Keratron
™
Onda Installation and operating manual
OPTIKON 2000
Cod. 161401EN
5-3
2015-03-27
Rev.C
mires reflected on the cornea of the Keratron
™ Onda are the same as the other
Keratrons (28, at a uniform rate for the arc-step algorithm) without any compromise
in the center (Figure 5-2 in the lower left).
To establish when the eye is at the ideal distance from the cone, the Keratron
™
Onda uses an advanced version of the
EPCS
(
Eye Position Control System
)
electronic circuit adopted in the other topographs produced by Optikon 2000 S.p.A.
The eye’s apex is intercepted when it is located at the preset distance from the
optical head to accept the acquisition only within a preset range
range
. If the eye is
not at the correct distance, it is not possible to capture the image.
Figure 5-3: EPCS with traditional Keratron
™ and with Keratron™ Onda
In the original version the photoelectric cells guarantee a precision of a tenth of a
diopter within a “
range
OK” of only ±0.05mm. Within this range the image is
accepted.
Topo-aberrometric acquisitions require more time; therefore, the eye may have
greater movement in the axial direction.
Therefore, the Keratron
™ Onda uses larger photoelectric cells and an A/D Converter
that measures and associates each image with its forward or backward
misalignment. Using such measurement to compensate these variations, the
measurement of the curvature of a normal eye is precise to within a tenth of a diopter
with axial movements of up to 0.55 mm (Figure 5-3).
If, instead, the pati
ent’s movement with respect to the cone is too fast, and the
relative movement exceeds these limits, the Keratron Onda notifies the operator that
the videokeratographic image is “too close” or “too far away” for a good topograph,
even if it can be adjusted for the measurement of the pupil.
5.1.2
THE BIVALENT CONE AND TOPOGRAPHY
The adoption in the Keratron
™ Onda of a double pair of photoelectric cells on the
external edge of the optical head makes it possible to choose between the near
