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Light Frame : Forrest Myers

Light Frame tower w cloud.jpg

Light Frame

A Technical description of Light Frame
by Elsa Garmire



At night the Pavilion and cloud bank are framed in a tilted square of white light.  Towers 130 feet apart set in almost a square configuration at the corners of the Pavilion, each contain two lights which shine toward the neighboring towers.  The tower heights vary from 65 to 80 feet presenting a skewed rhombus.


The eight high-intensity Xenon lamps atop the towers shine into each other in order to create what appear to be solid lines of white light.  These specially designed lights produce narrow beams as bright and collimated as those of large carbon-arc search-lights.


The 500-watt light beams are half a meter in diameter with a beam spread of one degree.  This means that the light beam has spread to only three times its diameter in traveling a length of the light frame.  The two lights pointing at each other produce a more symmetrical lighting for the sides of the light frame.  They are carefully adjusted to beam into each other and be as parallel as possible.


Such intense collimated beams are available only from the use of high-pressure Xenon arc lamps.  The lights used in this frame are 24V, 500-watt lamps which contain 18 atm of Xenon and produce an arc only 1/8" long.  Since the light source is almost a point, a good parabolic reflector can make an accurately parallel beam.  The reflectors used here are 5" deep, 14" in diameter and are accurately electroformed from nickel.  The master parabola from which replicas are electroformed was made by spinning liquid epoxy in a pan.  Gravity forms a natural parabolic curve which remains when the epoxy hardens.  In Appendix B is a discussion of the advantages of high-pressure Xenon arc lamps.


Since the lamp and reflector are mechanically separate, optical alignment necessitates moving only the lamp.  Air cooling and self-contained igniter have been specially designed for this project.  Both lamps atop each tower are contained in a single black housing, designed large enough to stop all light shining toward it from adjacent towers.


The means by which a light beam is made visible along its path is by scattering from small particles.  We cannot see actual light beams themselves.  What we do see is small particles illuminated by the light.  These can be dust, water droplets, or localized increases in the air density (clumps of molecules). This last effect causes so-called Rayleigh scattering, which makes our sky look blue.  If the particles which scatter the light beam are very small, then the beam looks much stronger looking down it rather than from its side.  The visitor can compare relative brightness of the different arms of the light frame to determine the size of the particles that make its beam visible.

A Brief Discussion of Searchlight Concepts
by Marlowe Pichel

The Xenon short arc light source, used in Japan and parts of Europe for projectors, etc., is relatively new to the United States.  It consists of two closely spaced electrodes, usually made of tungsten (an anode and a cathode) encased in a quartz envelope, with the envelope  filled to a number of atmospheres' pressure with the rare gas Xenon.  These lamps take a relatively high voltage pulse, usually of radio frequency, to cause ionization between the electrodes, which in turn allows current to flow.  A short period of high open circuit voltage is required (higher than the normal running voltage of the lamp) to allow ionization to occur to a sufficient level for steady-state operation.  In operation, the arc occurs between the electrode points creating a plasma ball of relatively small size and high radiant energy.


The color temperature of these lamps is approximately 6000° Kelvin, which, more closely than any other source, approximates the spectral distribution of natural sunlight.  Because of this characteristic, the light produced from the lamp is extremely white and is excellent for natural color photography and many other uses such as solar simulation, etc. where the characteristics of sunlight are required.  In these short arc lamps, the arc is stabilized by the electrode configuration, containing it to a small steady spot between the two electrodes, with the higher intensity zone of illumination occurring closer to the cathode. The lumen, or light output per electrical watt input, is substantially higher for these lights than for conventional filament sources such as tungsten filament lamps.


There is another type of Xenon lamp called a Xenon long arc which has the electrodes spaced at some distance from each other and encased in a quartz envelope similar to a fluorescent tube.  These arcs are unstable in operation unless contained by the close proximity of the walls of the quartz envelope.  In this configuration, the walls get very hot and must be water cooled for high power lamps.  This type of lamp, however, produces an extremely brilliant line source which is good for certain applications such as area illumination, etc., where a point source is not required.


The reasons why Xenon short arcs are ideal for searchlights, are as follows:


1.   Higher light output per electrical watt input than incandescent (filament type) lamps.

2.   Cleaner and safer than carbon arcs.  Carbon arcs have been used for years in searchlights such as the World War II - advertising-type, 5-foot searchlights.  The carbon arc also gives a 6000  Kelvin color temperature with good spectral match to sunlight but they are difficult to maintain in stable condition and require elaborate electrode feeding mechanisms to maintain the arc, and the arc sputter and smoke gets all over the mirror and mechanism, etc.


3.   The Xenon short arc is extremely small (.point source) , radiating relatively uniformly in all directions, thus making it possible to collect and utilize the radiant energy better.


Comparisons of geometry between a filament and Xenon short arc source:


The smallest production incandescent source in the 100 watt andabove category is a high performance tungsten iodide lamp made by Sylvania which has a coiled filament measuring 0.1 inch in diameter by 0.11 inches long.  In comparison to most high-power filament sources, this is a small filament.  An equivalent Xenon short arc, 100-watt lamp has a "source size" (the effective illuminated plasma ball between the two electrodes) of approximately 0.010 to 0.015 inches in diameter.  This is almost ten times smaller than the equivalent filament type source.  For any given reflector design (focal length, diameter, etc.) and assuming that the source is perfectly symmetrical and the mirror is geometrically accurate and perfect, the beam spread or angle at which the beam diverges is a simple geometric ratio between the size of the source and the distance from the source to the reflector. Therefore, for 100 watts of electrical input, a Xenon short arc source will produce a ten-times narrower or more collimated beam than an equivalent filament source will produce.


For larger filament lights such as the 1,000, 2,000, 5,000 and 10,000 watt tungsten lamps, the filament array is a relatively large area of many s'eparate filament coils.  When viewed from different directions, these coils subtend varying angles with any point on the mirror surface.  Due to this characteristic, it is impossible to have a searchlight system having a uniform beam spread, as the angle subtended from various points around the mirror varies as the projected size of the filament source varies.


Pichel Searchlight Optical Design:


The basic optical design concept is different from former search-light designs in one basic way.  That is, it employs an extremely deep, short focal length, parabolic reflector with the source coaxially located in such a manner that nearly 100 percent of all the light emitted from the source falls on the reflector and is redirected into a collimated beam.  Even when using filament type sources, the filament is located coaxially within the deep reflector, and a much greater percentage of the energy emitted from the source is captured and redirected by the reflector.  This same concept has been used in the Army's 30" searchlight which employs a 20,000 watt Xenon source.  This searchlight has a peak beam candlepower of 1,200,000,000 candles.  This is achievable only because of the relatively small Xenon short arc source combined with an extremely accurate Pichel Industries, Inc. mirror.

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