2. Aberrometry used to measure aberration in the eye by
analysing wave fronts.
Types Of Aberration
1. Lower order aberration
2. Higher order aberration
3. Lower Order Aberration
Aberration which can be corrected using ophthalmic
prescription is known as lower order aberration
Eg: Myopia, Hyperopia, Astigmatism
Higher Order Aberration
Failure to create a perfect image system
Eg: 3rd order and up.
4. Factors effect wave front
Age
Size of pupil
Accommodation
Chromatic aberration
Tear film
AligNment of eye
5. Indications
Refractive surgery
To measure lower order and higher order aberration
Contact lens fitting
Early diagnosis of keratoconus
Post graft fitting
Orthokeratology
Improving best corrected visual acuity by removal of
optical aberration
6. Principle of Aberrometer
Commercially available Aberrometers are based on any of the following
principle:
1.Scheiner’s disc principle:
Which quantify aberration by measuring visual angle between reference
ray and test ray
2.Shack Hartmann lens let array principle:
Which calculate aberration of the eye by projecting a low intensity laser
beam onto the retina through a lens let array.
The reflected light rays from the retina is influenced by several aberrations
are focussed on the photo detector.
Computer compare pattern of reflected rays with pattern formed by an
ideal eye.
7. 3.Tscherning Principle :
In which a grid pattern is projected on to the retina and
calculate amount of aberration by analysing distortion of the
pattern.
4.Ray Tracing Aberrometry:
Which use a single ray and calculate amount of aberration by
measuring deviation of the path of the single ray.
In which eye movement does not affect accuracy of
measurement.
8. Parameters Of Wave Front Analysis
Peak to valley error(PV error)
Root mean square error(RMS error)
Strehl ratio
Point spread function
Modulation transfer function (MTF)
Phase transfer function (PTF)
Optical transfer function(OTF)
