GB2077946A - Indirect ophthalmoscope - Google Patents

Abstract

An indirect ophthalmoscope consists of a single unit incorporating a light source 5, collimating lens 6, and a beam splitter 7 reflecting the light through a lens 8 serving as a projection lens and a viewing lens. The user views the aerial image produced by the lens 8, through a viewing window 13 close to the image plane. The light source and collimating lens are mounted in a handle 4. The ophthalmoscope can be used with an infra-red image converter for infra-red examination. <IMAGE>

Description

SPECIFICATION Indirect ophthalmoscope This invention relates to indirect ophthalmoscopes.

Conventional indirect ophthalmoscopes comprise a head band which carries a lightprojection system and viewing optics, usually a binocular eye pieee. The projection system forms a beam of light which is directed into the eye of a patient by way of a hand-held condensing lens, and the retina thus illuminated is viewed through the same condensing lens, and the viewing optics. Such instruments are widely used but have a number of disadvantages. The light projector and viewing optics are quite heavy, and a power supply lead for the light source must be provided.

The head band is consequently awkward and uncomfortable. It takes a substantial amount of practice, to learn to use such an instrument effectively. Careful co-ordination is needed to position correctly the hand-held lens and the observer's eye. There is usually a large amount of stray light which impairs the clarity of the image seen by the observer, and this makes it necessary to use relatively intense illumination, which may be uncomfortable or even dangerous for the patient. Because of the difficulty in eliminating stray light, inconvenient corneal reflexes may be produced.

According to the present invention, the light projection system, and a converging lens used to form an aerial image of the fundus of the eye illuminated by the projection system, are combined in a single unit, which can be designed to be hand-held. The viewing line of sight and the axis of the light beam are made substantially coincident, for example by the use of a beam splitter. With such an instrument, the aerial image formed can be viewed directly, or through auxiliary optics for example a binocular eye piece. Because the light source and the viewing and imageforming lens are combined in a single unit, accurate alignment of the light beam axis and the line of sight is ensured and the amount of stray light is very much less than in the case of a conventional indirect ophthalmoscope using a separate hand-held lens.The present instrument is therefore much easier to use than a conventional indirect ophthalmoscope, requiring little practice to secure a clear and well illuminated image, and the accurate control of the illuminating beam which it makes possible greatly reduces discomfort of the patient, both directly and because it becomes possible to use less intense illumination than in a conventional indirect ophthalmoscope.

In a preferred arrangement, the converging lens is placed in the light path from the light source to the patient as well as in the line of sight between the patient and the observer, so that this lens serves both to direct the light into the eye of the patient and to form an aerial image of the fundus, in a manner analogous to the hand-held lens of a conventional indirect ophthalmoscope.

Preferably, the aerial image is formed substantially at a viewing aperture of the instrument. This makes it possible to place graticules and other optical elements in or near the image plane, for measuring or mapping features of the retina.

The light source is preferably a light-emitting diode or similar source, or a fibre optic light guide.

The accompanying drawing show, in section, an indirect ophthalmoscope embodying the present invention. The eye of a patient is shown schematically at 1.

The instrument has a housing 2 which is generally T-shaped and comprises a cylindrical tube 3 mounted across the wider end of a tapered tube 4. The tube 4 has at its narrow end a fibre optic light guide 5 which illuminates a plano convex lens 6 to form a parallel beam of light which is projected into the tube 3. In the latter is a partly reflecting and partly transmitting plate 7 mounted obliquely so as to reflect the incident light along the tube 3 onto a plano-convex lens 8 at one end of the tube 3. This lens forms an image of the fibre optic light source, of unit magnification, and this image in use is made to coincide with the iris of the patient's eye. The lens 8 also forms an image of the fundus of the eye -thus illuminated, in a plane 9 at the opposite end of the tube 3.

A filter wheel 10 is mounted on the outer end of the light source tube 4, so that different filters and graticules carried by the wheel can be moved into the path of the light from the fibre optic light guide 5. The light guide is longitudinally adjustable for focusing.

On the output side of the lens 6 is a polariser 1 and another polariser 12 is mounted in the viewing window 13 at the end of the tube 3, close to the image plane 9. The polarisers are to eliminate unwanted reflections.

In use, the instrument is held by the observer or examiner, so that the light beam is focused at the iris of the patient's eye. The examiner observes the aerial image formed at the image plane 9. This image can be viewed directly or by way of further optics for example a binocular optical system (for stereoscopic viewing) mounted on a headband or on a spectacle frame. The instrument is very easy to use. It is only necessary to ensure that the light beam is accurately placed in the patient's iris. The optical system will then be necessarily correctly aligned to produce the desired aerial image, which is easily found by the examiner.

An important advantage of the instrument is the accurate imaging of the light source onto the desired position in the eye (usually the plane of the pupil). The object-image relation is one to one in the case of the lenses 6 and 8 being of the same power.

The lens 8 may be mounted so that lenses of different powers can be fitted. In this case, a dust plate consisting of an optically clear disc is provided behind the interchangeable lens 8.

Typically, a set of lenses of powers 1 6D, 20D, 30D and 36D is provided. The lenses 8 preferably have projecting hoods, which when the lens is attached to the body of the instrument projects forwards towards the patient. The hoods preferably have lengths which are in inverse relation to the lens power. This serves to identify the particular lens, and to establish an appropriate viewing distance from the instrument to the patient's head.

The instrument can be held by the user in whatever way is convenient for example by the tube 4 or by means of the region containing the lens 8 or a lens hood. Holding the instrument by the lens hood or lens-containing region has the advantage that the user's fingers, other than the index finger and thumb which grip the instrument, are free to rest against the forehead of the patient in order to steady the instrument.

The tube 4 may be attached to or form part of a hand grip which can contain a lamp bulb and battery, and optionally, a dimmer control.

Alternatively, light may be conducted from a remote lamp by the light guide.

Fibre optic light sources have excellent characteristics for use in the present instrument.

The light output falls within a well defined cone which fills the aperture of the collimating lens 6 and the resulting parallel beam can be focused to give a small image at the patient's iris. For example, a 2 mm diameter fibre bundle, with an optical system of unit magnification, produces an image of 2 mm diameter which results in very efficient and uniformly distributed illumination of the fundus. Because of the small size of the beam image, the eye can be examined with a small pupil. There is very little stray light in the focal plane of the fibre optic source, with the result that the reflex from the cornea is spatially well defined and can be easily avoided by the examiner.

The position of the light source is adjustable towards and away from the lens 6, so that the effective focal length of the illumination system can be continuously varied whereas the focal length for viewing remains unchanged. The instrument is normally set for viewing an eye with a "standard" iris-retina distance. To view an object closer to the front of the eye, the viewing lens 8 must be moved away from the eye. This would increase the diameter of the light beam at the iris, reducing the illumination with the eye and causing reflexes. By adjustment of the light source towards the lens 6, the narrowest part of the beam can be moved to coincide with the iris. Such focusing adjustment is also desirable to compensate for changes in the focal length of the lens when the instrument is used in different regions of the spectrum, for example by means of filters in the filter wheel 10.

The fibre optic bundle may have the simplest end profile namely a circle, or other bundle end profiles can be used for example further to alleviate problems of reflexes originating at refractive index boundaries on the surface of or within the eye. Any desired bundle end profile can be selected for example a rectangular or "slit" profile forming a corresponding image which can be rotated about the axis of the eye by rotation of the fibre optic bundle 5. Two or more fibre optic bundles may be provided to give a desired light distribution in the eye.

Alternative light sources are solid state emitters, for example of gallium arsenide. These also have the advantage that the light is directed forwards. Incandescent light sources are inferior, because they radiate light in all directions, and are therefore inefficient and cause stray light.

Conveniently, the collimating lens 6 is an aspheric lens of plastics material which is well corrected and has good optical transmission throughout the visible and near infra red regions of the spectrum.

The dual-function lens 8 can also be a plastics aspheric lens, but the optical layout enables a wide variety of lenses to be used. This lens should exhibit no birefringence. Since it is illuminated by parallel light, substitution of a lens of different power does not require any other changes in the system. The illuminating light will still be brought to a focus close to the plane of the iris, and will illuminate a larger or smaller area of the fundus as the power of the lens is increased or decreased. If the lens 8 is a high-power, small-aperture lens, light could be scattered from the lens mount or retaining ring which would cause a halo around the observed image. To eliminate this, the instrument may include an adjustable aperture 14 so that the illuminating beam can be stopped down so as just to fill the lens 8.

The polarisers are also to reduce stray light. The first polariser 11 preferably has its plane of polarisation perpendicular to the plane of incidence of the partial reflector 7. The second polariser 12 has its plane of polarisation set to give minimum transmission to the examiner of light reflected from the surface of the cornea and from the lens 8. The instrument described has a very low aura of stray light. The only significant light outside the instrument is that in the converging forward beam, which can be made to pass wholly through the iris of the patient as already described. The high background illumination typical of conventional indirect ophthalmoscopes is eliminated and the image quality is therefore greatly improved.It is therefore possible considerably to reduce the level of illumination of the retina without any loss of subjective brightness in the image compared with a conventional indirect ophthalmoscope.

The partial reflector plate 7 has a reflectivity chosen to maximise the brightness of the observed image. If the rear surface of the plate has an anti-reflection coating, the front surface should have a reflectance of 50%. If there is no coating,: the front surface should have a reflectance of approximately 47%.-The plate will in general be at 450 to the axis of the tube 3, providing a fixed lens/light source angle of zero. It may be advantageous to have a small fixed angle between the lens and light source, for example by mounting the reflector plate at 460 to the tube axis. The viewing/illumination angle is continuously variable from zero upwards, in contrast to conventional indirect ophthalmoscopes in which the angle is fixed at about 1 O or 20.

To enable several observers to view the image, a biprism or the like can be clipped or hinged to the instrument at the viewing window 13.

The instrument described has parallel surfaces at opposite ends of the tube 3. These can be used for accurately known alignment or relative orientation of the axis of the instrument and the examiner's eye, using graduation marks on the two surfaces. For example, a small reflecting or luminous point can be provided at the centre of the inner face of the viewing lens as a target, to be used in conjunction with a graticule of concentric rings.

The fact that the image is within the instrument makes it easier for the user to locate the image, without however restricting viewing, and makes the image accessible to viewing screens, graticules and other article devices, or the face plate of an image convertor or image intensifier. It should be noted that the image is not plane, and such auxiliary devices may have to be shaped accordingly to match the shape of the image.

It is not necessary to use visible light. One of the advantages of the instrument described is that, since it is not necessary for the user to wear the usual heavy light projector and viewing system on a headband, he can wear other components on a headband, for example an image converter for viewing an image produced by infrared light. In another possible arrangement, an image converter may be placed at the viewing window 13.

For infra-red use, a focusing target may be provided in or near the retinal image plane at the polariser 12. This target can for example be an illuminated ring, the illumination being channeled from the light source of the hand unit, whether a fibre optic bundle or a built-in source such as a gallium arsenide emitter.

The image intensification properties of an infrared image converter, used with the instrument, permit examination of interior portions of the eye at illumination levels significantly lower than those used at present in indirect ophthalmoscopy.

Conventional illumination levels are being increasingly recognised as hazardous to patients.

Because the image intensifier output depends on the properties of the screen phosphor, such devices are particularly suitable for low illumination level examination of the eye using monochromatic radiation of various wavelengths, infra-red or visible. The use of monochromatic radiation is a recognised technique in ophthalmoscopy but at present requires relatively high intensities of illumination. When the present instrument is used with an image intensification device, dilation of the pupil is not needed, which is an important practical advantage.

It will be understood that if the instrument is intended for use with infra-red illumination, its optical components must be made of materials transparent to infra-red wavelengths and must be appropriately designed for operation at such wavelengths.

The instrument can be used, with an appropriate accessory, for fluorescence angiography, with visible radiation (using for example fluorescein) or infra-red (using cardiogreen), using suitable filters and, in the case of infra-red, an image converter as already mentioned, preferably mounted on the examiner's head by means of a head band. The fluorescence excitation wavelength is selected by an appropriate filter in the filter wheel 10, and the fluorescence emission is selected by a filter for example clipped over the output face of the instrument.

The instrument described combines the ease of use of a direct ophthalmoscope, with the wider field of view, better illumination and possibility of stereoscopic vision, of an indirect ophthalmoscope, and has substantial advantages not found in either form of conventional instrument.

Claims (12)

1. An indirect ophthalmoscope comprising a light projection system for projecting a beam of light, and a converging lens arranged to form an aerial image of the region illuminated by the light projection system, the said light projection system and converging lens being combined in a single unit in which the viewing line of sight through the lens is substantially coincident with the axis of the projected light beam.

2. An indirect ophthalmoscope as claimed in claim 1 in which the light projection system includes a beam splitter and a light source, the beam splitter being arranged to direct light from the source along the said line of sight.

3. An indirect ophthalmoscope as claimed in claim 2 having a generally T-shaped housing of which the stem portion contains the light source, and the cross piece is a viewing tube which contains the beam splitter and converging lens.

4. An indirect ophthalmoscope as claimed in claim 1, 2 or 3 in which the said lens is placed in the light path from the light source to the region illuminated by the light beam as well as in the line of sight from an observer to the said region.

5. An indirect ophthalmoscope according to claim 1, 2, 3 or 4 having a viewing aperture and in which the lens is arranged to form the said aerial image substantially at the viewing aperture.

6. An indirect ophthalmoscope as claimed in any of claims 1 to 5 in the form of a hand-held instrument.

7. An indirect ophthalmoscope according to claim 6 having a handle which incorporates the light projection system.

8. An indirect ophthalmoscope according to any of claims 1 to 7 in which the light projection system includes a fibre optic bundle constituting a light source.

9. An indirect ophthalmoscope according to claim 8 in which the fibre optic bundle has a noncircular end profile.

10. An indirect ophthalmoscope according to any of claims 1 to 7 in which the light projection system includes a gallium arsenide light source.

11. In combination, an indirect ophthalmoscope as claimed in any of claims 1 to 10 incorporating an infra-red light source and having optical components transparent to infrared radiation, and viewing means including an infra-red image converter.

12. An indirect ophthalmoscope substantially as herein described with reference to the accompanying drawing.