ACR CREW DEPARTS BFI BELOW MINIMUMS AFTER JUMP SEAT CREW MEMBER CONVINCES CAPT THAT VISIBILITY IS LEGAL.

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WE PUSHED AND TAXIED FOR A RWY 13R DEP. AS WE TAXIED; THE WX WAS DROPPING. WHEN WE ARRIVED AT THE DEP END OF RWY 13R; THE VISIBILITY HAD DROPPED TO 1/4 MI RVR RWY 13R 1200 FT. WHILE WE WERE HOLDING SHORT; TWR ASKED WHAT WE NEEDED TO DEPART. I RESPONDED 1600 FT RVR. WE HAD A DEADHEADING CREW ON BOARD; THE DEADHEADING CAPT WAS IN THE COCKPIT JUMP SEAT; HE PULLED OUT THE OPS SPECS AND STATED THAT WE COULD DEPART WITH THE CURRENT WX. I DISAGREED AND STATED THAT WITHOUT (CTRLINE) WE REQUIRED 1600 FT RVR. AT FIRST; THE PIC AGREED WITH ME; UNTIL THE DEADHEADING CAPT SPOKE UP. THE PIC TOLD ME TO TELL TWR THAT WE COULD DEPART WITH 1200 FT RVR. I DISAGREED; BUT WE DEPARTED ANYWAY. THE TKOF AND THE TRIP WERE UNEVENTFUL. AT THE COMPLETION OF THE FLT; WE REVIEWED THE OPS SPECS AND CONFERRED WITH THE OTHER PLTS AND CAME TO THE CONCLUSION THAT I WAS CORRECT. CTRLINE LIGHTING IS REQUIRED TO DEPART WITH AN RVR OF 1200 FT. ALSO THAT DEADHEADING CAPT SHOULD BE SEEN AND NOT HEARD IF THEY ARE GOING TO PROVIDE WRONG INFO. I DO NOT BELIEVE THAT MY CAPT WOULD HAVE DEPARTED IF THE JUMP SEATER WOULD NOT HAVE OPENED HIS MOUTH.