ACR MLG LOST SIGHT OF TRAFFIC AFTER ACCEPTING TKOF CLRNC WITH RESTRICTION TO MAINTAIN VISUAL SEPARATION. ATC ADVISED AND ACFT GIVEN AN ALT RESTRICTION SO SEPARATION WOULD NOT BE LOST.

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OUR FLIGHT HAD REQUESTED TO DEPART RWY 15 AT DCA TO AVOID A LONG DELAY WITH DEPARTING RWY 18. (WE WERE RELATIVELY LIGHT AND COULD DO THIS ON THIS OCCASION). TWR CLRED US INTO POSITION AND SAID; 'BE READY FOR AN IMMEDIATE DEPARTURE - AS SOON AS ACFT (DEPARTING RWY 18) CLEARS THE INTERSECTION OF RWY 15/18. SURE ENOUGH AS SOON AS THE ACR LGT ROLLED THROUGH THE 15/18 INTERSECTION TWR SAID 'MAINTAIN VISUAL SEPARATION ON THE LGT CLRED FOR TAKEOFF' AT THIS POINT MY ONLY CONCERN WAS THAT OF JET BLAST FROM LGT AS WE WENT THROUGH THE INTERSECTION; BUT I RATIONALIZED THAT WOULDN'T BE A PROBLEM. WE TOOK OFF AND TURNED DOWN THE POTOMAC RIVER AS SPECIFIED IN THE NOISE ABATEMENT PROCEDURE. AT 1500'MSL WE REDUCED THRUST AS PRESCRIBED IN THE NOISE ABATEMENT PROCEDURE AND THEN WE WERE IN THE CLOUDS. THE F/O WAS CONCERNED AND SAID; 'WE'VE LOST THE LGT'. I RESPONDED - AT THIS TIME - RATHER ROUTINELY 'OK; BETTER TELL ATC'. ATC SAID VERY QUICKLY'LEVEL OFF AT 2000.' ABOUT THIS TIME WE BROKE OUT OF THE CLOUDS AND I SAW THE LGT AT 12 O'CLOCK - PERHAPS 2 - 3 MILES IN FRONT OF US. I SUSPECT THAT AIRSPACE SEPARATION WAS VIOLATED. HERE'S WHAT I THINK COULD PREVENT FUTURE OCCURRENCES: PLTS CAN USUALLY PROVIDE THEIR OWN SEPARATION IF ATC WILL INITIALLY PROVIDE THAT SEPARATION. HOWEVER WE WERE TAKING OFF (WITH HIGH DECK ANGLES; HIGH COCKPIT WORKLOAD DUE TO NOISE ABATEMENT) THEN ENCOUNTERED CLOUDS. THIS RESULTED IN A POTENTIAL LOSS OF SEPARATION BECAUSE THE LGT WAS ONLY APPROX. 30-45 SECONDS AHEAD OF US IN POINT IN TIME. IN SUMMARY; IT IS UNREASONABLE AND UNSAFE TO EXPECT DEPARTING JETS TO MAINTAIN VISUAL SEPARATION UNDER VERY CLOSE CONDITIONS THERE HAS TO BE AN ABSOLUTE MINIMUM TO AVOID PROBLEMS. CLOUD BASES WERE 2000 SCT; 6000 BROKEN; 7 MILES.