Note: Descriptions are shown in the official language in which they were submitted.
CA 02185267 2006-09-15
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HIGH INTENSITY ELECTRODELESS
LOW PRESSURE LIGHT SOURCE
Field of the Invention
This invention relates to electric lamps and, more particularly, to a low
pressure, high intensity fluorescent light source that can produce
considerably more light per unit length than conventional electroded
fluorescent lamps.
Background of the Invention
Very high output (VHO) fluorescent lamps and metal halide high
intensity discharge (HID) arc lamps provide efficient, high lumen output and
good color rendering. The VHO fluorescent lamp is based on conventional
electroded fluorescent technology. For the electrodes to have a long life
(about 10,000 hours), the buffer gas pressure in these lamps is about 2 torr,
and the discharge current is typically less than 1.5 amperes. To minimize
saturation in ultraviolet radiation and thus provide acceptable efficacy, VHO
fluorescent lamps operate with a relatively light gas, such as neon, at buffer
gas pressures of about 2 torr. The requirements for long life and efficacy
limit the parameter space in which these lamps can operate, and ultimately
this restricts the maximum axial light density that these lamps can produce
efficiently. Thus, VHO fluorescent lamps are relatively long for the amount of
light they produce, and their efficacy is moderate, typically no more than
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about 70 lumens per watt. However, because VHO fluorescent lamps can
be tailored to provide a uniform, stable and rich color spectrum, they are
widely used in large stores where good, stable color rendering and instant
turn on and turn off are required.
The metal halide HID lamp is an arc lamp that is considerably more
compact than the VHO fluorescent lamp. The overall length of the entire
lamp (including shroud) may be about 8 or 10 inches. The life of an HID
lamp is typically 7,000 to 10,000 hours. HID lamp operation is quite different
from that of low pressure fluorescent lamps in that the HID discharge
typically operates at a gas pressure of a few atmospheres. Since it takes
about 5-10 minutes to build up this gas pressure, the HID lamp does not emit
substantial light immediately. Additionally, if power is interrupted, even for
an instant, HID lamps may require 10 or more minutes to restart.
Furthermore, the color rendering and overall lumen output of HID lamps is
somewhat variable over life, and the lamps should be replaced at the end of
life to avoid possible catastrophic failure of the hot lamp. The HID lamp is
widely used in outdoor applications such as street lamps, tunnels and
stadiums.
An inductively coupled fluorescent lamp known as the QL lighting
system includes a lamp envelope having the shape of a conventional
incandescent lamp with a reentrant cavity, a power coupler positioned in the
reentrant cavity and a high frequency generator. The QL lighting system is
relatively complex in construction and requires cooling. In addition, the QL
lighting system typically operates at a frequency of 2.65 MHz, a frequency at
which care must be taken to prevent radio frequency interference.
Electrodeless fluorescent lamps are disclosed in U.S. Patent No.
3,500,118 issued March 10, 1970 to Anderson; U.S. Patent No. 3,987,334
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issued October 19, 1976 to Anderson; and Anderson, Illuminating
Enaineerina, April 1969, pages 236-244. An electrodeless, inductively-
coupled lamp includes a low pressure mercury/buffer gas discharge in a
discharge tube which forms a continuous closed electrical path. The path of
the discharge tube goes through the center of one or more toroidal ferrite
cores such that the discharge tube becomes the secondary of a transformer.
Power is coupled to the discharge by applying a sinusoidal voltage to a few
turns of wire wound around the toroidal core that encircles the discharge
tube. The current through the primary winding creates a time varying
magnetic flux which induces along the discharge tube a voltage that
maintains the discharge. The inner surface of the discharge tube is coated
with a phosphor which emits visible light when irradiated by photons emitted
by the excited mercury gas atoms.
The electrodeless lamp described by Anderson has a discharge
current between 0.25 and 1.0 ampere, and a buffer gas pressure between
0.5 and 5 torr. Argon was used as a buffer gas in the electrodeless lamp
described by Anderson. In addition, about 2.5 kilograms of ferrite material
were used to energize a 32 waft discharge in the electrodeless lamp
described by Anderson. The lamp parameters described by Anderson
produce a lamp which has high core loss and therefore is extremely
inefficient. In addition, the Anderson lamp is impractically heavy because of
the ferrite material used in the transformer core.
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Summarv of the Invention
According to the present invention, an electric lamp assembly
comprises an electrodeless lamp including a closed-loop, tubular lamp
envelope enclosing mercury vapor and a buffer gas at a pressure less than
about 0.5 torr, a transformer core disposed around the lamp envelope, an
input winding disposed on the transformer core and a radio frequency power
source coupled to the input winding. The radio frequency source supplies
sufficient radio frequency energy to the mercury vapor and the buffer gas to
produce in the lamp envelope a discharge having a discharge current equal
to or greater than about 2 amperes.
Preferably, the electrodeless lamp includes a phosphor on an inside
surface of the lamp envelope for emitting radiation in a predetermined
wavelength range in response to ultraviolet radiation emitted by the
discharge. The lamp envelope preferably has a cross sectional dimension in
a range of about 1 to 4 inches. In a first embodiment, the lamp envelope has
an oval shape. In a second embodiment, the lamp envelope comprises first
and second parallel tubes joined at their ends to form a closed loop. The
buffer gas is preferably a noble gas such as krypton.
The radio frequency power source preferably has a frequency in a
range of about 50 kHz to about 3 MHz and, more preferably, in a range of
about 100 kHz to about 400 kHz. The transformer core preferably has a
toriodal configuration that encircles the lamp envelope. Preferably, the
transformer core comprises a ferrite material. The core power loss is
preferably less than or equal to 5% of the total power supplied by the radio
frequency power source.
According to another aspect of the invention, an electric lamp
assembly comprises an electrodeless lamp including a tubular lamp
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envelope enclosing mercury vapor and a buffer gas at a pressure less than
about 0.5 torr. The lamp envelope comprises first and second parallel tubes,
which may be straight tubes, joined at or near one end by a first lateral tube
and joined at or near the other end by a second lateral tube to form a closed
loop. The electric lamp assembiy further comprises a first transformer core
disposed around the first lateral tube of the lamp enveiope, a second
transformer core disposed around the second lateral tube of the lamp
envelope, first and second input windings disposed on the first and second
transformer cores, respectively, and a radio frequency power source coupled
to the first and second input windings. The radio frequency power source
supplies sufficient radio frequency energy to the mercury vapor and the
buffer gas to produce in the lamp envelope a discharge having a discharge
current equal to or greater than about 2 amperes.
According to yet another aspect of the invention, a method is provided
for operating an electric lamp comprising an electrodeless lamp including a
closed-loop, tubular lamp envelope enclosing a buffer gas and mercury
vapor. The method comprises the steps of establishing in the lamp envelope
a pressure of the mercury vapor and the buffer gas less than about 0.5 torr,
and inductively coupling sufficient radio frequency energy to the mercury
vapor and the buffer gas to produce in the lamp envelope a discharge having
a discharge current equal to or greater than about 2 amperes.
According to a further aspect of the invention, an electric lamp
assembly comprises an electrodeless lamp including a closed-loop, tubular
lamp envelope enclosing mercury vapor and a buffer gas at a pressure less
than about 0.5 torr, and means for inductively coupling sufficient radio
frequency energy to the mercury vapor and the buffer gas to produce in the
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lamp envelope a discharge having a discharge current equal to or greater
than about 2 amperes.
Brief Description of the Drawinas
For a better understanding of the present invention, reference is made
to the accompanying drawings, which are incorporated herein by reference
and in which:
FIG. 1 is a schematic representation of a first embodiment of an
electrodeless fluorescent lamp in accordance with the invention;
FIG. 2 is a schematic diagram showing electrical connections to the
electrodeless ffuorescent lamp of the present invention;
FIG. 3 is a schematic diagram of an electrodeless fluorescent lamp in
accordance with a second embodiment of the invention;
FIG. 4 is a graph of lumens and lumens per watt as a function of
discharge power for the electrodeless fluorescent lamp of FIG. 3; and
FIG. 5 is a graph of discharge volts, core loss and power factor as a
function of lamp power for the electrodeless fluorescent lamp of FIG. 3.
Detailed Descri tion
A first embodiment of a discharge lamp in accordance with the
present invention is shown in FIGS. 1 and 2. A lamp 10 includes a lamp
envelope 12 which has a tubular, closed-loop configuration and is
electrodeless. The lamp envelope 12 encloses a discharge region 14 (FIG.
2) containing a buffer gas and mercury vapor. A phosphor coating 16 is
typically formed on the inside surface of lamp envelope 12. Radio frequency
(RF) energy from an RF source 20 is inductively coupled to the electrodeless
lamp 10 by a first transformer core 22 and a second transformer core 24.
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Each of the transformer cores 22 and 24 preferably has a toroidal
configuration that surrounds lamp envelope 12. The RF source 20 is
connected to a winding 30 on first transformer core 22 and is connected to a
winding 32 on second transformer core 24. A conductive strip 26, adhered
to the outer surface of lamp envelope 12 and electrically connected to RF
source 20, may be utilized to assist in starting a discharge in electrodeless
lamp 10.
In operation, RF energy is inductively coupled to a low pressure
discharge within lamp envelope 12 by the transformer cores 22 and 24. The
electrodeless lamp 10 acts as a secondary circuit for each transformer. The
windings 30 and 32 are preferably driven in phase and may be connected in
parallel as shown in FIG. 2. The transformers 22 and 24 are positioned on
lamp envelope 12 such that the voltages induced in the discharge by the
transformer cores 22 and 24 add. The RF current through the windings 30
and 32 creates a time-varying magnetic flux which induces along the lamp
envelope 12 a voltage that maintains a discharge. The discharge within
lamp envelope 12 emits ultraviolet radiation which stimulates emission of
visible light by phosphor coating 16. In this configuration, the lamp envelope
12 is fabricated of a material, such as glass, that transmits visible light.
One
suitable glass is Pyrex (tradename). Alternatively, the envelope may be
constructed from a soft glass, such as soda-lime, with an internal surface
coated with a barrier layer, such as aluminum oxide. In an alternative
configuration, the electrodeless lamp is used as a source of ultraviolet
radiation. In this configuration, the phosphor coating 16 is omitted, and the
lamp envelope 12 is fabricated of an ultraviolet-transmissive material, such
as quartz.
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The lamp envelope preferably has a diameter in the range of about 1
inch to about 4 inches for high lumen output. The fill material comprises a
buffer gas and a small amount of mercury which produces mercury vapor.
The buffer gas is preferably a noble gas and is most preferably krypton. It
has been found that krypton provides higher lumens per watt in the operation
of the lamp at moderate power loading. At higher power loading, use of
argon may be preferable. The lamp envelope 12 can have any shape which
forms a closed loop, including an oval shape as shown in FIG. 1, a circular
shape, an elliptical shape or a series of straight tubes joined to form a
closed
loop as described below.
The transformer cores 22 and 24 are preferably fabricated of a high
permeability, low loss ferrite material, such as a manganese zinc ferrite. The
transformer cores 22 and 24 form a closed-loop around lamp envelope 12
and typically have a toroidal configuration with a diameter that is slightly
larger than the outside diameter of lamp envelope 12. The cores 22 and 24
are cut in order to install them on lamp envelope 12. The cut ends are
preferably polished in order to minimize any gap between the ends of each
transformer core after installation on lamp envelope 12.
Because the ferrite material of the transformer cores is relatively
expensive, it is desirable to limit the amount used. In one approach, a small
section of the lamp envelope is necked down to a smaller diameter and a
transformer core of smaller diameter is positioned on the smaller diameter
section of the lamp envelope. The length of the smaller diameter section of
the lamp envelope should be kept to a minimum in order to minimize the
discharge voltage. In another approach, a single transformer core is used to
couple RF energy to the discharge.
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The windings 30 and 32 may each comprise a few turns of wire of
sufficient size to carry the primary current. Each transformer is configured
to
step down the primary voltage and to step up the primary current, typically by
a factor of about 5 to 10. Typically, the primary windings 30 and 32 may
each have about 8 to 12 turns.
The RF source 20 is preferably in a range of about 50 kHz to 3 MHz
and is most preferably in a range of about 100 kHz to about 400 kHz. By
way of example, a primary voltage in a range of about 100 to 200 volts and a
primary current of about 1 ampere may produce a discharge voltage of 20 to
30 volts and a discharge current on the order of about 5 amperes.
The electric lamp assembly of the present invention utilizes a
combination of parameters which produce high lumen output, high lumens
per waft, low core loss and long operating life. It has been determined that a
buffer gas pressure less than about 0.5 torr and a discharge current equal to
or greater than about 2.0 amperes produces the desired performance.
Preferably, the buffer gas pressure is equal to or less than about 0.2 torr,
and the discharge current is equal to or greater than about 5.0 amperes. At
large tube diameters, the performance of the lamp assembly of the present
invention meets or exceeds the lumen output and lumens per watt
performance of conventional very high output electroded fluorescent lamps.
It has been found important to minimize discharge voltage in an
inductively coupled discharge, because ferrite core loss increases sharply
with discharge voltage. The heavier atomic weight of the buffer gas, the
larger tube diameter and the higher current operation in comparison with
prior art electrodeless fluorescent lamps result in decreased discharge
voltage. The lamp of the present invention requires only 0.4 kilograms of
ferrite material to energize a 120 watt discharge. The core loss in this
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configuration is about 3%. In general, the transformer core power loss is
typically less than or equal to 5% of the total power supplied by the RF
source in the lamp of the present invention. Furthermore, the ratio of
transformer core volume to discharge power is typically less than 1 cubic
centimeter per waft in the lamp of the present invention.
Analysis of the lamp of the present invention indicates that the correct
choice of discharge current has a crucial effect on the ferrite core loss that
occurs when driving an inductive discharge. The issue of ferrite core loss
and discharge current can be understood from the following analysis.
Generally speaking, low pressure discharges have a negative
voltage/current characteristic. Thus, discharge voltage Vd is related to the
discharge current Id such that discharge voltage Vd is proportional to Id -k
Since voltage and current are approximately in phase, discharge power Pd is
proportional to Id 1-k. Ferrite core loss Pr is proportional to the nth power
of
discharge voltage Vd, which is equal to the primary voltage divided by the
number of tums on the transformer core. Thus, P. is proportional to V"
which in tum is proportional to Id *". The ratio of PC /Pd, can be written as
Pc/Pd a Itl -(kro-+> + 11
Typically, 0.2 < k < 0.4 and 2.5 < n< 3.1. Taking k = 0.3 and n =2.8 as
representative values, the expression for i; above reduces to
a Id -1.5
For a given ferrite core, increasing discharge current from 0.5 amp to 5
amperes provides a reduction in 5 by 10-' 5, or about 30 times less core loss.
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This analysis explains the greater coupling efficiency that is obtained at
higher discharge current. However, this does not imply that simply
increasing the discharge current in prior art electrodeless fluorescent lamps
would produce desirable lamp performance. It is also important to have the
discharge power efficiently converted to ultraviolet radiation. To obtain
efficient production of ultraviolet radiation from mercury at high current, it
is
important that the buffer gas pressure be less than about 0.5 torr. Thus, it
is
important to combine high discharge current with low buffer gas pressure.
Preferably, the discharge current Id should be equal to or greater than about
2.0 amperes, and the buffer gas pressure should be less than about 0.5 torr.
Starting of a discharge in the electrodeless fluorescent lamp of the
present invention is relatively easy. The output voltage of the RF source
prior to starting of a discharge is typically two to three times the operating
voltage. This voltage applied to conductive strip 26 on lamp envelope 12 is
sufficient to initiate a discharge. Other starting devices may be utilized
within
the scope of the present invention. If desired, the conductive strip or other
starting device may be switched out of the lamp circuit after initiation of a
discharge.
An example of an electrodeless fluorescent lamp in accordance with
the present invention is described with reference to the configuration of
FIGS. I and 2. A lamp envelope consisted of a closed-loop discharge glass
tube filled with a noble gas and mercury vapor, with the inside surface of the
lamp envelope coated with phosphor. The length of the discharge path was
66 centimeters (cm), and the tube outside diameter was 38 millimeters (mm).
The lamp envelope was filled with krypton at a pressure of 0.2 torr and
about 6 millitorr of mercury vapor. Two toroidal ferrite cores (P-type made by
Magnetics, a Division of Spang and Company) were cut into two pieces with
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the end of piece ground flat. Each toroidal core was assembled around the
lamp envelope with six primary turns of wire wrapped around each ferrite
core. The cores had an outside diameter of 75 mm, an inside diameter of 40
mm and a thickness of 12.6 mm, with a total cross section for the two cores
of 4.4 square centimeters. The lamp was driven with a sinusoidal signal RF
source at a frequency of 250 kHz. The performance of the lamp under one
set of operating conditions was as follows. Discharge current was 5
amperes; discharge power was 120 wafts, 1.8 wafts per centimeter; light
output was 10,0001umens; lumens per waft was 80; ratio of core power loss
to discharge power was 0.054; core volume was 80 cubic centimeters; ratio
of core volume to discharge power was 0.67 cubic centimeters per waft;
discharge voltage was 25 volts RMS; discharge field was 0.37 volts per
centimeter; core flux density was 500 gauss; core loss was 6.5 wafts, 0.08
watts per cubic centimeter; and total power was 126.5 watts.
A second embodiment of an eiectrodeless high intensity fluorescent
lamp in accordance with the invention is shown in FIG. 3. An electrodeless
lamp 50 comprises a lamp envelope 52 including two straight tubes 54 and
56 in a parallel configuration. The tubes 54 and 56 are sealed at each end,
are interconnected at or near one end by a lateral tube 58 and are
interconnected at or near the other end by a lateral tube 60. Each of the
tubes 58 and 60 provides gas communication between tubes 54 and 56,
thereby forming a closed-loop configuration. The straight tubes 54 and 56
have an important advantage over other shapes in that they are easy to
make and easy to coat with phosphor. However, as noted above, the lamp
can be made in almost any shape, even an asymmetrical one, that forms a
closed-loop discharge path. In a preferred embodiment, each of the tubes
54 and 56 was 40 cm long and 5 cm in diameter. The lateral tubes, 58 and
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60 were 3.8 cm long and 3.8 cm in diameter. Increasing the diameter of
tubes 54 and 56 decreases discharge voltage and thereby decreases ferrite
losses. Reducing the diameter of tubes 58 and 60 to 3.8 cm decreases
ferrite sizes and also decreases ferrite losses.
The lamp shown in FIG. 3 was filled with 0.2 torr krypton buffer gas
and 6 millitorr of mercury vapor. A transformer core 62 was mounted around
lateral tube 58, and a transformer core 64 was mounted around lateral tube
60. Each transformer core was a BE2 toroidal ferrite core that was cut into
two pieces with its ends polished. A primary winding of eight tums of wire
was wrapped around each ferrite core. Each core had an outside diameter
of 8.1 cm, an inside diameter of 4.6 cm, a cross section of 4.4 cm2 and a
volume of 88 cm3. The primary windings were driven with a sinusoidal RF
source at a frequency of 200 kHz connected as shown in FIG. 2.
Lumen output and lumens per waft for the lamp of FIG. 3 are plotted
in FIG. 4 as a function of discharge power. Lumen output is indicated by
curve 70, and lumens per waft are indicated by curve 72. The
measurements were made at 400C cold spot temperature after 100 hours of
lamp operation. As shown in FIG. 4, lumen output increases with discharge
power, while lumens per waft (LPW) peaks at 150 watts. At peak LPW,
14,000 lumens are produced with an efficacy (including ferrite core loss) of
92 LPW. The axial lumen density at this LPW is 415 lumens per inch, which
is 2.75 times greater than a conventional VHO fluorescent lamp. Discharge
current at 150 watts is about 6 amperes. Operation with the parameters
disclosed herein makes it possible for the lamp of the present invention to
achieve relatively high lumen output, high efficacy and high axial lumen
density simultaneously, thus making it an attractive alternative to
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conventional VHO fluorescent lamps and high intensity, high pressure
discharge lamps.
Selected electrical characteristics of the lamp of FIG. 3 are plotted in
FIG. 5 as a function of lamp power. Discharge voltage is represented by
curve 76; core loss is represented by curve 78; and power factor is
represented by curve 80. Discharge voltage and core loss are referenced to
the left ordinate, while power factor is referenced to the right ordinate. As
lamp power increases, discharge voltage decreases. The decreased
discharge voltage results in a corresponding decrease in core foss. FIG. 5
emphasizes the importance of keeping the discharge voltage low. The core
loss is 40% of total lamp power at 50 watts, while core loss is only about 6%
of total lamp power at 150 watts. The increase in LPW with discharge power
up to 150 wafts shown in FIG. 4 is primarily related to the corresponding
decrease in core loss. The remarkable overall performance of the lamp is
due to the choice of operating parameters (primarily gas pressure,
temperature, discharge tube diameter and discharge current). The BE2 core
material is not considered to be the optimum core material. Measurements
have indicated that the core loss may be reduced by almost a factor of two
by using a premium core material such as 3F3 manufactured by Philips.
At 150 watts, the average electric field in the discharge is about 0.75
volts per inch. Such a small electric field in an electroded discharge would
result in a rather inefficient light source, since the efectrode drop would be
appreciable (virtually no light comes from the electrode drop region) with
respect to the total discharge voltage. With regard to cathode evaporation
and efficacy, an electroded discharge could not operate for a long period
under these conditions. By contrast, the lamp of the present invention is
expected to have an extremely long life because of its electrodeless
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configuration.
While there have been shown and described what are at present
considered the preferred embodiments of the present invention, it will be
obvious to those skilled in the art that various changes and modifications
may be made therein without departing from the scope of the invention as
defined by the appended claims.
