Abstract:
An ophthalmoscope has viewing optics through which, in use, the user views an eye under examination. Those optics include one or more light receiving elements ( 62  and  64 ). The ophthalmoscope also has a centrally positioned camera ( 14 ) for detecting an image corresponding to that seen by the user. Light is reflected into the camera objective by means of a reflector ( 16 ) which is also centrally positioned on the ophthalmoscope and is situated in substantially the same plane as said elements of the viewing optics, but is laterally spaced therefrom. The central positioning of the camera does not adversely affect the lateral weight distribution of the ophthalmoscope, whilst the provision of a reflector at the same level as the optical elements helps to preserve the congruents between the image detected by the camera and that seen by the user if the condenser lens (used to form the virtual image of the eye under examination) is moved towards or away from the ophthalmoscope.

Description:
RELATED APPLICATION 
     This Application claims the benefit of U.S. provisional Patent Application No. 60/108,624 filed on Nov. 16, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to ophthalmoscopes, in particular to an indirect ophthalmoscope which is used by an ophthalmologist in the observation of an image of the retina of an eye under examination. The invention also relates to an attachment for an ophthalmoscope. 
     BACKGROUND TO THE INVENTION 
     It is known to fit an indirect ophthalmoscope with a camera for detecting an image of the retina being observed, through a condenser lens, by the user of the ophthalmoscope. Such ophthalmoscopes are often used in the instruction of students or to inform patients and their relatives, since the image of the retina being observed by an ophthalmologist using the device can be displayed on a separate video display unit in real time. The image can also be recorded for subsequent analysis. It is also envisaged that, with the advent of improved telecommunications systems, a non-specialist could use the ophthalmoscope to provide an image which is transmitted to a specialist at a remote location so that the specialist can perform an examination without visiting the patient. 
     Typically, an indirect ophthalmoscope is a binocular device having viewing optics which have two central mirrors arranged to direct respective images to the left and right eye of the person using the ophthalmoscope. In one known arrangement, the camera detects an image reflected from an angled half-silvered mirror disposed immediately in front of the two mirrors. 
     A disadvantage of this arrangement is that the half-silvered mirror reduces the intensity of light which reaches the viewing optics. in addition, the intensity of light reaching the camera is less than would be the case pith a fully silvered mirror. Given that there is, in practice, an upper limit to the intensity of light which can be used to illuminate the eye under examination, the image is seen by the person using the ophthalmoscope and detected by the camera may be less bright than is desired. 
     In addition, since the image detected by the camera has been reflected from the mirror, it is inverted relative to the image seen by the wearer, who would therefore have to take this into account when explaining features of the eye under examination to students viewing a display of the image detected by the camera. 
     Another known type of camera attachment for an ophthalmoscope is produced by Litechnica, and comprises a fully silvered mirror which reflects light into a camera attached to the side of the ophthalmoscope. However, this gives rise to a lop-sided arrangement and the mirror partially obscures the user&#39;s view of the eye under examination. It is also known to provide an ophthalmoscope with a centrally mounted camera (The Video Omega  2 C produced by Heine Optotechnik) but in this case the images detected by the camera and seen by the user may become incongruent as a result of changes in the working distance of the ophthalmoscope. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided an ophthalmoscope having viewing optics through which, in use, an eye under examination is viewed, the optics having at least one light receiving element via which light from an eye under examination is received by the viewing optics, the ophthalmoscope further comprising image detection apparatus for detecting an image of an eye under examination, wherein the image detection apparatus is substantially centrally positioned on the ophthalmoscope and is so positioned relative to the light receiving element that, in use, light from the eye under examination which is incident on the image detection means is received by the latter at a position which is laterally spaced from the paths of light from the eye to the light receiving element or through the viewing optics, wherein, said position is in the same horizontal plane as the light receiving element. 
     Thus, the image detection apparatus does not block the passage of any light from the eye to the light receiving element, and therefore does not reduce the intensity of the image viewed through the viewing optics. Similarly, the viewing optics do not affect the intensity of the image detected by the image detection apparatus. In addition, since the element and the position at which light is incident on the image detecting apparatus are at substantially the same level, the images seen by the user and detected by the apparatus are not displaced relative to each other by variations in the working distance of the ophthalmoscope. 
     Preferably, the image detection apparatus has a portion via which, in use, light from the eye under examination enters the image detection means, and which may be laterally spaced from the light receiving element. 
     Where the ophthalmoscope is a binocular instrument, having two spaced apart light receiving elements, the portion is preferably so situated that the elements are at least partially spaced in opposite lateral directions from the portion. 
     If the stereopsis of the ophthalmoscope is fixed, substantially all of each element is preferably laterally spaced from the portion. If, however, the separation of the elements is adjustable to adjust the stereopsis of the ophthalmoscope, substantially all of each element is preferably laterally spaced from the portion at least when the separation of the objectives is at its maximum. 
     Preferably, the portion is equidistant from the elements. 
     This helps to ensure that the image detected by the image detection system corresponds to that viewed through the viewing optics. 
     Preferably, the image detection apparatus comprises a camera, which is substantially centrally positioned on the ophthalmoscope, preferably in a position equidistant from the light receiving elements. 
     The substantially central mounting of the camera helps to reduce the effect that the camera has on the balance of the ophthalmoscope; the instrument is not lopsided, and is consequently easier to wear than the type of ophthalmoscope in which the camera is situated to one side of the viewing optics. 
     Said portion of the image detection means may comprise the objective of the camera. 
     Preferably, however, the portion comprises reflecting means which, in use, reflects the image of an eye under examination into the camera. Preferably the camera is positioned above the light receiving elements, and the reflecting means is at substantially the same level as said elements and reflects light up into the camera. 
     The reflecting means is conveniently so arranged as to cause two reflections of light from the eye under examination so as to eliminate any mirror inversion of the image detected by the camera. 
     The use of two reflections to avoid mirror inversions provides an effective and relatively cheap method of ensuring that the orientation of the image detected by the camera corresponds to that of the image seen through the viewing optics. 
     Conveniently, the reflecting means comprises a prism, preferably a pentagonal prism, for example a penta prism. 
     The pentagonal prism provides the double reflection and is particularly advantageous since the relative orientation of the prism surfaces which cause the two reflections is fixed and the prism provides is an efficient reflector of light. 
     Preferably, the prism and camera are carried by a frame adapted to be mounted on the front of the ophthalmoscope. 
     The invention also lies in an attachment for an ophthalmoscope, the viewing optics of which have at least one light receiving element via which light from an eye under examination enters the viewing optics, the attachment comprising retaining means for holding a camera and reflecting means operable to reflect light from an eye under examination into a camera held by the retaining means, the attachment further comprising mounting means for mounting the attachment on an ophthalmoscope, wherein the position of the reflecting means relative to the mounting means is such that, when mounted on an ophthalmoscope, the reflecting means is, in use, laterally spaced from, and at the same level as the light receiving element and the camera is substantially centrally positioned on the ophthalmoscope. 
     Preferably, the attachment is adapted for use with a binocular ophthalmoscope, and the retaining means comprises a frame having a cross-member on which the reflecting means is mounted in a generally central position such that, with the attachment mounted on the ophthalmoscope, the two light receiving elements of the latter are laterally spaced, in opposite directions, from the reflecting means. 
     Preferably, the reflecting means comprises a pentagonal prism, for example a penta prism. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of an ophthalmoscope and attachment according to the invention; 
     FIG. 2 is a schematic side view of the illuminating optics of the ophthalmoscope; 
     FIG. 3 is a more detailed, cut-away view of part of the ophthalmoscope; and 
     FIG. 4 is a simplified diagram illustrating the relative positions of parts of the viewing optics and image capture means of the ophthalmoscope. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, reference numeral  1  generally denotes an indirect ophthalmoscope which, in this example, is of the type currently supplied by the Applicants under the Trade mark KEELER VANTAGE. This ophthalmoscope can be retrofitted with an attachment, generally referenced  2 , to provide an ophthalmoscope in accordance with the invention. 
     The ophthalmoscope  1  is provided with a headset (not shown) to enable the ophthalmoscope to be mounted on the head of a user, and has a housing  4  for a light source, and illuminating optics. 
     With reference to FIG. 2, the illuminating optics comprise a light bulb  56 , light from which is reflected from an angled planar mirror  58  in the housing  10 , to an eye to be examined (indicated as the “eye of patient”). Light from the patient&#39;s eye travels back to the ophthalmoscope to enter the viewing optics of the latter. Reference numeral  60  denotes the viewing optics, some components of which are shown in more detail in FIG. 4, and are described below. 
     The front of the housing  4  includes a window  6  through which light from the illuminating optics exits the housing  4 , and light from the eye under examination reaches two spaced apart light receiving elements of the binocular viewing optics. 
     In use, the illuminating light beam is reflected from the planar mirror  58  and passes through a hand-held lens (usually plus  20 D) before reaching the patient&#39;s eye. The user of the ophthalmoscope views the illuminated eye through the binocular viewing optics. The illuminating beam path does not coincide with the path of light reflected from the eye into the viewing optics since there is vertical separation and an acute angle between those two paths. 
     The viewing optics  60  include two light receiving elements, each comprising a respective one of two mirrors  62  and  64  mounted on triangular mirror blocks  66  and  68  which are, in turn, supported on a platform  70 . The platform  70  also supports two further, triangular mirror blocks  72  and  74  positioned one on either side of the mirrors  62  and  64 . The further blocks carry corresponding further mirrors  76 ,  78  (also part of the viewing optics) and have apertures (eg  80 ) to allow light reflected from the mirrors  62  and  64  to travel to the reflective surfaces of mirrors  76  and  78  from which the light is reflected into eye pieces (not shown) forming part of the viewing optics and situated behind the platform  70 . 
     All the mirror blocks  66 ,  68 ,  72  and  74  are all slideably mounted on the platform  70 , and the ophthalmoscope includes mechanisms, not shown, for varying the separation between the blocks  66  and  68  (to adjust the stereopsis of the ophthalmoscope) and between the blocks  72  and  74  to enable the ophthalmoscope to accommodate a range of different interpupillary distances of its users. 
     The attachment  2  comprises a frame  8  having upper and lower generally rectangular portions, respectively referenced  10  and  12 . The upper portion carries a camera  14 , whilst a penta prism  16  is mounted at the bottom of the lower portion  12 . 
     The frame  8  is pivotally connected to a pair of opposed arms  18  and  20  which are in turn attached to a rectangular mounting frame  26  adapted to be attached to the front of the ophthalmoscope  1 . The top of the mounting frame  26  has two spaced apart tongues (not visible in FIG. 1) which slide up under the front vents of the ophthalmoscope  1 . The bottom portion of the frame  26  has two apertures, one on either side, for receiving respective screws, one of which is shown at  28 . The screws extend into screw-threaded bores in the front of the ophthalmoscope  1  so that the frame  26  is securely located on the latter. 
     As can be seen prom FIG. 1, both the frame  26  and the portion  12  have central openings in registry with the window  6 . 
     Two compression springs, one of which is shown at  30 , are situated towards the top of the portion  12  and the mounting frame  26 , and act between the frame  8  and the mounting frame  26  to bias the top of the frame  8  away from the ophthalmoscope  1 , and hence the portion of the frame  8  below the pivotable arms  18  and  20 , towards the mounting frame  26 . The extent of movement towards the mounting frame  26  is limited by an adjustment screw  32  which extends through a screw-threaded through bore in a cross-member  34  at the bottom of the portion  12  to bear against the mounting frame  26 . The screw  32  is thus urged against the mounting frame  26  by the springs  30 , and the rotation of the screw  32  will cause the frame  8  to pivot around the pivotal connections at the arms  18  and  20 , and hence about a horizontal axis passing through those connections. 
     The prism  16  is securely held within a bracket  36  from which an apertured lug  38  extends. The lug  38  is, in turn, screwed onto the bottom of the portion  12  using a screw  40 . The screw shaft is of a smaller aperture than the aperture, and a pin (not shown) extends from the bracket  36  into the bottom portion  12 . This mode of attachment of the prism  16  enables the position of the latter to be adjusted by loosening the screw  40  and rotating the bracket  36  about the axis of the pin before tightening the latter to fix the orientation of the prism  16 . The axis about which the bracket can rotate is indicated at  82  in FIG.  1 . 
     With reference to FIGS. 1 and 3, a further bracket  42  extends forwardly from the top of the portion  10  and includes a vertical passage  43  having a cylindrical upper portion and a frustoconical lower portion. The passage  43  accommodates a ball sleeve  44  through which the camera  14  extends and is securely fixed thereto by-means of two grub screws  46 . The ball sleeve  44  can move within the passage in the bracket  42  to allow the orientation of the camera  14  to be adjusted. Once a suitable orientation has been selected, the position of the ball sleeve  44  relative to the bracket  42  is fixed by means of a pair of radial grub screws  48  which have pointed ends for engaging the exterior of the ball sleeve  44  to hold the latter in position. 
     A compression spring and ball (not shown) act between the ball sleeve  44  and the passage  43  to hold the sleeve captive in the passage even when the screws  48  are removed. 
     As can be seen from FIG. 1, the prism  16  is centrally located on the bottom portion  12  of the frame  8 . When the separation between the mirrors  62  and  64  is at a minimum, the prism holder  36  does partially occlude the two mirrors  62  and  64 . However, when the stereopsis is set at a maximum, so that the separation between the mirrors  62  and  64  is also at a maximum, the mirrors  62  and  64  are laterally spaced, in their entireties, in opposite directions from the prism  16  and the holder  36 . Consequently, when the ophthalmoscope  1  is viewed from the front, the prism  16  would appear between the two mirrors. As a result, the user of the ophthalmoscope looks past either side of the prism  16 . 
     This can be seen from FIG. 4, in which the separation between the mirrors  62  and  64  is such that light travelling from the eye under examination to the viewing optics travels along paths, for example paths  84  and  86  which straddle the prism  16 , whilst the camera  14  receives light, reflected by the prism  16 , which has travelled along paths, for example path  88 , which are laterally spaced from the mirrors  62  and  64 , and hence from the path of light thereto. 
     Light from an eye under examination enters the prism  16  through a front face  50  and travels to a rear upper-angled face  52  which reflects the light down and forward onto a further angled face  54 . The face  54 , in turn, reflects light from the face  52  directly up into the camera  14 . Consequently, the image of the eye under examination is reflected firstly by the face  52  and then by the face  54  before entering the camera  14 . As a result of this double reflection, the image entering the camera  14  is not inverted relative to the view seen through the viewing optics of the ophthalmoscope  1 . 
     In this example, the prism is sold under the Trade Mark MELLES GRIOT and is identified by the reference 01PPA003. The video camera is identified by the Trade Mark “TELI” and produced by Tokyo Electronic Industry Co. Limited. The present example has a product number CS6100, and uses a PML 300-30 mms focal length lens. 
     It has been found, in practice, that not all users of the ophthalmoscope look straight into the eyepieces of the viewing optics For example, a wearer of half -moon spectacles, may tilt his/her head forwards relative to the ophthalmoscope so as to view the eye under examination (through the viewing optics) over the tops of the spectacles, Consequently, the image detected by the camera  14  may not correspond with that being viewed by the user. However, this difference can be eliminated by using the screw  32  to pivot the frame  8  about the pivotal connection to the arms  18  and  20  until the rays of light which the prism  16  reflects up into the camera  14  are substantially co-planar with those received by the viewing optics. Before the ophthalmoscope is first used with the attachment  2 , the attachment  2  is set up by locking the prism  16  and camera  14  in a suitable relative position. This can be achieved with, for example, a suitable jig. 
     The ophthalmoscope described above is one example of an embodiment of the invention, and various alterations or modifications may be made without departing from the scope of the invention as defined by the claims. Thus, for example, the prism  16  may be replaced by an alternative type of reflecting means, for example a mirror. In such a case, the camera will receive a laterally inverted image, which may be displayed on a monitor or may be electronically re-inverted by image processing circuitry connected between the camera and the monitor. 
     Furthermore, instead of the frames  8  and  26 , the ophthalmoscope may have an alternative type of mount, for example formations which attach the camera and reflector directly to the ophthalmoscope, which enable the position of the camera and reflector to be altered and which therefore also constitute the adjustment means.