A B737-700 PLT RPTS THE PREVAILING VISIBILITY AT THE RWY DEP END TO BE ONE-QUARTER MI WITH A 400 FT RVR RPTED. THE TWR CLRED THE CREW FOR TKOF AND THEY DEPARTED.

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THIS RPT IS A REQUEST FOR CLARIFICATION ON WHAT DETERMINES WHETHER RVR OR PV (PREVAILING VISIBILITY) IS CTLING. WHILE WE WERE APCHING #1 AT SMF RWY 16R FOR TKOF; THE TWR RPTED RVR AT 400 FT; 1400 FT; 1200 FT. NOT GOOD ENOUGH FOR US TO DEPART. HOWEVER; UPON ARRIVING AT THE #1 POS; IT APPEARED THAT THE VISIBILITY WAS CLOSE TO 1/2 MI OR BETTER. THE TWR ASKED HOW THE VISIBILITY LOOKED TO US; AND TO BE CONSERVATIVE WE SAID THAT IT LOOKED BETTER THAN 1/4 MI. TWR SAID 'PREVAILING VISIBILITY IS GREATER THAN 1/4 MI; CLRED FOR TKOF.' I DID A QUICK CHK OF THE DEP PAGE AND DETERMINED THAT GREATER THAN 1/4 MI WAS GOOD FOR TKOF. SO WE DEPARTED AND INDEED THE VISIBILITY WAS AT LEAST 1/4 MI THROUGH THE WHOLE TKOF ROLL. MY UNDERSTANDING IS THAT ONCE THE TWR RPTS PV; THAT IT IS NOW CTLING. FURTHER STUDY LEFT ME LESS SURE ABOUT WHEN PV IS CTLING VERSUS RVR. I HAVE ALWAYS TRIED TO OPERATE WELL WITHIN THE REGS; BUT THIS SEEMS A LITTLE GRAY. I WANTED TO BRING THIS TO LIGHT SO I OR ANYONE ELSE MAY BETTER UNDERSTAND WHAT IS CORRECT.