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More on the Magnificent Lens now at the Bruce County Museum and Cultural Centre |
The Saugeen and Kincardine Times Celebrate the Year of the Light |
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The lens in place at the Museum. Please Click on this image for a larger picture The light from Cape Croker that is now in 'residence' at the BCM&CC is unusual in a couple of ways: It's in very good condition after over 100 years in operation. The last Lighthouse Keeper left Cape Croker in 1986, so the care was in the hands of the Coast Guard. It used to revolve on a bed of mercury (2 1/2 gallons), which would rotate its more than 3700 pound weight. Later it was rotated by an electric mechanism. Over the more than hundred year operation, it was subjected to intense heat and cold and it stood up well due to its construction of pure European (probably French) glass and brass fittings. It is both a dioptric and catadioptric lens in that it includes both reflection and refraction.
The diagram above shows
graphically how the Fresnel lens works. To bend and focus the rays to
form a single, concentrated sheet of high intensity light, the
catadioptric prisms refract and reflect; the dioptric prisms and
center Bull's Eye lens refract. With just a 1000 watt bulb, a
first-order Fresnel lens can generate a 680,000 candlepower beam visible
up to 21 miles out to sea if set high enough over the water.
On the inside you can see the dioptric & catadioptric prisms of the
Cape Croker Light. The 'circular' ones are dioptric.
Click on the image for a larger picture
Cove Island Lens The lens at Cove Island is a dioptric lens. It is made out of green glass. Notice the characteristic 'Bulls Eye' effect. The centre circle has an area equal to each of the other annular rings. Cabot Head Compared to DeTour Light
Detour Light Notice the DeTour Light is different in two ways. It has white glass and it has 4 bulls eyes located at 90 degree intervals that don't partially overlap. This would mean that in a unit of time interval the rotating light would 'flash' with maximum intensity 4 times. So if there was a rotation of the light on a mercury bed every 4 seconds, then you would see a flash at a fixed position every second. The Cape Croker Light has six Bull's Eyes so in the same 4 second interval it would flash 6 times. The Eyes are not evenly spaces so you would get: Flash, Short Pause, Flash, L o n g P a u s e, Flash, Short Pause, Flash, L o n g P a u s e, Flash etc That is, you would see two close flashes, a pause, two more close flashes, a pause etc. The Cove light would show green bright, green dim, etc while the other two lights would never as dim between flashes as would the Cove Light. Notice you could put opaque panels in the Cove Light and shut off all light between flashes. 27/02/2009 12:19 AM
Marine Heritage Society Southampton
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Why the Funny Bull's Eye Shapes?
(L) Curator Director Barb Ribey & Vicki Cooper admire lens. Notice the Bull's Eyes that overlap One of the great things about Fresnel's original invention was that idea of how simple it was. He overcame many disadvantages all at once. They were:
Cutting a Watermelon
What Fresnel did is think about a big lens as though it were a melon and then slice it in equal increments, forget about the interior, just cut it out and put the whole thing together as shown in the 'Collapsing Lens' picture below
The Collapsing Lens
The cuts produce the Bull's Eye and the Annuli Notice that the first cut through the melon becomes the Bull's Eye and the remainder become annular rings. You can repeat this process as many times as practical. The curve of the shape creates the magnification, not its thickness! Other Examples of the Fresnel Lens
Inside the Pensecola Light Station
First Order Cape Canaveral
Cape Meares Oregon First order |
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Curator_Director_Barb_Ribey_&_Vicki_Cooper_admire_lens[1].jpg)
