How did they position Fresnel panels?

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Postby Terry_Pepper » Thu Apr 07, 2005 4:20 pm

In my files, I have a copy of an Historic American Engineering Record, published by the National Park Service, delineated by Mabel A. Baiges in 1988. This document shows cross sections and dimensions of Fresnel lenses of the six orders commonly used in the US, along with a table titled “Comparative Table of Lens Orders.”

One column of this table purports to show the relative brightness of each order over the base reference of a Sixth Order lens. A footnote on the document states that the brightness multipliers were obtained from a table in Léonce Reynaud’s book mentioned in my previous posting.

However, after reviewing my copy of Reynaud’s book, I am unable to identify or verify the specific table or numbers provided by Ms. Baiges, and thus assume that she extrapolated them in some way from the book in a manner not immediately evident to me.

While I am thus unable to succinctly verify the accuracy of the information, I have decided to post it in the event that it might stimulate further dialog on the subject:

Using the brightness of a Sixth Order lens as a base reference point:
A Fifth Order lens is 1.23 times brighter
A Fourth order lens is 2.31 times brighter
A Third Order lens is 3.85 times brighter
A Second Order lens is 11.54 times brighter
A First Order lens is 17.69 times brighter.
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Postby island » Fri Apr 08, 2005 8:38 am


This same information appears on page xv of the NPS 1994 Inventory of Historic Lights. The footnote describes the relative brightness as "comparative brightness of lights". What is not clear is if the comparison was made using the same source of illumination, i.e. identical lamp producing the same intrinsic brightness.

For the diagrams illustrating orders of lenses, the 1st Order diagram shows 15 upper and 8 lower reflecting prisms. The 2nd Order shows 15 upper and 5 lower. The 3rd Order has 11 and 4. There are also more annular "grooves" in the 1st Order central panel than 2nd Order, which has more than the 3rd Order. One would therefore expect more light to be emitted in the beam from the larger lens having the same source of illumination.

The greatest impact on lens output is the source of illumination. When IOVs were placed in Fresnels to replace earlier lamps it was found that the output of a 2nd Order equaled that a Hyper-radial. An example is in the early 1900s when Eddystone Light bivalve lens (Order ?) was changed from a 6 wick oil lamp to IOV and the output increased from 79,000 to 292, 000 candlepower. The IOV flame produced higher intrinsic brightness and greater flame compactness at the focal center of the lens.

So whatever the output difference there is between orders of Fresnels, the illumination source is most significant to obtain maximum candlepower from these lenses.

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Postby Optics » Fri Apr 08, 2005 12:56 pm


The lens put in the Eddystone when it was rebuilt by Douglass was a 1st Order BI-FORM lens. A bi-form lens uses only the dioptric bull's-eye panels (I think this lens had 4 panels) no upper or lower catadioptric panels and has 2 such lenses mounted one above the other. There were also tri-form and quadri-form lenses.
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Postby Fred » Sun Apr 10, 2005 12:34 pm

The following from an author who started as a aprentice with Chance Brothers in 1937
An extract from "PHAROS The Lighthouse Yesterday,Today and Tomorrow" by Kenneth Sutton-Jones

In the poor light one made out on both sides of the central
passage a score or more great rotating tables supported on heavy iron
frames. Power for these tables,and the undulating cranks and cams over
each was transmitted by belting and line shafts which provided an all
pervading drone.Amid this unreality the men themselves appeared pallid and worn out creatures of the dungeon.

The founding of the glass prisms and rings took place in a nearby
building within the 'glassus' by men simultaneously 'unrolling' blobs of glass from the ends of tubes into an annular iron mould.A period of
annealing followed in a special oven.The really large prismatic rings had to be cast in segments,a formidable task when one considers the hundreds of elements within a large lighthouse optic.

Then,after annealing,the rough looking elements were ground flat on one
face and stuck with pitch to form a ring on a turntable.

Each prismatic ring had three faces,two flat and one curved.The curved
face and one of the flat sides were ground to correct shape using grinding powders of diminishing coarness and rubbing pads which the machinery caused to abrade across the glass surfaces.

Conformity to shape was gauged by templates and feeler gauges and,when complete,the whole glass ring was inverted and a plaster mould made to accomodate the shape so that the remaining flat surface could be ground.The processes were repeated using polishing media until a
jewel-like finish was obtained.Each prism was tested on a special machine where very narrow parallel rays of light were made to traverse across the surface of the glass and conformity with the correct focal point was recorded.

What appeared to be a rough grinding process achieved of surprising
accuracy whereby a prismatic ring 1.5 metres in diameter displayed a focal conformity to within a circle only 2 millimetres in diameter.

He also described how the prisms were lined up,but Rosemary gives an explanation as follows.

How did they line up the prisms when they built the lens?
Extract from "Lighthouses" by Rosemary Garland
At the lighthouse workshops where the optics are made for export all over the world,each prism bar of the glass cage has to be carefully tested to make sure that it is at its correct angle.This is a difficult task as the testing has to be done in a small space.No workshops in Britain could possibly provide fifteen to thirty miles of space,which is the distance these lights are expected to shineThe problem is overcome by reversing the lighting.Instead of placing a light inside the glass optic and testing the focus of the beam outwards,a light is placed on a distant wall and directed inwards.Each prism bar is tested to make sure that the pin-point of light from the distant wall falls on to a small needle placed upright inside the very center of the glass cage.When the angle is judged to be absolutely accurate,the prism bar is sealed into position in its gun-metal frame with plaster of Paris.

So we now know of at least two ways to align the prisms?
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Postby Fred » Sun Apr 10, 2005 3:23 pm

There was a question concerning lampsize with order of lens,one concern was horizontal divergance,particularly important in sector lights and also flash length, in "A Few Notes on Modern Lighthouse Practice." by Chance Brothers in 1910.

85 m/m lamps gave
1st order 5o 18’ Horizontal divergance
2nd order 6o 58’
3rd order 9o 44’

55 m/m lamp
1st order 3o 24’
2nd order 4o 30’
3rd order 6o 18’

The more you read the more you find had to be taken into account?
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Postby island » Mon Apr 11, 2005 10:06 am


Thank you for joining this discussion and for posting the additional information. With several lens manufactures producing lens for lighthouses, it can be expected that there would be differences in the process methods and procedures for making the lens elements and for aligning those elements. French wine makers did not share process secrets with the competition and I expect this would have been the same for the lens makers.

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Postby island » Mon Apr 11, 2005 9:53 pm

Concerning the horizontal divergence numbers, what this tells us is the lens is out of focus by a greater amount with the larger diameter lamp, the flame being a greater relative distance from the focal point of the lens.

The horizontal divergance angles are theoretical and were calculated as per the following equation:

Sine of divergence angle = flame diameter / lens focal length

Applying this to a 250 mm 4th Order lens the divergence angle would be 12o 43' with a 55 mm lamp and 19o 52' for an 85 mm lamp. This would make for a rather fuzzy beam for a sectored or a revolving light.
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