Strobe light having reduced electromagnetic radiation

A high intensity strobe light having reduced electromagnetic radiation. A ash tube is mounted on a housing designed to both reduce electromagnetic interference and protect the flash tube from the environment, and light from the flash tube is piped to a reflector. The light pipe is surrounded by a waveguide proportioned to operate below cutoff frequency thereby providing attenuation to undesirable RF radiated from the flash tube.

BACKGROUND OF THE INVENTION 
The present invention relates to a strobe light and more particularly to a 
high intensity strobe light for anti-collision systems. 
An increased usage of strobe lights on aircraft, particularly military 
aircraft, is being made so that a high percentage of mid-air collisions 
and near-misses might be eliminated. A recent safety survey indicates that 
about 85 percent of mid-air collisions and near-misses have occurred 
during daylight with clear weather conditions and with about 80 percent of 
the aircraft having radar and/or radio contact with air traffic control at 
the time. These statistics tend to confirm that the present day 
incandescent rotating beacons on aircraft are adequate for night operation 
but are not bright enough for daylight operation in clear weather. 
The replacement of rotating beacons with presently available strobe lights 
has caused additional problems with the main problem being that of 
electromagnetic interference. Commercially available strobe lights 
generally have a zenon flash tube and reflector, a bank of capacitors, and 
power supply and timing circuit, all of which are encased in a housing and 
mounted outboard on an aircraft appendage, such as a wing or tail. These 
strobe lights, which might flash 60 to 70 times a minute, produce 
electromagnetic radiation which interfers with the aircraft's navigation 
and communication system to such an extent that the safety of the aircraft 
becomes of concern. 
A flash tube is generally comprised of two spaced apart electrodes within a 
sealed glass envelope having a rare gas fill, typically xenon, at a 
sub-atmospheric pressure. Such lamps are connected across a large 
capacitor charged to a substantial potential, which is, however, 
insufficient to ionize the xenon fill gas. Upon application of an 
additional pulse of sufficient voltage, the xenon is ionized, and an 
electric arc is formed between the two electrodes, discharging the large 
capacitor through the flash tube, which emits a burst of intense light, 
usually of short duration. In many cases the pulse voltage is applied 
between an external trigger wire wrapped around the envelope and the 
electrodes; this is referred to as shunt triggering. In other 
applications, the lamp may be internally triggered by applying the pulse 
voltage directly across the electrodes, a technique referred to as 
injection triggering. 
When the flash tube ignites upon being pulse triggered, it has been 
observed to inherently produce radio frequency (RF) interference from 14 
KHz and 1 GHz. Such radiated RF noise is extremely undesirable in a number 
of applications in the broadcast band (540-1770 KHz), the VHF band (50-500 
MHz) and the UHF band (300-500 MHz) where it interferes with direction 
finding, navigation and VHF and UHF communications equipment. 
In addition to being a source of undesirable radiation, the outboard 
mounting of these strobe lights subjects their flash tubes to other source 
of radiation which is of sufficient magnitude to flash the tubes. For 
example, a high-powered search radar might trigger a flash tube during 
every revolution of the radar's antenna and thus greatly reduce the life 
of the tube. As these flash tubes are relatively expensive and only have 
an average life of about 500 hours of operation, the accidental flashing 
greatly increases the costs of keeping these lights in operation. 
Various approaches have been made in an endeavor to reduce the 
electromagnetic interference caused by strobe lights. For example, in U.S. 
Pat. No. 3,840,766, which issued Oct. 8, 1974, to John A. Pappas and 
Robert J. Cosco, there is described an improved flash tube with reduced RF 
noise emission which is accomplished by providing a flashed barium deposit 
on portions of the inside surface of the flash tube. 
Another method employed to reduce electromagnetic interference involves 
coating the strobe light lens with a thin film, such as a film made of 
oxides of various metals, such as gold, tin, chromium and copper. Although 
these films reduce electromagnetic interference, they also reduce the 
amount of light that passes through the lens. 
SUMMARY OF THE INVENTION 
The present invention is for an improved strobe light which not only 
provides less electromagnetic interference than heretofore available 
lights, but also is designed to protect a flash tube so that it cannot be 
triggered by stray radiation. 
The flash tube is placed in a housing which protects the flash tube from 
adverse effects of the environment and provides radiation shielding. In 
addition, the housing with the flash tube therein, can be mounted below 
the aircraft skin which provides additional protection and shielding. 
Light from the flash tube passes through a light pipe to a reflector and 
the light pipe is surrounded by a waveguide which is designed to operate 
below cut-off frequency thereby providing attenuation to the RF radiated 
from the flash tube. 
It is therefore a general object of the present invention to provide an 
improved anti-collision strobe light which provides reduced 
electromagnetic interference. 
Another object of the present invention is to provide a high intensity 
strobe light having reduced susceptibility to stray radiation. 
Still another object of the present invention is to provide a strobe light 
which has its light system components protected from temperature, 
pressure, physical damage, moisture and electromagnetic radiation. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawing, a flash tube 11 and reflector 12 
are mounted in a housing 13 which is designed to be mounted beneath the 
aircraft skin 14, such as the skin of a wing. Housing 13 is designed to 
reduce the effects of the magnetic field produced by current pulses and is 
preferably made of high mu material, such as Mu-metal, Permalloy and 
Hypernom. Flash tube 11 might be of a commercially available type and be 
comprised of two spaced apart electrodes within a sealed glass envelope 
having a rare gas fill, typically xenon, at a sub-atmospheric pressure. 
Such lamps are connected across a large capacitor charged to a substantial 
potential, which is, however, insufficient to ionize the xenon fill gas. 
Upon application of an additional pulse of sufficient voltage, the xenon 
is ionized, and an electric arc is formed between the two electrodes, 
discharging the large capacitor through the flash tube, which emits a 
burst of intense light, usually of short duration. 
The spectrum of radiated energy from a commercially available flash tube 11 
extends from about 100 to 1000 nanometers with the visible portion of the 
spectrum extending from about 400 to 700 nanometers. The amount of energy 
radiated into the visible portion of the spectrum amounts to about 30 
percent of the total energy produced and the remaining 70 percent of the 
energy is radiated in the form of ultraviolet (UV) and infrared (IR). The 
optimal choice of flash duration appears to be about 0.2 second. A longer 
flash duration does not make the flash appear more intense and a shorter 
flash duration appears less intense. 
In addition to flash tube 11 and reflector 12, a strobe light system is 
comprised of a power supply, storage capacitors, and electronic circuits 
to set the pulse rate. A circuit board 15 is mounted on standoffs 16 
within housing 13 and supports the necessary components and circuitry for 
operating flash tube 11, and an RFI reducing connector 17 is provided on 
housing 13 for connecting the components and circuitry to a suitable power 
supply. 
Light from flash tube 11 is directed through a light pipe 18 to a reflector 
19 which disperses the light outwardly from light pipe 18. In the 
embodiments shown in FIGS. 1 and 2 of the drawing, the light pipe 18 is 
made of plastic material, such as lucite, and reflector 19 is made 
integral with light pipe 18 by machining the desired curvature or surface 
in light pipe 18 and then coating the surface with a reflective coating 
19, such as thin film metals, paint, and the like. It should be 
understood, of course, that various other shapes of reflectors might be 
used and also that a separate reflector might also be attached to light 
pipe 18, or to an aircraft, to provide the desired dispersion of light 
passing through light pipe 18. Also, it should be recognized that many 
other types of plastic material, as well as glass, might be used to make a 
suitable light pipe 18. 
In the preferred embodiment shown in FIG. 1 of the drawing, a circular 
waveguide 21 surrounds a portion of light pipe 18 and is attached to 
housing 13. By way of example, waveguide 21 might be made of aluminum 
tubing, but also might be made of other metals and have other shapes such 
as being square, rectangular, tapered and the like. Waveguide 21 is 
specifically designed to be an attenuator for the RF energy being 
proprogated from flash tube 11. It is well-known that when a circular 
waveguide is used at a frequency where the wavelength is equal to 1.7 
times the diameter, it will show little loss. This is the cut-off 
frequency. At frequencies below the cut-off, the waveguide becomes an 
attenuator. Typical applications of a waveguide as an attenuator assume 
the wavelength of the frequency attenuated to be 0.1 of the cut-off 
wavelength. 
The attenuation, A, in dB at such a gap, follows the 
wave-guide-beyond-cut-off criteria: 
EQU A.sub.dB = 0.0046 Lf.sub.MHz .sqroot. (f.sub.c /f.sub.MHz).sup.2 -1 (1) 
where 
L = waveguide length in inches; 
f.sub.MHz = operating frequency in MHz; 
g = largest diameter transverse dimension in inches; and 
f.sub.c = cut-off frequency of waveguide in MHz For a circular waveguide: 
EQU f.sub.c = 6920/g (2) 
When f.sub.c &gt;&gt; 10f.sub.MHz, equation (1) above becomes: 
EQU A.sub.dB .apprxeq. 0.0046 Lf.sub.c = 32 L/g dB 
for circular gap. 
By way of example, assume a circular waveguide has a diameter of 1.5 inches 
and a length of 4.5 inches. From equation (3) above, it can be seen that: 
##EQU1## 
and from equation (2) above, it can be seen that: 
##EQU2## 
For full attenuation, the wavelength of the frequency attenuated should be 
0.1 of the cut-off wavelength, thus from equations (4) and (5) it can 
readily be seen that the attenuation of 96dB would apply to frequencies of 
460 MHz and below. 
FIG. 3 of the drawing shows graphically the cut-off frequencies for various 
diameters of circular waveguides and also the attenuation per inch of 
length of waveguide. By way of example, assuming a circular waveguide 
having a 1 inch diameter and 4 inches long, it can be seen that the 
cut-off frequency is about 7000 MHz and the attenuation for 1 inch of 
length is about 32 dB. Thus, for 4 inches of length, the attenuation would 
be 128 dB. FIG. 3 of the drawing graphically illustrates that both the 
cut-off frequency and attenuation per inch of waveguide length, decreases 
with an increase of the waveguide diameter. 
From the above-listed equations and specific examples, it can readily be 
seen that a waveguide 21 and light pipe 18 can be selected that would have 
practical dimensions for use with a strobe light. 
Referring now to FIG. 2 of the drawing, there is shown a light pipe 18 
having a different shaped reflector and having a waveguide which is made 
by applying a conductive coating 22 around the periphery of the light pipe 
18. By way of example, coating 22 might be silver coated conductive paint, 
a conductive epoxy or a thin metallic film. 
It can thus be seen that the strobe light system described herein, not only 
protects the flash tube and associated hardware and circuitry from adverse 
effects of the environment, but also the waveguide provided therewith 
provides effective attenuation for frequencies one octave below the design 
cut-off frequency of the waveguide when the length to diameter ratio is 
kept 3 to 1 or more. 
Obviously many modifications and variations of the present invention are 
possible in the light of the above teachings. It is therefore to be 
understood that the invention may be practiced otherwise than as 
specifically described.