The Fresnel Lens on exhibit at the Trinidad Museum is a 375 mm. Fifth Order Fresnel drum lens, so named for the French physicist Augustin-Jean Fresnel (1788-1827), who originally designed this model of lens. It is on permanent loan to the Trinidad Museum Society from the U.S. Coast Guard, and has been since it was replaced in 2013 with a new LED light. Before this, it had served as the operating lens in the lighthouse since its installation in 1947. It replaced the 1871 Fourth Order Fresnel lens, which used to house a coal-oil lamp. The fog bell was also replaced in 1947, with a pair of compressed air horns. The old bell was then installed on the memorial site where the Memorial Lighthouse originally stood. The 1898 two-ton bronze Fog Bell is now located temporarily at the Memorial Lighthouse at the Trinidad Rancheria Harbor area. A project to permanently install both the Memorial Lighthouse housing the old Fourth Order lens and the Bell at the harbor is underway.
The electric drum lens was originally taken from an old acetylene buoy lantern, and it measures 14 and 3/4 inches across the interior. Its 120 VAC lamps were Mogul bi-post, using 1000 watts, which is equal to some modern microwaves, and 8.8 amps. These lamps had 3000 hour life-expectancy, and cost around $50.00 each. Inside the Fresnel lens, they had a lumen output of 17,200, which was visible at 14 nautical miles (16.11 miles or 25.93 km). In the lighthouse, it sat 196 feet above sea level.
The former light, the coal oil lamp, was replaced due to its high cost and maintenance demands. The old coal-oil light would give constant illumination: 60 seconds of white followed by 5 seconds of red. To accomplish this, it rotated on ball bearings which had to be painstakingly and carefully balanced. They were cleaned every week by the lighthouse keeper with kerosene, and then lubricated with clock oil and re-balanced. If they were the slightest bit off, the lamp would not rotate properly. When it was changed out, responsibility for the upkeep of the lighthouse was given over to the U.S. Coast Guard, thus ending a long and fascinating legacy of lighthouse keepers in Trinidad.
Patrick Duerr, TMS Intern, 2018
An exhibit at the Museum in 2014 featured the Trinidad Light House. Visit that exhibit on the Past Exhibits page: Trinidad Lighthouse 1871 – Present