Technics of the Lights

Technics of the Reflectors

---

The major problem with early lighthouses was the small output of light from the wood or coal fires then being used. Lighthouse designers were constantly looking for ways to produce more light. Experiments were made with the types of fuel being burned, with the design of the braziers, which held the burning fuel, and with ways to direct or concentrate the light from the fire. The first method of projecting and directing the light started with the design of the reflector. In this story we will discuss the various types of reflectors that were used to create the ‘Mirror of Light’.

Early history of reflectors

The first reflectors were whitewashed walls and later flat metal plates of polished brass or tin were used, set in the walls behind the open flames from the Coal braziers in early lighthouses. The earliest known use of these reflectors was on the Baltic Sea at the lighthouse of Gollenberg, now in Poland, which had a single candle lamp backed by a polished metal reflector in 1532.

In 1660, Johan D. Braun designed and manufactured cast steel reflectors in Sweden. They were used at the lighthouse at Landsort, Sweden, which used one reflector, shape unknown, and a single primitive oil lamp in 1669. Braun was given a patent for the use of these reflectors in lighthouses, in 1681. One of the more unusual reflector designs was invented about 1687 and used at the lighthouse of Orskar, in Sweden, which had 6 large Braun reflectors, shape unknown, each with 2 oil lamps.

Metal reflectors were also used in Germany at Travemünde in 1710, where the lighthouse had two reflectors and each reflector had a lamp with two wicks. The first reflectors in Germany were purchased from England or France. Later in the early 1800s, a German, Johann Georg Repsold, began making less expensive and optically improved reflectors for the German market.

A major attempt to intensify the light from a lighthouse wood or coal fire was made in 1727 by Monsieur Bitry at the Cordouan lighthouse in France (see picture right colomn). The mariners had complained they could not see the coal fire in the lighthouse from more than a few miles away. Monsieur Bitry covered a cone of wood with metal plates and suspended it; point down, above the fire. The reflection of the flames in the metal made the light appear brighter, but the system was not adopted elsewhere, probably because soot from the fire constantly dulled the reflector and made any increase in the light very difficult to sustain.

In 1738, the first mention of a parabolic reflector suggests that five parabolic Braun cast-steel reflectors, each with from two to six lamps were installed at the lighthouse at Orskar, in Sweden.

Reflector Designs

Reflection occurs when a ray of light hits a painted or polished surface and is reflected from the surface. Highly polished metal surfaces behave as mirrors, and when they are made in the shape of a parabola they concentrate the light from a light source placed in the focus of the parabola. There are three basic types of reflecting surfaces, Flat, Spherical and Parabolic as shown in the drawing.

Flat plate reflectors are seldom truly flat, nor are they always highly reflective. A flat plate reflector reflects light in many directions and only a small portion of the light is reflected forward in a direction useful to a mariner.

Spherical reflectors return the light rays to the focus of the reflector where the lamp is placed. A spherical reflector will appear to intensify the brightness of the flame, but will not form the light into a concentrated beam useful to the mariner.

Parabolic reflectors gather the light from the focus of the reflector and return it in a concentrated beam in the direction the reflector is facing. This concentrated beam of light can be directed to the horizon and can be manipulated to provide a very useful light for the mariner. However, approximately 30 percent of the total available light escapes from around the edge of the reflector and is not concentrated into the beam.

Problems with the manufacture of reflectors kept them from reaching their theoretical capabilities. The parabolic shape was very difficult to fashion by hand. Two general methods of construction were employed. In the early method(1), a sheet of metal, usually copper, was fashioned into a crude parabolic shape by hammering a flat sheet using only the skill of the craftsman as a guide.

Later method(2), wooden molds were carved by hand and checked against paper or brass templates. The sheets of metal were then formed into parabolic shapes by hammering into the mold, and still later, iron molds were used to create more refined parabolic reflectors. The copper reflectors were then polished to form a shiny surface to reflect the light.

However, plain copper was not a very reflective surface and was soon abandoned. A second construction method used heavy silver cladding, which was bonded to the copper to improve its reflective qualities. Unfortunately silver oxidizes (tarnishes) rapidly leading to the need for continual polishing. The removal of the tarnish and also of the soot left on the reflector from the lamp flame, when chimney-less lamps were used, caused the need for polishing the reflectors on a very frequent basis. In many cases the silver cladding was polished so much that it was removed from the copper surface altogether, returning the reflector to a poor state of light reflection.

With any metal surface, some of the light is absorbed into the surface by minor imperfections, scratches, soot and the metal itself. It was found through experimentation that the highest quality metal reflectors with the finest silver-plated and polished surfaces still reflected only 55 percent of the available light.

In addition, some of the light is lost around the edge of the reflector and it was found that only 70 percent of the total light from the lamp was actually captured by the reflector. Thus, the total light captured by a near-perfect reflector and directed in a usable direction is a combination of the two factors just mentioned or about 39 percent, while an average quality reflector produces only 20 percent.

Glass Facet Reflectors

The first attempt at a true parabolic reflector in England was a design by William Hutchinson in 1763. In his book 'A Treatise on Practical Seamanship' published in 1777, William Hutchinson described how his reflectors were constructed and employed:

We have had in use here at Liverpool, reflectors of 1, 2, and 3 feet focus, and 3, 5½, 7½ and 12 feet diameter, the three smallest made of tin plates soldered together, and the largest of wood, covered with plates of looking glass, fixed as near as can be to the parabolic curve, and using a copper lamp.
The cistern part of the lamp stands behind the reflectors, so that nothing stands before the reflector to interrupt the blaze from acting upon it, but the tube that goes through it, with a spreading burner mouth-piece, to spread the blaze of the lamp parallel to, and with the middle of it, just in the focus or burning point of the reflector.
We have a feeding can with oil to stand upon the cistern of each lamp, with a small brass cock that is turned to drop or run a little according to the consumption of oil.

The Hutchinson oil lamps could be seen from ten miles away, and needed to have their flat rope wicks trimmed (snuffed) every 4 hours.

Ezekiel Walker of Lynn, England rebuilt the lighting for the Hunstanton lighthouse in 1776, and installed lamps with 18-inch parabolic reflectors made of pieces of mirror glass. He used facets of mirror glass tapering in width arranged in long rows set into a parabolic plaster shell in a metal frame and fixed in position with varnish and white lead. The lamp in Walker’s reflectors used a single flat wick made of five cotton strands. Walker’s glass facet reflector’s gave about two thirds of the power of an equal diameter silvered metal reflector, and were estimated at about 1000 candlepower.

In 1777, a very unusual reflector and lamp was created and installed at the Lowestoft lighthouse in England. It was named the ‘Spangle Light’ because of its use of a concave drum on which 4000 glass mirror facets were placed. Surrounding the concave drum was a hollow tube ring with 126 holes into which short rope wicks were placed. This ring was fed from a common oil supply and when the 126 wicks were lit their light was reflected from the Spangle drum and out to the sea. The Spangle Light was first lit in 1778, but while the light was better than the previous coal fire, the 126 flames used a large quantity of oil, were difficult to maintain, and soot formed easily on the 4000 mirrors. The Spangle light was abandoned in 1796.

Thomas Smith, Engineer of the Northern Lighthouse Board in Scotland, built a parabolic reflector made of polished tin in 1786. During the following year, Smith changed from polished tin to octagonal reflectors twelve inches in diameter made of 48 pieces of mirror glass using Ezekiel Walker’s design process. He also a made larger reflectors with 350 pieces of mirror glass set in plaster on a brass plate eighteen inches in diameter, which were used at the Kinnaird Head lighthouse in 1787. These reflectors used a lamp with four rope wicks and this reflector and lamp combination produced about 1000 candlepower. The last glass facet reflector of this type was produced in 1801 and removed from use in Scotland in 1820.

An Argand lamp produced between 6 and 7 candle power at any single observation point. A high-quality 21-inch diameter silver-plated parabolic reflector collected and reflected 39 percent of the lamp's total available light, creating a concentrated beam of light that increased the lamp's power approximately 350 times, and thus the Argand lamp and reflector combination produced 2450 candle power.

The most powerful light from a parabolic reflector is concentrated in the central axis of the beam. At positions just 8 degrees either side of the central axis, the effective candle power is only 10 percent of that at the center, and in areas beyond 8 degrees the light drops out very quickly. This narrow divergence of the light beam required the sailor to line up almost directly with the front of the reflector in order to see the light beam from a distance. The divergence is a function of the size of the lamp flame and the focal distance of the reflector, with the average reflector’s useful beam covering about 14 degrees of the horizon, and intense light covering about 5 degrees.

Note: Candelpower

The determination of the light output of reflectors and lamps is usually measured in candlelight (see also: Candela). In the early years of reflector development, there were no accurate methods of measuring candlelight and later, in the early 19th century, the methods used were highly subjective and unscientific by our current standards.

Candle power is also dramatically affected by the size of the reflector, the type of oil used as fuel, the maintenance of the lamp and the brilliance of the reflector, and by factors such as the brightness of the sky and the alignment of the reflector with the horizon.

Robert Stevenson's Reflectors

From 1803 to 1810, Robert Stevenson worked to improve the design of Scotland’s copper parabolic reflectors and lamps. He chose an improved Argand lamp and a true parabolic reflector with a heavy cladding of silver. He completed his first silver clad copper reflector in 1804 and by 1809 he had succeeded in developing a design where the lamp was able to be released from the focus of the reflector by turning a locking ring and later a locking shaft, that let the lamp assembly slide down, allowing easy wick replacement or trimming, and allowing easy polishing of the inside of the reflector.

When the work was completed, the keeper raised the lamp back into its pre-set position, in the reflector’s focus, and locked it by turning the ring or shaft. Stevenson’s new reflectors were first installed in the Bell Rock lighthouse in 1811, and produced about 2500 candlepower (see 4th figure in the right column).

First Intermittent Light

The Catoptric intermittent light was introduced by Robert Stevenson in 1830 at the Mull of Galloway. The occultations are effected by the sudden closing and opening of two intercepting opaque drums, Bothe figures which include the apparatus, and are moved vertically in opposite directions by means of machinery.

Thomas Stevenson's Holophotal Reflector

In 1849, Thomas Stevenson invented a new reflector design, where no light escapes around the rim of the reflector, as was the case in all other reflector designs. He named his design the Holophotal Reflector, because it collected nearly one hundred percent of the available light and directed it forward in a single beam.

The Holophotal reflector-lens combination was first installed at the North Harbour light at Peterhead, Scotland. The Holophotal reflector is made from a spherical reflector behind the lamp, which collects all of the light rays and returns them to the lamp flame in its focus. The front portion of the reflector is a deep short focus parabolic reflector that captures the light from the lamp and sends it forward in a single beam.

Inset into the parabolic reflector is a small portion of a Fresnel lens that is positioned so that it will collect all light rays that would normally escape at the edge of the reflector.

.

This three-part reflector-lens combination collects nearly every one of the light rays and forms them into a single beam. The only light not collected is that obstructed by the lamp, chimney, lens framework, and that absorbed by the metallic reflecting surfaces. The Holophotal design uses both a reflector and a lens and it is known as a Catadioptric or reflective / refractive illuminator.

Both Thomas and Alan Stevenson proposed various modifications to the holophotal reflector and in the 1850s a spherical reflector was mated to a Fresnel bull’s-eye lens. This combination collected nearly all of the available light and was used in harbour and range lights. It was eventually replaced by the lens with spherical doubly reflecting prisms, which will be discussed in the section on Glass Doubly Reflecting Prism Designs.