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Refractive errors and correction

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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!
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