Zernike Fit Parameters; Zernike: Normal Values And Aberration Coefficient - OCULUS Easygraph 70620 Instruction Manual

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There are various
Parameters") that may be set before carrying out a
Zernike analysis ("Zernike fit"). One of these
parameters is the maximum degree (n). This
parameter determines
polynomials are used and hence how much
computing time is required. Furthermore, it is
possible to specify up to what diameter the
measured corneal data are to be included in the
Zernike analysis (∅max). In addition, one has the
option of specifying whether the height data should
be converted directly to Zernike coefficients or
whether the difference of the height data to a
reference body should be used as the basis for the
conversion. This reference shape can be a sphere,
an ellipsoid with an average eccentricity equal to that
of the measurement or an ellipsoid with an
eccentricity of 0.75. The reference body is always
rotationally symmetric, and its shape influences only
the rotationally symmetric polynomials (Z 0,0 / Z 2,0 /
Z 4,0 ...). The central radius of the reference body is
always taken as the mean central radius of the
measurement. In the absence of a reference body,
the greatest contribution to the shape of the cornea
is made by the Z 2,0 component (paraboloid), since
this component has the greatest resemblance to the
shape of the cornea.
If we take into account the corneal refractive index, a
cornea with optimal image forming properties must
theoretically have the shape of an ellipsoid with a
numerical eccentricity of 0.75 and therefore a
reference body of this shape is the obvious choice.
Rotationally symmetric polynomials can then be
interpreted as deviations from this ideal shape.
The current settings of the Zernike parameters
(reference body, maximum
diameter) are shown at the top in the field titled
"Zernike-Fit-Parameters". Clicking the "Change"
button opens a menu for changing the Zernike fit
parameters.

7.5.2.5.4 Zernike: Normal Values and Aberration Coefficient

A set of absolute values of Zernike coefficients
corresponding to the norm are stored in the
Easygraph. Abnormal absolute values are therefore
shown in red in the coefficients list. In addition, the
Easygraph calculates an aberration coefficient from
Instruction Manual Easygraph

7.5.2.5.3 Zernike Fit Parameters

parameters ("Zernike
how many Zernike
degree, maximum
Fit
Click "Calculate new" to confirm the new settings
and perform a recalculation.
The current settings (i.e. the Zernike fit parameters
and the selection of polynomials to be activated or
deactivated) can be saved by clicking "Save
settings". Once stored, these settings are loaded
automatically the next time the "Zernike Analysis"
menu is activated. To load the stored settings while
the program is running, click "Load settings".
Two additional check boxes are to be found at the
top of the field on the lower right titled "Reference
Shape" and "Diff.". The first, "Reference Shape",
specifies that the height data of a reference body are
to be added to the polynomial data. This provides a
realistic image of the cornea in the case where a
reference body is used.
"Diff." adds to the calculated height data their
deviation from the measured data. In calculating the
Zernike coefficients the selected polynomial terms
are fitted as accurately as possible to the measured
height data. The more polynomials that are used, the
more closely the calculated data approximate the
measured height data; however, the fit will never be
perfect. Adding these residual differences to the
Zernike polynomials gives an indication of the
magnitude of the discrepancy in relation to the
polynomials.
The mathematical standard deviation of the original
height data with respect to the Zernike polynomials is
also calculated and shown as a numerical value in
the "Diff" line.
the Zernike coefficients. If there is no abnormal
Zernike coefficient, the aberration coefficient will be
equal to 0.0. Values exceeding 1.0 are an indication
that the corneal surface contains untypical wave