Page 28
Instruction Manual Easygraph
•
Regular astigmatism
The second-order
wave represents a
regular sine wave of
double frequency,
meaning that it
achieves two minima
and two maxima over
a radius ring. This
function also yields
only relative values.
The axis position for
each zone is
represented by red
and blue circles. In addition, central and peripheral
astigmatism are indicated by red and blue lines. A
red line signifies a steep meridian, while a blue line
signifies a flat meridian.
Keratoconus is often associated with a rotation of the
astigmatic axis from the center to the periphery,
resulting in impressive whirl patterns.
Experience has shown that the magnitude and axis
position of central astigmatism obtained by Fourier
analysis are in closer agreement with subjective
refraction than are the central radii measured by an
integrated Easygraph. This is due to the fact that any
decentration or major corneal aberration (e.g. trefoil
or four-lobed defect) influences the amplitude and
axis position in keratometer measurements.
•
Irregularities
All remaining wave
components add up to
give the irregularities
of the corneal ring
under measurement,
again expressed as
relative values. In a
normal cornea the
arithmetic mean of all
irregularities is less
than
0.030 mm for
sagittal curvature
(0.141 for tangential
curvature)
. An inverse relationship obtains between
the degree of irregularity and the best corrected
visual acuity.
The “Irregularities” field contains only wave
components that cannot be corrected by means of a
sphere, cylinder or prism. If these are large, the
patient will only achieve good visual acuity with rigid
contact lenses.
Combining Fourier components
One option is to display the “Decentration“ and
“Irregularities“ components separately, as described
above. However, it is also possible to add the
spherical equivalent to each of these components.
Use the “Settings” menu to add or remove the
spherical equivalent. (see 7.5.4 page 62).
•
Combining decentration and spherical
equivalent
Adding the spherical
equivalent to the
decentration
component often
provides an intuitive
display of the conus
position, allowing the
apex to be more
readily discerned
than in the original
image.
•
Combining irregularities and spherical
equivalent
Combining the
irregular and the
spherical compo-
nents often makes it
easier to detect three
or four-lobed defects
than would a display
of irregular compo-
nents alone.
Fourier indices
Calculations based on the individual components
yield indices which permit a quick numerical
characterization of the corneal surface: