CABRA ISLAND LIGHTHOUSE - OCCIDENTAL MINDORO

The Cabra lighthouses are located about 75 km from Manila. The original lighthouse was built by the Spaniards in 1889, to mark the passage from the South China Sea, to the Verde passage through the center of the Philippines archipelago.

The lantern room pictured on the left is mounted on an approximately 20 m cylindrical tower originally painted white. The white light occulted about every 5 seconds.

Above the lantern is a dome made of metal and the interior and exterior of it is clad with copper. Above the dome is a ball vent through which air can pass. Finally there is a wind vane topped by a lightening rod

In the interior of the dome, beneath the top, can be seen a large concave dish surrounding the event tube. This concave dish caught any condensation that formed in the top of the lantern. Beneath the lens was a service room that contained clockwork mechanisms for rotating the optics.

Reference: ANATOMY OF A LIGHTHOUSE(http://www.seathelights.com/other/anatomy.html)

To get to Cabra Island from Lubang Island, there is a public boat from Tagbac that leaves daily at 1p.m. and returns the next day.

Most people hire a boat - for four people this costs about P1500 for the round trip from Tagbac.

For information on ferries from Batangas to Lubang see my page:
www.delahyde.com/lubang/pagesl/ferries.html

FOOTNOTE: FRESNEL LENSES

"A Fresnel lens is a compact, lightweight redesign of a conventional lens - an invention attributed to French physicist Augustin-Jean Fresnel."

"Where a conventional lens may be thin in places and thick in others, the Fresnel lens has rectangular chunks of "zone" - rings - of the thick parts of the lens, removed to give a flatter (and so lighter) lens performing the same function. A small redesign of the curvature of each chunk may be necessary to compensate for the thinner material."

"... Lighthouse lenses are usually of Fresnel design, and made in many small prismatic sections - to save weight and make construction simpler."

Reference: http://camera-wiki.org/wiki/Fresnel_lens

"The original light (Cabra Island Lighthouse), visible for 25 nautical miles (29 mi; 46 km), was shown from a 67-foot (20 m) high square tower on the west angle of the station. It is visible around the entire horizon except where obscured by Lubang and Ambil Islands."

Reference: http://en.wikipedia.org/wiki/Cabra_Island_Lighthouse

The lens pictured on the left is a first order Fresnel Lens - the same type as at Cabra Lighthouse. This has a focal length of 920mm (36.22 inches) and a height of 2.59 meters (8.5 feet).

There is a Fresnel lens on 8 sides of the lamp, with prisms (curved chunks of glass) mounted above and below the lens to pull in light rays that deviate further from the central axis.

"The lens rotates and has a number of bull’s-eye lens panels that create beams of concentrated light (an eight-panel lens produces eight beams). As the lens rotates, the beams successively pass the view of the mariner giving what appears as a flash of light followed by darkness."

"Beneath the flashing Fresnel lens are several unusual mechanisms used to rotate and control the lens. The lens will be found to be supported on wheels, ball bearings, or on mercury contained in a large metal tub."

Reference: http://uslhs.org/lens-rotation)