Retinal image motion during deliberate fixation: implications to laser safety for long duration viewing

With the easy attainability of hand-held laser devices and the burgeoning light emitting diode (LED) technology, safety standards for long-term viewing of continuous light sources are being scrutinized. One concern is with quantifying the effect of head and eye movements on the distribution of energy over the retina. This experiment describes target motion over the retina as a result of head and eye movements during a deliberate fixation task. Volunteers deliberately fixated, with (fettered) and without (unfettered) head and chin rest support, on LEDs that subtended a 0.1 min of arc visual angle. A Dual Purkinje Image Eyetracker measured eye position during each 100-s fixation trial. The data showed an elliptical retinal energy distribution, oriented on the temporal/ nasal retinal axis, with a major axis 1.5 times greater than the minor. The average half-maximum diameter for the major axis was 40 microm for the fettered and 107 microm for the unfettered condition. The retinal area illuminated from head and eye movements showed a positive linear relationship with time. Peak retinal radiant exposure at 100 s was reduced by a factor of 10 as a result of eye movements and by a factor of 30 as a result of head and eye movements. Although the net result was a reduction in radiant exposure from a "no-movement" baseline, the distribution of energy over the retina was well within the 200 microm foveal boundary. The data suggest that the laser permissible exposure limits for long term viewing of a small continuous wave source include a factor that adjusts for this reduction in retinal radiant exposure with time.