fresnel lens + 1kW lamp

Sorry somewhat OT but Bob Mizon [CfDS] at Ewell AS earlier this month
said that "some astronomers have 500W security lamps on their houses -
half the maximum power of lighthouses" - surely both statements can't
be right? Do you offend in the first part !!

Well the latter seems true - Portland Bill Lighthouse has a modest 1kW
lamp visible to mariners at 20+ miles - see my page
http://home.freeuk.com/m.gavin/ntrust.htm and a remarkable 'thin'
fresnel lens to project the horizontal beam.

The lighthouseman present implied the fresnel lens had reflective
qualities to capture the backbeam [like the mirror in a slide
projector] which seems pointless. Have I missunderstood?
Can anyone knowledgeable in lighthouse fresnels advise? A web search
didn't help.

In article , Maurice Gavin
writes

The lighthouseman present implied the fresnel lens had reflective
qualities to capture the backbeam [like the mirror in a slide
projector] which seems pointless. Have I missunderstood?
Can anyone knowledgeable in lighthouse fresnels advise? A web search
didn't help.


Portland Bill appears to be equipped with a '4 panel 1st order
catadioptric fixed lens' arrangement.

http://www.trinityhouse.co.uk/html/ts46.htm

From what I've read, it looks as though the 'catadioptric' part refers
way in which the outer part of the lens uses total internal reflection
to direct the light, as opposed to the inner dioptric portion, that
directs the light by refraction. So, the lens does indeed reflect some
light.

Diagram and details he

http://www.lanternroom.com/misc/freslens.htm

The above may only be part of the story though. As the lenses are fixed,
there must be another rotating part, presumably incorporating a
reflector inside the lenses.

--
David Entwistle

On Mon, 21 Jun 2004 17:16:58 +0100, David Entwistle
wrote:

Portland Bill appears to be equipped with a '4 panel 1st order
catadioptric fixed lens' arrangement.

http://www.trinityhouse.co.uk/html/ts46.htm

From what I've read, it looks as though the 'catadioptric' part refers
way in which the outer part of the lens uses total internal reflection
to direct the light, as opposed to the inner dioptric portion, that
directs the light by refraction. So, the lens does indeed reflect some
light.

Diagram and details he

http://www.lanternroom.com/misc/freslens.htm

Thanks David - all is now clear from the fresnel diagram. The outer
zone of the fresnel acts as an internal reflector - the inner zone as
a refractor - all projecting a forward horizontal beam. The complete
system must be ~ f/0.7 eg very fast. The whole fresnel lens rotates on
a turntable. An identical fresnel on the 'reverse' side projects an
identical beam 180o to the first.

The above may only be part of the story though. As the lenses are fixed,
there must be another rotating part, presumably incorporating a
reflector inside the lenses.

Only the lamp is fixed.

David Entwistle