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Using a phoropter to determine a prescription for eyeglasses. An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.
oculus dexter (right eye) OH Ocular history OMB Oculo motor balance ONH Optic nerve head Oph Ophthalmoscopy OS oculus sinister (left eye) OU oculus uterque (both eyes) PD Pupillary distance PERRLA Pupils equal, round, reactive to light and accommodation PH Pinhole see Pinhole occluder and Visual_acuity#Legal_definitions: PHNI Pinhole No Improvement
The phoropter measurement is made at a common vertex distance of 12 mm from the eye. The equivalent prescription at the patient's cornea (say, for a contact lens) can be calculated as follows (this example assumes a negative cylinder sign convention): Power 1 is the spherical value, and power 2 is the steeper power of the astigmatic axis:
The lenses were increased to 3 ⁄ 4 inch diameter, the permanent size, and the unit was much more massive, with a weight of 7 lb. 9 oz., and with a range of +16.87 to -19.12 sphere and 6.00 cylinder, with auxiliary lenses to increase these to +18.87/-21.12 sphere and 8.00 cylinder. All these models resembled the original DeZeng model in design ...
By measuring this zone, the autorefractor can determine when a patient's eye properly focuses an image. The instrument changes its magnification until the image comes into focus. The process is repeated in at least three meridians of the eye and the autorefractor calculates the refraction of the eye, sphere, cylinder and axis.
The values indicated in the sphere and cylinder columns of an eyeglass prescription specify the optical power of the lenses in diopters, abbreviated D. The higher the number of diopters, the more the lens refracts or bends light. A diopter is the reciprocal of the focal length in meters.
Distance PD is the separation between the visual axes of the eyes in their primary position, as the subject fixates on an infinitely distant object. [2] Near PD is the separation between the visual axes of the eyes, at the plane of the spectacle lenses, as the subject fixates on a near object at the intended working distance. [3]
The eye's Abbe number is independent of the importance of the corrective lens's Abbe, since the human eye: Moves to keep the visual axis close to its achromatic axis, which is completely free of dispersion (i.e., to see the dispersion one would have to concentrate on points in the periphery of vision, where visual clarity is quite poor)