Condensing-image forming optical system for indirect ophthalmoscopy

Claim 1

A compact condensing-image forming and image inverting optical system for indirect ophthalmoscopy for providing an upright aerial image of the fundus of the examined eye comprising a coaxial system of three bixconvex high powered double aspheric lenese of glass or plastic identified as lenses A, B and C fixed in predetermined spaced relation to each other along a longitudinal axis and with each lens being disposed with its major axis substantially perpendicular to a source of light rays, lens A being disposed to initially receive light rays from said source of light rays and to project said rays onto its adjacent lens B, lenses A and B being spaced from each other by a distance equal to the sum of the front focal distance f B front of lens B and the back focal distance f A back of lens A, lenses B and C being spaced apart at a distance equal to the sum of the front focal distance f C front of lens C and the back focal distance f B back of lens B, the said light rays incident upon lens A converging therefrom to the back focus f A back of lens A which focus is also the front focus f B front of lens B, said light rays from f B front of lens B being refracted through lens B to a parallel bundle of light rays which are incident upon lens C, said lens C converging said light rays toward its back focus f C back at the center of the entrance pupil of the eye, and after refraction by the cornea of the examined eye an image of said light source is formed at the center of the pupil of the eye and said light rays then diverge to illuminate the fundus, homocentric bundles of diverging light rays from points in the illuminated fundus passing through the pupil and emerging from the cornea as homocentric bundles of parallel light rays which are then incident upon the adjacent surface of lens C and refracted by said lens C whereby an inverted image of the fundus is formed in the anterior focal plane of lens C, said inverted image is then the object of lens B, and light rays from said inverted image being then refracted by lens B whereby said light rays then proceed as homocentric bundles of parallel light rays with the chief ray of all bundles passing through the anterior focus f B front of lens B and proceed to lens A, and after refraction by said lens A, a reinverted upright aerial image is formed in the anterior focal plane of said lens A which image may then be viewed by means of an indirect ophthalmoscope.