MINER WAS WELDING ON STAINLESS TANK DURING 3-4 HOUR PERIOD. WEARING PROPER HOOD/LENS. SUFFERED FLASH IN EYES, DISCOVERED THAT NIGHT/MORNING. TREATED BY EMERGENCY ROOM, MEDICATED CAUSE APPEARS TO BE POSSIBLE REFLECTION ENTERING SIDEL BOTTOM OF WELDING MAST/HOOD OVER PROLONGED TIME.