FLC CONCERN OVER VISUAL SEPARATION AT NIGHT.

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WE WERE TAXIED INTO POS AND HOLD AFTER OUR COMPANY ACFT BEGAN ITS TKOF ROLL. CTLR ASKED IF WE COULD MAINTAIN VISUAL SEPARATION WITH DEPARTING COMPANY ACFT. WE HAD HIM IN SIGHT AND WX WAS EXCELLENT. WE CONFIRMED WE COULD. WE WERE CLRED FOR TKOF. ON RWY 18R WE WERE TO MAINTAIN RWY HDG TO 1.6 DME THEN R TURN TO 200 DEGS. AFTER TKOF AT APPROX 800 FT AGL; TWR HANDED US OFF TO DEP. DEP ASKED IF WE HAD ACFT IN SIGHT AND WE ADVISED WE DID AT ABOUT 11:30 POS AS WE STARTED OUR TURN TO 200 DEGS. DEP THEN ADVISED THEY WERE TURNING ACFT AHEAD OF US TO A HDG OF 290 DEGS. THAT PUT THE ACFT ALMOST 90 DEGS FROM OUR FLT PATH AND CLOSING. I'M NOT SURE HOW FAR AWAY HE WAS AND AT NIGHT ALTHOUGH HE WAS IN PLAIN VIEW AT ALL TIMES IT IS TOUGH TO GAUGE DISTANCE. A TFC CONFLICT WAS NOT A CONCERN; HOWEVER; MAINTAINING REQUIRED SEPARATION WAS. NO CONCERN WAS VOICED BY THE CTLR BUT BOTH THE CAPT AND I FELT IT WAS UNWISE IN THE FUTURE TO ACCEPT VISUAL SEPARATION AT NIGHT; EVEN IN CLR CONDITIONS; DUE TO INABILITY TO JUDGE DISTANCES. I FEEL THE CTLR'S ACTIONS COULD ALSO HAVE BEEN HANDLED BETTER. WE WERE BOTH BEING GIVEN W HDGS AFTER TKOF AND IF HE HAD TURNED US TO A W HDG PRIOR TO THE ACFT IN FRONT OF US; NO CONFLICT WOULD HAVE BEEN POSSIBLE. MY POLICY FROM NOW ON IS NOT TO ACCEPT VISUAL SEPARATION ON NIGHT TKOFS AS IT IS TOO DIFFICULT TO JUDGE DISTANCES AND NO ASSURANCE OF ACFT SPD OR DIRECTION ON THE TFC YOU ARE FOLLOWING.