Page 42
Instruction Manual Easygraph
7.5.2.8 Height Data
This display mode uses the internal three-
dimensional height model of the cornea generated
by the measurement.
The use of height data offers the following
advantages:
•
Height data give a more accurate representation
of the true shape of the corneal surface. They
can be used directly for fluo image simulation as
well as for calculating the posterior surface of
contact lenses. Comparison with the fluo image
of a contact lens shows that the use of height
data for locating the position of the apex in
keratoconus gives far more accurate results than
one would expect from a display method based
on sagittal and tangential radii
•
Height data contain a variety of information,
which, however, only becomes accessible
through further calculations.
•
Height data can serve to define standards
independent of the type of instrument being
used.
•
Height data are relatively insusceptible to fixation
artifacts, thus lessening the probability of
pseudokeratoconus occurring.
•
Height data can serve as a starting point for
analyzing various aberrations of the corneal
surface by means of Zernike polynomials.
Furthermore they can be used for keratoconus
quantification
•
Height data make it possible to determine how of
the cornea would theoretically have to be
removed in order to produce the desired
reference surface from cornea as originally
measured.
Height data can be shown either in absolute or in
relative terms (in the latter case as differences in
height between measurement and reference
surface):
Absolute height, P
A,
(in a meridian section) is the
difference in height between a local point on the
corneal surface and a plane which makes contact
with the cornea in its center.
The difference in height, P
D
, between cornea and
reference body, may either be positive or negative:
negative: measurement below reference body,
positive: measurement above reference body.
In the Height Map menu (see Figure on next page)
the map at the top right shows the corneal surface in
the normal display mode based on sagittal or
tangential radii.
The map at the bottom right shows the topography of
a
rotationally symmetric reference body
, the
shape of which can be defined using the
“r”
and
“Ecc”
fields above the topographic map. The height
data map is updated automatically after every
change to the reference body.
For example, if
“Ecc”
is set to 0, the reference
becomes a sphere.
If
“r”
is set to 0, the map will show absolute height
data.
p
D
cornea
reference
Difference in height between cornea
and reference body
p
A
cornea
Absolute height
plane