Measurement, modeling and visualization
of the shape of the cornea
F.M. Vos et al.
[Abstract],
[Introduction],
[Image Processing],
[Surface Reconstruction],
[Conclusions and Future Work],
[References]
Section 1: Introduction
The anterior surface of the human eye (cornea) is responsible
for 84% of the refraction of light and thus of eminent importance for good sight.
Ophthalmologists have recognized the role of the cornea in the refraction
process.
A modern technique to improve someone's sight is by adjusting the corneal surface.
The need for accurate methods to evaluate the shape of the cornea has paralleled
the rapid development of this keratorefractive surgery.
To measure the shape of the cornea standardly a cylindrically symmetrical
ring-pattern (stimulus) is mirrored to the eye.
Analysis of the recorded reflection (Purkinje image) allows for reconstruction
of the corneal surface. An example of such a Purkinje image is given in
figure 1.
Available instruments employing this technique are known not to work ideally
(antalis93).
We have developed a prototype stimulus that exploits the virtues of Pseudo
Random Binary Arrays (PRBA's)
(vos96),
which, encoded in a coloured stimulus-pattern,
contributes to a very robust measurement-technique with uniquely characterized
points. Exploiting this approach we are able to overcome the shortcomings
of traditional devices. A more detailed description of the instrument is given
in vos96.
The prototype-instrument devised by us is depicted in
figure 2. It consists of a cylinder closed
at one side, containing a pattern (called stimulus or stimulator) brightly
lit from the back. The object to be measured is positioned in the front of the
open end; the reflected image is registered with a camera.
The essence of the novel technique lies in the use of uniquely characterized
positions both on the stimulus and in the recorded reflection. In this way a
one-to-one correspondence between points on the stimulus and on the registering
device is established which can be used to reconstruct the corneal shape.
In this paper we will discuss various aspects of the instrument design
especially in relation to the added value of visualization of (intermediate)
results.
The measurement process consists of three steps:
- First the actual image is registered. For subsequent steps it
is of crucial importance to have an accurately focussed image.
This requires a real-time focussing procedure. We will not discuss
details of this. It suffices here to say that we have developed a
set-up that runs on a dedicated PC, grabs in real-time images and calculates
a measure-of-focus (see e.g. Firestone91).
Of this series the one with the highest focus value is stored. The result
of the cycle is the creation of an
TIFF image which serves as input to the rest of the analysis software.
- Extraction of the graph and the PRBA (color) information from the image
makes up the second step of the process. This is discussed in
section two of this paper and falls apart into two sub problems.
First the positions of the grid have to be recovered
and then to be connected into a graph.
- The third step is the modeling of the corneal surface based on
the information thus deduced. Some elements of this process
will be discussed in section three of this paper.
In the last section, four, we will draw some conclusions and
indicate which points will be addressed in future research
[Abstract],
[Introduction],
[Image Processing],
[Surface Reconstruction],
[Conclusions and Future Work],
[References]