Transcript Evaluation of vision

In the name of god
Evaluation of quality of vision
Hamid Fesharaki M D
•Best corrected visual acuity for far &near
•uncorrected visual acuity for far &near
•Current prescription, obtained vision
•Sharpest vision with the least amount of minus
•Duochrom test
•Cycloplegic refraction (1% tropicamide, waiting 30-60 minutes )
•Vertex distance for eyes with ≥5 D of R E, pupil size, age
•Snellen chart
•Retinoscope
•Wavefront analysis
•Contrast sensitivity function
•Corneal topography
The basic of refraction
Snell,s law
The power of a curved refracting surface is based on paraxial
rays, according the First order optics for
Ignoring the image quality
P = n2-n1 ÷ r
Total lens power = P1+P2
Exact ray tracing in an spherical refractive surface:
The image is not stigmatic
LME is based on approximation considering paraxisl rays only
vergence rule : U + P = V
Deviations from stigmatic imaging are called aberrations
Bluurre circle (point spread)
Normal eye and most of optical systems are not free from aberration
(using aperture)
The Cartesian ellipsoid produces a stigmatic image of only one object point
The basic of refraction
Fermat principle
Natural processes occur in the most economical way,
The light choose a path with least required time
In a perfect optical system or a first-order optical system, all wavefronts are
plane or spherical.
Optical path length (OPL): light travel distance × refractive index
Wavefronts are surfaces of constant OPL (Optical Path Length) from the source
point.
Rays indicate the direction of energy propagation and are normal to the wavefront
surfaces.
Stigmatic image
Fermat principle: all runners of the race should
reach the finish line at the same time
O.P.L (optical path length) to be equal for all rays
Wave front is a photo of these runners when the
fastest runner reaches the finish line
At the reference sphere all runners are in the same
phase, and it's radius is the distance between the
sphere and the image
The difference between the wave front and the
reference sphere is the wave front aberration
Reference sphere:
a circular arc centered on the image
point with a radius equal to the image distance
Wave front aberration: the difference between the
reference sphere and the wave front
Aberrations & irregular astigmatism
Deviations from stigmatic imaging are called aberrations
(monochromatic, chromatic)
Monochromatic : Myopic & Hyperopic spherical errors,
regular astigmatism
Higher order aberrastions (irregular astigmatism): are
detected by wave front aberrometry (sperical aberration,
coma, trifoil)
Optical aberrations
Approximately 80% - 90% of visual aberration error can
be explained through the first and second order
aberrations :
1st Order Aberation =Tilt (prism)
2nd Order Aberation = Defocus (sphere) and cylinder
The less frequent high order aberrations represent the
residual 10% to 20% cases: (surgically induced)
3rd Order Aberation = Coma and trefoil
4th Order Aberation = Spherical and quadrefoil
5th Order Aberation = Distortions / irregular astigmatism
6th to 8th Order Aberation = Significantly increasing
levels of irregular astigmatism
Chromatic aberrations
Conclusion
This project was incredibly interesting, as it allowed me to explore the various methods of diagnosing eye
problems and obtaining corrective prescriptions with the tools and knowledge that we learned from the class. I
have always been conflicted about LASIK surgery - wanting it because of the promise of significantly better vision,
but nonetheless reticent about the actually procedure and the "permanence" of it all. Therefore, in researching this
product I confirmed that the Ophthonix lenses represents a perfect solution for me and others who have wanted
.
significantly better vision without the anxiety surrounding the LASIK corrective surgical procedure
Common wave front aberrations
Myopia (positive defocus)
Hyperopia (negative defocus)
Regular astigmatism (cylindrical aberration)
Spherical aberration
Coma
18 basic types of astigmatisms have been found by
optical engineers (five of clinical interest)
Clinical importance of aberrations
• Aberrations cause non stigmatic imaging (point spread
function) damaging the image quality
• Higher order aberrations are defined as any refractive
error that cannot be corrected by spherocylindrical
lense combinations
• Despite the fact that we can measure many orders of
aberrations, the human eye can only distinguish
aberrations up to the fourth or fifth order
Clinical signs and symptoms of higher order
aberrations
Loss of spectacle best corrected visual acuity
Poor cylinder axis determination
Inconsistency between refraction and uncorrected
vision
Usually get corrected by hard contact lens
Can not be detected by autorefractometer
can be present in spite of good snellen acuity
Clinical signs and symptoms of higher order
aberrations
Are usually accompanied by decreased contrast
sensitivity
Are mostly induced surgically
The symptoms get worse at night (much governed by
pupil size)
Are mostly accompanied by topographic irregularities
What causes higher order aberrations?
Refractive surgery may increase the higher order aberrations •
however, not always enough to be perceptible to the patient.
•
Spherical aberration (associated with night vision problems) •
is often blamed on excessive flattening of the cornea as a
result of laser eye surgery or on too small a treatment zone •
relative to the patient's pupil size
. Coma (associated with double vision) is often caused by an
off centre laser treatment (decentration)
•
Effect of high amount of induced aberrations on vision
How to represent wave front aberrations
.
By Zernike polynomials which are simply the
mathematical formulas used to describe
surfaces
. 3-dimentional shapes (using graphing software)
. 2-dimentional contour plots
.Mentioning the amount of each basic form of
astigmatism.
.The future prescriptions may consist of 8 or so numbers
Zernike Prescription,This case was best corrected using a spherocylindrical prescription of +0.19 –0.67 x 110
Zernike polynomials allow specification of sphere, astigmatism and higher-order aberrations
for any wave front prescription shown below includes twelve Zernike modes.
higher-order RMS = 0.51 µm
pupil diameter = 5.6 mm
total RMS = 0.76 µm
This chart reveals more common shapes of aberrations created when a wavefront of light passes through eyes with imperfect
vision. A theoretically perfect eye (top) is represented by an aberration-free flat plane known, for reference, as piston. (Photo
courtesy of Alcon Inc)
Measurement of optical aberrations
Wave front analyzer systems
.Hartman-shack aberrometry (outgoing aberronetry),
a low intensity laser beam is directed onto the retina, a lens array
the light rays onto a photoreceptor (CCD)
.Tscherning aberrometry (ingoing aberrometry), a
collimated beam is passed through a mask of holes, a high
magnification camera captures the image onto the retina
.Retina ray tracing technique (ingoing aberrometry), a
laser beam is used to scan across the pupil in a sequential manner,
each position focus a single point on fovea
.Optical path difference (OPD scan): combination of
topography and retinoscopy
With aberration-free optics, wavefronts exiting the eye are perfectly flat (top). Refractive errors, such as
myopia, distort the wavefront (bottom).
Other refractive errors, including higher-order aberrations, cause wavefront distortions that differ in
shape from those seen in simple myopia.
Hartman-shack aberrometer
Lenses placed at each aperture within the Hartmann screen
In the case of an aberration-free eye, each dot is centered with respect to its lenslet.
If aberrations were present, the dots will be shifted away from their aberration-free positions.
Tscherning aberroscope: A grid (top) is projected onto the retina, and the
resultant shape (lower left) reveals amount of aberrations in the eye.
Tscherning aberroscope. Modern techniques use a projected grid of dots (left)
and images the resultant pattern distortion on the retina right to determine
aberrations.
Tracey Technologies Ray Tracing Analyzer shines a thin light beam to create
a retinal spot diagram one point at a time, unlike the Tscherning Aberroscope, which
measures spots simultaneously
Ray Tracing
256 separate points have been projected through the entrance pupil, and
is completed in 400 milliseconds
The I Trace displays the resulting Retinal Spot Pattern. If the eye were
emmetropic, then all 256 points would fall on one spot in the center of
the macula.
In this example, the pattern is large and broad indicating a myopic
eye.
Patient's refractive (sciascopy) and corneal topography data are converted into a
combinedrefractive power map. This information is then utilizedby Nidek's
proprietaryFinal FitTM software for refractive surgery to created custom, individualized
ablation
This map expresses wavefront total aberrations in microns, illustrating the difference in
height between the wavefront of the eye and that of an aberration-free eye as
reference. The influence of each aberration on the patient's visual disorder can be
expected by utilizingthis map.
This map shows specific high order aberration components only, extracted from
the total wave front map. This map illustrates the location and degree of high-order
aberrations found to be present.
.
Zernike Graph with the coefficient factor of each
aberration
The Zernike graph illustratesthe coefficient factor of each
aberration (such as coma, trefoil, astigmatism, or spherical
aberrations)
Resolution Lenses address the vision problems associated with the higher order aberrations of
the eye.
The result is improved contrast acuity, improved night vision and generally sharp vision
This Visx PreVue lens was shaped by a wavefront-guided laser,
offering the patient a chance to first experience simulated vision correction before
committing to a procedure