Transcript OPH 141 - Edwin Y. Endo, OD & Associates

Optical Theory II
ABERRATIONS
Copyright 2001 -- Ellen Stoner, MALS, ABOM, NCLC
Aberrations
“When light from a point source goes through
a correctly powered spectacle lens yet fails
to create a perfect image, the cause is lens
aberration.”
Brooks & Borish, Systems for Ophthalmic Dispensing, 2nd edition,
page 501
Classifications of Aberrations
• Chromatic vs. Monochromatic
– Depends on the material of the lens
– Requires the beam of light to contain more than
one wavelength
Classifications of Aberrations
• In Focus vs. Out of Focus
– Out of focus aberrations cause fuzzy images
where clear sharp images should be
– In focus aberrations cause images to be the
wrong shape (distorted).
Classifications of Aberrations
• Wide Beam vs. Narrow Beam
– Wide beam aberrations are not as important
when the light goes through a narrow opening
or aperture, such as the pupil of the eye.
– Narrow beam aberrations are the important
aberrations when making glasses.
– Wide beam aberrations are important for optical
instruments such as telescopes.
Classifications of Aberrations
• On Axis vs. Off Axis
– On axis aberrations effect vision when looking
straight ahead through the lens.
– Off axis aberrations effect peripheral vision.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Chromatic Aberration
• The lens material
breaks white light into
its component colors
• Why? Index of
refraction varies by
wavelength.
Longitudinal (axial)
• The placement of the various focal points on the
axis.
• This is the source of the Abbé value
Lateral (magnification)
• Different image sizes
• Result in colored
‘ghost’ images
Chromatic Aberration
• Material dependent.
• Results in out of focus image.
• wearer complains of peripheral color fringes
(more pronounced off-axis).
• The higher the power of the lens, the more
the chromatic aberration.
Chromatic Aberration
• Abbé value :
 High Abbé, low aberration
 Low Abbé, high aberration
Relation between index of refraction and Abbé
value is not perfect. Within a material
classification it works somewhat: for example,
comparing types of glass: change of index
resulting from different amounts of barium in
the glass.
Chromatic Aberration
Abbé value
Crown glass
58
index
1.523
CR-39
58
1.498
PGX
57
1.523
Spectralite
47
1.537
1.6 PGX
42
1.60
Polycarbonate
30
1.586
Brooks & Borish, Systems for Ophthalmic Dispensing
2nd ed., page 503
Chromatic Aberration
• Correction:
Doublet lens (for instruments: cameras,
telescopes, microscopes).
Change lens materials.
AR coat.
Careful placement of OC’s:
Monocular PD;
OC height and pantoscopic tilt;
Short vertex distance and small frame;
Control edge thickness.
Consumer education.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Spherical lens:
Peripheral rays have shorter
focal length than paraxial rays.
• Peripheral rays refract more than paraxial rays.
• Correct with parabolic curves, aplanatic lens
design.
• Results in out-of-focus image.
• Wide beam aberration – not important in glasses
design.
• On-axis aberration.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Spherical lens, narrow beam
entering off-axis.
• Narrow beam aberration, therefore important in
glasses lens design.
• Beam enters obliquely to lens axis, therefore
effects peripheral vision.
• Creates excess power and cylinder
• Also called Oblique astigmatism or Radial
astigmatism.
• Correct Curve lens design for glasses corrects for
this aberration.
• Tscherning’s ellipse – a graph showing the best
base curve for every Rx, to minimize marginal
(oblique) astigmatism.
• Goes from about –23D to about +7D. Outside that
range there is no ‘perfect’ base curve.
• Tscherning’s ellipse gives two correct base curves:
one in the pl to +12 range, one in a higher plus
power. We traditionally use the lower one.
Correction for Marginal (oblique)
astigmatism, continued:
• Pantoscopic tilt / OC height combination.
– Lower OC 1 mm for every 2 degrees
pantoscopic tilt.
– Use face form in glasses where the OC’s are
decentered in.
• Aspheric design for high powers and large
lenses.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Image – cone
or comet shaped.
 Object, way off to the left)
• Wide beam aberration, so not important in
glasses design (except very high plus Rx).
• Corrected with parabolic curves, aplanatic
lens design.
• Results in out-of-focus image.
• Off-axis aberration, so a peripheral vision
problem when present.
• For very high plus lenses, aspheric designs
will improve coma.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Plane of focus when
Marginal astigmatism is corrected
Plane of focus when
Curvature of field is corrected
• Also called power error.
• Light does not focus on a flat focal plane.
The focal plane is curved.
• Remember the screens at drive-in movies?
They are curved, not flat, to focus the sides
of the movie as well as the center.
• The retina at the back of your eye globe is
not a flat plane. It is curved.
• Curvature of field is minimized with
corrected curve design base curves.
• This aberration effects peripheral vision.
• Petzval’s surface, or the image sphere is
the name for the curved surface when
marginal (oblique) astigmatism is correct.
• Far point sphere is where the image would
focus correctly.
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
Distortion
Object:
Distortion – pincushion – high plus lens
Distortion
Object:
Distortion – barrel – high minus lens
Distortion
Brooks Systems for Ophthalmic lens Work, 2nd ed, page 509
Distortion
• Image is in focus, but not shaped the same
as the object.
• Results from increased prism away from the
OC of the lens.
• Solution is aspheric design lenses.
• Minor importance for glasses lenses.
Lens Aberrations
•
•
•
•
•
•
Chromatic --------------- material dependent
Spherical
(the rest are not)
Marginal Astigmatism
Coma
Curvature of Field
Distortion ----------------- in-focus image
(the rest give blurred images)
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
wide beam
narrow beam
wide beam
narrow beam
Lens Aberrations
•
•
•
•
•
•
Chromatic
Spherical
Marginal Astigmatism
Coma
Curvature of Field
Distortion
on-axis
off-axis
off-axis
on-axis
Lens Aberrations
• Chromatic
peripheral
• Spherical
central
• Marginal Astigmatism peripheral
(Central when pantoscopic tilt incorrect)
• Coma
peripheral
• Curvature of Field
peripheral
• Distortion
peripheral
Lens Aberrations
In order of importance for lens
design:
• Marginal Astigmatism
• Curvature of Field
.....
• Distortion
• Chromatic aberration
References
Brooks & Borish, System for Ophthalmic
Dispensing, 2nd ed, ButterworthHeinemann, 1996.
Meyer-Arendt, Introduction to Classical and
Modern Optics, 4th ed, Prentice-Hall, 1995.