Optical Aberrations in Human Eye

1. Jagdish Dukre

2.  Optical aberration is an imperfection in the
image formation of an optical system.
 Aberrations fall into two classes:
 monochromatic and
 chromatic.

3.  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.
 Chromatic aberrations are caused by
dispersion, the variation of a lens's refractive
index with wavelength. They do not appear
when monochromatic light is used.

4.  One needs to keep in mind these important
points: unlike the standard eye model, an
actual eye is:
 An active optical system, with adjustable
components and aberrations varying in time,
 It is not strictly centered system,
 It is not a rotationally symmetrical system, and
 Final perception is the subject of neural
processing.

5.  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.

6.  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, which can be used to prescribe a
solution, correcting the patient’s specific
vision problem.

7.  Another way of characterizing the wavefront is to
measure the actual slope of light rays exiting the
pupil plane at different points in the plane and
compare these to the ideal; the direction of
propagation of light rays will be perpendicular to
the wavefront.
 This is the basic principle behind the Hartman-
Shack devices commonly used to measure the
wavefront.
 Wavefronts exiting the pupil plane are allowed to
interact with a microlenslet array.

8.  If the wavefront is a perfect flat sheet, it will form a
perfect lattice of point images corresponding to the
optical axis of each lenslet.
 If the wavefront is aberrated, the local slope of the
wavefront will be different for each lenslet and result in
a displaced spot on the grid as compared to the ideal.
 The displacement in location from the actual spot
versus the ideal represents a measure of the shape of
the wavefront.

9.  Wavefront maps are commonly displayed as
2-dimensional maps.
 The color green indicates minimal wavefront
distortion from the ideal.
 While blue is characteristic of myopic
wavefronts and red is characteristic of
hyperopic wavefront errors.

10.  Once the wavefront image is captured, it can be
analyzed.
 One method of wavefront analysis and classification
is to consider each wavefront map to be the weighted
sum of fundamental shapes.
 Zernike and Fourier transforms are polynomial
equations that have been adapted for this purpose.
 Zernike polynomials have proven especially useful
since they contain radial components and the shape
of the wavefront follows that of the pupil, which is
circular.

12.  Following the above division of the Zernike
expansion adopted in ophthalmology,
monochromatic eye aberrations are addressed as:
(1) lower-order aberrations, with the Zernike radial
order n<3, and
(2) higher-order aberrations, with n≥3.

13.  The important optical aberrations that affect
vision are:
 2nd Order optical aberrations – currently
measured in all eye exams providing sphere,
cylinder and axis corrections
 3rd and 4th Order optical aberrations – high
order aberrations currently not measured in
today’s eye exams but can account for up to
20% of the eye’s refractive error.

14.  5th and 6th Order optical aberrations –also high
order aberrations not currently measured in
today’s eye exam.
 These aberrations are of less significance
clinically, however they manifest in reduced
vision for a small percentage of eyes.

15.  The lower-order aberrations are
 Piston
 Tilt
 Defocus
 Astigmatism
 The 2nd order aberrations, defocus and
primary astigmatism - are the most significant
contributors to the overall magnitude of eye
aberrations
Lower-order aberrations

16.  Remaining lower-order forms, piston and tilt,
or distortion, are usually ignored.
 The former being not an aberration with a
single imaging pupil, and
 The latter being not a point-image quality
aberration).

18.  Higher order aberrations are measured with
wavefront aberrometers and expressed in
terms that describe the shape and severity of
the deviated light rays as they pass through the
eye's optical system and strike the retina.
 Coma, spherical aberration, and trefoil are the
most common higher order aberrations .

19.  Coma causes light to be smeared like the tail of a
comet in the night sky.
 Double vision is a common symptom of coma.
 Trefoil causes a point of light to smear in three
directions, like a Mercedes-Benz symbol.
 Spherical aberration is characterized by halos,
starbursts, ghost images, and loss of contrast
sensitivity (inability to see fine detail) in low light.

20.  Starbursts – Patterns of Small Lights Around
Light Sources
 Haloes – Circles of Light Around Light Sources
 Ghosting – A Faint Duplicate of Each Object
Similar to Double Vision
 Glare – Intensification of Light Sources.
 It's quite common for a patient to have an increase
in all of these aberrations, resulting in distorted
night vision when the pupil opens and allows light
to enter through a larger area of the irregular
corneal surface.

22.  A comet-like tail or directional flare appearing in the
retinal image, when a point source is viewed.
 Because the eye is a somewhat nonaxial imaging
device, and because the cornea and lens are not
perfectly centered with respect to the pupil, coma
generally is present in all human eyes.
 A large amount of coma (0.3 μm of coma alone) may
point to known corneal diseases, such as
keratoconus.

23.  Fortunately, spherical aberration is
relatively easy to understand.
 For a normal photopic eye, spherical
aberration may vary from
approximately 0.25 D to almost 2 D.
 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.

24.  This is why many lens systems
incorporate an aspheric grind, so
that the periphery of the lens
system gradually tapers and
refracts or bends light to a lesser
degree than if this optical
adaptation was not included.
 The variation in index of
refraction of the crystalline lens
(higher index in the nucleus, lower
index in the cortex) is responsible
for neutralization of a good part of
the spherical aberration caused by
the human cornea.

25.  Because the index of refraction of the ocular
components of the eye varies with
wavelength, colored objects located at the
same distance from the eye are imaged at
different distances with respect to the retina.
 This phenomenon is called axial chromatic
aberration. In the human eye the magnitude of
chromatic aberration is approximately 3 D.

26.  However, significant colored fringes around
objects generally are not seen because of the
preferential spectral sensitivity of human
photoreceptors.
 Studies have shown that humans are many
times more sensitive to yellow–green light
with a central wavelength at 560 nm than to
red or blue light.