A B737-500 SUPER PLT DECIDED TO DSND BELOW DECISION HT TO A LNDG EVEN THOUGH HE WAS ON HIGH MINIMUMS.

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THE RVR AT SFO HAD BEEN HANGING AROUND 700 FT. IT WAS; HOWEVER; VARYING. WE HELD AT PYE FOR APPROX 1/2 HR. IT THEN BECAME TIME TO DIVERT TO SJC. WHILE DIVERTING TO SJC; THE CAPT SAID THAT IF THE RVR COMES UP AT SFO; WE WOULD MAKE AN APCH. THE RVR CAME UP TO 1000 FT AND THEN 1200 FT. WE WERE LIMITED TO CAT II BECAUSE THE CAPT WAS STILL ON 'HIGH MINIMUMS.' THE AIRPLANE WAS EQUIPPED FOR CAT III AND I AM CERTIFIED FOR CAT III; WHILE ACTING AS SIC. WE WERE IMMEDIATELY VECTORED FOR AND CLRED FOR THE ILS RWY 28R AT SFO. WE CONTACTED SFO IN THE VICINITY OF BRIJJ. AT 7.1 DME THE RVR WAS RPTED AT 700 FT. WE WERE 0.3 MI INSIDE OF BRIJJ. WE CONTINUED THE APCH. AT APPROX 1500 FT THE RVR WAS AGAIN RPTED AT 700 FT. I CALLED 'APCHING MINIMUMS' AND THEN 'MINIMUMS.' THE CAPT DID NOTHING. MINIMUMS WERE 111 FT RADAR ALT. AS WE APCHED 70 FT RADAR ALT; I BEGAN CALLING 'BELOW MINIMUMS.' THE CAPT INTERRUPTED WITH 'APCH LIGHTS.' I LOOKED UP AT APPROX 50 FT RADAR ALT AND ONLY SAW 1 APCH LIGHT. IN THE EVENT THE CAPT WOULD NOT GAR; I MONITORED THE INSTS FOR A SUCCESSFUL AUTO-LAND. I THOUGHT THIS WOULD BE THE SAFEST COURSE OF ACTION (AS OPPOSED TO PRESSING FOR A GAR). TO MY BEST ESTIMATE THE RVR WAS AT BEST 700 FT. THE CAPT DSNDED 40 FT BELOW DECISION HT BEFORE MAKING ANY COMMENT. I COMMENTED ON THIS LATER THE CAPT SAID HE HAD THE LIGHTS AND DID NOT THINK HE COULD JUDGE THE RVR FAST ENOUGH. I SHOULD HAVE CALLED FOR A GAR EARLIER.