Eye Opticsoptics

8/13/2019 Eye Opticsoptics

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David AtchisonDavid Atchison

School of Optometry & Institute of Health and BiomedicalSchool of Optometry & Institute of Health and BiomedicalInnovationInnovation

Queensland University of TechnologyQueensland University of Technology

Brisbane, AustraliaBrisbane, Australia

ScopeScope

Optical Structure

Optical Structure and Image Formation

Refractive Components

Refractive Anomalies

The Ageing Eye


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Optical StructureOptical Structure

cornea and scleracornea and scleraThe outer layer of the eye is in

two parts: the anterior corneaand the osterior sclera

The cornea is transparent andapproximately spherical withan outer radius of curvatureof about 8 mm

The sclera is a dense, white,opaque fibrous tissue whichs approx mate y sp er ca

with a radius of curvature ofabout 12 mm

Optical StructureOptical Structure uveal tractuveal tract

The middle layer of the eye isthe uveal tract. It iscompose o t e r santeriorly, the choroidposteriorly and theintermediate ciliary body

The iris plays an importantoptical function through thesize of its aperture

The ciliary body is importantto the process ofaccommodation (changingfocus)


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Optical StructureOptical Structure

retinaretina

The inner layer of the eye is theretina, which is an extensionof the central nervous systemand is connected to the brainby the optic nerve

Optical StructureOptical Structure lenslens

The lens of the eye is about 3mm inside the eye

It is connected to the ciliary bodyby suspensory ligaments calledzonules


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Optical StructureOptical Structure-- compartmentscompartments

The inside of the eye is dividedinto three compartments

The anterior chamberbetween the cornea and iris,

which contains aqueoushumour

The posterior chamberbetween the iris, the ciliarybody and the lens, whichcontains aqueous humour

e v treous c am er

between the lens and theretina, which contains atransparent gel called the

vitreous humour

Optical Structure and Image FormationOptical Structure and Image Formation

Principles of image formation by the eye are same as for man-made optical systems

Light enters the eye through the cornea and is refracted bye cornea an ens. e cornea as e grea er power.

The lens shape can be altered to change its power when the

eye needs to focus at different distances (accommodation). The beam diameter is controlled by the iris, the aperture

stop of the system. The iris opening is called the pupil. Theaperture stop is a very important component of an opticals stem, affectin a wide ran e of o tical rocesses.


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(cont.)(cont.)

The image on the retina is inverted - like a camera


-- optic disc and blind spotoptic disc and blind spot

The optic nerve leaves the eye atthe optic disc. This region is

blind.The optic disc is about 5 wide and

7 high and is about 15 nasal tothe fovea

region in the visual field is theblind spot


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-- power of the eyepower of the eyeOne of the most important properties of any

optical system is its equivalent power easure o t e a ty o t e system to

bend or deviate rays of light The higher the power, the greater is the

ability to deviate rays Equivalent power Fof the eye is given by

F= n/PF

P is the second principal point, just inside

n

P F

the eyeF is the second focal point. Light enteringthe eye from the distance is imaged at Fn is the refractive index of the vitreousThe average power of the eye is 60 m-1 or60 dioptres (D)

Refractive error more importantthan the equivalent power


-- refractive errorrefractive error

Can be regarded as an error in thelength due to a mismatch with the

equivalent power If the length is too great for its

power, the image is formed infront of the retina and this results

n

P F

If the length is too small, theimage is formed behind the retinaand this results in hypermetropia

n

P F


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The eye has a number of axes.Two important ones are the


-- axesaxes

op ca ax s an e v sua ax s

Optical axis: Surfaces centresof curvatures are not co-linear,there is no true optical axis taken as the line of best fitthrough these points

Visual axis is one of the linesjoining the object of interestand the centre of the fovea

Temporal field: about 105


-- field of visionfield of vision

Nasal field: only about 60 becauseof the combination of the nose

and the limited extent of thetemporal retina

Superiorly and inferiorly: about90, except for anatomicallimitations


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The use of two eyes provides better


-- binocular visionbinocular vision

than one eye alone

Two eyes laterally displaced by~60 mm give the potential for a 3-D view of the world, including theperception of depth known asstereopsis

he total field of vision in the

horizontal plane is about 210 Binocular overlap is 120

Refracting components are cornea and lens

Refracting ComponentsRefracting Components

ements must e transparent an ave appropr atecurvatures and refractive indices

Refraction takes place at four surfaces - the anterior andposterior surfaces of the cornea and lens There is also continuous refraction within the lens


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40 D (2/3rds power) provided by the cornea40 D (2/3rds power) provided by the cornea


-- corneacornea

Supports the tear film and has a number oflayers

~ 0.5 mm thick in centre

Posterior surface is more curved than theanterior surface

The anterior surface has greater power (48 D)an e pos er or sur ace - ecause o

low refractive index difference between thecornea and aqueous

Frequently curvature is different in different meridians (toric)

In general, the radius of curvature increases with distance fromthe surface apex - aspheric

Refracting ComponentsRefracting Components cornea (cont.)cornea (cont.)

Corneal surface asphericity influences higher order aberrations(subtle optical defects)


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Lens bulk is a mass of cellular tissue ofnon-uniform refractive index, contained

within an elastic capsule


-- lenslens

Do not yet have an accurate measure ofrefractive index distribution

Most cells are long fibres which have losttheir nuclei

Lens grows continuously with age, withnew fibres laid over the older fibres

Anterior radius of curvature is about 12mm

The posterior radius of curvature is about-6mm (note negative sign)

Changes in shape with accommodationand aging, particularly at the front surface

In accommodation, when the eye changes focusfrom distant to closer objects: ciliar muscle contracts and causes the zonules

Refracting ComponentsRefracting Components lens (cont.)lens (cont.)

supporting the lens to relax

This allows the lens to become more rounded under

the influence of its elastic capsule, thickening at thecentre and increasing the surface curvatures,particularly the anterior surface

The anterior chamber depth decreases

In accommodation, when the eye changes focusfrom close to distance objects: reverse process occurs


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In a young eye ( 20 years), accommodation can increase thepower of the lens from about 20 to 33 D


lens (cont.)lens (cont.)

the far and near points

The difference between the inverses of their distances fromthe eye is amplitudeof accommodation (not quite the same asthe increase in lens power, but closely related)

Ideally, when the eyes fixates an object of interest, theimage is sharply focused on the fovea

An eye with a far point of distinct vision at infinity iscalled anemmetro ice e. This is re arded as the normal

Refractive AnomaliesRefractive Anomalies

eye, provided that it has an appropriate range ofaccommodation

A refractive anomaly occurs if the far point is not atinfinity. An eyes whose far point is not an infinity isreferred to as an ametropiceye.


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Common type of anomaly


-- myopiamyopia

The back focal point of the eye is in front of the retina

This eye can focus clearly on distant objects by viewing

through a negative powered lens of appropriate power

RF

far point

R

Another common anomaly

The far point of the eye lies behind the eye

Refractive AnomaliesRefractive Anomalies-- hypermetropiahypermetropia

Back focal point is behind the retina

The eye can focus clearly on distant objects

If sufficient amplitude of accommodation

by viewing through a positive powered lens of appropriate power

RFR

far point


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Distribution of myopia andhypermetropia in different studies


myopia and hypermetropiamyopia and hypermetropia

These are represented by the powers ofthe lenses that correct them, withmyopia having negative numbers andhypermetropia having positive numbers

For adult populations, the meanrefraction is slightly hypermetropic andthe distributions are stee er than Fre

quency(%)

30

40

50

60

70

Stromberg

Stenstrm

Sorsby

normal distributions The distributions are skewed - bigger

tails in the myopic direction than in thehypermetropic direction

Refractive error (D)

-7.5-6.5-5.5-4.5-3.5-2.5-1.5-0.5 0.5 1.5 2.5 3.5 4.5 5.5 6.5

0

10

The range of accommodation is reduced so that near


-- presbyopiapresbyopia

o ec s o n eres canno e seen c ear y


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The power of the eye changes with meridian Usually due to one or more refracting surfaces having a

toroidal shape. May be due to surface displacement or tilting.


-- astigmatismastigmatism

e usua y re ate t s to t e error n t e pr nc pa mer ansof maximum and minimum power.

Astigmatism may be related to myopia and hypermetropia.Hence we may have myopic astigmatism, hypermetropicastigmatism, and mixed astigmatism.

Ageing EyeAgeing Eye

Many of the optical changes takingplace in the adult eye produceprogressive reduction in visual

trethickness(mm)

4

4.5

5.0

5.5

6.0

t= 3.167 + 0.024age, p < 0.001

per ormance. ome o t ese can econsidered as pathological

The most dramatic age-relatedchanges take place in the lens. Itsshape, size and mass alter markedly,its ability to vary its shapediminishes and its light

e(mm)

10

15

=

Age (years)

10 20 30 40 50 60 70 80

lenscen

3.0

3.5

.

transmission reduces considerably.In unaccommodated state: centre thickness at 0.024 mm/year Anterior surface radius of curvature

at 0.044 mm/year

Age (y ears)

10 20 30 40 50 60 70 80

Radiusofcurvatur

-10

-5

0

5anterior surface

posterior surface

r= -6.83

r= . . age, p .


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Refractive errors are relatively stable between the ages


refractive errorsrefractive errors

,hypermetropic direction

error(D)

2

3

4

cross-sectional (Saunders, 1981)

longitudinal (Saunders, 1986)

Age (years)

0 10 20 30 40 50 60 70 80

Refrac

tive

-2

-1

0

1

The amplitude of accommodationreaches a peak early in life, thengradually declines

Ageing EyeAgeing Eye-- presbyopiapresbyopia

Becomes a problem for mostpeople in their forties when they

can no longer see clearly toperform near tasks -presbyopia

Accommodation is completely lostin the fifties

The cause of presbyopia has beenlitudeo

faccommo

da

tion

(D)

1

2

3

4

5

6

7

Hamasaki et al. (1956), stigmatoscopy

Sun et al. (1988), stigmatoscopy

controvers a n recent years, utthe majority of investigators believethat it is due to changes within thelens and capsule in which the lensloses its ability to change shape

Age (years)

10 20 30 40 50 60

Am

0


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Pupil diameter pupil size decreases with increaseda e. This is referred to as senile miosis


-- pupil diameterpupil diameter

Light adapted and dark adapted eyes

m) 8

10

Winn et al. (1994) 9 cd/m2

Winn et al. (1994) 4400 cd/m2

Age (years)

10 20 30 40 50 60 70 80 90

Pupildiameter(

0

2

4

6

Recent work (pioneered by Pablo Artal) indicates that thesubtler optical deffects of the eye increase with age for


-- higher order aberrationshigher order aberrations

xe pup s ze

The reduction in pupil size with eye acts as an

influence to moderate these

ns(micrometers)

0.4

0.5RMS= 0.106 + 0.000927age, p = 0.05 5 mm diameter

Age (years)

10 20 30 40 50 60 70 80

RMShigherorderaberratio

0.0

0.1

0.2

0.3


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-- transmissiontransmission

Retinal illumination decreases80

90

400 nm

w t age ue to g t osseswithin the eye, mainly in thelens (van de Kraats & vanNorren, 2007)

Age related loss greater atshort than at long wavelengths

20 30 40 50 60 70 80

ocu

lar

transm

ittance

(%)

0

10

20

30

40

50

60

70

500 nm

600 nm

800 nm

Age (years )

ConclusionConclusion

Optical Structure

Optical Structure and Image Formation

Refractive Components

Refractive Anomalies

Ageing Eye


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Optical Structure (cont.)Optical Structure (cont.)

The eye rotates in its socket by the action of six extra-ocular muscles

RetinaRetina

The light-sensitive tissue of theeye is the retina.

num er o ce u ar anpigmented layers and a nervefibre layer

Thickness varies from about100 m at the foveal centreto about 600 m near theoptic disc.

called photoreceptors at theback of the retina - light mustpass through the other layersto reach these cells


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Retina (cont.)Retina (cont.)

The receptor types are the rodsand the cones

e ro s assoc ate w t v s onat low light levels. They reachtheir maximum density atabout 20 from the fovea

Cones are associated withvision at higher light levels,including colour vision.Predominate in the fovea ns

ity(thousands/mm

2)

40

60

80

100

120

140

160

Osterberg (1935)

Curcio & Hendrickson (1991)

rods

which is about 1.5 mm across.Their density is a maximum atthe pit at the foveola in themiddle of the fovea (about 5from best fit optical axis).

Ang le fr om fovea (degrees)

0 10 20 30 40 50 60 70

D

0

20

Dimensions of the eye vary greatlybetween individuals

Optical Structure and Image FormationOptical Structure and Image Formation-- typical ocular dimensionstypical ocular dimensions

,accommodation and age

Representative results are shown

here. Starred values depend upon

accommodation:anterior chamber depthens c nessradii of curvatures of lens

surfaces Average data have been used to

construct schematic eyes.

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Eye Opticsoptics