Bob,
Fresnel lenses are found in 7 sizes- 1st thru 6th order, with a 3 1/2 thrown in to screw things up. My experience has been that many folks at lighthouses, including some of the nicer museums, do not realize there was a 3 1/2. This lense was made primarily for the Great Lakes.
I will quote some info from Kenneth G. Kochel's book "America's Atlantic Coast Lighthouses":
1st order. This lens, the most powerful & largest of the six, was installed in primary landfall (coastal) lighthouses to give warning to the approach of land. Inside diameter (ID) = 72 7/16".
2nd order. Aids marking coastal headlands and the approaches to bays and sounds usually received this lens. ID 55 1/8".
3rd order. This lens was assigned to aids covering wide bays and located on large lakes, coastal inlets & their entrances. ID = 39 3/8".
4th, 5th, 6th orders. These lenses, including a mesoradial 3 1/2 order, mark prominent headlands, points and shoals in large bays and sounds. They also are used to give warning of existing obstructions in rivers or mark piers and wharves. ID 4th= 19 11/16"; 5th = 14 3/4"; 6th = 11 3/16".
There is one active (and huge) hyperradial lens in the US. That one, on Makapuu Point, Oahu, Hawaii, has an 8 1/2' diameter and an overall height of 24 '.
He goes on to say that at the start of 1998 there were 14 active landfall 1st orders:
SEBI, Bodie, Cape Henry, Currituck, Jupiter, St. Augie, Seguin Island, Tybee, Pensacola, Destruction Is, Heceta Head, Makapuu Pt, Point Conception, Yaquina
I'm not sure where he comes up with the ID of the various sizes, and whether this is an average or what. 1st orders can be 9' tall, and are quite impressive when viewed in a tower or in a museum setting.
At the risk of being corrected, and without digging out the books, I don't think they ever put a 1st order in the GL. There are 2nd orders - think one at Grosse Point & White Shoals
So much for todays history lesson. Oh, name of lense is pronounced fra-nell.
Dave
[This message has been edited by Dave H (edited 05-31-2002).]
