Patent GB-1464729-A (GB)
Title: Refractometer
IPC: A61B3/10
Filed: 1974-11-07
Abstract: 1464729 Opthalmic refractometers HOYA LENS KK and SOKKISHA KK 7 Nov 1974 [26 Nov 1973] 48261/74 Heading G1A [Also in Division G2] A refractometer comprises: two infrared-emitting diodes; means forming respective images of the diodes offset from a central axis; a transmitting collimator directing rays from the images to a slit to enable an image of the slit to be formed on the retina of an eye; a receiver lens to collect beams from the retina and from the cornea of the eye; means to interrupt beams received from the cornea; means dividing the beams received from the retina to direct components to respective ones of a pair of photo-cells; a lens system, movable by driving means, for focusing a beam reflected from the retina; means causing alternating flicker in the emitting diodes; and means responsive to the photocells to cause the driving means to move. Light-emitting diodes ID 1 , ID 2  Fig. 1, radiating at a wavelength of about 0À9 micron, are alternately modulated with 1 kHz square waves derived from a tuning-fork oscillator SO, Fig. 5. Images S 1 , S 2 , Fig. 1, spaced about 1 mm apart, are formed on either side of the output apex of a double prism DP 1 . A collimator lens L 1  directs a substantially parallel beam from each image through a slit T 1 . Rays diverging from the slit T 1  are directed along parallel paths by a second lens L 2  spaced from the slit T 1  by its own focal length d = f 2 , and enter an eye E 1  under test. A normal relaxed eye will form coincident retinal images of the sources ID 1  and ID 2  at F 1 ; but if the eye is myopic or hyperopic the rays will either cross at F 3  or will fail to meet at the retina. A double image will thus be observable, to provide an alternating signal having a phase representing the kind of abnormality present. Rays reflected out of the eye from the retina, and those reflected at the surface of the cornea, are directed by a beam-splitter BS and a receiver lens L 3  into a dichroic mirror DM 1 . The infrared components reflected from the retina, forming an image at T 1 ', are collimated by a lens L4 and directed by a lens L 5  to the input apex of a double prism DP 2  which divides the light about a central axis, directing it towards respective photo-cells PC 1 , PC 2 . A phase detector, PD Fig. 5, responsive to any alternating signals from the photo-cells, gates a number of driving pulses to a motor M 1  which drives the slit T 1  to annul the signals by bringing the retinal images to coincide. The driving pulses are counted and used to compute and record the displacement of the slit T 1  as a measure of the eye abnormality. Rays reflected from the cornea, though stronger than those from the retina are not collimated by the lens L4 but focused, and interrupted by a narrow bar T 2  at the focal point. The patient may align, orientate and relax his eye by observing a visibly illuminated target T3 in a dichroic mirror DM 2  via a prism P 4  and lens L 6 ; preferably these elements are duplicated so that the patient may use both eyes. The operator may check eye alignment by observing lamps La 1 , La 2 , imaged in the cornea, in relation to a hairline R via the dichroic mirror DM 1 , prism P 5  and lens L 7 . Residual positional errors are eliminated by rotation of a parallel glass PG driven by a motor M 2  responsive to any amplitude differences detected by the photo-cells PC 1 , PC 2 . Astigmatism is measured by rotating the visual image of the slit T 1  through  60 degrees with a multiple mirror system positioned by a motor M 3 , illustrated in end view in Fig. 3.