astigmatisma / silindris

8/18/2019 astigmatisma / silindris

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Theme 8: Astigmatism

• Concept of Astigmatism

• Formation of the retinal image

• Quantifying Astigmatism• Classifying Astigmatism

• Retinal Image of the astigmatic eye

• Neutralizing Astigmatism


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Concept of astigmatism

Astigmatism: Refraction Anomaly thatappears when the eye presents differentmeridian powers in some of its refractive

surfaces

The retinal image is

blurry but with adistinct size indierent directions


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CAUSES OF AST!"ATS"Astigmatism of curvature:#on spherical surfaces $First

surface of the cornea%Astigmatism of nde&rregular Astigmatism

Low Astigmatism 0.2D < A < 1D

Medium Astigmatism A < 3D

High Astigmatism A > 3D


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THE IMAGE OF A OI!T I" !OTA OI!T

I# ge#e$a%& the $eti#a% image is a#

e%%i'se

Near farAstigmatic ametropia

!ight rays


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Formation of the retinal image

CO#O' OF STUR"

SPHERICAL

AMETROPIA

ASTIMATISM


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Formation of the retinal image

CO#O' OF STUR"

FOCA( #TER)A( OF ST!RM "tu$m I#te$(a%) 0.3*mm +o$ 1D o+ Astigmatism

,I-,LE OF LEA"T ,O!F"IO!


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"#antif$ing Astigmatism

yy

zz

R PtRe'H

'n

R PtRe'H

'n

+=

+=

zyyzPPR R A −=−=

Astigmatic ametropia

!ight rays


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!otatio# +o$ Astigmatism

%otation for astigmatismis &one '$ analog$ to the

spherical(c$lin&rical

lenses


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spherical*cylindrical lens: $Sph% $Cyl%$Sph% "'he$i/a% %e#s 'owe$$Cyl% ,%i#d$i/a% %e#s 'owe$

A#g%e whi/h the ais o+ the/%i#d$i/a% %e#s +o$ms with theho$io#ta%



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Astigmatism)"'h4 ,%4 α

Ry% $R+*Ry% y

$R+% $Ry*R+% + $Transposed%

R90=2D R0=1D A=-1

(+2) (-1) 90º

(+1) (+1) 0º


Transposed


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Classif$ing Astigmatism

,OSTO# OF T-E FOCA(S .T- RES,ECT TO T-ERET#A

Astigmatism can be classi"ed under twocriteria#

CUR)ATURE OF T-E ,R#C,A( "ER'A#S


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,OSTO# OF T-E FOCA(S .T- RES,ECT TO T-E RET#A

C!C


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Compound hyperopic - > 0

- > 0

C!C


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Simple hyperopic - > 0

- 5 0

C!C


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"i&ed - > 0

- < 0

C!C


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Simple "yopic - 5 0

- < 0

C!C


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Compound "yopic - < 0

- < 0

C!C


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CUR)ATURE OF T-E ,R#C,A( "ER'A#S

'irect: !reatest curvature in the vertical meridian $/01 201% ,y3,+ Ry4R+ A30

nverse: !reatest curvature in the hori+ontalmeridian $01 201% ,y4,+ Ry3R+ A40

O5li6ue: !reatest curvature in the meridian situatedat $781 981%


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'irect

,y3,+ Ry4R+ A30 ,y4,+ Ry3R+ A40

nverse


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Retinal Image of the astigmatice$e

,oint O5ectConoid of Sturm

In function of where the image forms on the

eye$ the image will ha%e a distinct form


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If the retina is on the focal of the %ertical

meridian the image will be a horizontal line

,O#T O;


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If the retina coincides with the focal point of the

horizontal meridian the image will be a %ertical

,O#T O;


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If the retina coincides with the circle of least

confusion the image will be a circle&

,O#T O;


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In general$ the retinal image of a point ob'ect will bean ellipse whose greater a(is corresponds to thedirection of meridian that is perpendicular to the

more emmetropic one&

,O#T O;


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if the retina is found in the zone mar)ed in the"gure$ the retinal image will be an ellipse withthe greater semi a(is in the horizontal direction

*perpendicular to the more emmetropicmeridian

,O#T O;


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If the retina is found in the zone mar)ed in the"gure$ the retinal image will be an ellipse with

the greater semi a(is in a %ertical direction&

*perpendicular to the most emmetropicmeridian

,O#T O;


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E=TE#S)E O;


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E=TE#S)E O;


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E=TE#S)E O;


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E=TE#S)E O;


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E=TE#S)E O;


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%e#trali)ing Astigmatism

!oal of neutrali+ation: The 'e$so# sees o67e/ts at i#+i#it /%ea$%

without the #eed to a//ommodate

,rinciple of neutrali+ation:

The image o+ a# o67e/t 'ositio#ed at i#+i#itth$ough a %e#s shou%d +o$m o# the $emote 'oi#to+ the ee +o$ ea/h me$idia#4


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The +o/a% 'oi#t image o+the %e#s shou%d 6esituated o# the $emote'oi#t o+ the ee +o$

ea/h me$idia#4


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Cylindrical lens

Near far

Near far


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zv

z

vp

yv

y

vp

R 1

R P

R 1

R P

z

y

δ

δ

+

=

+=


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zv

z

vp

yv

y

vp

R 1

R P

R 1

R P

z

y

δ

δ

+

=

+

=

"'he$i/a% ,%i#d$i/a% Le#s"'h4 ,%4 α

('4 '(8'(4 α

4 4 α T d4

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astigmatisma / silindris