Effect of incident light wavelength and corneal edema on light scattering and penetration: laboratory study of human corneas.
OBJECTIVE
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 of wavelength.


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.
Peyrot
Donald A
DA
Laboratoire d'Optique Appliquée, ENSTA ParisTech-École Polytechnique, Palaiseau, France.
Aptel
Florent
F
Crotti
Caroline
C
Deloison
Florent
F
Lemaire
Simon
S
Marciano
Tal
T
Bancelin
Stéphane
S
Alahyane
Fatima
F
Kowalczuk
Laura
L
Savoldelli
Michèle
M
Legeais
Jean-Marc
JM
Plamann
Karsten
K
D003315:Cornea / Q000528:radiation effects*; D015715:Corneal Edema / Q000209:etiology*; D006801:Humans; D008027:Light; D012542:Scattering, Radiation