NMAC BETWEEN CORP LTT ON ILS APCH TO BFI AND SMA CLIMBING UNDER THE FLOOR OF THE SEA TCA.

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ON IFR FLT PLAN AND APCH TO BOEING FIELD IN VMC CONDITIONS; WE STARTED DOWN GLIDEPATH TO BOEING FIELD. I LOOKED UP AND SAW AN SMA AT OUR 12 O'CLOCK POS AT 1/8 MI SLIGHTLY HIGHER. HE FIRST TURNED TO HIS LEFT AND THEN UP AND TO HIS RIGHT. WE DOVE DOWN TO OUR RIGHT. THE APCH CTLR THEN CALLED TFC AT 1 MI AND 1700'. HE HAD GIVEN US NO TFC CALLS PRIOR TO THIS. THE OTHER ACFT WAS AN SMA; WHITE WITH BLACK AND BLUE TRIM AND SMALL 'N' NUMBERS; SO COULD NOT READ THEM. THIS NEAR MISS WAS CAUSED BY THE LIGHT ACFT VIOLATING THE TCA AS I SEE IT ON THE TCA CHART; AND POSSIBLE LACK OF CTLR VIGILANCE. WE MAY STOP GOING INTO BOEING FIELD IN ORDER TO AVOID LIGHT ACFT TFC WHICH STAYS AWAY FROM SEATTLE. WE WILL CONTINUE OUR PRACTICE OF KEEPING OUR HEADS OUT OF THE COCKPIT AND SCANNING WHENEVER POSSIBLE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: PLT CHKED CHART AND FEELS SMA WAS PROBABLY LEGAL AND BELOW FLOOR OF SECTOR AT TIME OF INCIDENT. PROBABLY JUST STARTING CLB AFTER LEAVING LOWER FLOOR SECTOR. JUST TOO CLOSE FOR COMFORT. FEELS APCH CTLR VIGILANCE WAS LACKING. CURRENTLY DISCUSSING THIS AND ANOTHER NMAC SITUATION WITH FAA.