GB2211658A - High pressure discharge lamp - Google Patents

Description

2211659 Title: HIGH PRESSURE DISCHARGE LAMP This invention relates to high

pressure discharge lamps which are lighted by means of a high frequency power source.

The luminous efficiency of the discharge lamps of the type referred to can be improved by lighting them with an application of a voltage of more than several KHz by means of a high frequency power source, to be effectively utilizable as a long hour illumination source such as a metal halide lamp.

Conventional high pressure discharge lamps generally compr.ise, for example, an arc tube made of a quartz glass or the like housed within a gas-charged outer jacket, and a pair of electrodes mounted in the arc tube with a rare gas and luminous substance sealed therein. It is known that, in addition to the improved luminous efficiency achieved by lighting such lamps with the high frequency power source,' their lighting circuit can be electronically manufactured to allow the required stabiliser to be minimised in size and weight and even to be low in loss.

In these high pressure discharge lamps, on the other hand, there has been involved such a drawback that, upon coincidence of the natural frequency of the arc tube with a pressure variation inside the tube due to the lighting, a stationary wave thereby made to arise causes an acoustic resonance phenomenon to occur, whereby the 2 discharge arc column generated in the tube is caused to be warped, to flicker or eventually to be extinguished. Destruction of the tube can also occur and it has been a common demand that the lamps be improved in this 5 respect.

In attempts to eliminate the above problem, various measures have been suggested, examples of which are U.S. Patent 4373146 of R. P. Bonazoli et al and Japanese Patent Application Laid-Open Publication No. 48095/1981 of Y. Koshimura et al. The U.S. patent discloses a high intensity discharge lamp which is driven by an A.C. output through an inverter receiving a square wave kept at 20-30 KHz or 25-50 KHz from a square wave generator for a frequency modulation, whereas the Japanese publication discloses a high pressure discharge lamp based on a technical idea similar to the U.S. patent but in which, specifically, an output frequency of a high frequency power source device is modulated by a modulator in a zone including a stable lighting zone of the discharge lamp. While these suggestions are effective to eliminate the acoustic resonance phenomenon to some extent, they still involve such a problem that, when zones in which the acoustic resonance phenomenon occurs (which zones shall be hereinafter referred to simply as "unstable zone") are considered as based on. for example, two functions of a centre frequency of 3060 KHz and-a modulation frequency range of 0-10 KHz upon the lamp lighting by means of a high frequency power source, realisation of a sufficiently broad zone in which the acoustic resonance phenomenon is avoidable (which zone shall be hereinafter referred to simply as Ilavoidable zone") requires a remarkably complicated electronic circuit so that required costs for avoiding the acoustic resonance phenomenon will become extremely high.

A primary aim of the present invention is, therefore, to provide a high pressure discharge lamp which can reduce the unstable zone to a large extent so as to obtain the avoidable zone which is sufficiently broad and effective to lower the required costs for avoiding the acoustic resonance phenomenon.

According to the invention, there is provided a high pressure discharge lamp comprising- an arc tube having a substantially cylindrical portion closed by opposed end portions hermetically holding electrodes within said substantially cylindrical portion, said are tube being housed in an outer jacket and lighted by a frequency modulated power from a high frequency power source of an associated lighting circuit for avoiding the acoustic resonance phenomenon, wherein the apex of at least one of said end portions of the arc tube is positioned to be eccentric to the longitudinal axis of the tube.

Preferably, the apex of each of said end positions of the arc tube is positioned to be eccentric to the longitudinal axis of the arc tube.

The invention will now be further described, by way of example, wi th reference to the drawings, in which:- Figure 1 is a diagram for explaining the acoustic resonance phenomenon in a known arc tube for a high pressure discharge lamp; Fig. 2 shows an example of a frequency-mbdulation type high-frequency lamp lighting circuit used for observations of the acoustic resonance phenomenon; Fig. 3 is a graph showing the presence and absence of the acoustic resonance phenomenon presented on a plane of co-ordinates the abscissa and ordinate of which are the centre frequency (CF) and the frequency modulation range (FM), respectively, the phenomenon having been observed in the case of the arc tube shown in FIG. 1 Fig. 4 is a schematic sectional view of an embodiment of an arc tube for a small high pressure discharge lamp according to the present invention; Figs. 5a, 6a and 7a are schematic views of different embodiments of the arc tube according to the present invention; and Figs. 5b, 6b and 7b are graphs similar to Fig. 3 for showing the acoustic resonance phenomenon observed respectively in the case of each of the different arc tubes shown in Figs. Sa. 6a and 7a respectively.

For a better understanding of the present invention, reference will first be made to the acoustic resonance phenomenon (which shall be hereinafter referred to simply as "AR phenomenon") with which the present 4 invention is concerned. The AR phenomenon is induced by a stationary wave caused to occur upon coincidence of the natural frequency determined by the configuration and charged substances of the arc tube with the pressure variation inside the arc tube due to time variation under an application of input power to the tube f rom a high frequency power source. Here, an assumption is made that, as shown in Fig. 1, the are tube LT is of a cylindrical shape, and a system of cylindrical co-ordinates is set up with a radial direction "r", circumferential direction "e" and axial direction In this case, fundamental frequencies Fr, F9 and FZ of the AR phenomenon in the respective directions r, 8 and z will be as follows:- r-direction resonance: Fr = 3.83C/27FR e-direction resonance: Fe = 1.84C/27rR z-direction resonance: FZ = C/2L where L is the entire length of the arc tube LT, R is the radius of the tube, and C is the sound velocity within the tube determined by the temperature within the tube. That is, C = 7 -PT/m, where Y is a ratio 11specific heat at constant pressure/specific heat at constant volume". P is a gas constant, T is a temperature within the arc tubet and M is an average atomic weight of the charged substances.

The AR phenomenon occurs within the arc tube actually at frequencies corresponding to an integer multiple of the foregoing fundamental frequencies, and the energy of such AR phenomenon has an adverse influence upon an own 30. stable discharge-sustaining force of an arc column itself established between electrodes f and f' within the arc tube LT during its lighting.

For the measurement of the AR phenomenon, a lighting circuit such as shown in Fig. 2 as an example has been used for the high pressure discharge lamp, in which circuit a pair of switching elements 11 and 11a are connected in series with a D. C. power source 10 and in parallel to capacitors 12 and 12a, a series circuit of a current limiting element 13 and high pressure discharge lamp 14 is provided between a junction of the pair of switching elements 11 and 11a and a junction of the pair of capacitors 12 and 12a, and a frequency modulator 15 is connected at its output terminals to the respective bases of the switching elements 11 and 11a for supplying a high frequency power as frequency modulated to the high pressure discharge lamp 14. Further, the supplied power has been set to have a centre frequency of 20 to 100 KHz in order to avoid audible frequency zone and radiation electromagnetic waves.

The AR phenomenon measurement has been performed with respect to the arc tube shown in Fig. 1 which is curved or spherical at both end portions and this measurement is shown in the graph of Fig. 3 in which the abscissa represents the centre frequency (CF), the ordinate represents the frequency modulation range (FM) during the lighting of the are tube, and circle mark "0" and cross mark "x" represent the zones where the AR phenomenon could be avoided (the avoidable zone) and could not be avoided (the unstable zone), respectively.

According to the present invention, the electrodes are positioned eccentric with respect to the tube axis or in point symmetry as shown in Fig. 4. In this case,, the length of the curved or conical end portions of the 7 tube for the small high-pressure discharge lamp can be made sufficiently long and it is possible to keep the temperature at the end portions high and to avoid the AR phenomenon effectively, without deterioration in the 5 luminous efficiency nor in luminous colouring.

Alternatively. the apices of both curved end portions of the tube may be positioned to be eccentric. That is, when the apices are deviated to one side of the axis of the tube as shown in Fig. 5a, the avoidable zones can be highly favourably obtained as shown by the graph of Fig. 5b. In this connection, only one of the end portions may be so curved as to have its apex deviated and, when such apex-deviated end portion is provided at the lower end as shown in Fig. 6a, the avoidable zones can be attained in such wider range as shown in Fig. 6b than in the case where the apex-deviated end portion is provided at the. upper end as shown in Fig. 7a, the measurement of which is shown in Fig. 7b.

It will be clear from comparison of the graphs of Figs.

3, 5b, 6b and 7b with each other that, if the electrodes are positioned to be eccentric with respect to the axis of the arc tube, the AR phenomenon can be remarkably avoided at many zones of the centre frequency higher than 30 KHz and of the frequency modulation range of 0 to 10 KHz.

Various modifications and other measures are possible in order to achieve the aim of the present invention and, in this connection, reference is made to British Patent Specification No. 2169440A.

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Claims (3)

1. A high pressure discharge lamp comprising an arc tube having a substantially cylindrical portion closed by opposed end portions hermetically holding electrodes within said substantially cylindrical portion, said arc tube being housed in an outer jacket and lighted by a frequency modulated power from a high frequency power source of an associated lighting circuit for avoiding the acoustic resonance phenomenon, wherein the apex of at least one of said end portions of the arc tube is positioned to be eccentric to the longitudinal axis of the arc tube.
2. 2. A lamp according to claim 1, wherein the apex of each of said end portions of the arc tube is positioned to be eccentric to the longitudinal axis of the arc tube.
3. 3. A high pressure discharge lamp substantially as described herein with reference to Figs. 4 to 7 of the drawings.

   Published 1989 at The Patent Office, State House, 66171 Ingh Holburn, London 7.1C1R 4TP. Further copies may be obtained from The Patent Ofnee. Sales Branch. St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187