Effect of Incident Light Wavelength and Corneal Edema on Light Scattering and Penetration: Laboratory Study of Human Corneas
J Refract Surg. 2010-10-01; 26(10): 786-795
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PURPOSE: The outcome of ultrashort pulse laser surgery of the cornea is strongly
influenced by the light scattering properties of the tissue, for which little
data are available. The purpose of the present study is to provide quantitative
values for light scattering and its relation to the degree of edema.
METHODS: An experimental optical measuring setup based on confocal geometry was
used to measure the unscattered and scattered fractions of light transmitted by
eye bank corneas presenting various degrees of edema. From these measurements,
the effective light penetration depth in the cornea was calculated as a function
RESULTS: Corneal transparency depends on the pathological state of the cornea and
on wavelength. It may be predicted as a function of corneal thickness, ie, the
degree of edema. In healthy and edematous cornea, the percentage of scattered
light decreases with increasing wavelength. The total penetration depths at the
wavelengths of ~1050 nm (which is used in typical clinical systems) and 1650 nm
(which is recommended for future devices) are comparable; however, the former is
limited by scattering, which degrades the laser beam quality, whereas the latter
is only limited by optical absorption, which may be compensated for.
CONCLUSIONS: The use of longer wavelengths should help improve the surgical
outcome in ultrashort pulse laser surgery of the cornea when working on
pathological tissue. A wavelength of approximately 1650 nm appears to be a good
compromise, as it allows for reduced light scattering while keeping optical
absorption reasonably low.
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