Corneal-tissue absorption coefficients for 193- and 213-nm ultraviolet radiation

Abstract

The small-signal absorption coefficients of 193- and 213-nm nanosecond laser pulses in bovine corneal tissue have been studied. The absolute reflectance of a planar quartz-cornea interface was measured at various angles of incidence for low-intensity laser irradiation (i.e., pulse fluences 3 orders of magnitude below the ablation threshold). The reflectance-versus-angle data were analyzed by the use of Fresnel theory to estimate the effective complex index of refraction of the tissue. This analysis indicated corneal absorption coefficients of 39,900 ± 9800 cm(-1) at 193 nm and 21,400 ± 4900 cm(-1) at 213 nm.

Ablation rate of PMMA and human cornea with a frequency-quintupled Nd:YAG laser (213 nm)

“…Corneal tissues are composed of collagen fibrils with bound water. Water does not strongly absorb at 213 nm [32]. Ultraviolet laser pulses are absorbed by the collagen molecules and detach the molecules and macromolecules from their neighbors by breaking the molecular bond or mechanical stresses.…”

Experiments on PMMA models to predict the impact of corneal refractive surgery on corneal shape

“…2) that the reflectivity factor f(R) is similar in PMMA and corneal tissue, which is true provided that the index of refraction at the laser wavelength is the same on cornea and PMMA and 3) that the laser fluence and the ablation thresholds for PMMA and corneal tissue are known. The fact that the Beer-Lambert´s law appropriately describes photoablation of corneal tissue is fairly well established [20,25]. We have also tested assumption 2 by computing the difference in the reflectivity factor when changing the index of refraction by 1.52 instead of 1.49 and found no appreciable change.…”

“…While the refractive index at 193 nm is similar in cornea and in PMMA (n cornea = 1.52 [20]; n PMMA =1.49), and therefore we will assume f (R) to be the same for both materials, the ablation threshold differs (F th, cornea = 40 mJ/cm 2 [21]; F th, PMMA = 80mJ/cm 2 [22]). …”

Analytical optimization of the ablation efficiency at normal and non-normal incidence for generic super Gaussian beam profiles

“…For human corneal tissue irradiated with nanosecond pulses at 193nm, the ablation threshold takes values of about 40-50 mJ/cm 2 [27,28], and the absorption coefficient is about 3.33-3.99 µm 1 [27,28], although Fisher and Hahn [29], described a global ablation model that incorporates a dynamically changing tissue absorption coefficient and that substantially deviates from a static Beer-Lambert model. We chose values of 46.5 mJ/cm 2 for the ablation threshold, 0.5 mm radius for beam size and 3.485 µm 1 as absorption coefficient of the human corneal tissue.…”