theme 8: astigmatism
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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. Concept of astigmatism. - PowerPoint PPT PresentationTRANSCRIPT
Theme 8: Astigmatism
• Concept of Astigmatism• Formation of the retinal image• Quantifying Astigmatism• Classifying Astigmatism• Retinal Image of the astigmatic eye• Neutralizing Astigmatism
Concept of astigmatism
Astigmatism: Refraction Anomaly that appears when the eye presents different meridian powers in some of its refractive surfaces
The retinal image is blurry but with a distinct size in different directions
Concept of astigmatism
CAUSES OF ASTIGMATISMAstigmatism of curvature: Non spherical surfaces (First surface of the cornea)Astigmatism of IndexIrregular Astigmatism
Low Astigmatism 0.2D < A < 1D
Medium Astigmatism A < 3D
High Astigmatism A > 3D
Concept of astigmatism
THE IMAGE OF A POINT IS NOT A POINT
In general, the retinal image is an ellipse
Near farAstigmatic ametropia
Light rays
Formation of the retinal image
CONOID OF STURM
SPHERICAL AMETROPIA
ASTIGMATISM
Formation of the retinal image
CONOID OF STURM
FOCAL INTERVAL OF STURM Sturm Interval: 0.37mm for 1D of Astigmatism
CIRCLE OF LEAST CONFUSION
Quantifying Astigmatism
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zz
RPtRe'H
'n
RPtRe'H
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zyyzPPRRA
Astigmatic ametropia
Light rays
Quantifying Astigmatism
Notation for Astigmatism
Notation for astigmatism is done by analogy to the
spherical-cylindrical lenses
Quantifying Astigmatism
spherical-cylindrical lens: (Sph) (Cyl)
(Sph) Spherical lens power(Cyl) Cylindrical lens power Angle which the axis of the
cylindrical lens forms with the horizontal
Notation for Astigmatism
Quantifying Astigmatism
Astigmatism: (Sph) (Cyl) (Ry) (Rz-Ry) y
(Rz) (Ry-Rz) z
(Transposed)
R90=2D R0=1D A=-1
(+2) (-1) 90º
(+1) (+1) 0º
Notation for Astigmatism
Transposed
Classifying Astigmatism
POSITION OF THE FOCALS WITH RESPECT TO THE RETINA
Astigmatism can be classified under two criteria:
CURVATURE OF THE PRINCIPAL MERIDIANS
Classifying Astigmatism
POSITION OF THE FOCALS WITH RESPECT TO THE RETINA
CLC
Classifying Astigmatism
Compound hyperopic Rz > 0
Ry > 0
CLC
Classifying Astigmatism
Simple hyperopic Rz > 0
Ry = 0
CLC
Classifying Astigmatism
Mixed
Rz > 0
Ry < 0
CLC
Classifying Astigmatism
Simple Myopic
Rz = 0
Ry < 0
CLC
Classifying Astigmatism
Compound Myopic
Rz < 0
Ry < 0
CLC
Classifying Astigmatism
CURVATURE OF THE PRINCIPAL MERIDIANS
Direct: Greatest curvature in the vertical meridian (90º30º) Py>Pz Ry<Rz A>0
Inverse: Greatest curvature in the horizontal meridian (0º30º) Py<Pz Ry>Rz A<0
Oblique: Greatest curvature in the meridian situated at (45º15º)
Classifying Astigmatism
Direct
Py>Pz Ry<Rz A>0 Py<Pz Ry>Rz A<0
Inverse
Retinal Image of the astigmatic eye
Point ObjectConoid of Sturm
In function of where the image forms on the eye, the image will have a distinct form
Retinal Image of the astigmatic eye
If the retina is on the focal of the vertical meridian the image will be a horizontal line
POINT OBJECTConoid of Sturm
Retinal Image of the astigmatic eye
If the retina coincides with the focal point of the horizontal meridian the image will be a vertical
line
POINT OBJECTConoid of Sturm
Retinal Image of the astigmatic eye
If the retina coincides with the circle of least confusion the image will be a circle.
POINT OBJECTConoid of Sturm
Retinal Image of the astigmatic eye
In general, the retinal image of a point object will be an ellipse whose greater axis corresponds to the direction of meridian that is perpendicular to the
more emmetropic one.
POINT OBJECTConoid of Sturm
Retinal Image of the astigmatic eye
if the retina is found in the zone marked in the figure, the retinal image will be an ellipse with the greater semi axis in the horizontal direction
(perpendicular to the more emmetropic meridian)
POINT OBJECTConoide de Sturm
Retinal Image of the astigmatic eye
If the retina is found in the zone marked in the figure, the retinal image will be an ellipse with
the greater semi axis in a vertical direction. (perpendicular to the most emmetropic
meridian)
POINT OBJECTConoid of Sturm
Retinal Image of the astigmatic eye
EXTENSIVE OBJECTThe clearer meridian is perpendicular
to the more emmetropic meridian
The vertical meridian is emmetropic The image of a point object is a horizontal line.
The horizontal segment of the cross is the clearest
CLC
Retinal Image of the astigmatic eye
EXTENSIVE OBJECTThe clearer meridian is perpendicular
to the more emmetropic meridian
The horizontal meridian is emmetrope. The image of a point object is a vertical line.
The vertical segment of the cross is the clearest.
CLC
Retinal Image of the astigmatic eye
EXTENSIVE OBJECTThe clearer meridian is perpendicular
to the more emmetropic meridian
The horizontal and vertical meridians have the same ametropia.
The image of a point object is a circle.The two arms of the cross have the same clarity.
CLC
Retinal Image of the astigmatic eye
EXTENSIVE OBJECTThe clearer meridian is perpendicular
to the more emmetropic meridian
The vertical meridian is the more emmetropic. The image of a point object is an ellipse with the
greater semi axis in the horizontal direction.The horizontal segment of the cross is clearer.
CLC
Retinal Image of the astigmatic eye
EXTENSIVE OBJECTThe clearer meridian is perpendicular
to the more emmetropic meridian
The horizontal meridian is more emmetropic. The image of a point object is an ellipse where the
vertical is the greater semiaxis The clearer arm of the cross is the vertical one.
CLC
Neutralizing Astigmatism
Goal of neutralization: The person sees objects at infinity clearly
without the need to accommodate
Principle of neutralization: The image of an object positioned at infinity through a lens should form on the remote
point of the eye (for each meridian)
Neutralizing Astigmatism
The focal point image of the lens should be situated on the remote point of the eye (for each meridian)
Neutralizing Astigmatism
Cylindrical lens
Near far
Near far
Neutralizing Astigmatism
zv
z
vp
yv
y
vp
R1R
P
R1
RP
z
y
Neutralizing Astigmatism
zv
z
vp
yv
y
vp
R1R
P
R1
RP
z
y
Spherical Cylindrical Lens (Sph) (Cyl)
(Pvpy) (Ppvz-Ppvy) y
(Ppvz) (Ppvy-Ppvz) z (Transposed)