Electric lamp and discharge devices: systems – Pulsating or a.c. supply – Induction-type discharge device load
Reexamination Certificate
1996-07-03
2001-06-19
Shingleton, Michael B (Department: 2817)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
Induction-type discharge device load
C315S344000
Reexamination Certificate
active
06249090
ABSTRACT:
BACKGROUND OF THE INVENTION
Electrodeless inductively-coupled fluorescent lamps (ICFL) have longer life than conventional fluorescent lamps that employ hot cathodes. The plasma which radiates visible and UV light is generated in the lamp by an azimuthal electric field, E
ind
, induced in the envelope by an induction coil. The coil is a critical component in the operation and performance of such lamps. This invention is about a particular design aspect of such a coil.
DESCRIPTION OF THE PRIOR ART
In a typical ICFL, a spiral-shaped induction coil is positioned in a reentrant cavity of the lamp envelope and has an inductance, L
coil
, of 1-3 &mgr;H. In a U.S. patent application Ser. No. 08/538,239, by Popov et al. (owned by the same assignee as the present application), the induction coil had a squeezed shape and a value of L
coil
=1.5-3 &mgr;H. By squeezed shape, we mean there were no separations between the turns of the coil. In the application of Popov et al., the squeezed coil is inserted in an aluminum cylinder which removes heat generated by the plasma from the coil and the cavity walls. The bulbous envelope has a spherical shape and is filled with the mixture of rare gas (Ar, Kr) and mercury vapor. The mercury vapor pressure is controlled by temperature of an amalgam positioned in a tubulation. The coil is connected to a matching network located in the lamp base. Radio frequency (RF) power is delivered to the lamp from an RF driver via an RF cable.
The introduction of the cylinder necessitates the use of a smaller coil diameter, D
coil
, and, hence, a weaker coupling between the coil and the plasma, K≈D
2
coil
/D
2
pl
, where K is the coupling coefficient and D
pl
, is the diameter of the plasma. The diameter of the plasma, D
pl
, is twice the radius, of the plasma (2R
pl
). The plasma radius, r
pl
, is determined as the distance from the lamp axis to the point where the plasma current density, J
pl
, has the maximum value.
It is known a weaker coupling between the coil and the plasma results in higher coil RF current, thereby producing greater RF power losses in the coil. Consequently, a higher coil RF voltage is required for the transition from the capacitive discharge to the inductive discharge, V
tr
. At low ambient temperatures, T
amb
<0° C., the transition voltage is the highest coil RF voltage. The transition voltage is considered as a lamp starting voltage, V
st
. It is desirable to have V
st
as low as possible from the RF lamp driver point of view.
Moreover, as a result of lower coupling coefficient, K, the coil RF voltage needed to maintain the inductive RF discharge at normal operation (maintaining voltage, V
m
, at an RF power of about 40-100 W) is also higher. It is desirable from a lamp maintenance point of view for the lamp to have a low V
m
. The lower the maintaining voltage, the lower the energy of ions bombarding the cavity walls whereby less damage is done to phosphor coating on the cavity walls. This substantially improves the lamp maintenance and extends the life of the lamp.
SUMMARY OF THE INVENTION
It is known the plasma density and its spatial distribution in the inductively-coupled plasma depends on the coil dimensions. When the diameter of the coil, D
coil
, is smaller than the coil height, H
coil
, the plasma has a toroidal shape. As the ratio of H
coil
/D
coil
increases, the plasma changes its shape from toroidal to cylindrical. It is known from transformer theory that the coupling is better (higher K) when both primary (coil) and secondary (plasma) are cylinders than when the primary is cylindrical and the secondary is toroidal.
The increase of the ratio H
coil
/D
coil
can be achieved by an increase in the number of turns, i.e., by an increase of the coil inductance. We have found, however, the increase of the coil inductance causes an increase of the lamp transition voltage and lamp maintaining voltage.
We have found the reduction of the V
tr
and V
m
can be achieved by spreading the coil along its axis. Having the same inductance as the squeezed coil, the spread coil has a higher ratio H
coil
/D
coil
and, hence, better coupling with the plasma leading to a smaller transition voltage, V., and a smaller maintaining voltage, V
m
. The higher the ratio H
coil
/D
coil
, the lower is the transition and maintaining voltage, and the longer is the lamp life.
As shown above, the coupling efficiency of the spread coil, K, is higher than that of the squeezed coil. On the other hand, the spread coil has larger coil resistance, R
coil
, than that of the squeezed coil of the same inductance due to the longer length of the wire.
However, the RF current in the spread coil, I
c
, is also lower, so the amount of RF power “consumed” by each coil, P
c
=I
2
c
R
coil
, is the same, and the amount of RF power transferred into the plasma by the spread coil is equal (or close) to the power transferred to the plasma by the squeezed coil.
An object of the present invention is to provide an electrodeless inductively-coupled fluorescent light source which can be substituted for the incandescent light source, high pressure mercury light source, metal halide light source, or a compact fluorescent light source.
Another object of the present invention is to reduce the electrodeless lamp starting voltage.
A further object of the present invention is to reduce the lamp maintaining voltage thereby reducing the energy of ions bombarding the cavity walls and, therefore, improving the lamp maintenance.
Another object of the present invention is to reduce the RF coil current thereby reducing the RF losses in the coil and, hence, increasing the RF lamp efficiency.
An additional object of the present invention is to provide an RF electrodeless lamp which incorporates a Faraday shield, a spread induction coil and a matching network in the lamp base.
REFERENCES:
patent: 2030957 (1936-02-01), Bethenod et al.
patent: 4010400 (1977-03-01), Hollister
patent: 4568859 (1986-02-01), Houkes et al.
patent: 4622495 (1986-11-01), Smeelen
patent: 4704562 (1987-11-01), Postma et al.
patent: 4710678 (1987-12-01), Houkes et al.
patent: 4727295 (1988-02-01), Postma et al.
patent: 5325018 (1994-06-01), El-Hamamsy
patent: 5343126 (1994-08-01), Farrall et al.
patent: 5355054 (1994-10-01), Van Lierop et al.
patent: 5412280 (1995-05-01), Scott et al.
patent: 5412288 (1995-05-01), Borowiec et al.
patent: 5412289 (1995-05-01), Thomas et al.
patent: 5461284 (1995-10-01), Roberts et al.
patent: 5465028 (1995-11-01), Antonis et al.
Maya Jakob
Popov Oleg
Matsushita Electric Works Research & Development Laboratories In
Shingleton Michael B
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