KEYWORDS: Luminescence, In vivo imaging, Laser damage threshold, Gaussian beams, In vitro testing, Microscopes, Amplifiers, Sensors, Eye, Argon ion lasers
We performed measurements to validate damage threshold trends in minimum visible lesion (MVL) studies as a function of spot size for nanosecond laser pulses. At threshold levels, nanosecond pulses produce microcavitation bubbles that expand and collapse around individual melanosomes. This microcavitation process damages the membranes of retinal pigment epithelium (RPE) cells. A spot size study on retinal explants found cell damage fluence (energy/area) thresholds were independent of spot size when microcavitation caused the damage, contradicting past in vivo retinal spot size experiments. The explant study (ex vivo) used a top-hat beam profile, whereas the in vivo studies used Gaussian beams. The difference in spot size trends for damage in vivo versus ex vivo may be attributed to the optics of the eye but this has not been validated. In this study, we exposed artificially pigmented human RPE cells (hTERT-RPE1)-in vitro-to 7 ns pulsed irradiation from a Ti:Sa TSA-02 regenerative amplifier (1055 nm) with beam diameters of 44, 86, and 273 μm (Gaussian beam profiles). We detected the microcavitation event with strobe illumination and time-resolved imaging. We used the fluorescent indicator dye calcein-AM, with excitation by an Argon laser (488 nm), to assess cell damage. Our current results follow trends found in the in vivo studies.
The measurements on the nonlinear absorption coefficient for the whole retina and separated molecular components have been determined using open-aperture z-scan. Our recent retinal damage studies have shown that the threshold for retinal damage decreases below one nanosecond exposure. Laser-induced breakdown has been implicated in the threshold-level mechanism for damage, and the threshold is reduced below 100 fs where LIB is the damage mechanism. Our hypothesis for this experiment is that non-linear optical properties of the constituents of the retina will affect the absorption coefficient of the retina for ultrashort pulse laser exposure and lower the retinal damage threshold for these exposures. This suggests that nonlinear absorption effects should be considered in the analysis of any data that relate energy deposition rates from laser exposures in tissue to thermal or photomechanical damage mechanisms that explain cell death. We describe the impact of these measurements on retinal damage thresholds and damage mechanisms for various pulse regimes.
The measured optical density of various laser eye protection samples is presented as a function of irradiance using femtosecond laser pulses. We show that the protective quality of some eyewear degrades as irradiance increases. In previous studies this problem has been demonstrated for samples irradiated by nanosecond pulses, but the current study shows that some modern laser eye protection seems to be robust except for the irradiance level possible with ultrashort laser pulse exposure. We discuss the most likely saturation mechanisms in this pulse duration regime and its relevance to laser safety.
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