Alfredo Dubra's PhD thesis - Imperial College London

5. Preliminary experiments
The recorded patterns look very similar, if not identical, to what the subject sees when
the experiment is being performed. We believe the image projected on the retina not
only shows information about the tear topography, but also about floaters within the
eye, and although this was not explored further, it should also show opacity in the
eye due to for example cataracts. Typically when the subject participates for the first
time in the experiment, asked to concentrate on one particular spot of these images
and to remain looking always in the same direction, they would choose a floater or
a bubble on the tear. Then because this feature is rigidly attached to the eye, the
subject would start moving the eye trying (unsuccessfully) to bring that object to the
fovea, resulting in a very frantic eye movement (one could push the analogy with a
dog chasing its own tail to its very limit). The subject would then be asked to try to
fixate on one of the set of small Newton rings that are due to the optics (and therefore
static with respect to the optical setup). In any case, it is an interesting experiment
to participate in as a subject, not only because it allows one to see what happens on
ones own tear film but also to see other structures (e.g. floaters) within ones own eye.
5.1.2 Focused illumination and estimation of undesired eye reflections
Despite the failure of the previous experiment to provide a quantitative tool for studying
the tear topography, a similar experiment with the same optical setup was performed,
but with the eye further away from the experiment, so that the focus of the
beam incident on the eye coincided with the front surface of the tear. In this way, if
there was a reflection from a surface within the eye other than that from the tear surface,
it would show up by producing an interferogram with concentric circular fringes.
These fringes would carry information on the optical path between the front surface
of the tear and the second reflecting surface. This is a similar experiment to that performed
by Prydal, the only difference being that in this case the axis of the incident
beam is perpendicular to the surface of the tear rather than at 45 o .
For this experiment, a series of 100 images on two subjects (series coded bb focused
and lm focused) were recorded. The two images shown at the top in figure 5.3
are representative of the recorded series of images, and show three types of ringing:
those at the edges, caused by diffraction, small sets due to interference from reflections
within optical elements in the setup and the very dim set almost centered covering the
81