Instruction Manual Easygraph
Page 35
components which may be the cause of impaired
vision.
The function of calculating an aberration coefficient
and highlighting abnormal values in red is only
available if the following Zernike fit parameters are
used:
reference body: ellipsoid ecc. =0.75
•
max. degree: n=8
•
∅
max: 8mm constant
These are the parameter settings relevant to the
normal values.
7.5.2.5.5 Zernike 2D Display Mode
The
“Display”
field (near the middle of the display)
permits switching from the
3D
to a
2D
height display
mode. While the 3D display gives a very graphic
representation of height data, it is not as convenient
when it comes to determining the exact height at
certain positions on the cornea. The 2D display
enables you to determine the height of any desired
point on the cornea by simply clicking on it with the
left mouse button.
In addition, the software determines the highest point
(apex) which is then shown as a black cross in the
picture. The height of this point (
Max:
) is shown
along with its position (
X-Pos:
and
Y-Pos:
). This
height value quantifies the degree of a keratoconus.
The more the keratoconus progresses, the bigger is
gets this height usually. The most appropriate way to
calculate the cone height is to switch on the
reference body (with the eccentricity of 0.75) and all
Zernike-polynomials, except of the lower orders (Z
0,0 / Z 1,1 / Z 2,0 / Z 2,2), as a standard. To achieve
that the Zernike polynomials different from the
standard appear right in the menu window in red
colour, the setting “|Z| Vectors” must be used.
In this way keratoconi can be made visible even in
their early stages. In addition, this feature makes it
easier to recognize the apex position of a
keratonconus. As can be seen from the three maps
below, the conventional display mode using sagittal
radii is prone to errors in representing the apex
position. A clearer representation is achieved with
the Zernike 2D display or with fluo image simulation,
since these techniques are based on height data.
In higher keratoconus stages the x/y position of a
keratoconus apex shifts usually in the direction
temporal/down.