Abstract:
A compact digital imaging microscope includes a stage for holding a specimen, a lens assembly for collecting a beam of light transmitted through the specimen, and an adjustable beamsplitter assembly for splitting the beam between an eyepiece for viewing an image of the specimen and a CCD image detector to provide both a digital and an analog signal representing an image of the specimen. The use of a lens positioned along an optical path between the lens assembly and the image detector allows the image detector to be integrated compactly within the microscope and also allows the image detector to capture an image whose viewable area is roughly the same as that viewable through the eyepiece. Furthermore, all electrical connections (other than output and power supply connections which are located at the base of the microscope) are internalized within the microscope.

Description:
FIELD OF THE TECHNOLOGY 
     This invention relates generally to microscopes, and more particularly to a digital imaging microscope having an eyepiece for viewing and a digital camera for the electronic transmission of a digitized image of a specimen to a monitor or computer. 
     BACKGROUND 
     Digital imaging microscopy relates to the capture in digitized form of a magnified image of a specimen, for live viewing on a monitor or for processing and archiving by a computer. In certain known digital imaging microscopy systems, there is a microscope unit that resembles a standard microscope by having a light source, a specimen stage, optics for collecting and directing a light beam reflected from or transmitted through the specimen, and an eyepiece mounted at the end of an attached tube. Unlike a standard microscope, a conventional digital imaging microscope typically has a port for receiving a digital camera. The port is typically at the top of the microscope casing in the vicinity of the eyepiece. Furthermore, the optics are adapted to use a beamsplitter to direct a determined portion of the light to the camera port. A coupling adapter and a phototube provide a suitable physical connection between the microscope and the camera. As the camera and the eyepiece share the same optics, the coupling adapter together with the phototube must be dimensioned, and particularly must be of sufficient length, such that the camera lens can effectively image the beam onto the photosensitive image sensor. 
     Cameras employing a charge coupled device (CCD) are suitable for use in digital imaging microscopy. In a camera equipped with a CCD detector, the magnified image is digitized within the CCD camera and is transmitted as either a digital signal to a computer or as an analog signal to a monitor. The computer may be programmed with dedicated image processing and archiving software. 
     An example of such a digital imaging microscope is described in U.S. Pat. No. 5,933,513 (Yoneyama et al.). A commercially available example of such a system is the LEICA DC 100 Digital Imaging System manufactured by Leica Microscopy Systems Ltd. This system includes a DC 100 digital camera, a MZ12 stereo-microscope and a standard Windows-based PC computer running TWAIN and QWin image processing and archiving software. The system also has a phototube with C-mount optical adapter that is used to physically connect the digital camera to an opening at the top of the microscope casing within some suitable focal range for the digital camera. An electrical cable extends from the camera and connects to the computer via a digital frame grabber located in the computer. 
     Both the Yoneyama and LEICA microscopes are not compact. In particular, both systems include lengthy phototubes in part to provide the necessary focal length such that a peripherally connected camera can image the beam onto the photosensitive CCD image sensor, and in part to allow for the insertion of other optical devices such as filters between the magnifier and the camera. Furthermore, as the camera in each of these systems is a discrete device separate from the microscope, the camera and any other peripherally connected optical devices must be carefully calibrated each time the camera is mounted to the microscope. Further, each device may require its own power cable and data transmission cables which may interfere with the effective or convenient operation of the microscope. Lastly, these systems allow the camera to capture only a small portion of the image viewable from the eyepiece. While the Yoneyama and LEICA microscopes may be adaptable for a variety of uses, they are also cumbersome to operate for the novice or intermediate microscopist. 
     Other known digital microscopy systems include an adapter for affixing a digital camera over the eyepiece of a conventional microscope. The main drawback of such systems is that direct viewing and digital imaging cannot occur at the same time. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a relatively compact and relatively inexpensive digital imaging microscope having at least some of the following characteristics and, in a preferred embodiment, having all of them: 
     (1) a compact assembly of optical components; 
     (2) means to deliver all the available light from the specimen to the image detector thus maximizing the quality of the images produced; 
     (3) an integrated digital image detector which needs only be calibrated once; 
     (4) means to capture a relatively large portion of the image available from the magnifying objective lens, the captured image area being roughly equivalent to that viewable through the eyepiece; 
     (5) means to provide an electronic signal representing the captured image in both analog and digital form; 
     (6) a single power connector for all the system components; and 
     (7) a convenient means to connect power and data cables at the base of the assembly. 
     One embodiment of the invention provides a digital imaging microscope for viewing and capturing an image of a specimen. The elements of the preferred embodiment listed in this paragraph are conventional. The microscope assembly has a base, a hollow bent arm extending generally upward from the base, a light source located either in the base or attached to the arm, a stage for receiving a specimen to be viewed, an objective lens, an optical component housing in the vicinity of the top of the arm, and an eyepiece. The objective lens is mounted on the microscope on the underside of the upper part of the arm along an unimpeded optical path from the specimen. The objective lens may be one of several mounted in a carousel. The user sets the microscope&#39;s magnification by rotating the carousel so that a selected objective lens is positioned over the specimen. The objective lens collects light transmitted through the specimen and transmits it to a beamsplitter located in the housing. The beamsplitter splits the light arriving from the objective lens into two beams. One beam is directed to the eyepiece and the second beam to a digital image detector. 
     In a conventional digital imaging microscope, the digital image detector is located outside the optical component housing within a camera that is attached as a separate device by means of an adapter and a phototube. The eyepiece provides a magnified virtual image of the specimen observable with the human eye and the digital image detector converts the magnified image of the specimen into an analog electronic signal. 
     According to the one aspect of the invention, a beamsplitter assembly, positive lens and an image detector are provided within the optical component housing. This provides a compact design. 
     The beamsplitter assembly holds a plurality of prisms in a tray which may slide transversely to allow the user to select a prism to control the division of light between the eyepiece and the digital image detector. Preferably, at least three prisms are provided. A suitable set of prisms comprises a splitting prism, which splits the light collected equally between the eyepiece and the digital image detector, a columnar prism, which transmits all of the available light to the digital image detector, and a reflecting prism, which reflects all of the light to the eyepiece. This arrangement enables the microscope to be operated in three distinct modes: 
     (1) a direct viewing mode, in which all light is directed to the eyepiece; 
     (2) a digital imaging only mode, in which all light is directed to the image detector; and 
     (3) a dual viewing mode where light is directed to both the eyepiece and the image detector. 
     This arrangement provides the user with considerable flexibility. Direct viewing mode provides best viewing through the eyepiece as the image is of maximum intensity. Digital imaging only mode maximizes the light intensity available for image detection. Dual viewing mode allows one person, possibly a teacher, to operate the microscope with the aid of the eyepiece, while a number of other viewers, possibly students in a class, can watch on a standard monitor or on computer screens. 
     The positive lens is interposed between the beamsplitter assembly and the digital image detector so that it focuses the light from the beamsplitter onto the image detector. The use of the positive lens permits the image detector to be mounted within the optical component housing in close proximity to the beamsplitter, while still imaging a substantial portion of the image viewable through the eyepiece. This arrangement facilitates a compact design. The positive lens may preferably be mounted so that it can slide vertically within the optical component housing thus providing a means to focus the image onto the digital image detector. 
     A suitable image detector is a CCD. The CCD sensor captures an image of the specimen and converts this to an analog electronic signal which is transmitted by signal wires and electrical connectors to one or more RCA or S-type ports and to an analog/digital converter. The output of the analog/digital converter is directed to a USB output port. 
     According to a second aspect of the invention, the analog/digital converter, power supply, output ports and other electronic components are preferably located in the microscope base contributing to a compact and efficient design, free of cable connections at other parts of the microscope. 
     A microphone may optionally be provided on the microscope. The microphone is connected electrically to the image detector. The audio output of the image detector is sent either as an analog signal to, for example, an RCA audio port or, after digitization by the analog/digital converter, to a USB port, both output ports being located in the microscope base. 
     According to another embodiment of the invention, the beamsplitter assembly holds a single splitting prism mounted over the aperture. This splitting prism divides the light from the objective lens into two parts, one passing by reflection to the eyepiece and the other by transmission to the digital image detector. A suitable splitting prism comprises a semi-pentagonal shaped splitting prism joined to a compensatory prism having a reflectance-to-transmission split ratio of 1:1. 
     According to another embodiment of the invention, the microscope provides a stereoscopic, magnified image viewable through a pair of eyepieces and the means to capture a magnified image of the specimen with a digital image detector. This embodiment is generally the same as the preferred embodiment with the revisions described below to accommodate stereoscopic viewing. 
     The stereoscopic microscope suitably comprises a pair of eyepieces each having an eyepiece tube and a lens. In this embodiment, the carousel holds pairs of objective lenses. The carousel may be rotated to place a selected pair of objective lenses beneath the aperture at the bottom of the optical component housing such that light arriving from the specimen may be collected by the lens pair and transmitted to the beamsplitter assembly. The beamsplitter assembly has a pair of reflecting prisms, both placed over the aperture. For stereoscopic viewing through the eyepieces, both reflecting prisms direct light arriving from the objective lenses to the eyepieces. For imaging with the image detector, one reflecting prism is moved out of the light path thus allowing a beam to pass to a mirror, through a right-angle prism and the positive lens to the digital image detector. As in the earlier-described embodiment, the positive lens makes it possible to position the digital image detector close to the positive lens while at the same time capturing a substantial portion of the image viewable by means of the eyepiece. 
     According to another embodiment of the invention, a pair of objective lenses is provided to magnify the image of the specimen for stereoscopic viewing through the two eyepieces together with a third objective lens to capture an image on a digital image detector, Separate apertures in the optical component housing are provided for each of the three objective lenses. The beamsplitter assembly has two prisms. One prism receives light arriving from two objective lenses and reflects these beams to the eyepieces. The second prism redirects the light collected by the third objective lens through a positive lens to the digital image detector. As in the earlier-described embodiment, the positive lens makes it possible to position the digital image detector close to the positive lens while at the same time capturing a substantial portion of the image viewable by means of the eyepiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail with reference to the drawings. 
     FIG. 1 is a schematic partial cut away side view of an imaging microscope according to a first embodiment of the invention. 
     FIG. 2 is a schematic perspective view of a beamsplitter assembly according to a second embodiment of the invention. 
     FIG. 3 is a plan view of a the beamsplitter assembly of FIG.  2 . 
     FIG. 4 is a side elevation view of the beamsplitter assembly of FIG.  2 . 
     FIG. 5 is a sectional side view of an eyepiece and an optical component housing of the microscope in FIG.  1 . 
     FIG. 6 is a ray trace diagram of the optical path of the microscope in FIG.  1 . 
     FIG. 7 is a schematic circuit diagram showing the electrical connections between the image detector to the various components of the microscope of the invention. 
     FIG. 8 is a schematic view of the base of microscope having a plurality of output ports. 
     FIG. 9 is a sectional side view of an eyepiece and an optical component housing of the microscope according to a second embodiment of the invention. 
     FIG. 10 is a schematic perspective view of a beamsplitter assembly of the microscope in FIG.  9 . 
     FIG. 11 is a schematic partial cut away side view of an imaging microscope according to a third embodiment of the invention. 
     FIG. 12 is a schematic perspective view of a beamsplitter assembly according to a third embodiment of the invention. 
     FIGS. 13 and 14 are sectional side and front views of the beamsplitter assembly of FIG.  12 . 
     FIG. 15 is a ray trace diagram of the optical path of the microscope according to a third embodiment of the invention. 
     FIG. 16 is a schematic partial cut away side view of an imaging microscope according to a fourth embodiment of the invention. 
     FIG. 17 is a schematic side view of a fragment of a microscope according to a fourth embodiment of the invention, illustrating an image detector, positive lens, beamsplitter assembly and objective lenses of the microscope. 
     FIG. 18 is a schematic bottom view of a carousel and cluster of objective lenses of the microscope illustrated in FIG.  17 . 
     FIG. 19 is a schematic perspective view of a beamsplitter assembly of the microscope of FIG.  17 . 
     FIG. 20 is a ray trace diagram of the optical path of the microscope according to a fourth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments of the invention will be described with reference to the accompanying drawings. 
     Referring to FIG.  1  and according to the first embodiment of the invention, a digital imaging microscope  10  has a base  12  in which a light source  14  and collector lens  16  are provided in vertically stacked alignment, an arm  18  extending upwards from the base  12  having a focus control device  20 , and an optical component housing  22  attached to the upper end of the arm  18  and positioned in vertical alignment over the light source  14  and the collector lens  16 . 
     A stage  24  is connected to the arm  18  such that it is positioned between the optical component housing  22  and the collector lens  16 . A specimen S may be placed on the stage  24  such that the specimen is in vertical alignment with the light source  14 , collector lens  16  and the optical component housing  22 . The stage  24  is vertically movable relative to the arm  18  by adjusting the focus control device  20 . 
     An operator looking through an eyepiece  27  at the proximal end of an eye tube  29  sees an image of specimen S illuminated by the light source  14 , and magnified by an eyepiece lens  28  and an objective lens selected from a cluster of lenses  32  (for the sake of clarity, only one objective lens is shown in FIG.  1 ). The eyepiece lens  28  is located inside the eye tube  29 . As the eye tube  29  is mounted to the optical component housing  22  such that the view through the eyepiece  27  is not along a direct optical path from the stage  24 , a beamsplitter assembly  30  is provided to suitably redirect the light collected by the objective lens to the eyepiece. The beamsplitter assembly  30  is mounted inside the optical component housing  22  and is positioned along an optical path from the stage  24  and collector  16 . The objective lenses  32  are each removably mounted on a rotatable carousel  34  which is in turn affixed to the bottom of the optical component housing  22 . Each objective lens  32  may be rotated into an active position that is along an optical path with both the beamsplitter assembly  30  and the specimen on the stage  24 . 
     With reference to FIG. 2, the beamsplitter assembly  30  comprises three prisms, namely a composite prism  90  (further comprising splitting prism  36  and compensatory prism  37 ), columnar prism  92  and reflecting prism  94  that are mounted laterally spaced apart from each other on a sliding prism holder  96 . The prisms in the beamsplitter assembly all have the same refractive index. Composite prism  90  is constructed from a semi-pentagonal splitting prism and a  30  prism. The composite prism  90  splits the beam into two beams of equal intensity, one of which is reflected to the eyepiece (not shown) and the other for transmission to the image detector (not shown). Columnar prism  92  transmits all incident light to the image detector and no image is reflected to the eyepiece. Columnar prism  92  is necessary to ensure that the focal plane at the image detector does not move when the beamsplitter assembly is switched between image detector only mode (when all light rays are transmitted directly upwards towards the image detector) and dual mode (when light rays are split between the eyepiece and the image detector), thus avoiding the need to adjust the location of the image detector. Semi-pentagonal reflecting prism  94  reflects all incident light towards the eyepiece. No light is transmitted to the image detector. 
     With reference to FIGS. 3 and 4, the prism  90 ,  92 , and  94  on holder  96  may slide horizontally along a lateral channel in linking base  98 . Linking base  98  is affixed to the bottom of the optical component housing (not shown) and has an aperture therein  41 . The linking base  98  is positioned in the optical component housing such that the aperture  41  is directly over the active objective lens (not shown). The holder  96  may slide so that each one of the prism  90 ,  92 ,  94  may be placed into an active position directly over the aperture  41  with an uninterrupted vertical optical path with the active objective lens. This enables the microscope to be operated in one of three modes: an eyepiece-only mode wherein the reflecting prism  94  is in the active position, an image detector only mode wherein the columnar prism  92  is in the active position, and a dual mode with the composite prism  90  in the active position. This provides the following advantages: 
     (1) the user may select eyepiece only mode to maximize the intensity of the image at the eyepiece; 
     (2) the user may select image detector only mode to maximize the intensity of the image at the image detector; and 
     (3) the user may avoid undue aging of the image detector by reducing the image detector&#39;s unnecessary exposure to light. 
     With reference to FIGS. 1 and 5, a positive lens  46  is provided for imaging the beam transmitted through the beamsplitter assembly onto the image detector  35  such that the image detector may be placed in relatively close proximity to the beamsplitter assembly. Furthermore, by using a photographic lens of 0.36× magnification, approximately 90% of the available image can be captured by the image detector. 
     With reference to FIG. 1, the image detector  35  also emits a signal carried by an electrical conduit  61  to an LED emitter  62  at the top of the optical component housing  22  indicating that the microscope is operational. 
     FIG. 6 shows a ray trace of light through the composite prism  90 . Incident light enters prism  36  through the boundary plane  36 A. This beam is partially transmitted through surface  37 A and partially reflected by the angled plane  36 C through surface  36 B. Compensatory prism  37  is necessary to compensate for the different refractive indices of air and glass. In order that the light rays may continue along in the same direction as before passing through the prism medium, it is necessary that the outer boundaries of the prism medium ( 36 A and  37 A) be aligned. 
     Furthermore, in order to integrate the image detector  35  within the body of the microscope  10  and maintain the relatively compact dimensions of a conventional microscope, the positive lens  46  is provided to form an image plane close to the beamsplitter assembly  30 . Without such a positive lens, a video tube long enough to provide the sufficient focal length would be required, thus making a compact design impractical. 
     The image detector may suitably be a Panasonic CCD sensor (part no. MN37777PT) or a Sony CCD sensor (part no. ICX054AK/58AK). Such cameras output analog video signals. The analog video signal is carried by a dedicated electrical wire in a signal conduit  56  (FIG. 2) and split into three separate identical signal wires. Two wires transmit the video signal to a RCA video output jack  58  and S-type video output jack  60  respectively, both of which are located at the microscope base  12  (FIG.  8 ). An analog video monitor (not shown) may be connected to the RCA jack  58  to display the images captured by the image detector  35 . 
     The third video signal wire transmits the video signal to an analog/digital converter  47  (FIG.  7 ). The analog signal is converted to a digital signal then transmitted to a USB port  64  (FIG.  8 ). A computer (not shown) may be connected to the microscope  10  via the USB port  64 . The analog/digital converter  47  is a USB video grabber, comprising a Phillips SAA711A video decoder having an analog video in port and a YUV and IIC signal out ports, a NOGATECH NT1003-1 video camera I/F controller (with compression) electrically connected to and communicative with the video decoder video out ports and having a digital video signal out port connected to the USB port  64 , and DRAM electrically communicative with the video camera I/F controller. 
     The software interface for communicating with the USB port is Microsoft Video for Windows driver and TWAIN interface. This software may be suitably operated on a Windows based computer having a USB port. The computer and related software are beyond the scope of this invention and are not further described here. 
     Referring to FIGS. 1 and 7, a microphone  66  is mounted at the front of the base  12  to record the user&#39;s oral comments. An analog electrical signal is transmitted from the microphone to the image detector  35  via signal wire  68 . The microphone output signal from the image detector  35  is then split into two signals, one of which is directed to the analog/digital converter  47  for conversion into a digital signal, and then sent to the USB coupling  64 . The other signal (analog) is sent to a second RCA coupling  70  that is dedicated to audio signals. 
     Referring to FIG. 1, AC power is supplied to the microscope via a power socket  72 . Power lines  74  transmit the electricity to a voltage transformer  76  that converts the voltage to 12V AC. The 12V AC is used to power the light source  14 . An AC-to-DC converter  78  including a wavelength rectifier, a filter, and a stabilizer (not shown), converts the current into 12V DC. The 12V DC is then directed to the circuits which drive the image detector  35 . The DC circuit is closed by turning on a switch  80  located on the base  12 . Alternatively, an external source of 12V DC can be used to supply power to the circuits which drive the image detector (not shown). 
     As seen in FIG. 7, the microscope  10  can simultaneously transmit analog and digital video and audio signals. This enables a variety of devices to be connected to the microscope  10  at one time, such as a television monitor to the RCA video or S-type ports  58 ,  60 , an audio recorder to the RCA audio port  70 , and a computer to the USB port  64 . 
     FIGS. 9 and 10 illustrate a simplified embodiment of the microscope wherein a beamsplitter assembly  130  has a single splitting prism and is otherwise as in the first embodiment. The beamsplitter assembly  130  comprises a pair of prisms  136  and  137 . A semi-pentagonal prism  136  is stacked beneath a  30  compensatory prism  137 . Both prisms  136 ,  137  have a refraction index of 1.5163. The semi-pentagonal prism  136  splits the total light intensity of the beam collected by the objective lens (not shown) equally between the eyepiece (not shown) and the image detector  135  (via the  30  prism). A fixing screw  138  and cushion plate assembly (not shown) inserted between prism holder  139  and prisms  136 ,  137  serve to protect and to fasten the prisms  136 ,  137  within the prism holder  139 . The prism holder  139  rests on a prism bench  140  mounted to the bottom of the optical component housing  122  and above the carousel (not shown). An aperture  141  is provided through the prism holder  139  and prism bench  140  such that an uninterrupted optical path is provided between the prisms  136 ,  137  and the objective lens (not shown). 
     A lens holder  142  is mounted above the beamsplitter assembly  130 . An aperture  144  is provided through the lens holder  142  so that an uninterrupted optical path is provided between the compensatory prism  137  and image detector  135 . Mounted to the lens holder  142  in the aperture  144  and between the image detector  135  and the compensatory prism  137  is a positive lens  146  having a magnification of 0.36×. The lens holder  142  may slide vertically to enable focussing of the specimen image on the image detector  135  by vertically moving the lens  146 . Once the lens  146  has been satisfactorily positioned, it is fixed in place by a horizontally mounted fixing screw  138 . 
     FIGS. 11 to  15  illustrate a third embodiment of the invention. The microscope is as described in the first embodiment except: 
     (1) the microscope is modified to provide stereoscopic viewing of the specimen image; and 
     (2) the beamsplitter assembly is designed to provide simultaneous monocular imaging by the image detector  201  and stereoscopic viewing by the user. 
     The elements that provide stereoscopic direct viewing are conventional and may be implemented by a person skilled in the art. They do not require elaboration nor detailed explanation. In short, a pair of eyepieces are provided in respective eye tubes. A carousel holds a plurality of objective lenses such that separate pairs of objective lenses may be rotated into an active position wherein each objective lens of the pair collects and magnifies a beam of light from the specimen onto a beamsplitter assembly which in turn reflects the beams to the pair of eyepieces. 
     According to the invention, the beamsplitter assembly  200  reflects a beam of light to each eyepiece  205  and to the image detector  201 . The beamsplitter assembly  200  has a pair of splitting prisms  202 ,  204  and a right angle prism  208 . The splitting prisms  202 ,  204  are mounted laterally apart on a prism bench  212  such that each prism  202 ,  204  is over an aperture  213  in the bench  212  and is positioned along an uninterrupted vertical optical path with one of the pair of active objective lenses  209 . Prism  202  is fixedly mounted to the prism bench  212  by holder  206 . Prism  204  is mounted to the prism bench by holder  207 . Holder  207  may slide horizontally relative to the bench  212 . A horizontally extending channel  218  on holder  207  mates with horizontally extending flange  220  that is fixedly attached to the bench  212  to constrain the horizontal sliding to one dimension. 
     An arm  210  extends generally upwards from the bench  212  and is shaped like an inverse “L” such that the horizontal portion of the L extends laterally over the prism  204 . Mounted to the underside of the horizontally extending portion of the arm  210  is the right angle prism  208  and a reflecting mirror  214 . The right angle prism  208  and reflecting mirror  214  are positioned on the arm  210  by mounts  215  such that they are above splitting prism  204  and such that incident light is reflected by the reflecting mirror  214  onto the right angle prism  208  and further reflected by the prism  208  upwards through an aperture  216  in the laterally extending top portion of the arm  210 , and to the image detector  201 . 
     As shown in FIG. 15, the light beam collected by the active objective lens that is directed towards splitting prism  204  is split into two beams by splitting prism  204  as described in the first and second embodiments such that: 
     (1) one of the beams emerging from splitting prism  204  is transmitted upwards and is reflected by the mirror  214  and prism  208  through aperture  216  for capture by the image detector  201 ; and 
     (2) the second beam emerging from splitting prism  204  is reflected and redirected towards one of the pair of eyepieces  205 . 
     The light beam collected by the other active objective lens is directed towards splitting prism  202  where it is split into two beams. One of the beams emerging from splitting prism  202  is reflected towards one of the eyepieces  205 . The other beam emerging from splitting prism  202  is transmitted upwards and is not used. The use of splitting prism  202  is necessary to provide an image of equal light intensity to both eyepieces. 
     In order to increase the amount of light directed to the image detector  201 , splitting prism  204  may be moved in a horizontal direction so that all light otherwise incident on splitting prism  204  is transmitted to the image detector  201 . 
     As in the first embodiment of the microscope, a positive lens  211  is provided for imaging the beam transmitted through the splitting prism  204  onto the image detector  201  such that the image detector may be mounted in relatively close proximity to the beamsplitter assembly and the image detector captures a substantial portion of the image viewable by the eyepiece  205 . A 16 mm diameter photographic lens of 0.32× magnification provides a focal length of 40.77 mm, thereby enabling approximately 91% of the image to be captured by the image detector  101 . 
     FIGS. 16 to  20  illustrate a fourth embodiment of the microscope invention. The microscope is the same as described in the third embodiment except that the design has been modified to provide a dedicated pair of active objective lenses to magnify an image for eyepiece viewing, and another dedicated objective lens for imaging by an image detector. These modifications are described in the following paragraphs. 
     Referring to FIG. 17, a rotatable carousel  321  is provided for mounting a cluster of objective lens pairs  322 ,  323  as described in the third embodiment. Differing from the third embodiment, a further aperture  324  is provided in addition to a pair of eyepiece apertures  325  through prism bench  326 . The eyepiece aperture  325  is located such that a pair of objective lenses  322  can be rotated into an eyepiece active position beneath each aperture  325  (note that lenses  322  are omitted from FIG. 16 for the sake of clarity). The aperture  324  is located such that when the pair of objective lenses  322  are in the eyepiece active position, one objective lens in a pair of objective lenses  323  is in a CCD active position beneath the CCD aperture  324 . 
     A semi-pentagonal eyepiece prism  328  is mounted over the eyepiece aperture  325  by holder  329  so that the stereoscopic image picked up by the eyepiece active objective lenses  323  is redirected (as a beam) to the eyepiece  327 . The split ratio for this prism  328  is 1:0. All of the image is reflected to the eyepiece and there is no upwards transmission of light through the prism to reduce the image intensity. A pentagonal CCD prism  130  is mounted over the CCD aperture  324  by holder  331  so that the beam collected by the CCD active objective lens  323  is redirected to the CCD sensor of the camera  332 . The split ratio of this prism  330  is also 1:0; all of the light rays are reflected to the image detector  332 . As the image detector  332  does not require a stereoscopic image, the other objective lens in the pair  323  is idled. 
     A positive lens  334  is provided along an optical path between the image detector  332  and the CCD prism  330  for imaging the beam collected by the CCD active objective lens  323  onto the image detector  332  such that the image sensor  332  may be mounted in relative proximity to the CCD prism  330  and the image sensor  332  captures a substantial portion of the image viewable through the eyepiece. A 15 mm diameter photographic lens provides a focal length of 30.8 mm. 
     Modifications, variations and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims. For example, the carousel may be adapted to mount more or fewer objective lenses than described in the above four embodiments. Various magnifications may be selected for the objective, positive, and eyepiece lenses depending on the preferences of the designer. Different indices of refraction and split ratios may be selected for the variously described prisms.