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Introduction to Refractive Error and Prescription Writing

Introduction to Refractive Error and Prescription Writing. Walter Huang, OD Yuanpei University Department of Optometry. Vision. Optics Cornea Aqueous humor Lens Vitreous humor Retina. Refractive Error. The result of a mismatch between optics and the growth of the eye

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Introduction to Refractive Error and Prescription Writing

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

  2. Vision • Optics • Cornea • Aqueous humor • Lens • Vitreous humor • Retina

  3. 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

  4. Types of Refractive Error • Emmetropia • Myopia • Hyperopia • Astigmatism • Presbyopia

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

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

  7. Myopia • Blurry vision at distance • Clear vision at near

  8. 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 • Focal length of corrective lens needed = 20cm • Power of corrective lens needed = 1/f =1/0.2m = -5.00D • Unit for Power = Diopter (D)

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

  10. Hyperopia • Blurry vision at distance and near • Intermittent blurring of vision

  11. 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

  12. 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

  13. 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

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

  15. Astigmatism • It is corrected by cylindrical or spherocylindrical lenses

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

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

  18. 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

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

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

  21. Types of Lens • Spherical lens • Cylindrical lens • Spherocylindrical lens

  22. 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

  23. Spherical Lens • Power cross: • Prescription form: +2.50DS

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

  25. 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

  26. Cylindrical Lens • Power cross: • Prescription form: -4.00 x 180

  27. 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

  28. Spherocylindrical Lens • Power cross: • Prescription form: • +3.00 -1.00 x 180 • +3.00/-1.00 x 180

  29. Prescription Writing • Example 1 • Power cross: • Prescription form: +1.00 -0.50 x 120

  30. Prescription Writing • Example 2 • Power cross: • Prescription form: -3.00 -0.50 x 084

  31. Prescription Writing • Example 3 • Power cross: • Prescription form: +1.25 -2.50 x 005

  32. 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

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

  34. 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

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

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

  37. 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

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

  39. Conversion between Minus and Plus Cylinder • Example • Minus cylinder form: +1.00 -3.00 x 180 • Plus cylinder form: -2.00 +3.00 x 090

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