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Performance specification
EN 20_070_0009I v1.5
The wavefront map is colored uniformly for an emmetropic eye without
aberrations. If an eye has refractive errors, these are shown as color coded
in the wavefront map. Short sightedness (myopia) and far sightedness
(hyperopia) are expressed in wavefront maps showing spherical caps curved
inward or outward. Astigmatism is represented as elliptical contour lines
(areas of the same color) on the wavefront map with the axis of the ellipses
along the axis of the astigmatism. Other abnormalities (higher order
aberrations) result in characteristic irregular structures in the wavefront
map.
A wavefront measurement is only a snapshot of the optical condition of the
customer's eye. It is not a constant picture of the eye and depends on
accommodations.
Topography measurement
The topography of the cornea is measured using a process involving a
Placido disc. The diagram (Fig. 4) provides a simplified illustration of optical
imaging.
1
Camera
2
Lens
3
Eye
4
Placido disc with illuminated rings
Fig. 4:
Imaging beam path of a topographical measurement device with a Placido
disc. A light beam starting at point P is reflected by the reflective, convex
surface of the cornea. The virtual point P’ is depicted on the camera P” via a
lens.
