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 Amer ican 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 t o 530 nm. These studies report data corresponding to the smallest retinal i mage 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 wa velength for minimal retinal spot sizes. In this article we present measure ments of the retinal lesion threshold as a function of retinal image size f or 150 fs ultrashort laser pulses at 1060 nm. Retinal image size was varied from approximately 48 to 800 mum in diameter using external optics. Thresh olds were determined using probit analysis of the data. The retinal spot si zes were calculated using the Gaussian beam propagation and multiple-lens f ormulas. The thresholds as a function of retinal image size were then compa red to previously reported spot size studies. Results of our measurements s how that as the retinal image diameter is increased from 48 to 800 mum, the threshold at 24 h postexposure increases from 1 to 54.1 muJ, 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 sho wn for previous studies with pulse duration from 30 ps to 10 s. Thus, exten ded sources for 150 fs and 1060 nm show no deviation from the trend of decr easing radiant exposure for increasing retinal image sizes. We conclude fro m our data that the current correction factors used in the laser safety sta ndards also apply to femtosecond laser exposures between 400 and 1400 nm. ( C) 2001 Laser Institute of America.