LESS THAN LEGAL SEPARATION BTWN 2 ACR FLTS ON SUCCESSIVE TKOFS FROM SAME RWY DURING A NIGHT OP AT DFW; TX.

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CLBOUT RWY 35L; TKOF. FAA INSPECTOR WAS ON BOARD. HIS CREDENTIALS WERE CHKED. ISSUED TKOF CLRNC WHILE TAXIING TO #1 POS ON TXWY. OTHER ACFT DEPARTED BEFORE US ON SAME RWY. TURNED ONTO RWY AND STARTED TO APPLY TKOF PWR; BUT COULD NOT SEE PREVIOUS DEP. THEN NOTED PREVIOUS ACFT STILL ON RWY; AT ROTATION; AND CONTINUED OUR TKOF. AT APPROX 800 FT; HAD NOT CALLED FOR HALF RATE YET. TWR ISSUED; 'DO YOU SEE PREVIOUS ACFT IN FRONT OF YOU.' WE RESPONDED 'NEGATIVE.' TWR CAME BACK WITH ADDITIONAL CALLS FOR US TO ACCEPT VISUAL CLRNC FROM PREVIOUS ACFT. I REFUSED STATING; 'UNABLE VISUAL.' CTLR SOUNDED UPSET THAT WE WOULD NOT ACCEPT VISUAL. FLT CONTINUED UNEVENTFULLY TO LNDG AT ICT. FLT WAS AT NIGHT. OUR TKOF CLRNC WAS ISSUED WHILE THE PREVIOUS ACFT WAS STILL ON THE RWY. IT IS EXTREMELY DIFFICULT TO SEE ANOTHER ACFT; AT NIGHT; AT THE FAR END OF THE RWY ON A TKOF ROLL. THE WINGTIP LIGHTS BLEND WITH THE WHITE RWY EDGE LIGHTS. AFTER TKOF; WE HAD NO PRE-NOTIFICATION OF THE COMING VISUAL REQUEST. IS THIS A LEGAL REQUEST BY THE CTLR? THIS DISTR OCCURRED DURING A CRITICAL PHASE OF FLT. SHOULD WE ACCEPT THIS; ESPECIALLY AT NIGHT? THE DISTR CAUSED THE FO TO MISS THE HALF RATE AND I SELECTED IT MYSELF; TELLING HIM TO GET BACK INTO THE COCKPIT. WHAT ARE THE LIMITS OF THIS TYPE OF VISUAL CLRNC? WHERE DOES COCKPIT RESPONSIBILITY FOR SEPARATION END?