Changing The Light Bulb at Point Loma

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Postby BMR » Thu Feb 07, 2013 4:15 am

From The Union-Tribune
http://www.utsandiego.com/news/2013/feb ... house-led/

(Photo/John Gibbons)
There are two pips in a beaut, four beauts in a lulu,
eight lulus in a doozy, and sixteen doozies in a humdinger.
No one knows how many humdingers there are in a lollapalooza.
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Postby tinypiney » Fri Feb 08, 2013 10:11 pm

Once I was asked if lighthouses just had a giant lightbulb at the top of them. He proceeded to ask that when they need to change it, how do they get it out of the lighthouse? I told him it's not quite that simple... :lol:
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"Lighthouses are more useful than churches."
-Benjamin Franklin
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Postby island » Sat Feb 09, 2013 10:52 am

Interesting article but the data presented does not compute.

This lighthouse has a focal plane 88 feet above sea level. The normial range ( when viewed from 15 feet above sea level) is 15 nautical miles. Norminal range or geographic range is the standard for describing the range of visibility of a light.

To be seen at a distance of 24 nautical miles the atmosphere would have to be completely free of dust and moisture and the light refracting over the horizon. This is the luminous range that is based on a metorological visibility of 10 nautical miles. If the existing visibility is over 10nm in clear air then the visible range exceeds the norminal range or if less than 10nm such as in haze or fog.then the visible range would be substantially less than the norminal range.

Because the meterological visibility is highly variable luminous visibility is not a standard and should be such for describing range of visibility.

In the case of Point Loma with this new LED lamp just exactly what is this new 14nm visible range? And it was stated this new lamp is brighter than the former so why is the range reduced by 10 nm?

And another item
The new setup is smarter, as well. Thanks to a sensor, the beam switches on at dusk and blinks out at dawn. The old one ran around the clock.
So how much of the savings comes from reduced time of illumination resulting from this ambient light sensor compared to that from the LED lamp.?

The Coast Guard says that’s still plenty of warning for mariners ( this reduced range). Not so. Not if "a boater swamped in fog" with this light of less intensity.
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Postby Fred » Sun Feb 10, 2013 6:57 am

From the 1973 Reeds Nautical Almanac

Range of Lights

which is computed from the height of the object,the height of the observer and the curveture of the earth

which is computed from the intensity or candlepower under a standard set or meteorological conditions

is the luninous range when the meterological visibilty is 10 sea miles.

Maybe the reporter is mixing definitions thus the difference in range?

Luminous range appears to be one of thoses modern definitions that has no connection with reality?
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Postby island » Sun Feb 10, 2013 12:23 pm

The norminal range is the straight line distance that the light can be seen and without regard to the geographic range. This norminal range is a calculated number based on the effective intensity of the specific lamp. Thus the norminal can exceed the geographic range or may be less than geographical depending on lamp intensity and lamp elevation.

This new led at Point Loma is a Vega VLB-44 (7 tier) with an effective intensity of about 8500+ candela thus a 14nm norminal range with meterological visibilty of 10nm. Vega has a page on their website where one can calculate this stuff.

The normial range of the previous lamp asssembly was listed as 24nm which is 13nm beyond the 11nm geographic range. To be able to observe this light over the horizon at the outer end of the normial range one would have to be at an elevation about 130 feet above sea level, a tall ship indeed or low flying aircraft.

From a practical standpoint, the greater the intensity the light the more visible to the eye it will be when the meterological visibilty is less then 10nm in haze or fog, condititions of low atmopspheric transmissivity. This can be even more troubling with a light displaying a flash of less than two seconds. This involves the effective intensity, that which is observed by the human eye, the shorter the flash the weaker the effective intensity. This is the same as if you closed your eyes and then opened them. It takes moment of two to focus on a distant small object.
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