Ex-plant retinal laser induced threshold studies in the millisecond time regime

Excised bovine retinas were used as model for threshold determination of laser induced thermal damage in the pulse regime of 1 ms to 655 ms for a range of laser spot size diameters. The thresholds as determined by fluorescence viability staining compare very well with the prediction of thermal damage models. Both models compare well with published and new Rhesus monkey threshold data. A distinctive dependence of the threshold on laser spot size diameter for different pulse duration was found which indicates that current (ICNIRP, ANSI and IEC) laser exposure limits for large spots can be increased in this pulse duration regime. A time dependent αmax is proposed which only for the case of long exposure durations has the current value of 100 mrad, but decreases to smaller angles for short exposure durations, effectively increasing the permissible exposure level. An explanation based on intra-retinal scattering is offered for the unexpected spot size dependence for spot diameters less than about 80 µm. The time dependence and nature of damage is discussed for pulse durations shorter than 1 ms where bubble induced damage seems to lead to a threshold a factor of 10 lower than the thermally induced threshold, resulting in the need to lower the MPE values for this condition. Possible changes of the MPE values are offered and discussed.