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Performance specifications
000000-1520-928_GA_EN_021013
15
The Placido disc has ring-shaped, diffusely transparent apertures which
guarantee even projection of the rings on the eye. The eye positioned in
front of the Placido disc reflects the ring image of the Placido disc like a
convex mirror. The reflections of the surface of the cornea of the eye are
captured by a video (CCD) camera located behind the Placido disc.
The position of the rings on the cornea change depending on the form of
the corneal surface. An evenly curved surface of the cornea exhibits
concentric and undeformed rings (
A
, Fig. 5). However, their width and
spacing differs subject to the radius of curvature of the cornea: The smaller
the radius of curvature of the cornea, the narrower the rings and the closer
they are together. The flatter the cornea, the wider the rings and the farther
apart they are. If the corneal surface deviates from the shape of a sphere,
the shape of the rings will also change (
B
Software algorithms evaluate the form and position of the rings, providing
detailed information on the shape of the corneal surface. The distance of the
rings from one another is a measure of the radius of curvature of the corneal
surface, the deviation from a circle indicates the extent of the astigmatism.
The evaluated data is displayed in different two-dimensional, color-coded
charts which illustrate the different characteristics (optical effects, local
curvature) of the cornea.
Fig. 5
The image of the rings of the Placido disc on the camera for a normal cornea (A)
and a distorted cornea (B).
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