Indirect ophthalmoscopy is recognized as an important technique in diagnostic and evaluative examination of the central and peripheral retina and related structures of the eye. Degenerative retinal diseases may be quantitatively evaluated by tonometry, visual field analysis and other techniques, but may be subject to various limitations and possible errors. Other techniques using complex photographic analysis or computer analysis have been used to determine the size and dimensions of internal eye structures. These methods as well help to provide valuable information relating to retinal health, but require expensive equipment and are often time consuming and complicated. As the retina and associated structures are readily examined using indirect ophthalmoscopy techniques, the possibility of utilizing indirect ophthalmoscopy in topographic evaluations of the size and shape of the central and peripheral retina and related structures is becoming increasingly important. For example, detectable changes in the optic nerve head may reflect the loss of retinal ganglion cell axons, which has been found to occur at early stages in glaucoma. Similarly, dimensional variations in the neuroretinal rim and optic disc have been correlated with parameters of visual function and with assessments of the retinal nerve fiber layer in glaucoma diagnosis. Precise measurements of the neuroretinal rim, optic disc and associated structures may therefore facilitate examination and diagnosis of diseases in the central and peripheral retina.
In indirect ophthalmoscopy, examination of the fundus of the eye by the observation of a real aerial fundus image is performed. The observation of the aerial image of the fundus is performed using either a binocular slit lamp biomicroscope or binocular indirect ophthalmoscope. To generate the real aerial image of the fundus, various optical systems have been developed. For example, in U.S. Pat. No. 4,738,521, an optical lens for indirect ophthalmoscopy is described, wherein an objective lens is used as a condensing lens acting to converge the light from an ophthalmoscope or biomicroscope light source to the entrance pupil of the eye so as to illuminate the eye fundus and also as an image forming lens adapted to form an aerial image of the fundus to be viewed by an examiner. Such an optical system may be provided as a hand-held indirect ophthalmoscopy lens. Other examples of indirect ophthalmoscopy lenses and optical systems are shown in U.S. Pat. Nos. 4,721,378 and 4,627,694. In such hand-held devices of this type, a real, inverted and magnified image of the retina is formed in space at approximately the front focus of the condensing lens in the optical system. Generally, the position of the image as produced by the indirect ophthalmoscopy optical system is not immediately apparent to the examiner, and the indirect ophthalmoscope or slit lamp biomicroscope is adjusted so as to provide a clear fundus image. In the unaccomodated emmetropic eye, light rays emerging from the eye are parallel, and the size of the formed aerial image is dependent on lens position and power. On the other hand, any refractive error in the examined eye will result in a shifting in the position of the image plane.
More recently, other hand-held indirect ophthalmoscopy lens systems have been developed, which include one or more anterior lens elements used in conjunction with a contact element. In that the lens system is positioned directly on the cornea of the examined eye, the position the aerial image in relation to lens and examined eye remains generally constant. An example of such an indirect ophthalmoscopy optical device is shown in U.S. Pat. No. 5,046,836. Further, an adaptor for a lens retaining ring and associated indirect ophthalmoscopy lens as seen in U.S. Pat. No. 4,913,545, also acts to properly space the indirect ophthalmoscopy lens mounted therein at a predetermined distance from the examination lens and eye of the patient. In these systems, the image forming lens is positioned at a predetermined and constant location relative to the examined eye. Likewise, for a particular lens power and design, the front and back focus of the lens is known, and the position of the aerial image as formed by the lens system may also be known.