A new generation of high-pressure sodium lamps

Transcription

1 Philips tech. Rev. 39, ,1980, No A new generation of high-pressure sodium lamps C. A. J. Jacobs and J. A. J. M. van Vliet Although existing high-pressure sodium lamps (SON) have a very high luminous efficacy, Philips continue to look for ways of improving their performance. Research on the high-pressure sodium discharge - which has already been reported in detail in the special 'Light' issue of thisjournal in has enabled us to design a new generation of lamps, which differ very little from existing lamps in most of their characteristics - so that they can replace lamps in existing installations - but have a luminous efficacy about 15% higher. One type (150 W) is already in production, a second (50 W) is tofollow shortly and several other types are under development. Introduetion Lamps in which the light is generated by means of a high-pressure sodium discharge contain sodium vapour, a little xenon to ensure th~t the cold lamp ignites and the sodium discharge comes into operation, and mercury vapour at a relatively high pressure. By adding this heavy gas - the buffer gas - it is possible to give the discharge the electrical and spectral characteristics that are desirable in a lamp that is to combine a high luminous efficacywith a convenient size and an acceptable chromaticity and can be run economically from the lighting mains [1]. It has been known for some years that it would be more advantageous for the efficacy if xenon could be used for the buffer gas instead of mercury vapour [2], but for various reasons manufacturers were not able to do this. Replacing the mercury by xenon would result in an unsatisfactory colour, a longer discharge tube and, finally, an extremely high ignition voltage. It was this high voltage that appeared to be the insuperable problem. Research has shown that the first two objections can be overcome, and the advantage of the better efficacy retained, by adding a certain amount of mercury to the xenon. It has also been found that the ignition voltage can be reduced below the limit specified in the draft standards now in cir- C. A. J. Jacobs and Ir J. A. J. M. van Vliet are with the Philips Lighting Division, Eindhoven. culation, by fitting a simple auxiliary electrode around the discharge tube [3]. To give some idea of the nature of these problems and how serious they are, we shall first consider how the efficacy, chromaticity and the electrical characteristics of a high-pressure sodium discharge depend on the nature and pressure of the buffer gas. The final -section will summarize the characteristics of three of the new generation of lamps: the 150W lamp already in production, and experimental versions of 70 Wand 4(30W lamps. Luminous efficacy and chromaticity The luminous efficacy 11of a light source is proportional to the fraction of the electrical power supplied that is converted into visible radiation, multiplied by the luminous efficiency Vs of the visible radiation produced: 11= 683 Pvis Vs lmwatt. Pin [1] See for example J. J. de Groot, J. A. J. M. van Vliet and J. H. Waszink, Philips tech. Rev. 35, 334, [2] K. Schmidt, C. R. 6e Conf. int. sur les Phénomènes d'ionisation dans les gaz, Paris 1963, Vol. lil, page 323. [3] I. Iwai, M. Ochi and M. Masui,'}, Light & vis. Env. 1, No. 1, page 7, C. A. J. Jacobs, L. Sprengers and R. L. C. de Vaan, J. IlIum. Engng Soc. 7, 1, 1978.

2 212 C. A. J. JACOBS and J. A. J. M. VAN VLIET Philips tech. Rev. 39, No. 8 The ratio Pvis Pin (the radiant efficiency) is not very different for mercury and xenon. Fig. 1 indicates how the radiant efficiency for the two buffer gases depends on their pressure Pb' This pressure has been expressed as a multiple of the pressure PNa of the sodium , Pvis~n Hg a. 10.o.=-----:-1a'::----:2:'::-a---3='=a=-' - PbPlO Fig. 1. The radiant efficiency, i.e. the ratio of the power P vis radiated in the visible region to the electrical power supplied Pin, as a function of the pressure Pb of the buffer gas. The pressure is expressed as a multiple of the value PlO of the sodium vapour pressure when the maxima of the self-reversed sodium D line differ by 10 nm (about 15 kpa) [I] [4]. The discharge took place in a tube with an inside diameter of 4.8 mm and a wall temperature of 1500 K (at the hottest point), values that apply for the discharge tube of a 150 W SON lamp. At a buffer-gas pressure of about 20PlO or higher the radiant efficiency is much higher than at low pressure, but it makes little difference whether mercury vapour or xenon is used - the difference is less than the accuracy of the measurement. The ratio Pbl PNa is a measure of the ratio of the buffer gas to the sodium vapour, and hence of the thermal conductivity Of the mixture. As Pbl PNa increases, the thermal conductivity decreases; this is an important quantity for the designer [I]. The advantages of using xenon as the buffer gas instead of mercury come about because of the different effects the two gases have on the spectral energy distribution and hence on the luminous efficiency Vs. Both gases reduce the luminous efficiency Vs as compared with pure sodium vapour, but this is much smaller for xenon than for mercury; see fig. 2. An examination of the spectra (fig. 3) reveals that as a result of the addition of each of the gases the red 'wing' (the longer-wave side) of the broadened sodium doublet is raised; this results in a reduction of Vs because the eye is not very sensitive in this wavelength range and the power is taken from the range where the eye is more sensitive. This effect is most marked in mercury, and in addition the spectrum for mercury has two bands in this region (at 655 and 670 nm). In xenon, on the other hand, there is a strong band at about 560 nm just where the eye is very sensitive. Obviously the changes produced in the spectrum of the discharge as a result of the addition of mercury or xenon as the buffer gas have some effect on the colour of the emitted light. As fig. 4 shows, the addition of mercury improves the spectrum at first - the light source becomes more like a black-body radiator - but when the mercury pressure is increased further the colour coordinates of the source move towards the red. The addition of xenon makes it move towards the yellow-green and this is less desirable. Xe Electrical characteristics a PbPlO Fig. 2. The luminous efficiency Vs of a high-pressure sodium discharge - the conditions are the same as in fig. I - again as a function of Pbl PlO' Increasing the buffer-gas pressure gives a reduction in Vs, but this effect is much weaker for xenon than for mercury. vapour. In all our experiments the pressure PNa had the value PlO' at which the separation of the two maxima in the self-reversed D line is 10 nm; at this pressure 11 has the highest value for a discharge in pure sodium vapour [11. The advantageous effect of the buffer gas is unmistakable, but it is also clear that there is no point in making Pbl PNa larger than 20 to 30. One of the advantages of using mercury as the buffer gas is that the field-strength in the discharge is higher, which means that for a given operating voltage the discharge tube can be shorter - and thicker - than for a discharge in pure sodium vapour. Xenon does not offer this advantage; the addition of xenon to the sodium vapour has very little effect on the field-strength. To obtain a reasonable operating voltage it would therefore be necessary to make the tube longer but, to keep the 'wall loading' [11 at the same value there would have to be a simultaneous reduction in the diameter. However, this has an adverse effect on Pvis Pin. A lamp like this would also [4] J. J. de Groot and J. A. J. M. van Vliet, Determination of the sodium and mercury vapour pressure in high pressure sodium lamps, in: 2nd Int. Symp. on Incoherent light sources, Enschede 1979, page 30. J. J. de Groot and J. A. J. M. van Vliet, J. Physics D 8, 651, 1975.

3 Philips tech. Rev. 39, No. 8 NEW-GENERATION SODIUM LAMPS Wnm p 1.4 t 1.2 ' 0.2-, À Fig. 3. The spectrum - power P per nm plotted against the wavelength À - of high-pressure sodium discharges with mercury vapour (blue) and xenon (red) as the buffer gases. The dotted line gives the spectrum of pure sodium vapour. All the spectra relate to the sodium pressure PlO and are absolute spectral power distributions for an input power of 150 W. The dashed curve is the spectral luminous efficiency (see the scale on the right). The spectrum of the discharge with xenon as the buffer gas is clearly more suitable than the spectrum of a discharge with mercury as the buffer gas because the red 'wing' is lower, mainly as a result of the presence of a strong band at about 560 nm. Fig. 4. Chromaticity diagram (a) with detail (b), illustrating the chromaticity of the highpressure sodium discharge under different conditions. Cr, Rand V indicate the green, red and violet corners of the colour triangle. A discharge in pure sodium vapour (no buffer gas). Adding mercury vapour shifts the point in the direction Hg ; adding xenon moves it in the direction Xe. The solid curve gives the chromaticity of a black-body radiator each point corresponds to a given temperature. The chromaticity coordinates of the lamps of the first generation (SON) are x = 0.53, y = 0.42, and the chromaticity of the new lamps (SON') is close to it (x = 0.53, y = 0.4). y , t 0.75 Gr x y t Aq----- Xe \ 'o-son* SON~q_ t----..l~ \ Hg x

4 214 C. A. J. JACOBS and J. A. J. M. VAN VLIET Philips tech. Rev. 39, No. 8 be rather inconvenient for practical lighting fittings. A greater disadvantage is that a very high voltage is necessary to ignite lamps in which xenon is the buffer gas. The starters that have been developed for the SON lamps cannot ignite an Na-Xe lamp of the same dimensions when the xenon pressure is higher than 6 kpa (i.e. about 50 kpa in the operating lamp) [5]. Since mercury and xenon lamps should be interchangeable and there should be no flash over in the starter or between the supply leads in the base of the lamp, this is an unacceptable situation mm OT IW C The new generation of SON lamps As already stated briefly in the introduction, the various difficulties of using xenon in the new generation of high-pressure sodium lamps have largely been overcome. It has been found that the dimensions of the discharge tube can be kept the same as those in the existing SON lamps - and almost all the advantages retained - by using a mixture of xenon and mercury as the buffer gas instead of xenon alone. This also had the important advantage that the chromaticity shifted appreciably in the desired direction to a position very close to the chromaticity of the existing lamps. The ignition electrode that is used to overcome the starting problem is nothing more than a wire of tungsten or tantalum running the entire length of the discharge tube and connected to one of the main electrodes through a capacitor or a bimetallic strip. The wire makes starting easier because it considerably reduces the effective electrode spacing for the starting pulse. In the new 150W lamps the wire is capacitively coupled to one of the main electrodes and is mounted so that it lies against the discharge tube; see fig. 5. In lamps with a bimetallic strip this element moves the wire away from the discharge tube as soon as it starts to get hot. The purpose of both of these measures is to prevent a high electrical field-strength from building up in the wall of the discharge tube. This would cause migration of sodium ions and hence local blackening of the tube. The characteristics of three SON lamps of the new generation - the 70W lamp and the 400W lamp are still in the experimental stage - are set out in Table I. It can be seen that an improvement of 10 to more than in the luminious efficacy has been obtained. There are a number of other important improvements. The new lamps have a much shorter warmingup time than their predecessors (about a minute). Sputtering of the electrodes - in the first few moments after switching on - and hence blackening of the tube have been reduced considerably; see fig. 6. This sputtering effect is particularly important be- Fig. 5. Diagram of a new 150 W SON lamp with clear cylindrical envelope. (A version is also available with an oval envelope with light-diffusing coating.) DT discharge tube. IW ignition electrode, connected to one of the electrodes through the ceramic capacitor c. Fig. 6. AlSO W SON lamp of the first generation (right) and one of the new generation, both after hours of operation. The discharge tube of the older SON lamp reveals a definite blackening near the electrodes but the new lamp does not. The effect of the difference is that the luminous efficacy of the new SON lamps has hardly decreased at all during the hours of eperation. [51 J. A. J. M. van Vliet and C. A. J. Jacobs, New developments in high pressure sodium discharge lamps, in: CIBS Nat. Lighting Conf. 1980, Canterbury, page CA I.

5 Philips tech. Rev. 39, No. 8 NEW-GENERATION SODIUM LAMPS 215 cause it makes the lamp voltage increase - and hence the re-ignition voltage - and thus ultimately causes the lamp to fail. The luminous efficacy of the new SON lamps therefore decreases much more slowly than that of lamps with mercury alone as the buffer gas, and their life is longer. Table I. The most important- characteristics of three high-pressure sodium lamps of the new generation (SON*), compared with those of existing lamps (SON). type 70W 150 W 400W SON SON*[a] SON SON* SON SON*[a] Luminous flux (Im) (clear envelope) Luminous efficacy (ImW) Partial pressures (kpa; ± 20%) in the operating lamp: sodium vapour mercury vapour xenon [a] experimental unit loo ISO IS Summary. Extensive research into the high-pressure sodium discharge has resulted in the development of a new generation of highpressure sodium lamps, which have a luminous efficacy about 15% better than that of existing SON lamps and also a longer life, but in other respects differ very little. The improvement is based on the use of xenon as the buffer gas. The xenon is not used on its own, because this would result in a longer and thinner discharge tube, an unsuitable chromaticity and a very high ignition voltage. The first two of these disadvantages have been overcome by adding a certain amount of mercury to the xenon; the third disadvantage has been eliminated by using an auxiliary electrode consisting of a wire stretched along the outside of the discharge tube and connected to one of the main electrodes through a capacitor or a bimetallic strip. A 150W lamp of the new generation is already in production; other lamps for higher and lower powers are being developed.