Visible lesion threshold dependence on retinal spot size for femtosecond laser pulses

National laser safety standards have only recently been specified for laser pulse widths below 1 ns, with the publication of the ANSI Z136.1-2000 American National Standard for Safe Use of Lasers. A number of in vivo retinal injury studies using ultrashort laser pulses have been documented for pulse widths from nanoseconds to femtoseconds and having wavelengths from 1064 to 530 nm. These studies report data corresponding to the smallest retinal image diameters that can be achieved experimentally. The resulting data have been used to establish the exposure limits for small-source laser emitters. Data have shown that the thresholds decrease with pulse width and with wavelength for minimal retinal spot sizes. In this article we present measurements of the retinal lesion threshold as a function of retinal image size for 150 fs ultrashort laser pulses at 1060 nm. Retinal image size was varied from approximately 48 to 800 μm in diameter using external optics. Thresholds were determined using probit analysis of the data. The retinal spot sizes were calculated using the Gaussian beam propagation and multiple-lens formulas. The thresholds as a function of retinal image size were then compared to previously reported spot size studies. Results of our measurements show that as the retinal image diameter is increased from 48 to 800 μm, the threshold at 24 h postexposure increases from 1 to 54.1 μJ, corresponding to the fluence at the retina decreasing by a factor of five (from 56 to 11 mJ cm−2). Our results also show that as the retinal spot size increases, the radiant exposure necessary to cause a minimal visible lesion decreases, but not in proportion to the retinal image area. This decreasing radiant exposure for increasing retinal spot sizes at 150 fs follows the trends shown for previous studies with pulse duration from 30 ps to 10 s. Thus, extended sources for 150 fs and 1060 nm show no deviation from the trend of decreasing radiant exposure for increasing retinal image sizes. We conclude from our data that the current correction factors used in the laser safety standards also apply to femtosecond laser exposures between 400 and 1400 nm.