Eye movements during fixation: implications to long-term viewing of small optical sources

With the easy attainability of hand-held laser devices and burgeoning Light Emitting Diode (LED) technology, safety standards for long-term viewing of continuous light sources are being scrutinized. One concern is with formalizing the effect of head and eye movements on smearing energy from a small optical source 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 fixated, with (fettered) and without (unfettered) head and chin rest support, on an LED and laser source that subtended 0.1 minutes of arc visual angle. A dual Purkinje Eye-Tracker measured eye position during each 100-second fixation trial. The data showed a non-uniform retinal energy distribution with an elliptical footprint. The major axis was 2 times greater than the minor axis and oriented along the temporal/nasal retinal axis. The average half-maximum diameter measured along the major axis was 42 microns for the fettered and 108 microns for the unfettered condition. Although the eye is not a stable platform, the `smear' of energy over the retina from head and eye movements does not grossly affect spot size. The data suggest that the time dependent spot size correction of the current laser safety standards be more restrictive.