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 !