Abberations of eye. Higher order physiological abberations wavefront analysis, Zernike polynomials and its management

1. ABBERATIONS OF
REFRACTIVE ERROR
-Dr. Dhivya shri
23/10/20
DR.Dhivya shri s
23/10/20

2. ABBERATIONS
 Optical aberration is an imperfection in the image
formation of an optical system
 Abberation can cause difficulty seeing at night,
glare, halos, blurring, starburst patterns or double
vision (diplopia).

3. OPTICAL PATH DIFFERENCE
 Aberrations can be defined as the difference in optical
path length (OPL) between any ray passing through a
point in the pupillary plane and the chief ray passing
through the pupil center.
 This is called the optical path difference (OPD) and
would be for a perfect optical system

4. TYPES OF ABBERATIONS
CHROMATIC MONOCHROMATIC
LOWER
ORDER
HIGHER
ORDER

5. CHROMATIC ABBERATION
Due to variation in Refractive index of
transparent medium with wavelength of eye
Emmetropic eye is hypermetropic for red(long
wavelength) & myopic for green and blue(short
wavelength)

6. MINIMIZATION
• Narrow spectral sensitivity bands of Long and
mid wavelength cone
• Foveal lacking of blue cones
• In a focused eye rays of greatest intensity forms
sharp image while colours of long and short
forms low intensity circles which are neglected

7. MONOCHROMATIC ABBERATIONS
 Monochromatic aberrations are caused by the
geometry of the lens and occur both when light is
reflected and when it is refracted.
 They appear even when using monochromatic
light, hence the name.

8. WAVEFRONT ABBEROMETER
 Wavefront aberrometer shines a perfectly shaped wave
of light into the eye and captures reflections distorted
based on the eye’s surface contours.
 Thus, it generates a map of the optical system of the eye

9.  Basic principle behind the Hartman-Shack device by
measuring the actual slope of light rays exiting the pupil
plane at different points in the plane propagation to the
ideal the direction of propagation of light ray.
And the wavefronts exiting the pupil plane are allowed
to interact with a microlenslet array.

10.  perfect flat sheet wf= perfect
lattice of point images
 wavefront aberrated = displaced
spot on the grid as compared to
the ideal
 The displacement in location of
actual spot vs the ideal
represents a measure of the
shape


11. WAVEFRONT MAPS
Wavefront maps displayed as 2- dimensional maps.
GREEN = minimal wavefront distortion from the
ideal.
BLUE. = myopic wavefronts
RED = hyperopic wavefront errors.
The captured wavefront maps are analysed and
classified

12. ZERNIKE & FOURIER POLYNOMIALS
 Zernike and Fourier
transforms are polynomial
equations that have been
adapted for this purpose
 Based on this
monochromatic eye
aberrations are addressed as:
(1) lower-order aberrations,
n<3, and (2) higher-order
aberrations, n≥3.

13. LOWER ORDER ABBERATIONS
 Lower order aberrations include
 astigmatism,
 positive defocus (myopia), and
negative defocus (hyperopia).
 They are minimized by spherical
and cylindrical corrections

14. HIGHER ORDER ABBERATIONS
 Higher order aberrations (HOAs) of the eye
are unable to be corrected by cylinder or
spherical corrections and include
 spherical aberrations
 oblique abberations ,
 coma and
 trefoil.

15. SPHERICAL ABBERATIONS
 Light rays entering the central area
of a lens are bent less and come to
a sharp focus at the focal point of
a lens system.
 However, peripheral light rays tend
to be bent more by the edge of a
given lens system so that in a plus
lens, the light rays are focused in
front of the normal focal point of
the lens and secondary images are
created.

16. DIMINISHING SPHERICAL
ABERRATIONS
 Peculiar curvature of cornea i. e. Flatter
periphery than centre
 Peculiar structure of crystalline lens,
wherein the central portions have a greater
density and are arranged in layers of
greater curvature than the peripheral
portion
 Iris blocks the peripheral rays from entering
the eye, so only refraction of paraxial rays
of light takes place

17. COMA
Different areas of lens will form foci in planes other
than the chief focus. This produces in the image plane
a 'COMA EFFECT' from a point source of light.

18. As with spherical abberation, the limitation of rays to
the axial areas of the lens can reduce this effect

19. OBLIQUE ABBERATION
 Oblique astigmatism is an aberration of off-axis rays
that causes radial and tangential lines in the object
plane to focus sharply at different distances in the
image space.
 Occurs when a ray of light traverse a spherical lens
obliquely a toric effect is produced forming a
STRUMS CONOID

20.  Oblique astigmatism is much more evident
when biconvex
 or biconcave lens are used
 Reduced by use of periscopic or meniscus
lenses

21. NATURAL MECHANISMS TO
DECREASE ABBERATION
• Cutting of peripheral rays by iris
• High refractive index of the core of nucleus of the
lens than that of peripheral cortex
• Low sensitivity of peripheral retina
• Stiles-Crawford effect, I. e. More sensitivity of the
retina to perpendicular rays than the oblique rays.

22. THANK YOU