optics, refractive errors -...
TRANSCRIPT
Optics, refractive errors
János Németh
Topics
• Eye as an optical system – Refraction
– Accommodation – Presbyopia
• Visual acuity
• Refractive errors & their corrections – myopia
– hypermetropia
– astigmia
– higher order aberrations
Eye: combined optical system
cornea
aqueous humor
lens
vitreous body
Schematic eye models
• Donders, Helmholtz, Gullstrand
1862-1930
Swedisch ophthalmologist Nobel Prize in 1911
• Glass lens
• Fish lens
Total refraction of the eye
43 + 20,5 = 63,5 D
Lens refraction is changing FAR distance:
Ciliary muscle relaxes
Zonula fibres tighten
Lens becomes flat
SHORT dist.:
Ciliary muscle contraction
Zonula fibres relax
Lens becomes more rounded
Accommodation decreases with age
Accommodation capacity (diopters)
Age (years)
Range of accommodation
Far point Punctum remotum
Near point Punctum proximum
Dacc = 1/n – 1/f
If far point in the infinite: Dacc = 1/n
n f
Presbyopia
Accommodation capacity (diopters)
Age (years)
Measurement of refraction
• Subjective – with trial lenses
• Objective – retinoscopy
– refractometry
– wavefront analysis
Visual acuity
• Minimum separabile: 1 minute of arc = 1.0
Far distance visual acuity
Decimal tables ETDRS table
Near visual acuity
Reading charts
Contrast sensitivity
Refractive errors of the eye
• Emmetropia
• Ametropia
– Myopia
– Hypermetropia
– Astigmia
– Higher order aberrations
– Anisometropia
Axial & refractive ametropia
• Mismatch between the axial length of the eye and the refractive power of the lens and cornea
– refractive ametropia
– axial ametropia
Prevalence of refractive errors
• In childhood: hypermetropia more frequent • Szeged: at age 7 - 83,1%-a hypermetrop
• Young adults: equalisation of
myopia/hypermetropia ratio,
later myopia becomes more frequent
• Far-East: myopia far more frequent • Children in Hong-Kong : 85% myop
Ametropia: USA, EU, Ausztrália
0
5
10
15
20
25
30
USA EU AUS
Myopia (1 D fölött)
Hyperopia (3 D fölött)
%
Myopia - Shortsightedness
• Diverging (concave, minus) lens
Hypermetropia - Farsightedness
• Converging (convex, plus) lens
Hypermetropia
Astigmia, astigmatismus
• Cylindrical lens
Types of astigmia
• As. myopicus simplex
• As. hypermetropicus simplex
• As. myopicus compositus
• As. hypermetropicus compositus
• As. mixtus
Anisometropia
• Larger difference between the refraction of the two eyes than 4 Diopters (prevalence: 1%) – aniseiconia
Myopia - Shortsightedness
• Globally: 2017: 23% 2050: 50%
• In Asia: already increased to 90-95%
– Pathologic myopia: increased to 20%
Myopia - Shortsightedness
• Prevention / Stop progression
– More outdoor activities, sports
– Less near work & more breaks (20-20-20 role) – focusing 20 feet away for 20 seconds every 20 minutes
– Optical correction (special contact lenses)
– Small dose atropin eyedrops – anti-muscarinic receptor binding slows eye growth
Corrections of refractive errors
• Glasses
• Contact lens
• Refractiv surgery
– Intraocular lens implantation
– Refractiv corneal surgery
• PRK, LASIK, LASEK
Glasses
• Bifocals, trifocals, progressive lens
Why everybody has no supervision?
• Higher order aberrations:
– cornea
– lens
– fundus
Aberrations
Aberrometry – wavefront analysis
Correction: excimer laser
Summary
• Eye as an optical system – Refraction
– Accommodation – Presbyopia
• Visual acuity
• Refractive errors & their corrections – myopia
– hypermetropia
– astigmia
– higher order aberrations
Thank you for your kind attention !