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Robert W. Knighton, Xiang-Run Huang; Linear Birefringence of the Central Human Cornea. Invest. Ophthalmol. Vis. Sci. 2002;43(1):82-86.
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purpose. To determine the polarization properties of the central cornea at
perpendicular incidence in a normal human population on the assumption
that the cornea behaves as a linear retarder.
methods. A corneal polarimeter provided a view of the fourth Purkinje image of a
yellow (585 nm) light-emitting diode through crossed polarizers and a
variable retarder. The Purkinje image was extinguished by adjusting the
fast axis and retardance of the retarder to match the slow axis and
double-pass retardance of the cornea. Both eyes of 73 normal subjects
(49 women, 24 men; ages, 21–71 years) were measured. Correlations were
expressed as Pearson’s r.
results. In most corneas the slow axis pointed nasally downward, with the peak
of the axis distribution falling between 10° and 20° nasally
downward. Double-pass corneal retardance varied widely (range, 0–250
nm); 80% of retardance values were uniformly distributed from 40 to
140 nm. Retardance was moderately correlated with axis
(r ≈ 0.5), such that weaker retardance was
associated with axes that were more nasally downward. Corneal
birefringence was well correlated between the two eyes of a subject in
both axis (r = 0.77) and retardance
(r = 0.75).
conclusions. The variation of corneal birefringence among individuals is substantial
enough to produce large, uncontrolled differences in the polarization
state of a measuring beam, differences that can introduce variability
in newer technologies for ophthalmic diagnosis. The interocular
similarity of corneal birefringence suggests deterministic control of
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