DHC8 FLC TOOK OFF BELOW FAA APPROVED WX MINIMUM FOR THEIR COMPANY.

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ATCT WAS NOT YET OPEN SO WE MONITORED AWOS WHICH WAS RPTING 1/4 SM VISIBILITY. WE BRIEFED FOR A REDUCED VISIBILITY TKOF PER STANDARD COMPANY PROCS. WE THEN STARTED AND TAXIED TO RWY 24 FOR DEP. REDUCED VISIBILITY TAXI PROCS WERE FOLLOWED AS OUTLINED IN OUR COMPANY MANUAL. AFTER REACHING THE RUN-UP AREA WE COMPLETED ALL REQUIRED CHKLIST ITEMS. AT THIS TIME THE TWR OPENED AND RPTED TWR VISIBILITY AS 1/16 SM IN FOG. WE LOOKED DOWN THE RWY AND; BASED ON RWY LIGHTING AND MARKING; DETERMINED WE HAD AT LEAST 1600 FT AND 1/4 SM VISIBILITY. WE RECEIVED OUR CLRNC TO PHOENIX AND NOTIFIED THE TWR WE WERE READY FOR TKOF. TWR CLRED US FOR TKOF AND THEN PROVIDED AN RVR RPT. WE WERE ON THE RWY; AGAIN VERIFIED WE HAD AT LEAST 1/4 SM RVV; AND DEPARTED. SUBSEQUENTLY IT WAS DETERMINED THAT THE RPTED RVR MAY HAVE BEEN BELOW ALLOWABLE TKOF MINIMUMS OF 1600 FT RVR. NEITHER MY FO NOR MYSELF CAN RECALL WITH CERTAINTY WHAT THE RPTED RVR VALUE WAS AT DEP. IF IT IN FACT DOES TURN OUT THAT WE INADVERTENTLY DEPARTED BELOW MINIMUMS; IT IS MY BELIEF THAT WE DID SO AS A RESULT OF DISREGARDING TWR VISIBILITY. TWR VISIBILITY IS NON CTLING WHEREAS RVR IS. WE HAD HEARD ASOS AND TWR RPTED VISIBILITY AND WE FAILED TO CATCH THE SIGNIFICANCE OF A SUDDEN AND UNEXPECTED RVR RPT. WE MAY HAVE BEEN SO OCCUPIED WITH VERIFYING THAT WE HAD 1/4 RVV THAT WE SUBCONSCIOUSLY DISCOUNTED THE LAST MIN RPT AS NON CTLING AS WELL. AVOIDANCE OF THIS SAME SIT REQUIRES CLOSE ATTN TO ALL INFO SO FIXATION DOES NOT BECOME A FACTOR.