Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-04-03
2001-08-14
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S237000, C315S246000, C315S302000, C315S340000
Reexamination Certificate
active
06274986
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to discharge lamps, and in particular to the electrical control and construction of such lamps with a view to obtaining desired emission wavelength characteristics.
2. Description of the Related Art
A widely used lamp in interior lighting, the fluorescent tube, exploits the properties of a low-pressure discharge in mercury vapour (typically 7×10
−3
torr, corresponding to a wall temperature of about 40° C.) and argon gas (typically 3 torr) produced by the application of a mains, or higher, frequency alternating high voltage to a pair of either cold or heated electrodes at either end of a sealed glass tube. Such a plasma emits a number of discrete mercury emission lines by far the strongest of which is the 254 nm resonance line (up to 60% of the total lamp input power can appear in this line). The intense 254 nm UV radiation is converted to useful broadband visible radiation by a coating of red, green and blue phosphors on the inside walls of the glass envelope.
A major known disadvantage of the fluorescent lamp is the large difference in energy (inversely proportional to the wavelength of the radiation) between the exciting radiation at 254 nm and the range of visible wavelengths from 400 nm to 700 nm, i.e. there is a very large “Stokes shift”. In theory the 254 nm photon has sufficient energy to produce two visible photons, e.g. two at greater than 508 nm, and a process that achieved this would bring about a major advance in overall lamp efficiency. A practical process to achieve this has to date been neither implemented nor described in principle. As a consequence a large proportion (typically 75%) of the energy delivered to a standard design fluorescent lamp is wasted as heat.
Recently it has been demonstrated that the colour of the emission from mercury/rare-gas discharges can be altered significantly by replacing the standard alternating (sinusoidal) power supply with a pulsed power supply. M. Aono, R. Itatani et al. (J. Light & Visual Environment, vol. 3, no. 1, p. 1-9, 1989) demonstrated that the relative intensity of emissions from the rare gas itself, usually insignificant, could be greatly enhanced by pulsed excitation. This was exploited to produce lamps whose phosphor emissions changed colour according to whether alternating (sinewave) or pulsed electrical excitation was used. Hitachi have demonstrated electrical control of the colour of the emission from mercury/rare-gas lamps and from xenon lamps; see for instance JP-A-5-135744 (Shinkishi et al).
The effect has been exploited to meet particular commercial requirements. For instance, OSRAM Sylvania have described (EP-A2-700074) the pulsed excitation of a neon discharge to produce a lamp suitable both as a flashing indicator light and as a brake light for automobiles.
Matsushita have reported (JP-A-7-272672) a fluorescent lamp driven by an alternating high frequency supply supplemented with a pulsed power supply. The advantage cited was an increase in the radiant intensity of the 254 nm emission and an increase in fluorescent lamp efficiency.
EP-A1-334356 (VEB NARVA) also discusses the use of pulsed discharges to produce a desired spectral emission, though here the emphasis is on the use of high-pressure caesium and/or rubidium discharges, with possible additives, and phosphors are not used.
SUMMARY OF THE INVENTION
The invention makes use of the technology of pulsed voltage application in a somewhat different way.
To explain the invention reference will first be made to
FIG. 1
, which shows the main energy levels and transitions of the mercury atom. In a normal AC discharge by far the most intense emission is that corresponding to the transition from 6
3
P
1
to the ground state. As a result, the spectrum of the continuously excited mercury discharge is dominated by the line at about 254 nm.
The present invention arose as a result of a detailed investigation of the temporal behaviour of a mercury rare-gas discharge subject to such excitation. The basic observation is exemplified in
FIG. 2
which shows the time-resolved emissions of mercury vapour to which a voltage pulse is applied, at 254 nm (two transitions: resonance line at 253.65 nm and the
3
D
1
−
3
P
0
transition at 253.48 nm) and at 366 nm (four transitions:
1
D
2
−
3
P
2
at 366.33 nm;
3
D
1
−
3
P
2
at 366.29 nm;
3
D
2
−
3
P
2
at 365.48 nm; and
3
D
3
−
3
P
2
at 365.02 nm). Both sets of transitions show a step increase in intensity as the voltage pulse is applied. Subsequent behaviour, however, differentiates them. The 254 nm intensity continues to increase for a period, stays high and then falls with a characteristic time as the voltage falls at the end of the pulse. In contrast the 365-366 nm emission shows an immediate drop during the pulse-on period but shows a step increase as the pulse is turned off. After peaking at a time after the end of the pulse it shows a decay with a characteristic time which turns out to be longer than that describing the 254 nm emission immediately after the pulse. The remarkable consequence of the latter, post-pulse, effects is that integrated over an entire cycle of the repetitive pulse sequence the total intensity of the 366 nm emission exceeds that of the 254 nm emission if the cycle time is long enough.
Investigation over a wide range of conditions showed that the behaviour of these two discharge emissions at the termination of each pulse described above was characteristic of the prevailing conditions of wall temperature, and gas composition and pressure, i.e. the competing processes controlling the populations of the emitting electronic states involved.
The sustained enhancement of the 254 nm emission during the pulse arises probably from the transfer of net population from mercury ground states,
1
S
0
, to the manifold of excited states, thus reducing the radiation trapping of 254 nm radiation that is a strong feature of the operating mechanism of a fluorescent lamp. (Nute: there is probably an increase also in the emission of the other mercury resonance line at 184.96 nm during the pulse). The burst in the 366 nm emission at pulse termination arises possibly from the rapid increase in the population of highly excited mercury states produced by the neutralisation of the high density of mercury ions present during the intra-pulse period. Excited rare-gas states may also play a part.
The recognition of the relative importance of the trailing edge of applied pulses can be exploited in interesting ways. For example, a mercury/rare-gas discharge can be operated in such a way that the intensity of the 366 nm emission can significantly exceed the intensity of the 254 nm emission (in the plasma emission of a typical fluorescent tube this ratio favours the 254 nm line by a factor of over 20; see FIG.
3
). Thus by optimising the design of a pulsed-operation mercury/rare-gas discharge in respect of the inter-pulse behaviour it is possible to increase the ratio of 366 nm to 254 nm radiation by a factor of at least 100.
This 366 nm to 254 nm relative enhancement, and in general the shift in emission ratios brought about by the use of pulsed excitation, can be exploited in various ways.
In a first aspect of the invention therefore there is provided a discharge lamp comprising a tube for containing the discharge medium, and a control means for applying a field to the medium so as to cause a discharge within the tube, wherein the discharge in the medium when excited by a simple alternating field contains two lines at first and second wavelengths, the first wavelength predominating, in which the control means is adapted to apply a waveform consisting of relatively short excitation pulses (“marks”) and relatively long substantially non-excitation intervals (“spaces”) such that the integral over one waveform period of the intensity of the light emitted at the second wavelength is greater than the corresponding integral for the first wavelength.
In the corresponding method an electrical signal is app
Devonshire Robin
Healey Timothy James
Stone David Andrew
Tozer Richard Charles
Morse, Altman & Martin
Philogene Haissa
University of Sheffield
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