Lens for indirect ophthalmoscopy

Claim 1

An indirect ophthalmoscopy lens for use in examining a patient's eye comprising a homogeneous transparent optical material having two aspheric surfaces of revolution of conoid type on opposite sides thereof and means for holding said lens to enable the same to be supported in the hand of the examiner before a pateint's eye at a distance substantially equal to the secondary focal distance of the lens with the optical axis of the lens passing through the center of the entrance pupil of the patient's eye, one of the aspheric surfaces of revolution being defined as the front surface which faces the examiner and the other aspheric surface of revolution being defined as the back surface which faces the eye being examined, and wherein the nominal dioptric power of the lens is defined as the sum of the dioptric powers of each of the two surfaces at its apex and which nominal dioptric power is within the range of approximately 10-55 diopters, and wherein the dioptric power at the apex of the front surface is within the range of approximately 6.666 to 36.666 diopters and wherein the dioptric power at the apex of the back surface is selected to be within the range of approximately 3.333 to 18.333 diopters, and wherein for any lens the selected dioptric power of the front surface at its apex is approximately twice the dioptric power of the back surface at its apex, and wherein the eccentricity of the front surface is in the range of 0.80 to 1.45 and the eccentricity of the back surface is in the range of 0.50 3.80, and wherein for a given selected eccentricity for the front suface within said range the eccentricity of the back surface is so selected as to provide an aerial image of the fundus of the examined eye which least departs from a substantially flat astigmatism free image of said fundus and which enables clear observation of the full extent of the extent of the aerial image of the fundus of the eye by the examiner.