An observer would exceed present industrial safety standards* for the area of the retina used - probably the vital fovea - if he fixated the sun for longer than .84 second, when viewed at the zenith from California. The safe time would increase with more northern latitudes, towards winter and at earlier and later times of day because the rays would pass through the earth's atmosphere on a longer slant path, resulting in greater absorption of energy and, important for our problem, much greater scattering of blue light. We estimate the zenith -3 2 summer sun at 77 X 10 W/cm . Sliney (ref. 12) states that values of -3 2 10 X 10 W/cm , are subjectively still too painful to look at, but when the sun declines to less than 5° from the horizon in clear weather - corresponding to -3 2 below 3 X 10 W/cm , it is comfortable for most people to watch the sunset. However, we should not rely in our analysis on this avoidance response to bright light, since people have repeatedly burned their retinas by gazing at the sun during eclipses. -4 For light sources smaller in solid angle than 1 X 10 steradian, the ACGIH plans to recommend a different formula for calculating maximum permissible exposure for prolonged viewing for the blue light hazard: Where J is Joules (watts/sec.) and En is the irradiance of the source at the D measuring instrument weighted for the blue light hazard action spectrum Notice that these units are independent of the solid angle subtended by the 2 source at the eye. The value of 10 m J/cm is also identical to the maximum permissible exposure for continuous viewing of a point-source, blue laser (13). *It should be noted that since the ACGIH Maximum Permissible Exposure Value is an industrial standard, it provides a safety margin. Thus, we can say that exposures which do not exceed the standard are safe, but we cannot say that ones that exceed the standard will necessarily result in retinal damage.
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