Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Patent
1993-04-21
1996-03-12
Sikes, William L.
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
351205, 351246, 359362, A61B 312, G02B 1500
Patent
active
054990661
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates generally to an optical instrument which has particular but by no means exclusive application as an ophthalmic instrument, eg in ophthalmoscopy.
Although the present invention will be described with particular reference to its application to ophthalmoscopes, it is to be noted that the present invention is not so limited and is more extensive m scope, extending in its application to other areas of optometry, ophthalmoseopy in ophthalmic surgery, and to diagnosis and treatment in other areas of medicine e.g. gynaecology and dentistry.
BACKGROUND ART
At present there are two basic types of ophthalmoscopes; the direct ophthalmoscope and the indirect ophthalmoscope. Both types have advantages in certain circumstances and disadvantages in other circumstances.
In the direct ophthalmoscope a light source is used to directly illuminate a patient's eye and the optical system of the patient's eye functions as a simple magnifier, thereby allowing the observer to see an erect, magnified image of the patient's fundus. The direct ophthalmoscope provides a means of illuminating the fundus and of observing the illuminated area. The illumination is usually provided by means of a light source and a reflecting prism or mirror whereby the beam of light can be conveniently directed through the pupil. It will be appreciated that the illuminated fundus is viewed under conditions analogous to a magnifying glass. The retina is at the focus of the dioptric system of the eye and consequently an erect, magnified, clearly focussed image of the fundus is observed. The magnification is about fifteen times depending on the condition and characteristics of the observer's and patient's eyes, but the field of view is quite small, only about 61/2or thereabouts. The actual area of the fundus in view at any time also depends upon the distance of the observer from the pupil of the patient's eye, and to a small extent on the patient's pupil diameter. Thus, while the direct ophthalmoscope provides an erect and relatively highly magnified image, the actual area of the eye being viewed is quite small and an extensive scan of the entire surface of the eye is required in order to gain a complete picture of the eye at this magnification.
In the indirect ophthalmoscope, which can be either binocular or monocular, a condensing lens positioned in front of the patient's eye produces an inverted image of the patient's retina. This image can then be viewed by the observer either with or without the aid of additional lens systems. With most commercial instruments, the condensing lens is hand-held and the lens system is mounted on a head band, although a hand-held device and a spectacle configuration are available. In most cases, the observed image is inverted, Ln some it is not, The retinal image is viewed with a magnification in the range 2.5 to 5 times and a field of view generally around 35.degree. to 65.degree., depending on the precise optical configuration. Thus, the indirect ophthalmoscope, when provided with a suitable relay lens system, provides an inverted or erect image which is only moderately magnified but entails a relatively extensive field of view compared with direct ophthalmoscopes.
From the foregoing it is clear that there is a compromise in the use of both types of ophthalmoscopes. Whereas the direct instrument produces a greatly magnified image, it has a very small field of view and the indirect instrument has a large field of view but produces an image which is only slightly magnified. Accordingly, both instruments must be used in conjunction with each other for a full comprehensive study of a patient's eye when attempting to detect an abnormal condition. One problem with using two instruments is that when an area of interest with respect to an abnormal condition is located, say using the indirect ophthalmoscope because of its large field of view, the magnification is not great enough to investigate fully the precise nature of the abnormality. However, when an observer changes to t
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Pomerantzeff, et al., "An Optimized Zoom Ophthalmoscope", Opthalmology, vol. 91, No. 2, pp. 197-203, Feb. 1984.
Brook John F. H.
Farmer John W.
Gin Malcolm
Powell Ian
See Brian A.
Parsons David R.
Remraf Pty. Ltd.
Sikes William L.
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