The document discusses optical aberrations, which are imperfections in image formation that can lead to visual impairments such as night blindness, glare, and double vision. It categorizes these aberrations into chromatic and monochromatic, with lower and higher order types, and describes how tools like wavefront aberrometers can measure and analyze these distortions. Furthermore, it explains the natural mechanisms of the eye that help minimize aberrations.

1. ABBERATIONS OF
REFRACTIVEERROR
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 PATHDIFFERENCE
 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
CHROMAT
IC
MONOCHROMA
TIC
LOWE
R
ORDE
R
HIGHE
R
ORDE
R

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 oflight rays exiting the pupil
plane at different points in the plane propagation to the
ideal the direction of propagation of lightray
.
Andthe wavefronts exiting the pupil plane are allowed
to interact with a microlensletarray
.

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. WAVEFRONTMAPS
Wavefront maps displayed as 2- dimensional
maps. GREEN = minimal wavefront distortion
from the ideal.
BLU
E.
RED
= myopic wavefronts
= hyperopic wavefront
errors.
The captured wavefront maps are analysed
and classified

12. ZERNIKE & FOURIERPOLYNOMIALS
 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)

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

14. HIGHER ORDERABBERATIONS
 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. COM
A
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