 Greens Ledge Lighthouse is a <unk> lighthouse in Connecticut , United States , off the southwest end of the Norwalk Islands , Long Island Sound , near Norwalk , Connecticut . It is on north side of the west end of Greens Ledge , west of Norwalk Harbor a mile south of the entrance to Five Mile River at <unk> , and just over a mile southwest of Sheffield Lighthouse . Completed in 1902 , it was constructed by the Philadelphia Construction Company . The light is 52 feet ( 16 m ) tall and is made of five courses that make up its four stories . The lantern measures 7 feet ( 2 @.@ 1 m ) in diameter . The Greens Ledge Light replaced the Sheffield Island Light . Originally , the light had a fifth @-@ order Fresnel lens , but a fourth @-@ order Fresnel lens was installed in May 1902 , just three months into its operation . Currently a <unk> @-@ 25 is in use and it has alternating white and red flash every 24 seconds . The light was added to the National Register of Historic Places as Greens Ledge Lighthouse on May 29 , 1990 . 
