Alfredo Dubra's PhD thesis - Imperial College London

5. Preliminary experiments
5.1.1 Normal illumination experiment
For this particular experiment, the eye was placed so that the spherically convergent
illumination was everywhere normal to the surface of the tear film. Two sequences
of 100 images at a 2.5 Hz rate were recorded from each of the two subjects (series
coded bb1, bb2, dm1 no wedges and dm2 no wedges). Figures 5.1 and 5.2 show two
sequences of 12 images each, where the illuminated area over the tear was around
3.1 mm in diameter. Note the diffraction rings at the pupil edge, which decrease in
thickness towards the center of the pupil, the bubbles and other tear features, and
finally the small sets of Newton rings with very low contrast (in comparison to the
diffraction rings) that are due to interference within optical elements and/or their
coatings.
These particular sets of images were selected because they illustrate some of the most
interesting things that can be seen using this technique. The first sequence (figure
5.1) starts with what is probably the most representative situation, a smooth tear
surface with some small bubbles distributed pretty much randomly over the pupil.
Before the next frame the top eyelid moved down and then up a fraction of the pupil
diameter, leaving a clearly delimited region full of bubbles which smooths out slowly in
the following 5 frames (approximately 2 seconds). The subject felt the need to blink
and tried to prevent it in the eighth frame of the sequence, where the tear surface
becomes very rough. Finally the subject blinked between the eighth and ninth frame,
producing again a smooth tear surface that remains so for the rest of the sequence
apart from the displacement of a small line of bubbles.
The second sequence (figure 5.2) shows a phenomenon that could be seen every time
this subject blinked. After the blink shown in the first image, a set of relatively
large bubbles are left near the top of the image pupil, and as time passes these set
of bubbles move downwards while their size is reduced until they almost disappear
before the next blink.
We failed to identify clearly an interference pattern on any of the 400 images recorded
with this experimental configuration that we could attribute with confidence to interference
between the reflections from the front surface and another surface within the
eye.
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