Abstract:
An indirect ophthalmoscope is used in the observation of the eye, and comprises viewing optics through which, in use, at least part of said eye is viewed. Light from the eye travels to and through the viewing optics along a viewing path, and the ophthalmoscope includes a lens ( 36 ) which is movable between an operative position (in which it is situated in the viewing path) and an inoperative position (in which it is situated clear of the viewing path). The lens  36  can be used to enable the observer to see a magnified image of the eye under examination.

Description:
FIELD OF THE INVENTION 
     This invention relates to indirect ophthalmoscopes. 
     BACKGROUND TO THE INVENTION 
     In diagnostic and operative retinal practice, binocular indirect ophthalmoscopy is widely accepted as a fundamental method of examination of the retina of an eye. Typically, an indirect ophthalmoscope includes illuminating optics for illuminating the retina and viewing optics through which the image of the retina is viewed, and is mounted on a headband which enables it to be attached to the head of a user. The ophthalmoscope is used in conjunction with a hand held condensing ophthalmoscopy lens which the user positions between the viewing optics and the eye under examination so as to form an image of the retina between the lens and the viewing optics. 
     Compared with direct ophthalmoscopes, indirect ophthalmoscopes provide a relatively wide field of view and bright illumination of the retina under examination. However, this view is of a relatively low magnification. 
     It has been proposed to improve the magnification of an indirect ophthalmoscope by re-configuring the viewing optics so that the latter act as telescopes. However, this increases the complexity of the viewing optics and can adversely affect the size and weight of the ophthalmoscope. In addition, such an ophthalmoscope is no longer suitable for providing low magnification views. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided an indirect ophthalmoscope for use in the observation of an eye, the ophthalmoscope comprising viewing optics through which, in use, at least part of said eye is viewed, light from the eye travelling to and through the viewing optics along a viewing path, wherein the ophthalmoscope includes a lens so mounted thereon as to be movable between an operative position, in which it is situated in said viewing path so as to refract light from the eye under examination, and an inoperative position, in which the lens is situated clear of the viewing path. 
     Thus the movable lens can enable the user to alter the size of image viewed through the viewing optics. If the viewing optics incorporate telescopic systems, the lens may, for example, be a diverging lens which reduces the magnification of the final image. 
     Preferably, however, the lens is a converging lens which, when in its operative position, acts as a magnifying lens for the image viewed through the ophthalmoscope. 
     When this lens is in its operative position, and the image formed directly by the hand held ophthalmoscopy lens is closer to the movable lens than the focal length of the latter, the movable lens acts as a magnifying glass which creates an enlarged virtual image of the retina further away from the viewing optics than the image formed by the ophthalmoscopy lens. This enables the user to see a focused image of the retina from a closer position than would be possible without the movable lens. 
     Preferably, the movable lens is a meniscus lens, preferably of a power of approximately +3 dioptre. 
     The lens may to advantage be mounted in front of the viewing optics so that, when in its operative position, it is interposed between the viewing optics and the eye under examination. 
     Preferably, the lens is pivotally mounted on the front of the ophthalmoscope so that movement between the operative and inoperative positions is achieved by pivoting the lens. 
     In this case, the lens is conveniently connected to a manually operated control member, preferably a rotary member, movable to cause said pivotal movement. 
     Preferably, the lens is mounted on the ophthalmoscope through a mounting bracket. 
     Preferably, the ophthalmoscope is a binocular device, the viewing optics being one of a pair of such viewing means for enabling an eye under examination to be viewed along corresponding left and right eye viewing paths, wherein the lens, when in its operative position, is situated in both viewing paths. 
     Preferably, the ophthalmoscope is attached to mounting means (for example a headband or headgear) for mounting it on the head of a user. 
     Another advantage of the movable lens is that it can be retrofitted to existing ophthalmoscopes without having to make any adjustment or modification to their viewing optics. In addition, since the ophthalmoscope can be positioned closer to the eye under examination, the user will obtain a better three-dimensional image (if the ophthalmoscope is a binocular device) because of the greater angle of convergence of the left and right eye viewing paths of the viewing optics on the retina under examination. 
    
    
     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 embodiment of a binocular indirect ophthalmoscope in accordance with the invention; 
     FIG. 2 is a schematic side view of the viewing optics and illumination optics of the ophthalmoscope; and 
     FIG. 3 is a schematic view of part of the viewing optics of the ophthalmoscope. 
    
    
     DETAILED DESCRIPTION 
     The drawing shows one of the present applicant&#39;s ophthalmoscopes, identified by the Trade Mark KEELER VANTAGE, which has been modified by the addition of a further lens assembly, generally referenced  1 . 
     The ophthalmoscope comprises a housing  2  which is in the general shape of an inverted T. The housing contains illumination and left and right eye viewing optics, and is attached to a headband  4  which enables the ophthalmoscope to be mounted on the head of a user (for example, an ophthalmologist). The housing  2  is attached to the front of the headband  4  via two arms  6  and  8 , which are pivoted together and to the headband and housing  2  respectively, and which enable the distance between the housing  2  and the face of the user to be adjusted. 
     The connections between the two arms  6  and  8  and the headband  4  and the arm  6  and the arm  8  and the housing  2  are provided with nuts, for example  18  and  20 , which can be tightened to fix the housing  2  at a desired position relative to the headband  4 . 
     The vertical stem of the housing  2  contains the illumination optics for illuminating a retina to be examined. The illumination optics comprise a light bulb  7  (FIG. 2) at the top of the housing  2 , and an angled mirror  9  positioned underneath the light source so as to reflect light from the latter out through a window  10  in the front face of the housing  2 . The mirror is positioned immediately above the viewing optics  11  (situated in the cross-piece  12  of the housing), and is mounted in the housing  2  via an adjustment mechanism which enables the angulation of the mirror  9  to be adjusted by rotation of either of two control knobs  14  and  16  projecting horizontally from the sides of the stem of the housing  2 . Vertical separation between the mirror  9  and the viewing optics can be varied by vertically moving control knobs  14  and  16  or by raising or lowering plates  43  and  44  on the opposite side of the housing stem from the plate  42 . This adjustment of the height of the mirror  9 , in combination with an adjusted angulation of the mirror  9 , enables the vertical angle between the path along which light illuminates the retina under examination and the viewing path to be adjusted. When the ophthalmoscope is used at a relatively short distance from the eye under examination, the mirror  9  can be moved further towards the viewing optics to decrease the angle between the illumination path and the viewing path so as to allow both paths to pass through the pupil of the eye under examination. When the ophthalmoscope is used at greater distances, however, the mirror  9  can be moved further from the viewing optics so that corneal reflexes can be reduced. 
     With reference to FIG. 3, the viewing optics  11  comprise a pair of laterally spaced mirrors  13  and  15  which are mounted on triangular mirror blocks  17  and  19 . The mirrors  13  and  15  are angled so as to direct light travelling into the window  10  (along the viewing paths  21  and  23 ) horizontally outwards to the reflective surfaces of the mirrors  25  and  27  mounted on triangular mirror blocks  29  and  31  which have apertures ( 33  and  35 ) to allow the light to reach the mirrors  25  and  27 . Further apertures ( 37  and  39 ) in the blocks  29  and  31  enable the light reflected by the mirrors  25  and  27  to travel to eyepieces (not shown) situated behind the platform  41  on which the mirror blocks are mounted. The spacing between the mirrors can be adjusted by means of a lever  22  in order to adjust the stereopsis of the ophthalmoscope. 
     Since the illuminating and viewing optics within the housing  2  are part of a proprietary ophthalmoscope (i.e. the KEELER VANTAGE), they have not been shown in the drawing. 
     The front of the housing  2  is provided with screw holes at  24  and  26  via which a generally U-shaped mounting bracket  28  is screwed onto the housing  2 . 
     The bracket  28  has a pair of opposed side arms  30  and  32  through which a horizontal shaft  34  extends. 
     The shaft  34  carries a radial meniscus converging lens  36 , and is terminated at each end by a respective knob  38  and  40  which can be turned to rotate the shaft  34 , and hence the lens  36 , in either direction indicated by the double-headed arrow  42 . 
     The frictional interaction between the arms  30  and  32  and the portions of the shaft  34  passing therethrough is sufficient to hold the shaft  34  and hence the lens  36  in a selected angular position. 
     The power of the lens  36  is + 3  dioptre. Since the shaft  34  extends over the top of the front of the window  10 , the lens  36  can be moved in the directions indicated by the arrow  42  between a lowered position, in which the lens  36  covers the window  10  and a raised position in which the lens  36  presents no obstruction to the window  10 . When the lens  36  is raised into this position, it is in its inoperative position, and as a result the ophthalmoscope functions in the same way as a standard KEELER VANTAGE indirect ophthalmoscope. Consequently, the ophthalmoscope has the same magnification power and minimum working distance (i.e. between the real image of the retina formed by the hand held lens (not shown) and the mirrors of the viewing optics) below which the user would not be able to focus on the image of the retina. 
     If the user wishes to obtain a “closer” view of the retina, he or she can simply use the hand not holding the ophthalmoscopy lens to rotate the lens  36  into its operative position (in which it covers the window  10 ). The user positions the ophthalmoscope close to the eye under examination, so that the image formed by the ophthalmoscopy lens will be closer to the lens  36  than the focal length of the latter. As a result, the lens  36  will form an image of the retina at a position at or greater than said minimum working distance. 
     It will be appreciated that, when in its operative position, the lens  36  magnifies the images seen by both the left and right eye of the user, since it encompasses both the associated viewing paths  21  and  23  from the eye under examination. 
     The image formed by the lens  36  will be linearly magnified compared with that formed by the ophthalmoscopy lens, but will be further away from the viewing optics than the latter. Thus, the lens  36  has the effect of magnifying the image seen by the user, since the user can focus on the retina from a closer distance than-would otherwise be possible.