"The
Shining Eye of the Lighthouse"
America's
first lighthouse used a system of silvered
reflectors to intensify the main light source,
a whale-oil lamp. But, by the 1850's, the
government authorized use of a technology
new to U.S.: the glorious, multiprismed lens
invented in France by Augustin Fresnel (pronounced
Fray-nell) in 1822.
It
was a marvel....a complex array of dazzling
glass prisms and bull's-eye lens mounted in
a gleaming brass framework. Each lens cost
$12,000 at the time plus shipping costs from
France.
The
Fresnel lens was much more efficient at collecting
and directing the light rays and produced
a beam five times more powerful than the reflector
system used previously. But, to take maximum
advantage of the higher light intensity, the
light had to be placed high enough to compensate
for the curvature of the earth. When mounted
at 100 feet above sea-level, it had a visible
range of up to 18 miles at sea.
The
new lenses were ranked in six sizes called
orders. The weakest, ranked sixth, was used
in lights on lakes and in harbors while the
largest, first-order lenses were used in lighthouses
on fogbound coasts. A first-order lens, made
up of over 1000 prisms, stood up to 10 to
12 feet tall and measured 6 feet in girth
and could weigh up to 3 tons. Many lighthouses
have their original Fresnel lenses in place
though many are now unused having been replaced
by aero-marine beacons. Many of these beautiful
lenses have been removed from the lighthouses
and placed in museums and other display areas
where the public can view and appreciate the
workmanship that went into them. Others, unfortunately,
have been vandalized when the lighthouses
were abandoned and left unguarded.
Light
source for the early lens was a lamp made
up of up to 5 concentric wicks and fueled
originally by sperm oil or lard then kerosene
and, finally, replaced by the incandescent
lamp. The Fresnel prisms could focus the rays
of such a lamp into a beam of 80,000 candlepower.
By the 1930's, most of the lamps had been
replaced with Incandescent bulbs as the light
source which brought beam intensity to as
high as 4.5 million candlepower.
Figure
1
Some lights employ a fixed or stationary lens
(Figure 1) projecting a steady, uninterrupted
beam of light in all directions. Others show
a set of flash-and-eclipse (dark) intervals
called the "light characteristic" (Figure
2) with the repetition rate of the intervals
called the period. When viewing a lighthouse
you can determine its period by timing the
flash and eclipse which continuously repeat
at 5 to 15 second intervals. Each light in
an area has a unique characteristic and mariners
distinguish one light from another by checking
its period on a chart called a 'Light List'.
Figure
2 
To
create the flash pattern, multiple lens panels
were mounted around the cirumference of the
Fresnel lens assembly which was mounted on wheels
on a circular track or floated in container
of mercury, reducing rotational friction to
a minimum, and rotated at a precise rate controlled
by a clockwork mechanism. In this way, even
a 6000 pound assembly could be rotated with
the touch of a finger. The clockwork drive,
powered by a weight which often traveled the
interior height of the tower, was wound by hand
using a crank in the lanternroom. The weight
required winding as often as every 4 hours which
meant the keeper had to make the trip to the
lantern room several times each night.
The
characteristic of the light was determined by
the number of lens panels, their placement around
the Frenel lens assembly, and the speed at which
it was rotated.
Figure
3 
Figure
3 shows graphically how the Fresnel lens works.
To bend and focus the rays to form a single,
concentrated beam 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.
Fresnel lens, in many different forms, find
many application in today's modern world.
They are used in the lens of traffic signals
and to shape the light beam in overhead projectors
as well as in molded plastic versions which
are sometimes placed on the rear windows of
motorhomes to broaden the drivers rearward
field of view.
* Refraction
is defined as the phenomenon whereby a ray
of light, heat etc. is deflected or diverted
from it's previous course in passing obliquely
out of one medium (material) into another
of different density. The amount of deflection
is determined by the characteristics of the
two materials and is defined as the refractive
index which is the ratio of the sines
of the angles of incidence of a ray of light
passing from one medium (usually air) into
another medium (material). A refractometer
is the instrument used to measure the index
of refraction of various materials.
