Trends in melanosome microcavitation thresholds for nanosecond pulse exposures in the near infrared

Abstract

Abstract. Thresholds for microcavitation of bovine and porcine melanosomes were determined using nanosecond laser pulses in the near-infrared (1000 to 1319 nm) wavelength regime. Isolated melanosomes were irradiated by single pulses (10 or 50 ns) using a Q-switched Spectra Physics Nd:YAG laser coupled with an optical parametric oscillator (1000 to 1200 nm) or a continuum laser at 1319 nm. Time-resolved nanosecond strobe photography after the arrival of the irradiation beam allowed imaging of microcavitation events. Average fluence thresholds for microcavitation increased nonlinearly with increasing wavelength from ∼0.5 J∕cm 2 at 1000 nm to 2.6 J∕cm 2 at 1319 nm. Fluence thresholds were also measured for 10-ns pulses at 532 nm and found to be comparable to visible nanosecond pulse values published in previous reports. Calculated melanosome absorption coefficients decreased from 925 cm −1 at 1000 nm to 176 cm −1 at 1319 nm. This trend was found to be comparable to the decrease in retinal pigmented epithelial layer absorption coefficients reported over the same wavelength region. Estimated corneal total intraocular energy retinal damage threshold values were determined in order to compare to current and proposed maximum permissible exposure (MPE) safe levels. Results from this study support recently proposed changes to the MPE levels.

Temperature dependence of nanosecond laser pulse thresholds of melanosome and microsphere microcavitation

“…3. In addition, the nucleation temperature of 1064 nm was used to re-estimate absorption coefficients at other NIR wavelengths from 800 to 1319 nm, based on the 20°C NIR threshold data published in our previous studies 12,13 and summarized earlier in Table 1. Table 5 lists the calculated absorption coefficients (method B values) based on the nucleation temperatures of 116 and 184°C at 532 and 1064 nm, respectively.…”

“…2,3,8,11 In these studies, the nucleation temperature is often used to estimate the absorption coefficient of melanosomes in the visible exposure range. However, little data existed in the NIR regime until our previous studies, 12,13 which measured room temperature, ns pulse, melanosome microcavitation thresholds at wavelengths from 532 to 1540 nm. Since temperature-dependent cavitation data in the NIR were not available at the time, NIR melanosome absorption coefficients were estimated in these studies using nucleation temperatures obtained from previous 532-nm temperature-dependence studies.…”

“…The nucleation temperatures at both wavelengths were extrapolated through the linear relationship of the temperature increases versus decrease in radiant exposure threshold. Melanosome absorption coefficients for visible and NIR wavelengths were estimated from temperature and threshold data using the model of Brinkmann et al 3 In addition, these absorption coefficients were compared to estimated values previously reported by Schmidt et al 12,13 2 Materials and Methods…”

“…All threshold radiant exposures used in Eq. (2) to calculate absorption coefficients, both in this work and in our previous studies, 12,13 were those measured at an ambient temperature of T ¼ 20°C. Model parameters (found in Table 6) for the microspheres were also used to determine absorption coefficients from the radiant exposure threshold values measured as a function of temperature.…”

“…In our previous studies, 12,13 an estimated threshold temperature (or nucleation temperature) of T th ¼ 150°C was used to calculate both visible and NIR absorption coefficients. This value was based on the temperature-dependent, ns pulse, 532-nm melanosome microcavitation studies of Neumann and Brinkmann 2 and Kelly.…”