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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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Optical Structure and Image FormationOptical Structure and Image Formation
(cont.)(cont.)
The image on the retina is inverted - like a camera
Optical Structure and Image FormationOptical Structure and Image Formation
-- 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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Optical Structure and Image FormationOptical Structure and Image Formation
-- 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
Optical Structure and Image FormationOptical Structure and Image Formation
-- 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
Optical Structure and Image FormationOptical Structure and Image Formation
-- 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
Optical Structure and Image FormationOptical Structure and Image Formation
-- 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
Optical Structure and Image FormationOptical Structure and Image Formation
-- 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
Refracting ComponentsRefracting Components
-- 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
Refracting ComponentsRefracting Components
-- 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
Refracting ComponentsRefracting Components
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
Refractive AnomaliesRefractive Anomalies
-- 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
Refractive AnomaliesRefractive Anomalies
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
Refractive AnomaliesRefractive Anomalies
-- 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.
Refractive AnomaliesRefractive Anomalies
-- 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
Ageing EyeAgeing Eye
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
Ageing EyeAgeing Eye
-- 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
Ageing EyeAgeing Eye
-- 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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Ageing EyeAgeing Eye
-- 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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