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 Optics and refraction

Light : Visible portion of electromagnetic spectrum

400-700 nm

VIBGYOR
Lowest wavelength Highest wavelength

Power of the eye : + 60D

Cornea Lens
43-45 D 13-17 D

Refractive Index : Cornea : 1.376

Lens Cortex/ surface= 1.39

Nucleus/ cortex = 1.42

Aqueous humor = Vitreous humor = 1.33

Refractive surfaces of eye : 4 1 2 3 4

1 2
Anterior and posterior surface of cornea
Aqueous humor Vitreous humor
3 4
Anterior and posterior surface of lens

Most imp refractive surface : Anterior surface of cornea

Max difference in Refractive Index at anterior surface of cornea

Purkinje image based test

No of images formed = No of refectory surfaces

Normal eye = 4 images

Aphakia = 2 images
Pseudophakia = 4 images
Cataract

IMSC 4 images

Mature 3 images

Out of 4 images, first three are visual and erect

While 4th is real and inverted

Hirschberg corneal reflex test : Used first Purkinje image

 Emmetropia

Normal state of refraction

Parallel rays of light coming from infinity

focused at retina with accommodation at rest

Ametropia / Refractive errors

Abnormal refraction
MC cause of low vision/ ocular morbidity

Myopia Hypermetropia
Short sightedness Far sightedness

DOV for distant objects DOV for near objects

Etiology : 1. Increase in Axial length : Most imp Etiology : 1. Decrease in axial length

Increase in axial length by 1 mm will produce 3D myopia Decrease in axial length by 1 mm will produce 3D of hypermetropia
Ratio = 1:3 Ratio = 1:3

Eg : Buphthalmos Eg : Congenital Rubella syndrome

Axial myopia Micro ophthalmos

2. Curvature of refractive surfaces Cornea 2. Curvature of refractive surfaces Cornea

Lens Lens

If Increase in curvature : Myopia If decrease in curvature : Hypermetropia

Ratio = 1:6 Ratio = 1:6
Increase by 1 mm : 6D of myopia Decrease by 1 mm : 6D of hypermetropia

Eg : Keratoconus Eg : Cornea plana

3. Refractive index 3. Refractive index

Increase in RI will cause Myopia Decrease in RI will cause Hypermetropia

Eg : Nuclear sclerosis Eg : Cortical cataract

Increase in RI Over hydration

Progressive index Myopia Decrease in RI

Index hypermetropia

4. Posterior displacement of lens 4. Anterior displacement of lens

 Signs : 1. Large eyeball Signs : 1. Small eyeball
2. Large cornea 2. Small cornea
3. Deep anterior chamber ( POAG ) 3. Shadow anterior chamber ( PACG )
4. Large and pale optic disc 4. Small and shiny optic disc/ Silk shot appearance/ pseudopapillitis

Treatment : Simple Myopia Treatment : Simple Hypermetropia

Concave/ diverging/ minus lens Convex/ converging/ plus lens

Minification of image Magnification of images

Refractive surgery Refractive surgery

Age : 21 years Age : 21 years

Minimum corneal thickness : 450 um Minimum corneal thickness : 450 um
Stable refraction for at least 1 year Stable refraction for at least 1 year

MC : LASIK Hypermetropia

Lifting a flap with hinge : 180 um Latent/ hidden Manifest

Minimum thickness of stromal bed : 250 um
Laser applied on stromal bed
MC laser used : Eximer laser Kept hidden by inherent tone of Facultative Absolute
ciliary muscles approximately 1D
MOA : Photoablation
For Myopia : flattening of Central cornea Compensated by patient’s Remaining hypermetropia
own accommodative efforts

Myopia : Diverging lens

Total hypertropia = Latent + Facultative + Absolute
Trouble in near vision
Especially small printed stuff Remove ciliary tone Remove accommodation

Preferable to under correct Myopia Cycloplegics Mydriatics

Dilates pupil
High myopia / Degenerative/ Pathological
For hypermetropia : refraction under cycloplegics and Mydriatics
Due to over stretching of eyeball outside normal biological variation
Give full treatment
Degenerative changes in eyeball

>6 D of myopia, progressive

Posterior staphyloma

1. Sclera : Posterior ectasia of eyeball

Crescentic degradation
Known as Posterior staphyloma

Staphylos = bunch of grapes

Cresentric edge here

Lacquer cracks

2. Choroid

Hemorrhages Breaks in Bruch’s membrane

Foster Fuch spots Lacquer cracks

 3. Retina : Peripheral retinal degeneration (Lattice)

Night blindness
Retinal hole

Rhegmatic Retinal Detachment

4. Vitreous Degeneration - floaters known as Muscae volitantes

5. Lens : Nuclear cataract

Complicated cataract
Pigmentory glaucoma

6. Fundus
Tigroid Fundus

Peri papillary degeneration

Vessles Optic disc

Myopic / temporal crescent

Tessellated or Tigroid Fundus

Visible choriocapillaries due to atrophy of RPE

Treatment of high myopia

TOC : Contact lenses

Because of anisometropia Anisokonia

Difference Refractive error between 2 eyes Difference in image size between 2 eyes

Dissimilar images

For specs 1D anisometropia leads to 2% Anisokonia No fusion

5% of anisokonia : best tolerated Diplopia

2.5D of anisometropia : Best tolerated

Types of contact lenses

Hard Rigid/ semisoft Soft

O2 permeability Little / no Average Maximum

Refractive quality Best Average Good

Material PMMA PMMA + Silicon HEMA

TOC : Rigid/ semi soft

MC infection in contact lens users is pseudomonas

 Aphakia

• Absence of lens from pupillary plane

• MC etiology : Cataract surgery

• Optics : 1. Jet black pupil

2. Limbal scar
3. Deepening of anterior chamber
4. Loss of accommodation
5. Power of eye decreases : Highly hypertrophic

TOC : PCIOL : Posterior Chamber Intra Ocular Lens

Specs usually 10 D convex lens

1. 1D = 2% images magnification
10D = 20% images magnification : Not tolerable

2. Spherical or chromatic aberration High plus High minus

3. Pin cushion distortion

4. Jack in the box phenomenon/ Roving Ring scotoma

5. Cosmesis

Astigmatism
Cylindrical error Different refractive power in at least 2 principle meridians

Spherical errors Myopia Power is same in all meridians

Hypermetropia

Etiology : Uneven Refractive surface Cornea MC

Lens
Retina

Known as Toric surface

Strum’s conoid

The rays are not coming to a point focus rather forms focal lines

Distance between 2 foci known as focal interval of Sturm.

FV
FH
 Types

Regular Irregular

2 meridians ( M1 & M2 ) are perpendicular to each other 2 meridians ( M1 & M2 ) are perpendicular to each other

Eg. Keratoconus

Types of regular astigmatism

1. With the rule :

M1 Vertical
Vertical > Horizontal

M2 Horizontal

Eg : Pterygium

2. Against the rule :

M1 Vertical
Vertical < Horizontal

M2 Horizontal

3. Oblique astigmatism
M1 M2

M1 and M2 are perpendicular but not horizontal and vertical

-2 +4
In terms is power
Myopia > emmetropia > hypermetropia
ATR ATR
More the myopia more is the power
Lesser the hypermetropia more is the power -5 0

Power of cylinder acts as perpendicular axis

Classification of astigmatism

Simple Compound Mixed

One meridians is emmetropic Both meridians have different power with same signs One meridians myopic and other is hypermetropic
Other is myopic or hypermetropic

Simple myopic astigmatism Both myopic Both hypermetropic

Simple hypermetropic astigmatism

M2 M1 M1 M2 M1 M2 M1
M1 M2 M2

Simple myopic astigmatism Simple hypermetropic astigmatism Compound myopic astigmatism Compound hypermetropic astigmatism

O +2 -3 +2 +2

-2 O -2 +4 -4
 Strum’s conoid

A. Vertical : Converging ( hypermetropia )

Horizontal : Converging ( hypermetropia )
Vertical > Horizontal
Compound astigmatism with the rule

B. Vertical : Emmetropic
Horizontal : Converging
Simple hypermetropic astigmatism

C,D,E. Vertical : Diverging ( Myopia )

Horizontal : Converging ( Hypermetropia )
Mixed astigmatism Least symptoms due to circle of diffusion

F. Vertical : Diverging ( Myopic )

Horizontal : Emmetropic
Simple myopic astigmatism

G. Vertical : Diverging
Horizontal : Diverging
Compound hypermetropic astigmatism

Type of astigmatism from power given

1. -2DC and -3DC at 180

-2 -3 -5
Power of cylinder acts at perpendicular plane
Compound myopic astigmatism with the rule
-2 -2

2. -2DS and -1 DC at 90

-2 -2

Compound myopic astigmatism against the rule

-2 -1 -3
 3. -2DS and +2DC at 90

-2 -2

Simple myopic astigmatism with the rule

-2 +2 0

4. +3 DC at 180

+3

Simple hypermetropic astigmatism against the rule

0

How to prescribe spectacles

Refractive error = Retinoscopy - Distance factor - Tonus allowance of mydriatics and cycloplegics

Distance factor = 1
Distance ( in meters )

Tonus allowance of mydriatics and cycloplegics A C H

Atropine Cyclopentolate Homatropine
+3

With Retinoscopy at 1 meter, Atropine

+5

+3 -1 -1 +1

+5 -1 -1 +3

Take one meridians as spherical

Hence Other will be cylinder

+1DS and +2DC at 90

+3DS and -2DC at 180

Retinoscopy

Dry Wet

Without mydriatics and cycloplegics Refraction under dilatation with mydriatics and cycloplegics
Refraction undilated

1. Atropine : 1% eye ointment : used in infants : Strongest and longest acting mydriatic
Onset : 3 days only at night
DOA : 2-3 weeks

2. Cyclopentolate

3. Hometropine
 4. Adults : mc : Tropicamide : Shortest acting

DOA : 4-6 hours 30-40 mins

5. Phenylephrine same onset and DOA as Tropicamide

• Only mydriatic no cycloplegic effect
• Preferred in old patients
• Contraindicated in systemic Hypertension due to vasoconstriction

Interpretation of Retinoscopy

1. Neural/ no movement : Myopia = 1D

2. With movement = Myopia < 1D or Emmetropia

3. Against movement = myopia > 1D

Refinement of refraction

Spherical Cylindrical

Duochrome test Jackson cross test

Astigmatic fan test
More clearer
Myopia or over correction of hypermetropia

- by Edward Jackson
More clearer - Combination of sphere and cylinder
Hypermetropia over Correction of myopia - Power of sphere = half the power of cylinder with opposite sign

If equal = Emmetropic Eg : -0.5DS and +1DC

+0.5DS and -1DC

-0.5 -0.5

-0.5 + 1 +0.5

2 cylinders of equal power in 2 meridians with opposite signs

First check axis then power

Technique known as flip cross technique

If one line looks more clearer : astigmatism present

Pin hole test 1. If visual acuity improves : Refractive error Present

2. No improvement : Organic pathology

3. In visual acuity worsens : macular pathology

 Accommodation

Power to see near objects clearly

Note Diverging rays from near object

During accommodation

Contraction of ciliary muscles

Loosening/ relaxation of ciliary zonules

Change in ROC of lens

10 mm 6 mm
While posterior ROC remains same (6mm)

Increase lens thickness

Accommodate

Amplitude of accommodation (A) = P - R

P = Power of eye in full accommodating eye

R = Power of the eye at rest

A=P-R

1 1
A=
Near point ( meter ) Far point ( meter )

Usually at infinity 1 =0

In this case

1
A=
Near point ( meter )

Spasm of accommodation : Involuntary, excessive, sustained accommodation

More convergence

Pseudo myopia

Etiology : 1. Uncorrected hypermetropia in children

2. Mitotics

Treatment : Cycloplegics

Insufficiency of accommodation
Due to ageing Lens
Ciliary muscle

Specially after 40 years of age known as presbyopia ( Not a Refractive error )

Treatment : A. Standard correction according to Age
40-45 years = + 1D
45-50 years = +1.5 to +2D • Increase by 0.5 upto 1D
51-55 years = +2.5 to +3D • Never should be over corrected
56-60 years = +3.5 to 4D
B. Near addition over distance correction
Bifocals

Distance

Near

Distinctive line Seam

Should always pass through pupillary plane

Trifocals
Distance
Intermediate
Near

Multifocals : No distinctive line