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Calculating a light's visible range

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Postby Forchu » Fri Apr 01, 2005 9:07 pm


island wrote:Ron.


Weather conditions obviously impact visibility. Thick Maine Coast fog for example. Dust and or smoke particles will reduce the range.

Moisture, salt and dirt on the lens or on the storm panes reduces the output of the light, a common problem now with no keepers to attend to the daily cleaning.

A light with a red sector may have 14 miles visibility in the white and only 11 miles in the red.


Perfect lighthouse DX conditions
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Postby island » Fri Apr 01, 2005 9:14 pm


What can be more perfect than thick Maine Coast Fog?
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Postby ron » Sat Apr 02, 2005 8:27 pm


i guess in reading this again you are referring to the daytime visual range of a structure.
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Postby island » Sun Apr 03, 2005 8:36 am


Ron,

The above calculation is the visual or geographic range.
As you said, the visual daytime range of visibility.

The luminous range will vary depending on strength of the light and on conditions at any given time. Smoke, haze, and dust will reduce the luminous range. The luminous range of a light with weaker output might have a range less than the geographic range.

For a strong light the luminous range exceeds the geographic range when atmospheric conditions cause refraction of the light beam and the beam bends over the viewing horizon.

A very common example of this phenomenon occurs at moonrise. When one first sees the moon appear at the horizon it is actually below the horizon but it is visible because of refraction of the light in the earth's atmosphere.

Not only will light refract over the horizon but it can also refract over an obstacle such as an island between the light source and the viewer. I have seen this on several occasions when viewing Matinicus Rock Light from Whitehead, a distance of about 18 miles. Not only could I see the light, but with binoculars, the astragals of the lantern and reflected light from the lighthouse dwelling and unused second light tower.
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Postby Terry_Pepper » Sun Apr 03, 2005 9:45 am


To eliminate the need calculate distance of visibility, Light Lists have included a table showing the maximum visible distances of lights at varing elevations for many years. Here is an example of such a table:

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This table comes from my website Seeing The Light, and can be viewed along with the accompanying explanation at the following URL:
http://www.terrypepper.com/lights/lists/visibility.htm
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http://www.terrypepper.com

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Postby island » Sun Apr 03, 2005 11:42 am


This table is for statute miles.
Does that mean it applies only for lighthouses when viewed over land? :)
And for a 15 foot tall observer. Hmmm. :?
Maybe Ross standing on the cab of a pickup with Hersh standing on his head and Hersh observing, :) and Leah driving the truck. :roll:

To be serious, the observer columm was based on an assumed eye level 15 feet above sea level when standing on a sailing vessel deck. To use this table accurately one must know approximately how high their eye level is above sea level and add the appropriate number obtained from the distance in miles column.

Example:
Lighthouse 75 feet and eye level 25 feet. The distance for the lighthouse is 11.7 miles. Add to this for the observer from this table a distance of 6.6 miles for 25 ft of height. Therefore the distance of visibility from the observer to the lighthouse is 11.7 + 6.6 = 18.3 miles.

If the eye of the observer is at 50 feet then add 9.4 miles to the 11.7 miles for the lighthouse. The distance seen now becomes 21.1 miles. It would be 25 miles if observed from 100 ft elevation.

So how many miles can a lighthouse be seen? Depends on where you are standing as much or more so than the height of the lighthouse.
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Postby island » Mon Apr 04, 2005 4:10 pm


Range information requested.

Ranges of lights in NM, tower height in ft, geographic range (calculated), listed range (per CG 2005 Light List)

characteristic, height, (geographic), listed range
Sankaty FlW 7.5s 158 (18 ) 24
Gay Head AlWR 15s 170 (19) 24 white 20 red
Chatham Fl(2)W 10s 80 (15) 24
Nauset Al(2) WR 10s 114 (17) 24white 20 red
Cape Cod FlW5s 170 (19) 18
Graves Fl(2)W 12s 98 (16) 15
Thatchers FlR 5s 166 (19) 17
White FlW 15s 82 (15) 20
Monhegan FlW 15s 178 (20) 20
Mt. Desert FlW 15s 75 (14) 18
Petit Manan FlW 10s 123 (17) 19

Note: Gay Head and Nauset the listed white range is 24NM and the listed red range is 20NM. The Al means alternating. Fl for the other lights means flashing and s is seconds. W is white. R is Red.
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Postby Terry_Pepper » Mon Apr 04, 2005 5:14 pm


Great thread folks. I look forward to many more such interesting and informative exchanges in the Technical Forum.
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