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How did they position Fresnel panels?

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Postby island » Tue Apr 05, 2005 12:13 pm


The reflecting panels in a 3rd Order Fresnel are positioned about 20 inches from the center (focal point) of the lens. It is desired that the output beam of light is strongly visible 20 miles or more distant. Very exact positioning was required when mounting these panels.

When Fresnel lenses were fabricated, how did they determine the exact positions when mounting the lens panels so they were properly focused to produce the desired horzontal beam of light of maximum intensity?
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Postby ron » Tue Apr 05, 2005 6:29 pm


wouldn't they be fabricated with the bath as a unit? they must have known the radius requirement as per the lens order
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Postby Optics » Wed Apr 06, 2005 6:06 am


An interesting question. The lens was actually focused in reverse - you will understand what I mean in a minute. The lens frame elements were first cast then machined to quite accurate tolerances. (not by today's standards) and each piece of frame had a number cast or stamped into it on each corner. The prisms and dioptric glass elements were rough set in the frames with wooden shims and a little litharge (a kind of putty). Then the frames were put into position and attached with countersunk machine screws, which accurately positioned the panels. Then a small red ball, about the size of a golf ball, was hung from the top center inside of the lens and placed at the focus. At this point each of the prisms was twisted by moving the shims until the red ball could be seen from a specific location outside the lens. There was a long square pole with graduations on it where each prism should be viewed to see the ball. When the prism was twisted in the frame so that it could be properly seen from the marked position on the pole it was final litharged into position. As a final check the lens was set up on a stand at one end of a long building (sometimes a courtyard) and a lamp was placed in it. At the other end of the building about 200-300 feet away the graduated pole was placed in a fixed position. At this point the light from each prism should be seen exactly at each marked line on the pole. Again, if it was not the specific prism would be loosened and twisted and then re-litharged. The pole simulated the horizon at 20 miles and would produce alignment to the actual horizon. The lens panels could then be taken apart and would hold their focus sufficiently due to their construction and the accuracy of the machine screw construction. The numbers on the lens panels matched the number on the adjacent panel so that they could be reattached in the exact position they were in when first built.
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Postby vacastle » Wed Apr 06, 2005 8:04 am


The numbers on the lens panels matched the number on the adjacent panel so that they could be reattached in the exact position they were in when first built.

And this we saw quite distinctly during the last month, as we worked on the Henry Lapaute Hatteras lens, and especially during the assembly.

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Postby island » Wed Apr 06, 2005 12:55 pm


Thank you for the explanation, Tom. This is very interesting. The lens makers quite literally had to be "on the ball'. I would have liked to have been there to observe this activity.

Applying a little math to this lens calibration scenario, at a distance of 20 NM a directional or focal plane error of +/- 1 degree of the focal plane equates to +/- 2123 feet. For a one hundredth of a degree error this focal plane error would be +/- 21 feet.

With the calibration pole positioned 200 feet from the lens, a +/- 1 degree error equals +/- 41.9 inches on the pole. A one hundredth of a degree error would be +/- 0.42 inches on the pole and equates to +/- 21 feet at 20 NM.

From these calculations the pole method of calibration appears potentially very accurate but might be a lengthy process. These guys could have made good use of a laser had such existed back then.

Tom. Do you know what accuracy the lens makers endeavored to obtain or did obtain when positioning the lens prisms?

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Postby Terry_Pepper » Wed Apr 06, 2005 2:05 pm


Tom, please feel free to correct me if I am wrong, but I believe that in order to assist the Lampist in accurately locating the focal point after lens reassembly at the final site, a small tapped hole was drilled on the inside edge of each frame section at the point at which the focal plane intersected the frame.

Machine screws equipped with eyes were then threaded into these holes and a fine string passed through them in such a way that the string crossed diagonally through the lens from two pairs of opposing frames. The point at which these stings intersected in the center of the assembly indicated the focal point of the lens, thereby showing the Lampist the exact point at which the light source should be located.
Last edited by Terry_Pepper on Thu Apr 07, 2005 5:56 am, edited 1 time in total.
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Postby plebetkin » Wed Apr 06, 2005 3:46 pm


for each lens order, what is the distance in inches from the focal point of the lens that would be necessary? Also for each order, is the 20 NM a constant? That's what I've been getting at in my other post, there should be formula work to dictate this process that could be performed in calculations to assist the lampist in her/his accuracy for aiming the light
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Postby Optics » Wed Apr 06, 2005 4:47 pm


Terry,

You are correct about the holes and setting in a string to form across the center of the focal point of the lens. This was used mainly to adjust the top of the lamp in relation to the focal point.
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Postby Optics » Wed Apr 06, 2005 4:57 pm


David,

I do not know what accuracy they attempted to get nor what they achieved. The light band at 200 or 300 feet would preclude anything like laser accuracy today. Probably 1/50 of a degree was the maximum accuracy that could even be thought of and I doubt if it was more than rarely achieved. Because only the human eye was being used to try to identify the exact brightest line on the pole. Remember also that the glass was not true optical quality glass just the best "Crown" glass they could make. That means optical distortion within the glass that may have accounted for another 1/10 degree of variation as each prism within the lens rotated past the observer.
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Postby island » Wed Apr 06, 2005 7:52 pm


With respect to Peter's question concerning lens order, would not the luminous output desired impact the size of the lamp required and therefore the space, the radius to accomodate the lamp within the confines of the lens structure?

This leads to another concern. Assuming no lantern diameter and height limitations. how would one determine what order lens to purchase and install in any given lighthouse application?
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Postby vacastle » Wed Apr 06, 2005 7:59 pm


This leads to another concern. Assuming no lantern diameter and height limitations. how would one determine what order lens to purchase and install in any given lighthouse application?


My simple answer to this would be by how far out to sea the light was needed to project. Part of the reason you don't see any 1st orders on the Great Lakes.

But that's my "blondie" answer for the moment. Now, take it away, you techies!

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Postby Optics » Thu Apr 07, 2005 5:40 am


The lamps were sized to the lens order:

1st Order 4 or 5 wicks usually 4
2nd Order 3 or 4 wicks usually 3
3rd Order 2 or 3 wicks usually 2
4th Order 2 wicks
5th and 6th Order 1 wick

All the lenses from 1st through 4th could send light to the horizon if placed at the correct height.

The real lens order selector was money. The Lighthouse Service chose to establish categories of importance.

1st order Sea Coast - usually 1st Order lenses.

Large sea coast or lake coast lights - usually 2nd Order

Sea coast or lake coast lights - 3rd or 3.5 Order

Small sea coast or lake coast lights - usually 4th Order

Harbor lights - 5th Order

Beacon pierhead and breakwater lights 6th Order

Some variation existed, but the major selection was based on cost and only partially on need.
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Postby island » Thu Apr 07, 2005 7:38 am


Tying this back to Peter's inquiry,

How much more luminous output might one expect from a 2nd order lens compared to a 3rd order lens, for example, and each lens equipped with an identical size oil lamp with the flame of each producing the same intrinsic brightness and focal compactness, or if with electric power, the same size and wattage bulb?

More precisely what I am wondering----is there a significant difference in light intensity amplifaction between the lower and higher order Fresnels?
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Postby Terry_Pepper » Thu Apr 07, 2005 9:16 am


To augment Tom’s comment that "The real lens order selector was money," I provide the following:

In 1899, J. B. Mansfield compiled and edited what is considered to be the first accurate and in-depth maritime history of Great Lakes. Volume 1 of this seminal work, featured an excellent synopsis of the operations of the Lighthouse Establishment in the Great Lakes, and included the following listing of prices of French fresnel lenses at that time.

Sixth order lenses – from $190 to $315.
Fifth order lenses - from $230 to $840
Fourth order lenses – from $350 to $1,230
Third order lenses – from $1,475 to $3,650.
Second order lenses - from $2,760 to $5, 530.

Naturally, the value of the dollar has decreased significantly over the 116 intervening years, and I thought it might be interesting to convert these 1899 dollars to 2005 dollars in order to give a better understanding of the cost of these lenses, and why price would have been such a key factor.

The Political Science Department at the Oregon State University has an Excel Spreadsheet containing conversion factors to bring past dollars to 2005 dollars for every year from 1800 through the present. According to this spreadsheet, the conversion factor for the year 1899 is 0.045. Thus, converting Mansfield’s numbers, we come up with the following purchase cost of such lenses in today’s dollars:

Sixth order lenses – from $4,222 to $7,000.
Fifth order lenses - from $5,111to $18,667
Fourth order lenses – from $7,778 to $27,333
Third order lenses – from $32,778 to $81,111.
Second order lenses - from $61,333 to $122,889.

Since there were no First Order lenses installed on the Great Lakes, Mansfield did not report on the cost of lenses of this order. However, I did find a reference to a First Order lens costing $65,000 in 1899, which equates to a value in 2005 dollars of approximately $1,444,444.

Clearly, there were real economies to be obtained by purchasing the smallest order of lens that would effectively serve the intended purpose at a specific location.

Note: for those interested, the conversion spreadsheet I mentioned above may be downloaded from the Oregon University website at the following URL:
http://oregonstate.edu/Dept/pol_sci/fac/sahr/cv2005.xls
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Postby Optics » Thu Apr 07, 2005 1:04 pm


I don't think I have ever seen a comparison of the light magnification of the various lens orders, and especially not using the same size lamp.
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