Lens
The station includes a first-order Fresnel lens that has a focal plane 52.5 feet (16 m) above ground (258 feet (79 m) above sea level). The tower originally contained a white light until 1929, when it was replaced with a green light with a range of 22 miles (35 km). This light was 12 feet (4 m) high by 6 feet (2 m) wide and stayed in service until 1990, when it was replaced by the United States Coast Guard over environmental concerns stemming from its use of mercury. In 1994 the Coast Guard agreed to relight the lighthouse and relocated the lens in the Cape Lookout Lighthouse in North Carolina to the Southeast Lighthouse. The lens is flashed every five seconds.
Read more about this topic: Block Island Southeast Light