  302  is modulated in that different portions  310  of the light along dimension  120 ′ are passed through slit  302  and some of the radial slits  304  at different times, so that not only does this generate an array of light beams for illuminating an object, but also causes the array of light beams to scan the object along dimension  120 ′. In order that the confocal aperture array  140  of  FIG. 3A  will pass the returning light from the object after interaction of the object with the beams, preferably the confocal aperture array  140  has a profile that matches the profile of the array of beams  310 , and moves in synchronism with beams  310 , such as by means of a mechanism similar in construction to the apparatus of  FIG. 5A ,  5 B,  5 C or  5 D described herein. In the embodiment of  FIG. 4A , the matching is done automatically by the star aperture array  240 , since the same star aperture array  240  generates the scanning array of illumination beams and also transmits the returned light from tissue. 
         [0045]    The light source  112  of  FIG. 3A  may be implemented as shown in  FIG. 5B , in which light source  112  includes a laser or lamp (not shown) that provides a light beam. This beam can be one similar to the beam generated by means of spherical and cylindrical lenses  202  and  204  of  FIG. 4 , with the length of the beam cross-section along dimension  120 ′. Light source  112  of  FIG. 3A  may also include a disk  320  with pin holes along its perimeter where some of the pin holes overlap the light beam, so that portions  324  of the beam passing through the overlapping pin holes (shown as dark spots in  FIG. 5B ) are passed in the forward direction for illumination. 
         [0046]    Disk  320  is rotated by a motor or other means (not shown) along arrow  326 , so that the light passing through pin holes  322  is modulated in that different portions of the light along dimension  120 ′ passes through the pinholes  322  at different times in a substantially one dimensional array of beams, so that not only does this generate an array of light beams for illuminating an object, but also causes the array of light beams  324  to scan the object along dimension  120 ′. In order that the confocal aperture array  140  will pass the returning light from the object after interaction of the object with the beams, preferably the confocal aperture array  140  has a profile that matches the slightly curved profile of the array of beams  324 , and moves in synchronism with beams  324 , such as by means of a mechanism similar in construction to the apparatus of  FIG. 5A ,  5 B,  5 C or  5 D described herein. 
         [0047]    As shown in  FIG. 5C , light source  112  of  FIG. 3A  may include a laser or lamp (not shown), an elongated plate  350  with its length along dimension  120 ′. The plate  350  has pin holes along its length aligned along the first dimension  120 ′ that passes light from the laser or lamp shown as dark areas  352  in  FIG. 5C . The light source  112  of  FIG. 3A  may also include a mechanism (not shown) for moving plate  350  in linear motion or oscillating motion along arrow  354  parallel to dimension  120 ′. 
         [0048]    Thus light passing through pin holes in plate  350  generates an array of light beams  352  for illuminating an object, and the linear motion of plate  350  causes the array of light beams to scan the object along dimension  120 ′. In order that the confocal aperture array  140  of  FIG. 3A  will pass the returning light from the object after interaction of the object with the beams, preferably the confocal aperture array  140  has a profile that matches the profile of the array of beams  352 , and moves in synchronism with beams  352 , such as by means of a mechanism similar in construction to the apparatus of  FIG. 5A ,  5 B,  5 C or  5 D described below. 
         [0049]    As shown in  FIG. 5D , light source  112  of  FIG. 3A  may include a laser or lamp (not shown) and a linear array  380  of electronically controlled individually addressable apertures  382  (e.g. liquid crystal pixels) aligned along dimension  120 ′. A control device (not shown) may be used to cause, such as by electro-optic mechanisms, different ones of the individually addressable apertures to pass light from the laser or lamp at different times, similar to the scanning achieved by the schemes in  FIGS. 5A-5C . As shown conceptually in  FIG. 5D , only every other aperture  384  along dimension  120 ′ is rendered light transmissive, with the remaining apertures remaining opaque to light, so that an array of light beams  384  is transmitted through selected apertures for illumination. In actual implementations, it may be only one aperture for a larger number of apertures that is rendered transmissive, with the remaining apertures remaining opaque to light. 
         [0050]    Thus this manner of generating the light beams  384  not only generates the beams, but also causes the array of light beams  384  to scan the object along dimension  120 ′. In order that the confocal aperture array  140  of  FIG. 3A  will pass the returning light from the object after interaction of the object with the beams, preferably the confocal aperture array  140  has a profile that matches the profile of the array of beams  384 , and moves in synchronism with beams  384 , such as by means of a mechanism similar in construction to the apparatus of  FIG. 5A ,  5 B,  5 C or  5 D described herein. 
         [0051]    As described above, all of the arrays of illumination beams  310 ,  324 ,  352  and  384  are substantially one dimensional along dimension  120 ′. The embodiment of  FIGS. 4A and 4B  employs the scheme of  FIG. 5A . However, the scheme of  FIG. 5B ,  5 C or  5 D may be used instead and is within the scope of the invention.  FIGS. 6A ,  6 B are graphical plots of the lateral and axial interquartile range (IQR) performance in μm as a function of imaging depth for each of four types of apertures. 
         [0052]      FIGS. 6A ,  6 B show how the pinhole aperture has an axial performance of about 1 micrometer and lateral performance of about 4 micrometers down to d·μ s =1. The linear array and Nipkow aperture maintain performance comparable to the pinhole aperture down to a depth greater than d·μ s =0.5. The slit aperture has substantially worse performance. Its lateral performance is about 5 μm and the axial performance is about five times worse than the other three apertures. 
         [0053]    To maximize the scanning speed performance and light efficiency of the linear array, the highest possible aperture density should be used. To determine the maximum possible density that can be used, the maximum imaging depth d·μ s  is specified a priori. Since the axial and lateral performance monotonically degrades as the imaging depth is increased (except very close to the surface), the center to center pinhole spacing need only be optimized to obtained the minimum acceptable performance at the maximum imaging depth. 
         [0054]    While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalents.