JFK CTLR EXPERIENCED CONFLICT WITH AN IFR ACR ARR AND VFR TFC WATCH ACFT BECAUSE OF LATE USE OF VISUAL SEPARATION TECHNIQUES.

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THE B757 WAS INBOUND ON THE VOR/DME RWY 22L APCH. THE C152 WAS SURVEYING TFC AND PASSING SEBOUND THROUGH THE FINAL APCH COURSE OF THE VOR/DME RWY 22L APCH AT 11000 FT MSL. WITH THE B757 ON FINAL AT 1800 FT MSL; WITH THE C152 XING R TO L ABOUT 1 1/2 MI AHEAD; I ISSUED TFC TO THE C152; WHICH DID NOT HAVE THE B757 IN SIGHT. I ISSUED TFC TO THE B757; WHICH HAD THE C152 IN SIGHT; AND INSTRUCTED THE B757 TO MAINTAIN VISUAL SEPARATION WITH THE C152. I ADVISED THE C152 THAT THE B757 HAD HIM IN SIGHT AND WOULD MAINTAIN VISUAL SEPARATION. BY THIS TIME; THOUGH; IT WAS QUESTIONABLE AS TO WHETHER RADAR SEPARATION WAS MAINTAINED PRIOR TO THE APPLICATION OF VISUAL SEPARATION. I HAD COUNTED ON THE C152 SEEING THE LARGER B757 WITHOUT A PROB. THAT DIDN'T HAPPEN; PROBABLY BECAUSE THE C152 WAS LOOKING TO THE E; TOWARD THE RISING SUN. THIS; COMBINED WITH LIMITED VISIBILITY; PREVENTED THE C152 FROM SEEING THE B757. FORTUNATELY; THE B757 SAW THE C152 BUT NOT AS SOON AS I HAD ANTICIPATED. I HAD KEPT OTHER TFC SURVEY ACFT CLR OF THE FINAL EARLIER THAT MORNING; BUT I FELT I HAD ENOUGH TIME TO GET THIS C152 THROUGH FINAL WITHOUT THE B757 BEING A FACTOR. I WAS WRONG. IN THE FUTURE; I WILL WEIGH ADDITIONAL FACTORS SUCH AS REDUCED VISIBILITY (HAZE) AND A RISING/SETTING SUN; BEFORE I COUNT ON A PLT SEEING ANOTHER ACFT.