Refractive errors and correction The optics of an eye Fig.1. Optic path of an eye Fig.3. Basic anatomy of an eye Fig. 2. Eye front – pupil, iris, sclera http://www.m2sys.com/blog/scanning-and-efficiency/common-misunderstandings-between-iris-recognition-and-retinascanning/attachment/three_main_layers_of_the_eye/ „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. Emmetropia • The ideal state of an eye • Refractive power = 60 D • Focal point on the retina • High visual acuity • If the eye has some refractive error (power ≠ 60D), it is ametropic Fig.4. Emmetropia: Parallel rays of light focused as a single point on the retina without accomodation „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. Myopia • Focal point in front of the retina • The eye is too long or its refractive power is too big • The patient is nearsighted • Corrected by a minus (diverging) lens Fig.5, fig. 6. Myopia and correction „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. „Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles „ Richard J. Kolker, MD Hyperopia • Focal point behind the retina • The eye is too short or its refractive power is too weak • The patient is farsighted • Corrected by a plus (converging) lens or accomodation Fig.7, fig. 8. Hyperopia and correction „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. „Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles „ Richard J. Kolker, MD Astigmatism • There are two focal points • The cornea has a „football” shape (whereas emmetropic cornea would have „basketball” curvature) • There are two refractive powers corresponding to two perpendicular axis • Corrected by a cylinder / toric lens Fig.9. Emmetropic and astigmatic cornea shapes „Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles „ Richard J. Kolker, MD Fig.10. Astigmatism types Fig.11. Cylinder lens correction „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. Presbyopia – „old-age vision” • Age-related decrease in accommodative ability • Crystalline lens gets too rigid to change its shape and power • It gets hard to see near objects ("arms aren't long enough") • Corrected by reading glasses (independent plus lenses) or bifocal or progressive addition to the distance correction Fig.12. Presbyopia, accomodation and corrective reading glasses „Podstawy refrakcji oka i korekcji wad wzroku”, M. Jarzębińska- Vecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. Reading glasses ● separate pair of glasses for the near vision Bifocals ● Top part = distance correction ● Bottom part = distance + near correction Progressive lenses ● graduated multifocal lens with gradient refractive index change Bibliography • „Podstawy refrakcji oka i korekcji wad wzroku”, M. JarzębińskaVecerova, D. Tuleja. Wrocław 2012. © Górnicki Wydawnictwo Medyczne. • „Subjective Refraction and Prescribing Glasses: Guide to Practical Techniques and Principles „ Richard J. Kolker, MD Thank you for your attention!
