RPTR BELIEVES HE MADE A TKOF FROM IAH IN WX RPTED BELOW RVR MINS FOR TKOF.

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ON OCT/WED/91 I WAS PIC IN AN MLG ACFT OPERATING UNDER PART 121 AT IAH ARPT. THERE WAS PATCHY FOG AROUND THE ARPT; BUT THE AREAS WE HAD OPERATED IN THE FOG WAS VERY THIN AND VISIBILITY WAS AT LEAST A MI. DURING OUR TAXI FROM RAMP TO RWY 8 ALL OF RWY 14L WAS VISIBLE. THE TWR CLRED US TO HOLD IN POS ON RWY 8. I THEN ASKED THE FO FOR THE 'BEFORE TKOF CHKLIST'. THE CTLR THEN CLRED US FOR TKOF. AT SOME POINT DURING THIS TIME THE CTLR GAVE THE RVR AS 3500/1700/400 BUT BECAUSE THE VISIBILITY WAS SO GOOD; WE COULD SEE THE VERY END OF RWY 8; AND BECAUSE I WAS DISTRACTED BY THE BEFORE TKOF CHKLIST; I THOUGHT THE RVR RPT WAS FOR RWY 9 (AND FOR ANOTHER ACFT). BECAUSE I COULD CLRLY SEE THE END OF THE RWY; THE RVR RPT MADE NO SENSE FOR RWY 8; FURTHERING MY BELIEF THE RVR WAS FOR ANOTHER RWY. DURING TKOF ROLL VISUAL CONTACT WAS MAINTAINED WITH THE END OF THE RWY AND BEYOND. JUST BEFORE BEING HANDED OFF TO DEP CTL; I HEARD THE TWR GIVE THE RVR FOR RWY 8 TO ANOTHER ACFT AS 3500/1700/400. AT THAT POINT I REALIZED WE HAD TAKEN OFF WITH THE ROLL OUT RVR BEING RPTED BELOW 600. THE TKOF WAS OTHERWISE NORMAL AND NO PROBLEMS WERE ENCOUNTERED DURING THE FLT.