MD80 CAPT IS UNSURE OF OP SPEC REQUIREMENTS FOR LOW VISIBILITY TKOF.

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I TOOK OFF RWY 31 WITH 800 FT RVR TOUCHDOWN; 700 FT RVR ROLLOUT BELIEVING I WAS BOTH SAFE AND LEGAL TO DO SO. UPON FURTHER REVIEW I AM UNSURE OF THE LEGALITY; AND AM FURTHER CONFUSED WITH COMPANY OPS SPEC. I UNDERSTAND THE OPS SPEC TO STATE: 1) IF TAKING OFF WITH STANDARD OR LESS THAN STANDARD MINIMUMS; USE THIS OPS SPEC AS SUPPOSED TO ARPT DIAGRAM. 2) LOW RVR TKOF REQUIRES 2 OPERATING RVR SYS WHICH ARE CTLING FOR VISIBILITY. 3) YOU MUST BE ABLE TO DETERMINE THE CTRLINE OF THE RWY EITHER WITH CTRLINE LIGHTING OR WITH CTRLINE MARKINGS (I HAD CTRLINE MARKINGS). UPON FURTHER REVIEW OF THE OPS SPEC (AFTER TKOF) I AM UNSURE IF I NEEDED CTRLINE LIGHTING; AND IF I NEEDED ARPT DIAGRAM TO SAY THIS WAS APPROVED. FACTORS: VISIBILITY HAD DROPPED FROM 1/2 TO 1/4 MI ON ATIS. 10 MINS PRIOR TO DEP AND GND CTL RPTED RVR 1200 FT. DISPATCH WAS CONTACTED WITH THIS INFO; AND A TKOF ALTERNATE WAS DETERMINED. CLRED TO TAXI TO RWY 31 WITH RVR RPTED PRIOR TO TKOF AS STATED ABOVE. BELIEVING ADEQUATE LEGALITY AND SAFETY; AN UNEVENTFUL TKOF WAS INITIATED WITH THE FO AS PF. AFTER AIRBORNE; I DECIDED TO CONFIRM MY DECISION TO TAKE OFF WITH REDUCED VISIBILITY AND BECAME UNSURE OF THE LEGALITY.