VISUAL TFC SEPARATION AT NIGHT IS DIFFICULT.

Narrative

WE WERE TRYING TO DSND INTO TAMPA; AFTER HAVING TO DEVIATE FROM NORMAL ARR TO AVOID TSTM ACTIVITY. APCH COULD NOT LET US DSND OUT OF 15000 FT BECAUSE OF AN ACFT LEVEL AT 14000 FT AT OUR 10 O'CLOCK POS; WHICH ATC HAD ADVISED US OF. I CALLED THE ACFT IN SIGHT AND I THOUGHT THE OTHER PLANE'S TRACK PARALLELED OURS. ATC ASKED IF WE COULD MAINTAIN VISUAL SEPARATION AND DSND TO 13000 FT. THE ACFT'S LIGHTS (POS ONLY) LOOKED AS IF WE WERE NOT GOING TO INTERCEPT FLT PATHS; SO I ADVISED ATC THAT WE COULD MAINTAIN VISUAL SEPARATION AND DSND. THE CLRNC TO 13000 FT WAS GIVEN AND WE STARTED A SLOW DSCNT TO 13000 FT. AS WE WENT THROUGH 14300 FT THE FMS SHIFTED TO LEVEL CHANGE WHEN THE FO COMMANDED A GREATER RATE OF DSCNT. THE AUTOPLT WAS ONLY GIVING US AN 800-1000 FPM DSCNT RATE AS THE FMS TRANSITIONED TO LEVEL CHANGE MODE. IT WAS BECOMING APPARENT THAT OUR COURSE AND THE OTHER ACFT'S COURSE WOULD INTERCEPT SO WE WANTED TO HURRY ON THROUGH HIS ALT. THE FO SELECTED FMS TO VERT SPD AGAIN TO HASTEN OUR DSCNT; BUT THE AUTOPLT RESPONDED SLOWER THAN ANTICIPATED TO THE FMS COMMANDED 3000 FPM DSCNT RATE COMMAND. THE FO DISENGAGED THE AUTOPLT AND MANUALLY FLEW OUR ACFT TO 13000 FT. THE OTHER ACFT PASSED BEHIND US WELL CLR. ALTHOUGH WE MAINTAINED VISUAL CONTACT WITH THE OTHER ACFT; I WAS SURPRISED THAT OUR PATHS WOULD CROSS. MY CONCERN IS THAT MAINTAINING VISUAL SEPARATION AT NIGHT IS DIFFICULT DUE TO RELIANCE ON AN ACFT'S POS LIGHTS AND ANTI-COLLISION LIGHT MAKES RECOGNITION OF THE OTHER ACFT'S SPD AND DISTANCE AND TRACK HARD TO JUDGE. I WOULD RECOMMEND THAT CTLRS NOT BE ALLOWED TO ISSUE 'MAINTAIN VISUAL SEPARATION' CLRNCS AT NIGHT OR AT LEAST NOT WHEN THE ACFT IN QUESTION HAVE GND TRACKS WHICH MIGHT INTERCEPT.