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Introduction to Refractive Error and Prescription Writing Walter Huang, OD Yuanpei University Department of Optometry

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Vision Optics Cornea Cornea Aqueous humor Aqueous humor Lens Lens Vitreous humor Vitreous humor Retina Retina

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Refractive Error The result of a mismatch between optics and the growth of the eye It is due to a combination of genetic and environmental influences It is NOT considered an eye disease Treatment includes spectacles, contact lenses, and refractive surgery

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Types of Refractive Error EmmetropiaMyopiaHyperopiaAstigmatismPresbyopia

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Emmetropia The average emmetrope has a VA of 20/20 or better

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Myopia When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus in front of the retina

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Myopia Blurry vision at distance Clear vision at near

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Myopia It is corrected by divergent or minus lenses Power of corrective lens needed can be estimated by finding the far point where the patient can achieve clear vision Example Far point is at 20cm Far point is at 20cm Focal length of corrective lens needed = 20cm Focal length of corrective lens needed = 20cm Power of corrective lens needed = 1/f =1/0.2m = -5.00D Power of corrective lens needed = 1/f =1/0.2m = -5.00D Unit for Power = Diopter (D) Unit for Power = Diopter (D)

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Hyperopia When parallel rays of light enter the eye (with accommodation relaxed) and come to a single point focus behind the retina

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Hyperopia Blurry vision at distance and near Intermittent blurring of vision

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Hyperopia It is corrected by convergent or plus lenses A young patient with low hyperopia can accommodate to focus the distant image on the retina Since accommodation decreases with age, a low hyperopic patient tends to wear corrective lenses for near work at an earlier age

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Astigmatism When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina

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Astigmatism It is due to a distortion of the cornea and/or lens The refracting power is not uniform in all meridians The principal meridians are the meridians of greatest and least refracting powers The amount of astigmatism is equal to the difference in refracting power of the two principal meridians

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Astigmatism Distorted vision Letter confusion P versus F P versus F A versus R A versus R H versus N H versus N

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Astigmatism It is corrected by cylindrical or spherocylindrical lenses

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Presbyopia Presbyopia = “old man’s eye” (Latin)

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Presbyopia Decrease in the amplitude of accommodation or loss of accommodative ability with age

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Presbyopia It is a natural part of the aging process The onset of presbyopia is at approximately 40 years of age and over though it may be earlier in low hyperopes

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Presbyopia Blurry vision at near Difficult or impossible to accommodate sufficiently for near work

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Presbyopia It is corrected by convergent or plus lenses for near work only (near Add)

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Types of Lens Spherical lens Cylindrical lens Spherocylindrical lens

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Spherical Lens A plus or minus lens where the refracting power is equal in all meridians Diopter Sphere (DS) is the measuring unit used to differentiate the spherical lens from lenses with cylindrical component Power cross and prescription writing for a spherical lens require the specification of the spherical power component only

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Spherical Lens Power cross: Prescription form: +2.50DS

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Cylindrical Lens A flat or plano (pl) axis meridian perpendicular to a power meridian

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Cylindrical Lens Diopter Cylinder (DC) is the measuring unit used to differentiate the cylindrical lens from lenses with spherical component Power cross and prescription writing for a cylindrical lens require the specification of both the cylindrical power and axis components

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Cylindrical Lens Power cross: Prescription form: x 180

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Spherocylindrical Lens A toric lens consists of two perpendicular principal meridians Power cross and prescription writing for a spherocylindrical lens require the specification of the spherical power, cylindrical power, and axis components

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Spherocylindrical Lens Power cross: Prescription form: x x /-1.00 x /-1.00 x 180

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Prescription Writing Example 1 Power cross: Power cross: Prescription form: x 120 Prescription form: x 120

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Prescription Writing Example 2 Power cross: Power cross: Prescription form: x 084 Prescription form: x 084

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Prescription Writing Example 3 Power cross: Power cross: Prescription form: x 005 Prescription form: x 005

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Rules for Power Cross Specify both power and axis Power is always represented by plus or minus sign in front and contains two digits after the decimal point Power is presented in 0.25D steps

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Rules for Power Cross Axis meridian starts counter-clockwise from 0 to 180

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Rules for Power Cross The cross orientation is drawn to the actual meridian When axis is at the 0 to 180 horizontal, use 180 instead of 0 Degree notation may or may not be used for axis If degree notation is NOT used for axis, three digits must be used for axis, except in the case of 0

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Rules for Prescription Writing Always include power, cylinder, and axis, except for spherical lenses (specified as DS) Degree notation is NOT used for axis

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Minus versus Plus Cylinder In Optometry, prescription writing is in minus cylinder (-cyl) form Sphere and axis specified is the most plus principal meridian

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Minus versus Plus Cylinder In Ophthalmology, prescription writing is usually in plus cylinder (+cyl) form Sphere and axis specified is the most minus principal meridian

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Conversion between Minus and Plus Cylinder Be sure to know how to convert between minus and plus cylinder form and back

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Conversion between Minus and Plus Cylinder Example Minus cylinder form: x 180 Minus cylinder form: x 180 Plus cylinder form: x 090 Plus cylinder form: x 090

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