Technology (The Fresnel Lens)
When the present Fire Island Light began operation on November 1, 1858, it was illuminated by a state-of-the-art first order Fresnel (pronounced fray-nel') lens, an intricate beehive-shaped mass of brass and glass prisms, rotating on a clockwork assembly. This lighting apparatus served as a beacon to mariners from the top of the Fire Island Lighthouse from 1858 to 1933.
The current Fire Island Lighthouse, completed in 1858 with a state-of-the-art revolving first order Fresnel lens and hydraulic lamp using whale oil, had a focal plane of 166 feet above sea level, so that this light could be seen from ships at least 21 nautical miles at sea. This was a part of a national initiative to improve maritime safety. It is significant today as a symbol of the commercial and maritime heritage of the United States, and the evolution of communication and navigational technologies. The Fresnel lens helps tell that story.
This original first order Fresnel lens, an intricate beehive-shaped mass of brass and glass prisms, served as a beacon to mariners from the top of the Fire Island Lighthouse from 1858 to 1933. After being taken out of service, it was displayed at the Franklin Institute in Philadelphia from 1939 to 2000.
Source : nps.gov
The current Fire Island Lighthouse, completed in 1858 with a state-of-the-art revolving first order Fresnel lens and hydraulic lamp using whale oil, had a focal plane of 166 feet above sea level, so that this light could be seen from ships at least 21 nautical miles at sea. This was a part of a national initiative to improve maritime safety. It is significant today as a symbol of the commercial and maritime heritage of the United States, and the evolution of communication and navigational technologies. The Fresnel lens helps tell that story.
This original first order Fresnel lens, an intricate beehive-shaped mass of brass and glass prisms, served as a beacon to mariners from the top of the Fire Island Lighthouse from 1858 to 1933. After being taken out of service, it was displayed at the Franklin Institute in Philadelphia from 1939 to 2000.
Source : nps.gov
Fresnel lens More Info
In 1822, Augustine Fresnel invented the Fresnel lens, also known as the diopter lens, which is a large glass beehive-looking object with a light at the center. The actual lens consists of a central glass disk and concentric rings, which are decreasing in thickness from the center so as to bend the light into a narrow beam. The central glass disk is shaped like a magnifying glass so that the intense beam is even more powerful. In order to retain the rings’ ability to focus the beam toward the central disk, the angle of each ring is different. A flashing Fresnel lens like the one from the Fire Island lighthouse, produces a flash each time a rotating lens aligns with an observer’s eye. The Fresnel Lens used in the Fire Island lighthouse was a first-order lens, which was the most powerful in its category at the time, being about 16 feet tall and having a diameter of over six feet. Unlike the bonfires built before, which usually lose about 97% of its light, the Fresnel lens only loses about 17% of its light. This is because an open flame, when it passes from an optically less dense medium such as air, and goes into an optically denser medium like water, light interacts with the water molecules and suspended particles to cause the loss of light. However the Fresnel lens, by being able to collect the light that would potentially be lost by being emitted in other directions while decreasing the divergence of the resulting beam, it was able to increase the distance over which a lighthouse’s beam could be seen to the line of sight.
It could convey its light to 20 or more miles to the horizon. In addition, taller lighthouses were now built to further convey light across the sea and overcome the curvature of the earth.
The Fresnel lens, which was put into the lighthouse, originally emitted a white flash at one-minute intervals. However, in 1933, the lens was moved to Philadelphia in anticipation of electricity powering the lighthouse. Eventually, the light was converted to electrical power, now using a high power spotlight called DCB-224 Aero-Beacon. The tower is now lit by two 1000-watt bulbs rotating in a counter-clockwise direction, gives a visibility of 21-24 miles across the sea and has a flash every 7.5 seconds.
It could convey its light to 20 or more miles to the horizon. In addition, taller lighthouses were now built to further convey light across the sea and overcome the curvature of the earth.
The Fresnel lens, which was put into the lighthouse, originally emitted a white flash at one-minute intervals. However, in 1933, the lens was moved to Philadelphia in anticipation of electricity powering the lighthouse. Eventually, the light was converted to electrical power, now using a high power spotlight called DCB-224 Aero-Beacon. The tower is now lit by two 1000-watt bulbs rotating in a counter-clockwise direction, gives a visibility of 21-24 miles across the sea and has a flash every 7.5 seconds.