Note: Descriptions are shown in the official language in which they were submitted.
1142Z15
This application is a division o ~anadian Application 333,599
filed August lO, 1979.
TEE INVENTION
This invention concerns a mercury-containing arc discharge device
for converting electrical energy into resona~ce radiation. It is
particularly concerned with improving the efficiency of such conversion.
An example o~ such a device is a fluorescent lamp. Such a lamp comprises
a tubular glass envelope having electrodes at its ends, containing a
fill of mercury and an inert gas, and having a phosphor coating on the
inner envelope wall. In fluorescent lamps, electrical energy is converte
into the kinetic energy of free electrons which in turn is converted into
the internal energy of atoms and molecules, which in turn is converted
into radiant energy, and chiefly into the resonance radiation at the
254 nanometer (nm) region of the electromagnetic spectrum, which in turn
is converted into luminous energy by the phosphor. A great deal of
effort has gone into improving the luminous efficacy of such lamps by
improving the phosphor blent, the fill gas pressure~ and tube geom~try~
Such effort has, fund~mentally, been direc~ed toward optimizing the
number density of mercury atoms in the aggregate and opt~mizing the
photon conversion efficiencies of the fluorescent materials.
Defining a quantum of resonance radiation energy as the energy of a
single mercury atom excited to its Pl state, in its esca?e from the
discharge tube such a quantum may exist either as an e~cited atom or as a
photon emitted by an excited atom. Because of the presence of mercury
atoms in their lowest energy state (ground state) in tbe plasma which
can absorb such photons, thereby becoming excited atoms, which may
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D-21,56~ ¦ subsequently re-emit a photon of substantially the same energy as they
I absorbed, a quantum of resonance radiation energy (created by electron
impact excitation of a mercury atom) escapes the discharge tube by a
series of stepwise emissions and absorptions, alternately changing its
form from excited atom to photon and vice versa before it finally escapesi
the discharge tube as a photon.
Each time the quantum is absorbed and becomes an excited atom, a ~ I
period equal to the natural life time of the excited atom (about 1.17 x ¦
10 7 second) must elapse on the average before it can be re-emitted.
Thus, the multiple emission, absorption and re-emission process, known ac
imprisonment of radiance radiation, greatly prolongs the length of time
the quantum spends as an excited atom before it can escape the tube eo
many times the single natural lifetime it would reside as an excited
àto~ if the photon escaped without re-absorption.
Whilo the quantum resides as an excited atom, there is a finite
probability that some non-radiative process may OCCUl' to dissipate its
energy. T~e longer the imprisonment time, that is, the time required for , `~
the quantum to escape, the greatsr is the total probability of such non-
radiative loss and the lower the efficiency. The proble~ of imprisonment
time and quantum escape has been considered theoretically; see, for
example, "Imprisonment of Resonance Radiation in ~ases. Il" by T. Holstein
(Physical Review, Volume 83, Number ~, September 15, 1951) and "Electric
Discharge Lamps" by John F. Uaymouth, The ~.I.T. Press (1971), Cambridge,
Massachusetts, and London, England, pages 122-126. Lamp optimization
relating, for example, to envelope diameter, fill pressure or operating
temperature, has been based on prior art treatments of the proble~ of
radiation transfer. A common feature of all of these treatments Xnown to
the prior art is that imprisonment time increases on the average as the
concentration of total mercury atoms in the vapor phase increases, and
this act is responsible for the declining efficiency of such lamps for
mercury pressures higher than 6xlO 3 torr, corresponding to the pressu~e ¦
of saturated vapor above liquid mercury at 40 C, which is about the
pressure in fluorescent lamps.
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D-21,564 ll As previously stated, the fluorescent lamp operates by using resonance
radiation from a plasma to excite a phosphor which emits visible light.
Previous improvements ;.n the performance of the discharge have been
l attained by changing lamp structure, fill gas composition and pressure, I
¦ and mercury pressure. We have discovered that the efficiency of fluo-- ¦
rescent;lamps, and of any mercury-containing arc discharge device for
converting electrical energy into resonance radiation, can be improved byl
altering the content of the mercury in tbe device. This i~vention is bas!d
on the recognition that the imprisonment time of mercury resonance radiat~on
depends not only on the number density of mercury atoms in the aggregate,
but dso on the number den~ity of the var~ou~ mercury isotopes. If, for
example, the 254 nm emissions of the indivitual isotopes have the same
spectral shape but lie in distinct, non-overlapping, wavelength regions,
and if each of the isotopes ha~ the same probability of being excited and
subsequently emitting 254 nm radiation, then each isotope could only
absorb radiation emitted by an isotope of identical mass nu~ber, and one ~-
would expect minimum imprisonment and maximwm 254 nm radiation if ali
is~topes were equally abundant. Such an isotopic distribution stands in ¦
marked contrast to that in naturally-occurring mercury, which is as follo
20Isotope ~Mass Number)Natural Abundance
196 0.1467.
198 10.0 %
199 16.8 %
25 . 201 13 21 7
202 29.8 %
204 6.8S %
In fact, the 254 nm spectral emissions of some of the isotopes do
overlap, but the emission of the Hgl96 isotope is not one of them. We
3~ have discovered that the entrapment time of 254 nm mercury resonance
radiation can be reduced and the output of 254 nm resonance radiation
can be increased in a device which incorporates relatively more of ehe
Hg lsoeope than is found in naturally-occurring mercury.
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According to the present invention there is provided a
mercury-containing arc discharge device for converting electrical
energy into resonance radiation, the Hgl96 content of the mercury
within the device being greater than that in natural mercury in order
to increase the efficiency of converting said electrical energy into
said resonance radiation.
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D-21,564 l The drawing shows a mercury-containing arc discharga device fabricat~d
so as to permit measurement of the 254 nm resonance radiation. The
device comprises a sealed 4 foot envelope 1 having electrodes 2 at each
!l end thereof. Envelope 1 contains mercury and an ir.ert gas such as argon.
I An lntermediate short length 3 of envelope 1 is made of fused silica
instead of the usual soft glass which ccmprises the rest of envelope 1
in order to transmit 254 nm radiation, soft gLass being opaque to such
radiation.
Three such devices were made and about 5 mg of mercury were added
to each device. In the first device, used as a control, ~he mercury
was naturally-occurring mercury, having the isotopic distriroution
prevlously mentioned. In the secont and thirt devices the zmou~t of
Hg isotope in the 5 mg of mercury was increased as follows. Enriched
Hg 6 was obtained from Oak Ridge National Labs, Oak Ridge, Tennessee,
in the form of mercuric oxide the mercury content o~ which was 33~97Z
Rg 96. The isotopic distribution of said mercury content was as follows:
Hgl96 _~33 ~977O; Hgl98 ~ 17.597o; Hgl99 ~ 16.02%; Hg200 _ 14-7270; Hg ~ ~ i
5~93%; Hg2 _ lO~lg7o; Hg 4 - 1.5~to. The mercuric oxide was thermally
decomposed to yield elemental mercury, 2.25 mg of which was added to the
second device and 0.55 mg of whi~ was added to the third device. In
each device, sufficient naturally-occurring mercury was added to bring thl a
total mercury charge to about 5 mg. Thé individual mercury compositions
were as follows:
Isotope Control #2 #3
196 0.146%15 ~3% 3 ~75%
198 .10~0 13~4 10~8
199 16.8 16.5 16.75
200 23.1 19 ~35 22 ~2
201 13.2 9. 95 12.4
202 29.8 21.0 27.7
' 204 6.85 4.5 6.3
The devices were operated at 430 milliampere constane current and the
relative outputs of 254 nm radiation were measured using a monochromator
ant photomultiplier tube by techniques well known in the art. The output :
of devices 2 and 3 were 4.2% and 4.~/O greater, respectively, than that of
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-21,564 ~ the controL. This is a significant gain. In a 4 foot fluorescent lamp,
it represents an improvement of better than 100 lumens. At a constant !
wattage of 40 watts, devi-e ~3 yielded a 3.6Z increase in output over thel
l control. ¦
¦ It is apparent that substantial enhancement of the efficiency of
I generation of the 254 nm resonance radiation emission has been achieved,
¦ and surprisingly, that such increase in efficiency has occurred for Hg
isotope enrichments which are well below the equal proportion value.
Since the c~-~ercial practicality of this invention will ultimately
depend on the cost of enriching natural mercury in the ~g 9 isotope,
ant that cost will strongly depend on the level of enrichnent required,
it is clear that this is a highly significant finding. On the basi~ of
the results of devices 2 and 3, it is expected that an enric~ment of
Hgl96 isotope as little as l~/o would yield a significantly economic increa :e
in efficiency. ~ i
The only prior art teachings of which we are aware regardin8 isotope
effects on the imprisonmene time of 254 nm resonance radiation in mercury
vapor are those in "Isotope Effect in the Imprisonment of Resonance
Radiation" by T. HolsteinJ D. Alpert, & A.O. McCoubrey (Physical Review,
Volume 8S, Number 4, March 15, 1952). The authors investigatedthe impris~ ~n- ,
ment time of a mercury vapor mixture consisting predcminantly of the
single isotope Hg , with small impurities of Hg and Hg . ~hey
tetermined that about a six fold longer imprisonment time occurred at
vapor pressures in the vicinity of 6xlO torr than in natural mercury.
In no case did they observe an imprisonment time shorter than that of
natural mercury
Although the improvement in efficiency of converaion of electrical
energy to mercury resonance radiation has been demonstrated primarily
for 254 nm radiation, it is equally applicable to mercury resonance
radiation at other frequencies, for example, 185 nm~ The 254 nm ratia-
tion is of primary importance in fluorescent lamps ~hile 1~5 nm radiation
is of importance in ozone producing devices as well as in some types of ¦
fluorescent lamps. -
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