AN SF34 CREW; DEPARTING GRR; ACCEPTED A TKOF CLRNC WITH THE RVR RPTED BELOW THEIR LEGAL LIMITS.

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OUR CREW WAITED FOR THE RVR TO REACH 1600 FT TO DEPART. WE RECEIVED A GOOD RVR RPT 1800 FT AND WERE INSTRUCTED TO TAXI INTO POS AND HOLD. (A CARJ HAD JUST DEPARTED IN FRONT OF US.) AS WE TAXIED UP TO THE APCH END OF THE RWY; TWR ADVISED US THEY HAD BAD NEWS -- RVR HAD JUST DROPPED TO 1000 FT. ACCORDING TO OUR OPS SPECS; WE CAN DEPART WITH 600 FT RVR WITH APPROPRIATE MARKINGS; IE; CTRLINE LIGHTS. WE THOUGHT WE HAD THEM AND ACCEPTED THE DEP. DURING THE TKOF ROLL; I REALIZED WE; IN FACT; WERE LACKING THE REQUIRED CTRLINE LIGHTS; BUT WE COULD SEE THE END OF THE RWY (5000+ RVR). I CALLED THE TWR CTLR LATER AND HE CONFIRMED THAT THE 1000 FT RVR RPT WASN'T ACCURATE; IN FACT; WE HAD 1800+ FT RVR. IN ESSENCE; ALTHOUGH WE WERE LEGAL TO DEPART; WE ACCEPTED AN ILLEGAL TKOF CLRNC BASED ON ASSUMED/RPTED RVR DUE TO FLAWED THINKING. OUR MISTAKE WAS ONE OF INTENT; NOT OF ACTUALITY. IN SUMMARY; ALTHOUGH TECHNICALLY; NO FARS WERE BROKEN; WE NEEDED TO BE MORE FAMILIAR WITH REQUIRED MARKINGS/LIGHTS FOR LOW VISIBILITY DEPS. MAJOR CONTRIBUTING FACTOR: FATIGUE. WE HAD COMPLETED 2 PREVIOUS CONTINUOUS DUTY CYCLES.