Fluorescent lamp with non-scattering phosphor

A fluorescent lamp comprises a source of near ultraviolet radiation together with an outer shell at least partially surrounding the ultraviolet source and comprising an ultraviolet transmissive material, the shell having embedded or dissolved therein a phosphor material having an indexed refraction approximately, but not quite equal, to the index of refraction of the shell.

BACKGROUND OF THE DISCLOSURE 
The present invention relates to fluorescent lamps and, more particularly, 
to fluorescent lamps employing near ultraviolet radiation sources together 
with phosphor material which is index matched to an outer envelope. 
In conventional fluorescent lamps, most of the visible light output 
originates in a thin layer of phosphor which is disposed immediately 
adjacent to a mercury discharge. This fact is a consequence of the high 
optical absorption coefficients that most phosphors exhibit at the 254 
nanometer wavelength of mercury resonance radiation in the far ultraviolet 
spectral region. Since much of this light is directed back into the lamp 
be scattering, useful light output is lost by multiple scattering within 
the outer portion of the phosphor material and by absorption at the ends 
of the lamp. 
It would be desirable to minimize this visible light scattering loss by 
optimizing the scattering characteristics of the phosphor for escape of 
the visible length radiation from the lamp. However, in conventional lamps 
where the excitation occurs as a result of the mercury resonance radiation 
at 254 nanometers, it is not possible to imbed phosphors in a suitable 
optimizing matrix since no low melting point material which is transparent 
to this short wavelength radiation is available. 
However, in U.S. patent application Ser. No. 288,822, filed July 31, 1981, 
there is disclosed a compact fluorescent lamp with copper arc excitation. 
The lamp disclosed therein is an efficient producer of near ultraviolet 
radiation. For example, if a copper halide is used in the ionizing 
discharge medium, ultraviolet radiation at 324.7 and 327.4 nanometers is 
produced. It is the use of a practical, near ultraviolet source for 
phosphor excitation which makes possible the consideration of employing 
materials, particularly plastic materials and low melting point glasses, 
in which a phosphor may be embedded to optimize optical properties. More 
particularly, the present invention discloses a fluorescent lamp in which 
the phosphor and embedding matrix material comprises substances which are 
matched to one another with respect to their indices of refraction or in 
which an optically homogeneous solution of phosphor and matrix is 
employed. 
It is noted, however, that the use of phosphor materials embedded in a 
light-transmissive matrix which is matched to the phosphor in terms of 
their respective indices of refraction has been employed in the past in 
significantly different applications. In particular, the use of such 
phosphors in index matched plastic material has been proposed as a 
substitute for certain crystal X-ray detecting phosphor bodies. However, 
consideration of such materials in fluorescent lamps was not possible 
until the development of the fluorescent lamp described in above-mentioned 
application Ser. No. 288,822, submitted in behalf of the same inventor as 
herein, and assigned to the same assignee as herein. Accordingly, the 
above-described patent application is hereby incorporated herein by 
reference. 
In short, since conventional fluorescent lamps employ a mercury resonance 
radiation at the far ultraviolet region of the sprctrum, around 254 
nanometers, it has not been possible to dispose the phosphor within a 
matrix medium since no low melting point material transparent to such 
short wavelength radiation was available. Furthermore, even though 
phosphors have been index matched to matrix media in the past, for the 
purposes of applications such as X-ray detection, index matching of 
fluorescent lamp phosphors was not generally thought to be available 
because of the lack of a suitable matrix material which was transparent to 
far ultraviolet radiation. However, a newly-developed, practical 
fluorescent lamp producing ultraviolet radiation in the near ultraviolet 
range has now made it possible to consider embedding phosphor materials in 
a matrix medium which is transparent to the near ultraviolet radiation. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, a 
fluorescent lamp comprises a source of near ultraviolet radiation having a 
wavelength between approximately 280 and approximately 400 nanometers 
together with a shell at least partially surrounding the ultraviolet 
source. The shell comprises a material transmissive to ultraviolet 
radiation in the above-mentioned range and transmissive to visible length 
radiation, the shell having embedded therein a phosphor material having an 
index of refraction approximately equal to the index of refraction of the 
shell material. In one embodiment of the present invention, the outer 
surface of the shell is provided with an irregular texture as by roughing, 
etching, molding, grinding, machining, or casting so as to enhance the 
escape of visible light. In still another embodiment of the present 
invention, the shell or outer envelope is plastic and the phosphors are 
organic phosphors which are dissolved within the plastic material. 
Accordingly, an object of the present invention is to provide a more 
efficient fluorescent lamp. 
It is also an object of the present invention to optimize the amount of 
visible wavelength radiation emitted from a fluorescent lamp. 
Additionally, it is an object of the present invention to provide a 
fluorescent lamp having excitation in the near ultraviolet region.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates one embodiment of the present invention in which outer 
jacket or shell 40 and arc tube 20 are configured in the general size and 
shape of a lamp envelope which serves as a replacement for conventional 
incandescent lamps. However, the lamp of the present invention may assume 
a large range of sizes and shapes. Furthermore, electronic ballast means 
are not illustrated, since they are not part of the present invention. 
However, conventional electronic or other ballast means are typically 
employed in such lamps. In the Figure, lamp 10 is seen to possess outer 
jacket or envelope 40 having phosphor particles or material disposed 
therein. Exterior envelope 40 is preferably gas-tight and is transmissive 
to light at visible wavelengths. Outer shell 40 may be conveniently 
fastened to glass base 17 through which leads 16 are disposed. Shell 40 
surrounds arc tube 20, which is more particularly described and discussed 
below. Arc tube 20 is a source of radiation at specific ultraviolet 
wavelengths and may comprise either relatively hard or relatively soft 
glass depending upon the operating temperature of the lamp. Arc tube 20 is 
supported within shell 40 by means of stiff electrode leads 18 and 19 as 
shown. Leads 18 and 19 are also electrically and mechanically connected to 
leads 16, which pass through base 17. Base 17 also serves as a stable 
platform for mounting arc tube 20 within shell 40. As is conventionally 
known in the art, electrodes 16 typically comprise or are coated with a 
metal composition to which the glass in base 17 is particularly adherent. 
In this way, the gas-tight integrity of outer shell 40 may be maintained. 
As is also well known in the lamp arts, electrodes 16 are coupled in a 
conventional manner to an appropriate electronic ballast which operates to 
supply starting and running voltage for the lamp. 
A number of phosphors may be embedded in shell 40. The principal 
requirement for these phosphors is that they absorb radiation in the near 
ultraviolet region and reradiate visible wavelength radiation. For 
example, but without limitation, these phosphors may include compounds 
such as yttrium vanadate doped with europium, zinc silicate germanate 
doped with manganese, and magnesium germanate doped with manganese. 
Organic phosphors which may be dissolved in shell 40, particularly if it 
comprises plastic, include, but are not limited to: perylene, chrysene, 
fluorescein, rhodamine, ethylviolet and malchite green. 
Space 15 between arc tube 20 and outer shell 40 preferably comprises a 
vacuum for the above-described inorganic phosphor materials. A vacuum is 
also preferred whenever the phosphor employed is one which is most 
efficient when operating at or near room temperature. In other embodiments 
of the invention in which the phosphor employed exhibits a higher 
efficiency at elevated temperatures, space 15 preferably includes inert 
gas such as nitrogen or argon so that some convective and/or conductive 
heat flow may be provided to the phosphor to permit arc tube 20 to provide 
the desired operating temperature for the phosphor. 
The Figure also illustrates the construction of arc tube 20 which 
preferably comprises a gas-tight ultraviolet transmissive arc tube 20 
having electrodes 24 disposed at either end thereof. Electrodes 24 may 
comprise enlargements of the ends of electrode leads extending into arc 
tube 20. In order to provide a gas-tight seal, a portion of leads 18 and 
19 may comprise metal foil strip 21 comprising a metal, is specifically 
chosen to form a gas-tight adhesive bond to the glass of arc tube 20 and 
electrical connection to the metal of the electrode leads. Such foil 
typically comprises molybdenum. 
A significant aspect of the present invention which is related to the 
production of the proper wavelength ultraviolet radiation is the inclusion 
within arc tube 20 of an appropriate amount of a vaporizable discharge 
medium. For example, this medium may be disposed within arc tube 20 as 
pellet 30. In accordance with one preferred embodiment of the present 
invention, the discharge medium comprises either copper halide, rhenium 
halide, magnesium halide or silver halide. However, copper halide is the 
preferred choice since rhenium and silver are not as easy to work with in 
their halide forms. However, either copper bromide or copper iodide may be 
employed to produce the desired ultraviolet output from arc tube 20. 
Copper from the halides exhibits a strong and efficient output at near 
ultraviolet wavelengths of 327.4 and 324.7 nanometers. Furthermore, even 
though their halides may be difficult to work with, both rhenium and 
silver exhibit ultraviolet radiation at appropriate wavelengths and may be 
employed in the lamp of the present invention. In particular, rhenium 
exhibits ultraviolet radiation having a wavelength of 346.5 nanometers. 
Likewise, silver exhibits ultraviolet radiation at a wavelength of 328 
nanometers. Magnesium has been suggested in the past as a radiating 
species. Magnesium has a strong resonance line at 285.2 nanometers, but 
this radiation is absorbed by many plastics and glasses making it less 
desirable for use in the present invention. 
For example, if copper iodide is employed in pellet 30, and if the lamp is 
operated at the reservoir temperature between approximately 500.degree. C. 
and approximately 900.degree. C., the copper iodide exhibits respectively 
correspondingly vapor pressures of between approximately 1 and 
approximately 100 torr. However, the preferred operating temperature of 
the present lamp is approximately 600.degree. C. At this temperature, an 
arc tube comprising relatively expensive fused quartz is not required and 
a less expensive, hard glass may be used for arc tube 20. Additionally, it 
is also desirable to add small amounts of argon or other noble gas to arc 
tube 20 for the purpose of facilitating lamp starting. However, such noble 
gases may be added for other purposes in amounts which vary according to 
the function desired. Optionally, mercury may be added to obtain desired 
electrical characteristics. Mercury is added to produce a partial pressure 
of from about 1 to about 20 atmospheres at operating temperatures. 
A significant advantage to be noted in the fluorescent arc lamp shown in 
the Figure is the fact that the phosphor is isolated from the discharge. 
As opposed to conventional fluorescent lamps, the environment for the 
phosphor, the vacuum and the inert gas can be selected independently, 
thereby eliminating phosphor deterioration due to the presence of mercury 
atoms or ions, electrons, or short wavelength ultraviolet radiation. 
In one embodiment of the present invention, phosphor material is embedded 
in plastic or low melting point glass which is, in turn, used as the outer 
jacket or envelope 40 of lamp 10. In another embodiment of the present 
invention, the phosphor comprises organic phosphors which are dissolved, 
rather than embedded, in the plastic material. In both cases, the 
resultant outer envelope acts as an optically homogeneous medium. The 
present invention also includes the use of alternating layers of phosphor 
and plastic materials. Insofar as the phosphor-containing shell is 
optically clear and homogeneous, a significant fraction of the radiation 
emitted is trapped by total or partial internal reflection of the surfaces 
of the layers. For instance, in a solid with index of refraction n-1.54 
the critical angle is 41.degree. and, thus, 25% of the 
isotropically-emitted radiation is totally internally reflected. In the 
case of the embedded particulate phosphor, it is desirable to employ 
phosphor in plastic materials having a slight mismatch between the index 
refraction of the phosphor and that of the embedding medium. This slight 
mismatch permits a greater amount of visible wavelength radiation to 
escape but does not significantly increase multiple internal reflections. 
In the embodiments in which organic phosphors are dissolved in a solid 
plastic solution, an effect similar to slight mismatch of the indices of 
refraction is obtained by embedding scattering particles in the plastic 
material. In both cases, a certain fraction of the light is still 
scattered back into the interior of the lamp where some of it is lost by 
absorption. However, additional modification can still be made to further 
optimize the escape of light from the lamp. In particular, the outer 
surface of shell 40 may be roughened or configured to have a roughened 
surface. Roughening may be accomplished by sand blasting or grinding, 
machining, molding or casting. Thus, escape of light is assisted by 
forming a multiplicity of prismatic-shaped surfaces on the exterior 
surface of shell 40, these surfaces not being parallel to the inner 
surface of shell 40. It is nonetheless desirable to have the inner surface 
optically smooth to inhibit, so far as possible, return of emitted visible 
wavelength radiation to the interior of the lamp where it may be absorbed. 
However, the outer surface preferably possesses an irregular texture. 
FIG. 2 shows a cross section of a portion of shell 40 more particularly 
being hatched to indicate that phosphor particles may be embedded in a low 
melting point glass. In the case of a close matching of indices of 
refraction for the glass and the phosphor, FIG. 2 is also illustrative of 
the fact that light scattering particles may also be included. FIG. 3 
shows a cross section of a portion of shell 40 more particularly being 
hatched to indicate the situation in which an organic phosphor is in 
solution with a plastic material. In this case, separate species are not 
present. However, in all embodiments of the present invention, shell 40 is 
characterized by being substantially optically homogeneous and containing 
phosphor for absorbing incident UV and reradiating at a different 
frequency. 
From the above it may be appreciated that the present invention provides an 
efficient fluorescent lamp which is optically optimized for the production 
and radiation of visible wavelength radiation. It is further seen that the 
lamp of the present invention is made possible by the use of ultraviolet 
sources emitting radiation in the near ultraviolet region. 
While the invention has been described in detail herein in accord with 
certain preferred embodiments thereof, many modifications and changes 
therein may be effected by those skilled in the art. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.