Previous studies determining laser-induced damage thresholds show inconsistent injury results due to measurement techniques as well as methodological challenges using top hat profiles. One reason that decisively influences the ED50 value is the appearance of so-called intensity modulations of a top hat associated with the quality of the energy uniformity on the irradiated tissue sample. By using two different square core fiber lengths (2 m, 20 m) to produce a top hat, we can on one hand ensure the focus control and the correlated homogenized intensity distribution and on the other hand vary and determine intensity modulations in the profile. This means that the accuracy of the measurements is significantly enhanced and the influence of energy variations in the profile can be evaluated explicitly and quantitatively. Our laser-induced irradiation experiments were performed with 1.8 ns pulses at a wavelength of 532nm on porcine RPE by means of a Q-switched, frequency-doubling Nd:YAG laser. Intensity modulations are characterized by a modulation factor that describes the ratio of maximum irradiation to the average. The generated factors in our measurements are 1.78 (high intensity variation) and 1.15 (low intensity variation). The calculated ED50 values are 52.15 μJ for the higher modulations and 33.90 μJ for the lower modulations inducing a discrepancy factor of 1.54. Hence, the determined thresholds highlight the importance of the influence of intensity modulations within the beam profile during irradiation. Therefore, we examine this influence and its possible consideration and handling with determined threshold data. Furthermore, a strategy for ensuring a high top hat quality for the assessment of prospective threshold data is shown.
Previous studies of laser-induced multiple-pulse damage measurements of the retina determined inconsistent thresholds in the thermo-mechanical damage regime. This inconsistency might be due to biological variability, measurement techniques as well as methodological challenges which occur during the use of explants. For better comparability with state-of-the-art measurements, we use a top hat irradiance profile to produce a uniform energy density and monitor the beam profile at the retinal pigment epithelium layer. This approach is applied in order to determine damage thresholds in the lower nanosecond regime for pulse sequences. The irradiation experiments were performed using a Q-switched, frequency-doubled Nd:YAG laser (532 nm wavelength, 1.8 ns pulse, 25 Hz, 319 μm-diameter, squared top hat). Freshly isolated porcine eyes were used as models for measurements of laserinduced thermo-mechanical damage. After removal of the sensory retina the retinal pigment epithelium layer attached to the sclera was irradiated. Four pulse trains (N = 1, 10, 100 and 1 000) were examined and evaluated concerning damage threshold via fluorescence microscopy. The ED50, expressed as energy per pulse, decreases from 33.9 J for N = 1 to 20.3 J for N = 1 000. The multiple-pulse threshold trend which was measured can neither be described by current methods such as the probability-summation-model nor by the N−0,25- approach which is also partly used within the laser safety standard. Therefore we discuss possible pulse interactions which could explain the multiple-pulse behavior in the thermo-mechanical damage regime. Their implications might present the build-up of a new, more accurate theory to describe the reduction factors of thermo-mechanical damage-inducing doses with increasing pulse numbers.
Novel products using nanosecond laser pulses such as LiDAR sensors gain more interest in the consumer industry. These products are in line with the coming trend and are assumed to gain economic importance in the future. The performance of such devices conforms to the maximum permissible exposures (MPE) of the laser safety standard. The accuracy of the standard can be improved by laser-induced damage measurements. Approaching this subject we present our own first ns-measurements of tissue damage in the mammalian eye and the interpretation of this data. The irradiation experiments were performed by means of a Q-switched, frequency-doubling Nd: YAG laser (532nm wavelength, 6 ns pulse). Freshly isolated bovine eyes were used as models for threshold determination of laser-induced thermo-mechanical damage. After removal of the neural retina the retinal pigment epithelium layer attached to the sclera was irradiated. Three spot diameters (150 µm 250 µm and 290 µm) were used. The evaluation of the data leads to the following ED50 values: 3.55 µJ (150 µm), 6.50 µJ (250 µm) and 16.34µJ (290 µm). Based on the measurements, different options of data analysis are explained and used.
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