Wave front Aberrometry

Wave front Aberrometry
Hamid reza Jahadi Hosseini
Professor of ophthalmology shiraz university of
medical sciences
Feb. 2014
Few decades ago , we considered the
Eye as a simple optical system.
Just learned the myopia , hyperopia
,astigmatism and correct them with
sphere and cylinder.
Most of the cases were happy.
But still some of the pt could not be
corrected to the 20/20 !!!
WHY ??
Few years later we started to do
conventional LASIK.
 Most of the cases were satisfied post
op with UCVA = 20/20
 But few of them were Un happy
20/20 cases !!!
 Complaining of halo, double vision
and night vision disturbance

WHY ???
The Answer is
ABERROPIA
Astronomists had encountered similar
problems
when looking at the galaxy through their
telescope,the aberrations leads to blur
images of the stars
1970 Dr Bille /Heidleberg/germany
Used deformable mirrors ( adaptive Optics)
In the telescope to neutralize the aberration
And produce sharp images.
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wikipedia
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Aberration is something that deviates
from the normal way ( ‫) ﺍﻋﻮﺟﺎﺝ‬
Optical aberration
 is an imperfection in image formation by
the optical system

Optical aberrations can be measured by 2
different methods
 1) only analyzing the aberration
 2) measuring effect of aberration on
image
in 1) we check the Optical quality of the
system , while
In 2) we check the image quality

Image quality
is defined by Strehl Ratio
 It is the ratio of Maximum intensity of
actual image/to the maximum intensity of
fully diffraction limited image


Strehl Ratio = 1- ( 2 π RMS )
Pi (π)= 3.14
Optical quality
of the eye is measured by the
aberrometers.
& is reported as RMS(Root Mean Square )

Another way of looking into image quality
is by

POINT SPREAD FUNCTION (PSF)
And
MODULAION TRANSFER FACTOR ( MTF)
MODULATION TRANSFER FACTOR
is a measure of system ability to
transfer contrast from the object
plane to the image plane at a specific
Resolution
PSF POINT SPREAD FUNCTION

Is measure of how
well a single object
point ,makes an
image point on the
retina after passing
through the optical
system of the eye
WAVEFRONT
‫ﺟﺒﻬﻪ ﻣﻮﺝ‬
Defocus ( Myopia)
wavefront
physics of Aberrometers
1) Out going wavefront Aberrometry
2) In going wavefront Aberrometry

Out going wavefront Aberrometry
Is Hartmann – shack
These are the most popular type of
Aberrometers used
100 miliscond ingoing
diode laser turns on
ZYOPTIX is a Hartmann – shack type
Technisian first measure the WF at dim
light un dilated pupil and then several
times with dilated pupil at Dark room or
dilatedwith non cycloplegic mydriatics.
 And select the WF with PPR which is in
better agreement with subjective
refraction reported by surgeon
 With dilated pupil aberrometer can detect
more amount of aberrations of globe


Shortcoming of hartmann shack systems
In highly aberrated eyes( HOA>1.5) because
of intererence of multiple beams which are
outgoing in diferrent direction, it fails.
So other systems like Tracy which uses single
Ray seems superior

2) In going wavefront Aberrometry
A- Retinal Image Aberrometer
Tscherning Aberrometer
Retinal Ray tracing (Tracy)
B- Double –pass Aberrometer
C – Ingoing subjective adjustable
Aberrometry
TSCERNING
ABERROMETER
Uses ingoing grid rays
WAVELIGHT
is a Tscherning Aberrometer
TRACY is an ingoing Aberrometer based on
Retinal RAY TRACING
It uses at each time a
single ingoing ray and
Traces it to retina &
checks how its image
formed On the retina
.then emits another
ingoing ray and
repeat

TRACY
Can measure
WF of the entire eye = Global WF
WF of Cornea alone
WF of internal optics of the eye
So can easily show abnormal WF is
origionating from which part of the eye?
NIDEK OPD SCAN


OPTICAL PATH
Difference to
Emmetropia
Is a scanning slit
autorefractometer
using skioscopy
technology
NIDEK OPD SCAN
Also has placido disc
for topography
 Actually is a hybrid
system
AUTO, TOPO and WF
Aberrometer
Uses 1440 data points

NIDEK OPD SCAN


Can measure the
largest range of
Sphere ( -20D to
+22D)
and cylinder up to
12D
Finally these abnormal wave front
detected by each of these systems should
be calculated.
 Diferrent formula can be used Taylor,…..

But the most popular is ZERNIKE
Polynomial formula
 Which can measure up to 10th order HOA
 And 35 coefficients

It measures the square root of sum of
square of all deviated distance from ideal
point
 And reports it as RMS
 Is reveals the TOTAL aberration,including
both low-order and higher –order
aberration

Then zernike has the ability to decompose
the wavefront into its components.
 ie. separareLower order and higher
orders
 And break the HOA into its different types

Majority of aberromtery machines usually
report the GLOBAL HOA ie HOA which
arise from the entire globe. Tear film,
cornea, lens, even the retina.
 Some aberrometers have the ability to
measure the Corneal HOA or
Lenticular HOA separately too, like
TRACY

The computer makes these calculations and
converts it into different maps and
displays which can be interpreted easily
by clinicians.
HOA can be displayed by different maps
eg 2D map , 3D maps, normal band
display , Refractive map,………..
HOA can be symmertical like spherical
aberration or asymmetric like coma.
 different HOA ,s do not have equal effect
on image degradation. Some have more
effect. Coma is one of the worst types.

In coma , the image of a point is comet
shape.
 It is more sharp at one edge than the
other edge.
 It causes symptoms like bluring of vision,
monocular diplopia, and glare

Coma occurs in human eye by 2 main
mechanism
 1) pupil decenteration
 2) when optical components are not
coaxial like decentered lens,IOL,
decentered ablation, Keratoconus,
decentered PK

Spherical Aberration
can arise from normal crystalline
lens,when pupil dilate at dark ( night
myopia) or from high power conventional
myopic ablation.
It causes night glare and halo and myopia
ZERNIKE PYRAMID
NORMAL BAND VIEW
Zero order is an ideal perfect optical
system which has no aberration and is
called Piston
 Lower order aberration include first order
and second order aberrations which can
be measured by retinoscopy or
autorefractometer too,
 First order includes Tilt ( Prism)

Second order includes DEFOCUS (Myopia
, Hyperopia ) and Astigmatism
3rd order and above are called Higher
order aberration

3rd order includes coma , Trefoil
4th order includes spherical Aberration ,
quadrefold
5th order includes pentafold,secondary
coma, secondary trefoil

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Z200 Defocus
Z220/Z221 stigmatism
Z300 coma
Z330/Z331 trefoil
Z400 Spherical aberration
Z420 Z421 secondary astigmatism
Z440/Z441 Quadrafoil
Z510 , Z511 secondry coma
Z 530, Z531 Secondary Trefoil
Z550 Z551 Perntafold
can the tear film cause HOA ??
Yes Dry eye , tear
debris,..


Wavefront technology has enabled we
physicians to detect different types of HOA
and quantify them
Correlate them with patients symptom.
Make clinical decision when to treat them by
WF guided ablation.
 or avoid iathrogenic HOA
by doing Aspheric ablations, or using Aspheric
IOL.
and Well centered ablation,……
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