Basics of Refractive Surgery

1. Presenter : Dr Samuel
Ponraj
Moderator : Dr Shrinivas
K Rao
Optical Aberrations

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

3. Ideal Optical System
 Stigmatic imaging
 Geometrical similarity

4.  No Field Curvature

5. SHACK HARTMAN TEST

6.  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.

7.  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.

8. WAVEFRONT ANALYSIS
 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.

9.  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.

10.  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.

11.  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

12.  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.

13.  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

15. Wave front technology

16.  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.

17.  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.

18.  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.

19.  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

20.  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).

21. Higher order aberrations
 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 .

22.  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.

23.  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.

24. Coma
 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,

25. Coma
 Spherical aberration applied to light coming from
points NOT lying on the principal axis.
 Rays passing through the periphery of the lens
are deviated more than central rays & come to a
focus nearer the principal axis.
 Results in unequal magnification of the image
formed by different zones of the lens.
 Differs from spherical aberration in that the image
formed is laterally displaced.

27. Ocular application
• May be avoided by limiting to the axial area of the
lens.
• Not of clinical significance due to the same
reasons for oblique astigmatism… which are:
1. Aplanatic surface of the cornea
2. Retina is a spherical surface
3. Coma image falls on peripheral retina which
has poor resolving power compared to the
macula; visual appreciation of astigmatic image
is limited

28. Spherical Aberration
 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

29.  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

30.  It was seen that the prismatic effect of a
spherical lens is least in the paraxial zone and
increases towards the periphery of the lens.
 Thus, rays passing through the periphery of the
lens are deviated more than those passing
through the paraxial zone of the lens

31.  In other words, the parallel light rays of incoming light
do not converge at the same point after passing
through the lens. Because of this, Spherical
Aberration can affect resolution and clarity, making it
hard to obtain sharp images.
 Results in out-of-focus image.

33. Spherical lens:
Peripheral rays have shorter
focal length than paraxial rays.

35. Correction of Spherical
Aberration
 Spherical aberration may be reduced by occluding the
periphery of the lens by the use of 'stops' so that only
the paraxial zone is used.

36.  To achieve the best results, spherical surfaces
must be abandoned and the lenses ground with
aplanatic surfaces; that is, the peripheral
curvature is less than the central curvature .
 Aspherical lenses are lenses with complex curved
surfaces, such as where the radius of curvature
changes according to distance from the optical
axis.

37. aspheric doublet lens
 Another technique of reducing spherical
aberration is to employ a doublet. This consists of
a principal lens and a somewhat weaker lens of
different refractive index cemented together .
 The weaker lens must be of opposite power, and
because it too has spherical aberration, it will
reduce the power of the periphery of the principal
lens more than the central zone.
 Usually, such doublets are designed to be both
aspheric and achromatic.

38. aspheric doublet lens.

39. Ocular application
 The effect of spherical aberration in the human eye is
reduced by several factors:
 (1) The anterior corneal surface is flatter peripherally
than at its centre, and therefore acts as an aplanatic
surface.
 (2) The nucleus of the lens of the eye has a higher
refractive index than the lens cortex… Thus the axial
zone of the lens has greater refractive power than the
periphery.

40.  (3) The iris acts as a stop to reduce spherical
aberration. The impairment of visual acuity that
occurs when the pupil is dilated is almost entirely due
to spherical aberration (Optimum pupil size is 2–2.5
mm.)
 (4) Retinal cones are much more sensitive to light
which enters the eye paraxially than to light which
enters obliquely through the peripheral cornea
(Stiles–Crawford effect).
 This directional sensitivity of the cone
photoreceptors limits the visual effects of the
residual spherical aberration in the eye.

41. Oblique astigmatism
 Occurs when rays of light traverse a spherical
lens obliquely… a toric effect is introduced
forming a Sturm’s conoid

43. Chromatic aberration
 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.

44. Chromatic aberration
 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.

45.  When white light is refracted at an optical
interface, it is dispersed into its component
wavelengths or colors .
 The shorter the wavelength of the light, the more
it is deviated on refraction.
 Thus a series of colored images are formed
when white light is incident upon a spherical lens

47. Correction of Chromatic
Aberration
 The dispersive power of a material is independent
of its refractive index.
Thus, there are materials of high dispersive power
but low refractive index, and vice versa.

48. Achromatic Lens
 Special optics design of two mated lens –
concave and convex – which more precisely
focus the wavelengths of light onto the same
plane.
 Achromatic lens systems are composed of
elements (lenses) of varying material combined
so that the dispersion is neutralized while the
overall refractive power is preserved

49.  The earliest achromatic lenses were made by
combining elements of flint and crown glass.