Self-luminous light source

A self-luminous light source is disclosed having a light-permeable shell, a body disposed within the shell, defining a space between the shell and body, a member connecting the shell and body, radioactive gas disposed in the space, and a phosphor coating on at least one surface of the shell and body. The shell and body may be coaxial glass tubes defining an annular space of restricted width, connected to one another by annular end members.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of self luminous light sources 
employing radioactive gas to activate phosphors deposited on surfaces of 
the light source, and in particular, to an elongated tubular light source 
charged with radioactive tritium. 
2. Description of the Prior Art 
Use of a radioactive gas and a phosphor coating responsive to the emissions 
of the gas, is known in the art. Such light sources generally take the 
form of simple glass tubes enclosing the radioactive gas in a cylindrical 
space. In connection with self-luminous light sources for watch dials and 
the like, rectangular or other enclosing shapes are employed. In any 
event, the conventional teachings of the art are to take a simple, sealed 
glass body having a phosphor coating on the internal surfaces thereof, and 
to charge the body with the radioactive gas. Particle emissions incident 
to radioactive decay of the gas within the body activate the phosphors on 
the inner surface of the external shell, producing light emissions by the 
phosphors. 
Competing concerns such as safety on the one hand, and maximizing 
brightness and efficiency of the light source on the other hand, require 
the designer to make a number of choices in the configuration of a light 
source. A larger enclosed volume results in a larger active surface area, 
producing increased light. In order to further increase brightness, it has 
sometimes been found necessary to employ a radioactive gas of a type in 
which particles released during radioactive decay are emitted at 
relatively high energy, for example, krypton-85 (.sub.36 Kr.sup.85). Gas 
of this variety will cause photon emissions in phosphors at a distance on 
the range of tens of centimeters from the decaying atoms, however, such 
relatively-powerful emissions are not healthful for humans. 
Safer levels of particle emission energy are obtained with use of tritium 
(.sub.1 H.sup.3) as the radioactive gas. Particles emitted by decaying 
tritium will activate phosphors within a range of millimeters from the 
decaying atoms. Accordingly, tritium is a preferred source of 
phosphor-activating emissions in light sources intended for use in 
proximity with persons. Unfortunately, such low energy particle emissions 
are also only able to produce a relatively weak level of light emission in 
the phosphors. 
Production of any radioactive gas is a relatively expensive procedure. It 
is sometimes the case that the expense of mechanical construction, 
packaging and the like is small relative to the cost of radioactive gas 
used in self-luminous light sources. Use of a larger volume of gas and the 
resulting additional phosphor surface area will, up to a point, 
proportionately increase the total brightness of the light source. There 
is a limit, however, to gains in brightness per unit of radioactive gas 
achievable from using increased amounts of radioactive gas. 
As the gas space becomes larger, more of the emitted particles are absorbed 
by neighboring atoms in the gas, and never reach the phosphor coating to 
produce light. With respect to tritium, for example, a glass tube having a 
phosphor coating on its inner surfaces and a diameter greater than several 
millimeters, i.e., the transmission range of tritium, will be of lower 
total brightness per unit of tritium (i.e., a lower efficiency) than a 
group of tubes each having a diameter within the transmission range and 
enclosing the same total amount of gas. In other words, with the single 
large diameter tube, emitted particles which happen to be directed 
radially inward are unlikely to ever reach the phosphors on the far side 
of the tube. These particles will be absorbed in the gas itself, and will 
not help produce light. 
The relatively low energy of tritium emissions frequently makes tritium 
unattractive as a phosphor-activating element in a large light source. The 
low power emission capabilities of tritium means that increasing the 
diameter of the light source tube in order to increase surface area in 
fact makes an inefficient use of the tritium. In this respect efficiency 
is the total light emitted per unit of tritium. 
U.S. Pat. No. 3,038,271--MacHutchin et al teaches a self-luminous sign in 
which a plurality of glass tubes of relatively small diameter are used to 
activate phosphor coatings over the area of the sign. If the diameter of 
each of the tubes is kept small, namely within the transmission range of 
tritium, the disclosed sign can be expected to be relatively efficient in 
production of light, that is, achieving a reasonable total brightness per 
unit of tritium, and therefore per unit of cost. Use of a plurality of 
separate closed glass tubes causes other problems, such as difficulty in 
production, handling, mounting and the like. 
U.S. Pat. No. 3,566,125--Linhart, Jr., et al teaches a light source having 
a particular contour for the gas-holding space. Light emission is said to 
be improved by a parabolic facing surface on the phosphor-bearing body, 
which is enclosed within the light source. It is believed that the 
increase in luminosity of the Linhart device is due to the increase in 
phosphor-bearing surface area of a curved area over a flatter one. 
Linhart's respective embodiments include a number of arrangements in which 
the transmission range of radioactive emission is clearly exceeded, 
particularly as to emissions directed toward the rear of the parabolic 
surface of the phosphor-mounting body. 
In the embodiment of FIG. 6, Linhart uses a collimating lens having a 
convex rear surface. The convex lens has a contour at least partly 
complementing the parabolic, phosphor-coated surface. Such restriction on 
the depth of the gas-enclosing space should be a relatively efficient use 
of radioactive gas. The construction has a number of drawbacks. The 
efficiency is achieved at expense of a need for multiple parts of 
dissimilar materials, and the need to connect the parts in a seal which 
will be impermeable to tritium. Tritium is, of course, a form of hydrogen, 
which is prone to difficulties with leakage and will diffuse directly 
through many materials. 
U.S. Pat. No. 3,005,102--MacHutchin et al teaches simple gas-enclosing 
phosphor-coated tubes, but also discloses one embodiment in which a 
flashlight bulb is simulated using a gas-enclosing plenum of 
relatively-restricted depth. Reference may be made to MacHutchin's FIG. 3, 
in which the gas-charged plenum is laid over a hollow glass bulb, with 
expected increase in efficiency. MacHutchin U.S. Pat. No. 3,005,102, like 
Linhart Jr., appears to teach an arrangement restricting the depth of the 
gas space to a distance approaching the transmission range of the 
radioactive gas. Both patents, however, teach a plurality of dissimilar 
parts in complex constructions. The constructions, using complex 
geometrical shapes, and requiring gas-tight junctions, will certainly be 
difficult and expensive to manufacture. Increased manufacturing expenses 
may increase the product cost to an extent that safety and gas 
conservation gains are outweighed. Moreover, a number of usual 
junction-making materials are simply not feasible due to diffusion and 
loss of radioactive tritium through the seals. 
U.S. Pat. No. 3,176,132--Muller teaches a refinement of the usual glass 
tube. A central tube, holding a souce of radioactive emissions, is mounted 
within a casing tube, and a plurality of coaxial phosphor-bearing tubes, 
or a spirally wound sheet of phosphor-bearing material, is disposed 
between the central axial radioactive tube and the casing. This 
construction is said to be useful to confine the radioactive emissions. 
While emissions may be confined, such a construction merely aggravates the 
difficulty with the low transmission range of tritium. Not only will the 
transmission range be possibly exceeded between the central source of 
radioactive emissions and the peripheral phosphors, but intermediate 
layers of phosphor-bearing material, phosphors and gas will themselves 
absorb emissions. Moreover, photon emissions from the excited inner 
phosphors must pass through multiple surrounding layers to reach the 
casing, in order to be released from the lamp as useful light. 
Accordingly, although Muller teaches a structure including coaxial tubes, 
the teachings emphasize safety over efficiency, and are more appropriate 
for high energy particle emitting gases and the like. 
The present invention employs a central body and an external casing, the 
body and casing together defining a gas-enclosing space of restricted 
width around the device. The inward-facing walls of the enclosed space are 
preferably all coated with phosphors. The invention therefore conserves 
gas by not exceeding the transmission range of the gas, for example 
tritium. Inasmuch as the device is preferably formed by a pair of coaxial 
glass tubes, sealed to form an annular glass-bounded area, an integral 
glass body results in which no possibility of leakage or diffusion loss is 
presented. 
Apart from radioactive self-luminous devices, in connection with electric 
discharge devices and chemically-operated self-luminous lamps, a number of 
coaxial tube constructions are known. In electric discharge devices, outer 
tubes are structured and intended as optical filters or for mechanical 
protection, and are not arranged to form a confined space for a 
radioactive gas. On the contrary, the operative gas and the electric 
discharge elements are almost invariably mounted in the central axial 
space. There is therefore no particular requirement of a complete 
enclosure around the inner tube. In addition, there is no difficulty with 
any safety consequences of radioactive emissions and no need for spacing 
of elements because the most dangerous emission expected from the light 
source (and the most often blocked via a shield) is ultraviolet radiation. 
U.S. Pat. No. 3,358,167--Shanks teaches a jacketed electric discharge lamp 
in which an outer casing physically protects the operative electric 
discharge light source mounted along the axis. A resilient plug having a 
central circular opening for receiving the light source, and an annular 
groove for receiving the casing is disclosed. 
U.S. Pat. No. 2,080,919--Ihln et al teaches a spring-like form of resilient 
spacer in an electric discharge device. The space between the light source 
and the casing is evacuated, to decrease heat loss by 
conduction/convection. 
A third category of interest is light sources powered by chemical reaction. 
Unlike either electric discharge devices or radioactive light sources, 
chemically self-luminous devices employ sealed containers of reagents 
within an external casing. The containers frequently are tubular, and 
means are provided to break or otherwise open the containers and thereby 
mix the reagents. These devices seldom have any direct connection between 
inner and outer tubes. 
The present invention involves a coaxial tube arrangement particularly 
adapted for self-luminous radioactive light sources. An optimum width gas 
enclosure is produced by a relatively inexpensive and easy to manufacture 
construction. The light source as so constructed can be further mounted in 
a casing with resilent shock-absorbing means and/or provided with a 
mounting as desired for a given use. 
SUMMARY OF THE INVENTION 
It is an object of the invention to optimally increase the active surface 
area of a radioactive light source in order to achieve a high total 
brightness per unit of radioactive material. 
It is an object of the invention to conserve radioactive gas and thereby 
minimize the expense of radioactive self-luminous light sources. 
It is a further object of the invention to achieve high total brightness 
without degrading safety of a radioactive light source. 
It is another object of the invention to produce a bright, inexpensive and 
long lasting light source which is easy to manufacture, safe and 
convenient. 
These and other objects are accomplished by a self-luminous light source 
comprising a light-permeable shell, a body disposed within the shell, a 
space being bounded by facing surfaces of the shell and the body, at least 
one member spacing the shell and the body, and holding the shell and body 
with respect to one another around the space, a radioactive gas disposed 
in the space, and, a phosphor coating on at least one of the surfaces of 
the shell and/or body, the phosphor coating emitting light in response to 
exposure to emissions of the radioactive gas. The light source preferably 
comprises coaxial gas tubes defining an axial space within the inner glass 
tube, the axial space being left open, and an annular space between the 
tubes, sealed at the ends by glass members, for enclosing the radioactive 
gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The light source of the invention may be packaged in various ways. It will 
be appreciated that a static or permanently-mounted installation will 
require less in the way of shock absorbing means, while mobile or exposed 
locations may require additional protection, and the like. The invention 
will be described with reference to a protected but otherwise unmounted 
light source, or to a hand-held light source, as shown in FIGS. 1 and 6, 
respectively. It should be appreciated that the invention is applicable to 
various mounting arrangements, other constructions and uses. 
The basic light source of the invention, as shown in FIGS. 2 and 3, 
preferably comprises a light-permeable glass outer shell 38, enclosing an 
inner body of slightly smaller diameter, for example, inner tube 32, 
coaxial with the outer shell. The inner tube need not be a coaxial glass 
tube, but may be a solid body spaced from shell 38. The shell and/or body 
may also be of irregular cross-section, or the like. It is presently 
preferred that coaxial glass tubes, preferably of borosilicate glass or 
the like, be integrally joined at their ends using annular glass portions 
of complementary shape. 
A radioactive gas such as tritium (.sub.1 H.sup.3) is disposed in the 
annular space 44 between the outer shell 38 and the inner body 32. A 
phosphor coating 36, responsive to the emissions of the tritium or other 
radioactive gas, is applied to the external surfaces of inner body 32, 
and/or to the internal surfaces of outer shell 38. Beta particles emitted 
by the tritium during radioactive decay excite the phosphor which in turn 
releases photons. 
In order to take maximum advantage of light emitted by the excited 
phosphors, and for ease of construction, it is preferred that inner body 
32 be a tubular glass shell of slightly smaller diameter than the external 
shell 38. The central axial space 34 encompassed by the inner shell is 
unused and may be left open to the air. Alternatively, the axial space 
could be used to attach the device to a mounting, for example using an 
axial dowel. An elongated annular space bearing tritium and bounded by the 
phosphor-coated transparent bodies, is the primary source of light. 
Inasmuch as particles emitted by tritium have relatively low energy 
levels, the relatively narrow width of gas space has the effect that a 
larger proportion of the total emissions of radioactive decaying gas will 
strike the phosphors, rather than be absorbed in the neighboring molecules 
of gas. 
In addition to the beneficial effect of bringing the phosphor-bearing 
surfaces to within the transmission range of the gas, the geometry of the 
invention has further beneficial consequences. The use of a narrow 
gas-enclosing space is also a means of increasing the surface area for 
bearing phosphors. For a given gas volume, the invention provides more 
phosphor area per unit of enclosed volume than does a simple cylinder. 
Besides allowing this additional amount of phosphor surface, the invention 
brings the outer surface of the inner body and its phosphor coating to 
close proximity to the exterior shell, whereby photon emission from the 
inner body phosphors is ultimately less attenuated when emitted from the 
light source. Of course narrowing the annular gas enclosing space also has 
the effect of reducing the number of particles of radioactive decay which 
strike the phosphors. 
The particular optimum width of the enclosed space which will produce the 
most light per unit of gas is subject to a number of variations such as 
the variation in transmission range depending on the type and pressure of 
the particular gas used. For example, at higher gas pressure, the 
transmission range will be reduced due to the increased probability that 
emitted particles will strike neighboring gas molecules. Similarly, the 
choice of phosphor and thickness of phosphor coating will impact on the 
particular optimum by altering the light transmission properties of the 
outer shell. 
The optimum dimensions of the overall unit will be likewise dependent upon 
the transmission range of the particular choice of radioactive gas. 
Tritium, an emitter of low energy particles, has a transmission range on 
the order of millimeters, and a tube 30 designed for use of tritium would 
therefore have an enclosed annular space 44 of that range of width. On the 
other hand, krypton-85 (.sub.36 Kr.sup.85) has a transmission range in the 
tens of centimeters, and a large light source could thereby be prepared 
having an annular gap in that range, and a higher luminosity. 
Suitable phosphors for radioactive light sources are known in the art. 
Examples of suitable phosphors are zinc-cadmium-sulfide, zinc sulfide, 
zinc silicate, cadmium sulfide, and the like. The phosphors may be applied 
to the inner surfaces in the form of powder, by use of a suitable glue, 
binder or other vehicle, as also known in the art. The light emission can 
be directed efficiently outwards by coating the internal surface of the 
central tube with reflective material such as white paint. 
In order to charge the vessel defined by the inner body and outer shell 
with tritium, capillary seal tubes 40 are provided, for example at either 
or both ends of the enclosed space. One tube 40 will suffice, depending on 
the process used. The enclosed space is simply charged with tritium, and a 
portion of the tube melted, whereupon collapse of the tube or the bead of 
melted glass thereby formed closes capillary tube 40. This procedure 
entirely closes the bounded space 44 in glass. Capillary tubes 40 can be 
melted down as close as convenient to the covered ends 46. Short capillary 
tubes, of course, are more convenient and less prone to breakage. 
The overall assembly may be formed by a number of procedures, as known in 
the art of glass working. The bodies may be formed by lamp working to join 
complementary flanged tubes of the needed diameters, with or without use 
of frit. It may also be desirable to employ tubes having one closed end 
for either or both tubes 32, 38, the annular space being closed on the 
other end. 
Glass is preferred for use with the invention due to the particular 
properties of glass, and the radioactive gases employed. Glass is, of 
course transparent. Glass is also sufficiently dense to confine tritium 
gas which, as an isotope of hydrogen, would diffuse through many 
materials. Borosilicate glass is preferably used in a light source with 
tritium. For krypton-85, it may be necessary to employ a different glass, 
as known in the art, in order to avoid browning of the glass over time as 
a result of exposure to radiation. 
Unlike electric discharge devices and/or chemically operative self-luminous 
light sources, radioactive light sources do not generate substantial 
temperature variation in use. Once the unit is charged, the phosphors are 
excited and the lamp stays lighted until the phosphors are eventually 
broken down by the radioactive emissions, or until an unacceptable 
proportion of the tritium decays into helium 3 (.sub.1 He.sup.3). The unit 
should be designed to withstand the usual temperature variations and 
mechanical shocks expected in the particular environment. 
Light source tubes 30 can be used for various applications, including those 
known in the art. Self-luminous exit signs for buildings and vehicles, 
highway markers, aircraft markers (both inside and outside) and stationary 
mobile units may be constructed by employing the tube of the invention in 
place of conventional light sources or simple hollow cylinders or 
radioactive, phosphor-coated glass. When used as a mobile or 
manually-carried light source, additional protective features or means for 
conveniently manipulating the light source are recommended, for example as 
shown in FIGS. 1, 4 and 6. FIG. 1 shows a completely-assembled 
shock-resistant unit. Lamp unit 30 (comprising co-axial tubes) is disposed 
within yet another outer coaxial member 50. Transparent member 50 is 
preferably formed of a relatively resilient transparent plastic, or a 
thick layer of glass, to decrease the possibility of breakage. Plastic may 
be expected to decrease the incidence of breakage, and also facilitates 
attachment of additional mounting features. An exploded view of the 
assembly is shown in FIG. 4. 
Transparent external casing 50, for example of plastic, is internally 
threaded at its ends 52, 52, for receipt of end plugs 54, 54. End plugs 54 
are threadably fitted into threaded ends 52 of casing tube 50. One or both 
of the end caps may be threaded or provided with other attachment means to 
secure the device to a mount. 
Casing 50, and for that matter glass tubes 32, 38, may be transparent, 
translucent, frosted, colored or otherwise adapted to the needs of a 
particular situation. The basic color emitted is substantially governed by 
the choice of phosphor, however, a certain range of modifications are 
possible. For example, a more attractive light source may be produced by 
employing a frosted external casing 50, and a brighter or more-focused 
light may be produced by a completely-transparent casing. A frosted or 
translucent outer tube and/or external casing will also tend to 
attractively conceal details of internal construction, for example the 
existence of shock absorbing discs 64. 
The external dimension of tube 30 may be of smaller diameter than the 
inside of casing 50, or may be nearly the same diameter. In order to 
minimize the possibility of breakage, a relatively larger space can be 
allowed between casing 50 and tube 30 and that space filled with a 
resilient shock-absorbing pad or spacer such as disc 64. With reference to 
FIGS. 1 and 4, suitable shock-absorbing mounting means 64 may be provided 
from sponge rubber or the like in the form of a wafer. The spacer is 
axially cut to fit tightly around tube 30, and externally dimensioned to 
fit tightly within tube 50. Additional shock-absorbing means (not shown) 
may be placed between the ends of tube 30 and caps 54. The shock absorbers 
supply resilience to cushion tube 30 against impact should be unit be 
dropped or struck against a hard surface. 
With reference to FIG. 5, other possible constructions for the tube and 
spacer are possible. For example, a tube 70 of rectangular cross section 
can be formed from a pair of elongated square tubes, 82, 84. Like the 
embodiment of FIGS. 1-4, only the space between tubes 82, 84 is charged 
with radioactive gas, and the space is entirely sealed in glass by melting 
capillary tube 90. This completed glass "lightbulb" can be further 
packaged as needed. 
FIG. 5 also shows an alternative embodiment of a spacer 76. Spacer 76, as 
above, employs a resilient body, for example of sponge rubber, for 
surrounding and cushioning tube 70. In addition, a central plug 86 is 
integrally formed with spacer 76, the plug 86 being inserted into the 
axial space inside inner tube 82, further connecting spacer 76 and tube 
70, and also protecting any extending portion of capillary tube 90. It 
will be appreciated that inasmuch as internal plug 86 is formed integrally 
with the spacer, this mounting means may be employed only over the ends of 
the tube. If desired, one or more additional spacers, lacking the central 
plug 86, or possibly to be used with separate plugs, may be employed for 
use at intermediate portions along the length of tube 70. Further 
mechanical support can be provided, or some protective features omitted, 
as needed in given uses. 
Another possible variation on the invention is shown on FIG. 6. In FIG. 6, 
a handle 74 is mounted along the side of the light source. In addition, a 
reflective shield 72 is provided along one side, for example under the 
handle, allowing the user to direct the emission of light. The reflector 
can be mounted inside the external casing, and held in its position by the 
inward pressure of caps 54. Handle 74 can be likewise provided with 
members engaging end caps 54, or may be simply glued to the casing. If 
desired, a movable cover can be provided to enclose the device when light 
emission is not desired. 
Further variations on the present invention are possible, and will be 
apparent in light of this disclosure to persons skilled in the art. 
Reference should be made to the appended claims rather than the foregoing 
specification as indicating the true scope of the invention.