Fluorescent lamps and low pressure sodium lamps

Fluorescent lamps and low pressure sodium lamps

A.G. Jack, B.Sc, Ph.D., C.Eng., M.I.E.E., and L.E. Vrenken

Indexing terms: Fluorescent lamps, Discharge lamps

Abstract: Both the fluorescent lamp and the low pressure sodium lamp are based on the same type of dis-charge, namely the low pressure metal/rare-gas discharge. The paper deals with recent advances and possiblefuture developments for both types of lamps. For the fluorescent lamp, recent advances include a betterunderstanding of the discharge processes, the introduction of narrow emission band phosphors, the use ofmore efficient narrower diameter discharges and improvements in the control of the mercury vapour pressure.Possible future developments include the more general use of electronic control gear, electrodeless dischargelamps, and more compact lamps realised, e.g. by using a recombination structure in the discharge vessel. Forthe low pressure sodium lamp recent advances include a better understanding of the discharge processes,improvements in the thermal insulation of the discharge tube, leading to better arc voltage control, and amodified current waveform leading to an improved combination of lamp and control gear. Possible futuredevelopments are likely to be similar to those for the fluorescent lamp, e.g. a further introduction of elec-tronic control gear, and also a further miniaturisation of the lamp.

1 Introduction

This paper deals with two types of lamps based on the lowpressure gas discharge, namely fluorescent lamp and lowpressure sodium lamp. Although the two lamp types areoutwardly rather different, there are quite a lot of simi-larities as far as the basic mechanisms of the lamp are con-cerned. Throughout the paper, emphasis will be placedupon the developments which have taken place during thelast few years, and possible future developments.

2 The fluorescent lamp

2.1 Brief introductory description

The lamp is based on the low-pressure mercury/rare-gasdischarge where the mercury vapour pressure is of the orderof 1 Pa, and the rare gas, e.g. argon, has a pressure in therange 102 to 103 Pa. In the discharge there is excitation andionisation of mercury, and the main radiation output comesfrom the two mercury resonance lines which lie in the ultra-violet at 254 and 185 nm. As the mercury density increases,the radiation output at first increases due to the increasednumber of excited mercury atoms; then, however, itdecreases. This latter effect is due to the fact that resonanceradiation is absorbed and re-emitted many times before itfinally escapes from the discharge vessel. Every time aphoton is absorbed there is a possibility that it is notre-emitted, but that the energy is released via a non-radiativeprocess. Thus, for an efficient discharge, the amount ofradiation trapping, and hence the mercury particle density,is limited. In practical situations a mercury pressure in therange 0 1 to 1 Pa is necessary. In the discharge there is littleor no excitation and ionisation of the rare gas. The functionof the rare gas is to regulate the diffusion processes in thedischarge so that the most efficient discharge is realised,and also to provide an environment conducive to a longelectrode life.

In order to realise a light source, the ultraviolet radiationmust be converted into visible radiation. This is realised bycoating the inside of the discharge tube with a fluorescent

Paper S34A, first received 3rd July and in revised form 10thSeptember 1979Dr. Jack and Mr. Vrenken are with the Lighting Division, N.V.Philips' Gloeilampenfabrieken, Eindhoven, The Netherlands

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

powder. Fig. 1 shows, very schematically, the constructionof a typical fluorescent lamp. The power balance for a 40 Wfluorescent lamp is given in Fig. 2. It is seen that the con-version of electric energy into radiation in the positivecolumn is high, ca. 74%. The electrode losses, and to agreater extent the energy loss in the fluorescent powder,where at most one visible photon is produced for eachenergy rich ultraviolet photon, lead to an overall conversion

visible light ultraviolet radiation

phosphur crystals \ electron /mercury atom electrode

Fig. 1 Sketch showing the main features of a fluorescent lamp

input power AOW

power in discharge column3AW

discharge radiation25W

visible1W

ultra violetradiation 2AW

phosphor layer

visible 10W(3A00lm,Ra85)

electrodevolume \ \ losses

and \ \ 6Wwalllosses

9W

heat dissipated by tube30W

fluorescent lamp

Fig. 2 Power balance for a40W fluorescent lamp

149

0413-702X/80I030149 + 09 $1-50/0

of only about 25% of the input electrical energy into usefulvisible radiation. The quality and colour of the visible radi-ation emitted by the lamp depend on which fluorescentpower is used in the lamp. A much more detailed descriptionof the fluorescent lamp and the progress since its intro-duction in the late 1930s is to be found in, for example,the books of Elenbass,1 Henderson and Marsden2 andWaymouth.3

2.2 Recen t developmen ts

2.2.1 Understanding of the discharge processes: Our under-standing of the low-pressure mercury/rare-gas discharge andthe numerical models used to describe the discharge is dealtwith, for example, in the book by Waymouth.3 Morerecently, two aspects have been considered; in previousmodels of the discharge, a Maxwellian distribution was usedas a first order approximation for the electron energy dis-tribution. Studies have recently been made of the electronenergy relaxation process in the low pressure dischargeusing a two group model approach rather than a Maxwelliandistribution for the electron energy.4 The application ofthe two electron group model gives better agreement withthe experiments, particularly at low current values.

Experimental measurements by Franck and Liidemann5

and Franck6 have shown that significant concentrations ofHg2 and ArHg+ ions can be formed in the mercury-argondischarge. Very recently Vriens, Keijser and Ligthart7 havecalculated the various rates for electron impact and excitedatom collisional ionisation. They find that 63P —63Pcollisions become even more important for ion formationthan electron impact ionisation at higher mercury vapourpressures. Thus the inclusion of excited state collisionalionisation leads to much better agreements as comparedwith prior calculations, in particular at higher mercurypressures. Fig. 3 shows some results where these twoadditional aspects have been included.8

2.2.2 Lamps with a more efficient discharge: The rare gasused in the great majority of fluorescent lamps is argon oran argon-neon mixture. From experiments and theoreticalstudies it is known that a more efficient discharge could berealised by switching over to krypton, or a krypton rich

80

60

20 experimentsnew calculationsolder calculations

20 40 60 80wall temperature,degrees centigrade

100

Fig. 3 The calculated column efficiency (r\) for the two mercuryultraviolet lines as a function of tube wall temperature comparedwith earlier calculations and measurements

For full details see reference22

150

mixture. The rise in the discharge efficiency is mainly dueto the diminished energy-loss when electrons collide withkrypton atoms in the discharge. However, krypton hassome drawbacks in that it is significantly more expensivethan argon and the ignition properties of the lamp arepoorer, especially in rapid start circuits. The last disadvan-tage can be overcome by placing a transparent conductinglayer on the inside of the discharge tube.9'10 Provided theresistance of the layer is within certain limits, reliableignition can be realised. During the last few years therehas been an increasing use of these energy saving lamps,especially in North America.11 Compared with the standardrange of lamps, they typically consume about 10% lessenergy while the reduction in the luminous flux is some-what less because of the increased discharge efficiency.Other, more efficient, discharges in narrow diameter dis-charge tubes will be discussed in Section 2.2.4.

There is a limit to the wattage reductions possible byvariation of rare-gas filling. Recently, impedance modifiedfluorescent lamps have been introduced in North America,12

thus markedly increasing the practical range of wattagereduction. It is achieved by using a shorter discharge tubelength and integrally mounting auxiliary ballasting in thelamp end. However, unlike varying the rare-gas filling, thisapproach does not lead to an increase in the lamp luminousefficacy.

2.2.3 New phosphor developments: In the last few yearsmajor advances have been realised with the phosphorcoating. Theoretical studies13'14 have shown that bylimiting the visible radiation to specific narrow bands it ispossible to obtain a high luminous efficacy together with ahigh colour rendering index. Phosphors with the necessarynarrow emission bands have been developed, thus trans-forming the theoretical predictions into working fluorescentlamps.15'16> 17 Fig. 4 shows the spectrum of a deluxe lampiwith the conventional halophosphate phosphor and thespectrum of a lamp with the newly developed very efficientnarrow band phosphors BAM [(Ba, Eu) MgAl1002o](blue, emission peak at 450 nm), CAT [(Ce, Tb) MgAlu019]

800r

600-

400"

200-

i/

/- /i

i\\

\

400 500 600I nm

700

Fig. 4 Comparison of spectral power distribution of old and newgeneration 'deluxe' lamps, superimposed on the c.i.e. V\ curve

new lamp (code 84)old 'deluxe white' lamp (code 34)CIE standard observer relative luminous efficacy curve V\


(green, emission peak at 543 nm) and the already known(Y, Eu)203 (orange-red, emission peak at 611 nm). Furtherimprovement of the properties of the phorphors hasled to a further improvement in the luminous efficacy to85 lmW"1 for 1 -2 m 40 W lamps with a nominal outer diam-eter of 38 mm produced by mass-production techniques.Because of their high price, efforts have been made toreduce the total cost of the fluorescent powder layer. Onesolution is to apply two phosphor layers18 as shown in Fig.5. Both phosphor layers have nearly the same chromaticallyco-ordinates, while the amount of the expensive rare-earthactivated phosphors is reduced. Since most of the absorptionof the ultraviolet radiation takes place in the region closestto the discharge, the emission spectrum of this double layerphosphor is dominated by the spectrum of the phosphorclosest to the discharge, i.e. the new rare-earth activatedphosphors.

discharge

expensive narrow band phosphor

inexpensive halophosphate phosphor

glass

Fig. 5 Sketch showing the positioning of two phosphor layers sothat good use is made of the expensive narrow band phosphor whilereducing the cost of the total phosphor layer

Advances have also been made in phosphors for standardfluorescent lamps. This has been achieved by replacingthe usual calcium halophosphate phosphor with a blend ofa narrow-band blue-emitting phosphor, and a brighter,yellow-emitting phosphor (europium-activated strontiumchlorapatite with manganese and antimony coactivatedcalcium fluorapatite phosphors).19 This combination ofa narrow and broad emission band results in a substantialincrease in the lamp luminous efficacy compared with astandard lamp with a calcium halophosphate phosphor.The general colour rendering index is somewhat lower.

2.2.4 Further optimisation of phosphor and discharge: Thedevelopments described in the above paragraph were withlamps with a nominal external diameter of 38 mm. Recently,further increases in the luminous efficacy have been realisedby exploiting the properties of the new narrow band phos-phors (BAM, CAT, (Y, Eu)2O3) in fluorescent lamps with anominal external diameter of 26 mm.20-21 The increase inthe lamp voltage due to the narrower diameter lamp can becompensated by replacing the usual argon-neon mixturewith a krypton-argon or krypton-neon mixture. As has beenmentioned in Section 2.2.2, this leads to a more efficientdischarge. Indeed, in order to realise the same luminousflux from this narrower diameter lamp, the input power isreduced from 40 W to 36 W. This change to a narrowerdiameter lamp has been possible because the new narrowemission band phosphors are able to withstand the moreintense ultraviolet radiation which is generated in thenarrower diameter discharge tube. With the older halo-phosphate phosphors, the more intense ultraviolet radiationcaused a much more rapid decrease in the luminous efficacy


during life in a 26 mm tube compared with a wider 38 mmtube21 - see Fig. 6. A detailed study of the 26 mm lamphas been carried out22 in which changes in the dischargeparameters are quantified. Fig. 7 shows the trends in lumi-nous efficacy over the last 25 years and dramatically showsthe step wise improvement due to the introduction of thenew phosphors.

2.2.5 Improved control of the mercury vapour pressure: InSection 2.1 it was briefly stated why there is an optimum inthe mercury vapour pressure for the low pressure mercurylamp. Since excess mercury is always present in the dis-charge vessel, the mercury vapour pressure is controlled by

100

90

80

improved three-band phosphor

38mm <f>

2 5 m m <fr

halophosphate

25mm $

0 1000 3000

burning time.hrs

5000

Fig. 6 Maintenance curve for some 4 ft 4000 K lamps with 26 and38 mm diameter, in which halophosphates and improved narrowemission band phosphors have been used

1001m/Wlaboratory, jz525

1950 1960year

1970 1980

Fig. 7 Trends in luminous efficacy of 40 W, 4000 K fluorescentlamps and what may be expected in the near future

151

the coolest spot of the discharge vessel. If the fluorescentlamps are operated in higher temperature environments, e.g.closed luminaires, then the mercury vapour pressure risesand the lamp luminous efficacy decreases. Some years agoamalgam lamps were developed which were far less depend-ant on the ambient temperature.23 The temperature rangeover which the luminous flux is more than 90% of theoptimum is extended from 30°C to about 60°C — see Fig.8. More recently newer mercury alloys have been studiedfor use in fluorescent lamps.24 They have improved charac-teristics, especially where ignition behaviour is concerned.

From environmental considerations it is essential thatthe mercury content of the lamp should be kept as low aspossible. Also, for lamps with an amalgam, it is essentialthat the correct amount of mercury is introduced duringthe production process. This has led to new ways of intro-ducing the mercury into the lamp; for example, the use of amercury dispenser strip2S or of a glass capsule which is

amalgam

10 20 30 40 50 60 70 80ambient temperature degrees centigrade

Fig. 8 Relative luminous flux, operating in free air, of standardlamps and amalgam lamps as a function of ambient air temperature

The regions where the luminous f lux > 90% of maximum areindicated

mercury capsule

indium for theauxiliary amalgams

indium for themain amalgam

Fig. 9 Sketch showing the locations of indium and how themercury capsule is mounted

filled with an accurately determined amount of mercury.16

Fig. 9 shows how the mercury capsule is mounted. Thecapsule is fixed by clamping it between one end of themetallic ring of the electrode assembly, and a thin tantalumwire. Once the lamp has been sealed off, a high frequencycurrent causes the wire to become incandescent, thus open-ing the small glass tube containing the mercury.

2.3 Possible future developments

For fluorescent lamps that are already on the market therewill be the usual regular improvements in performance dueto improvements in materials, design and fabrication tech-niques. However, there are other possible developmentswhich are further removed from today's products.

2.3.1 Electronics: Because the gas discharge has a negativevoltage-current characteristic it must always be connectedto the supply via a current limiting element, usually aninductor or a capacitor and inductor in series. Often it isnecessary to take measures to ignite the lamp. During thepast few years the possibilities of electronics taking oversome or all of these functions have come nearer to realis-ation. At the present time electronic circuits are being usedfor some important secondary functions of ballasts whichmade them cumbersome and expensive, functions such asignition and re-ignition of the lamp.26 An interesting recentdevelopment of this hybrid type of circuit is the use ofelectronics to short-circuit the fluorescent lamp for abrief period during each half-cycle on the normal mainsoperation.27 Normally the lamp voltage has to be nothigher than about half the supply voltage. However, withthis new technique the lamp voltage-can be about the samevalue as the supply voltage, thus leading to a more compactballast with lower power losses. At each half-cycle, thelamp is re-ignited with the energy that is stored in theballast during the short circuiting of the lamp.

It has been known for some considerable time that ifthe fluorescent lamp is operated at higher frequencies, say10—100kHz, then there is an increase in the luminousefficacy. This is due to the fact that above a frequencywhich lies typically between 1 and 10 kHz the electrodelosses are diminished,28 and with increasing frequency thereis a slight increase in the discharge efficiency.29 About 20years ago Campbell30 described high frequency fluorescentlighting installations, but their use has been limited to asmall number of specialised applications.

With the rapid development of electronics during thelast years, new possibilities for viable high frequencyoperation of fluorescent lamps may emerge. Indeed, veryrecently there has been an extensive comparative test ofconventional and solid state ballasts.31 Such solid stateballasts can offer additional features such as dimming. Thiscould, for example, be coupled with the daylight level togive a more constant illumination level with a reducedpower consumption.

2.3.2 More compact fluorescent lamps: One disadvantageof the fluorescent lamp, especially if it is to replace the lessefficient incandescent lamp, is its relatively large dimen-sions. Thus there is a continuing interest in modifying thegeometry of the lamp, and also increasing the power dis-sipated per unit volume in the discharge.

As the current density in the discharge is increased, thereis an increase in collisions between electrons and excitedatoms, which leads to a depopulation of excited states with-

152 IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

out the generation of radiation, resulting in a less efficientdischarge. Recently Hasker32 has shown that the currentdensity can be kept constant but the electric field andhence also the input power can be increased by the intro-duction of a recombination structure, for example finelydivided glass wool. This is placed in the discharge vesseland increases the surface available for the recombination ofelectrons and ions. The increase in the ionisation loss resultsin an increase in the axial electric field without seriouslydegrading the efficiency of the discharge. A possible com-pact fluorescent lamp which makes use of such a recombi-nation structure is shown in Fig. 10. The lamp and ballastform one unit which can be directly connected to the mainssupply. In general, such compact lamp systems operate athigher temperatures than those needed for an optimummercury pressure. Thus the use of an amalgam, as describedin Section 2.2.5, is usually necessary.

ballaststarter switch

\-i\ i :3

adaptor

=70mm =150mm

Fig. 10 Sketch of a one-thousand lumen prototype 20-watt,small-size fluorescent lamp with a recombination structure, com-bined with an adaptor. System efficacy is about 40lmW~l

Fig. 11 A proposed configuration for an electrodeless highfrequency fluorescent lamp34

In this case the coil and ferrite core are placed inside the dischargevessel

2.3.3 Electrodeless discharges: Up to this point in thesurvey, all the discharge vessels have been supplied withelectrodes and the power is coupled into the dischargethrough them. However, it is also possible to couple thepower into the discharge via varying magnetic or electro-magnetic fields. One example of this work is the systemproposed by Anderson33 where the discharge vessel wastoroidal in form, and formed the secondary 'single-coil'winding of a transformer. The operating frequency is typi-cally in the 10-100 kHz range and a ferrite material wasused as the transformer core. Further developments byAnderson have led to a system which has a more suitablegeometry for a lamp,34 see Fig, 11 for one example of this.

Other approaches which have been considered are theuse of an air cored coil at significantly higher frequencies,e.g. greater than 1 MHz as described by Hollister,35 or else ashift to the microwave frequency range with the dischargevessel located in a tuned cavity as described by Haugsjaaetol.36

Although detailed studies of electrodeless mercury/rare-gas discharges are not at present available there are reasonsto believe that their efficiency as a light source can be simi-lar to that of conventional fluorescent lamps. The prob-lems which have to be solved in order to obtain a viableelectrodeless light source are the realisation of an efficienthigh frequency generator and an efficient way of couplingthe high frequency power into the discharge.

3 The low pressure sodium lamp

3.1 Brief introductory description

This lamp is based on the low pressure sodium rare-gasdischarge where the sodium vapour pressure is of the orderof 1 Pa and the rare gas, e.g. neon, has a pressure in therange 102 to 103 Pa. These are conditions also found in thefluorescent lamp, see Section 2.1. Unlike the mercury dis-charge in the fluorescent lamp, the sodium resonance linesare in the visible wavelength range (589-0 and 589 6 nm)and thus no phosphor is necessary. Thus the low-pressuresodium discharge (SOX) lamp is characterised by its nearlymonochromatic yellow radiation and its high luminousefficacy. It finds application where high luminous efficacyand contrast recognition counts, but where colour render-ing is unimportant, e.g. highways, harbours, etc.

Because of the fact that the sodium vapour pressure hasto be of the order of 1 Pa for an efficient discharge, a coldspot temperature of about 260°C is necessary in order torealise the necessary sodium vapour pressure. All this resultsin the need for good thermal insulation and a glass which isresistant to sodium at the operating temperature. As far asthe positive column of the discharge is concerned there arenaturally many similarities with the mercury discharge, butin the sodium discharge, depletion of the sodium groundstate is a dominant effect. These three differences, namelythe need for a sodium resistant glass, thermal insulation andthe occurrence of depletion, will first be discussed.

3.1.1 Sodium resistant glass: Before a sodium/rare-gas dis-charge can be studied and optimised, it is necessary to havea discharge vessel which does not react with sodium attemperatures of up to about 300°C. In initial experimentsin the early thirties a borate glass was used, but such glassesare relatively expensive, are susceptible to attack by atmos-pheric moisture and are not particularly amenable to form-

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980 153

ing normal glass fabricating techniques. Very soon a two-ply tube was developed which consists of a soda lime tubecoated on the inside with a thin layer, typically 50/im thickof sodium resistant borate glass.37 More recently otheralternatives have been suggested such as a sodium resistantgehlenite glass.38

250

200

a*ai/)c<nE3

150

100

50

fixed bore and lamp length

8* @) JITL

- 7# fib OH improved ref lector^

O O

500 1000optimum power dissipation,watts

15000

Fig. 12 Improvements in luminous efficacy of a low-pressuresodium discharge lamp due to improvements in the thermal insulation

3.1.2 Thermal insulation: In order to limit the energy lossfrom the hot discharge tube there have been a succession ofimprovements in the thermal insulation. A review of thedevelopment has been given by Elenbaas39 and summarisedhere in Fig. 12. A most significant improvement was the useof a tin oxide film and its qualitatively better successor, theindium oxide film. Such films are electrically conductive,transparent in the visible range of the spectrum and have anexcellent reflection, in the near infra-red.40 Fig. 13 showsthe characteristics of a present day indiifm oxide filter.

Fig. 14 shows the construction of a present day lowpressure sodium lamp (SOX lamp). In addition to theindium oxide film, the discharge tube is often U shaped,resulting in a higher dissipation per unit lamp length.

There is also a linear form of the low pressure sodiumlamp.41 The high dissipation per unit length is obtained byusing a non-circular cross section for the discharge tube.This results in a relatively large surface area, and a small dis-charge volume leading to a fairly efficient lamp design.

indium oxide filmouter bulb(evacuted)

^~1—^T~-discharge tube dimples filled

with sodium

Fig. 14 Sketch showing the main features of a low pressuresodium discharge lamp

3.1.3 Radial depletion of the sodium ground state: In thelow pressure sodium/rare-gas discharge the current densityis often significantly higher than in the mercury dischargeand this often leads to a high degree of ionisation of thesodium atoms. Depletion of the ground state arises becauseof the relatively rapid ambipolar diffusion of sodium ionsto the wall where recombination takes place and a slowback diffusion of neutral atoms into the discharge space.

1-0r NaD

A,urn

Fig. 13 The spectral reflection R and transmission T of a tinoxide film (dashed line) and its qualitatively better successor theindium oxide film (solid line)

154

input power 180W

power in discharge column158W

dischage radiation68W

visibleradiation

63W

(32 AOOlm)

nearinfraredradiation

5W

volumeand wall

losses90W

outer bulb112W

SOX 180W lamp

Fig. 15 Power balance for a 180W low-pressure sodium lamp


If radial depletion exceeds a certain level, then anyfurther increase in the discharge current cannot be realisedby a further ionisation of sodium atoms which are already100% ionised at the axis. In such a situation, the rare gas isionised leading to an increase in the electric field and adecrease in the discharge efficiency. The effects of de-pletion will be discussed further in Sections 3.2.2 and 3.2.3.

3.1.4 Power balance for the sodium lamp: Fig. 15 gives thepower balance for a typical low-pressure sodium lamp. Herethe conversion efficiency of electrical energy into radiationin the positive column is ca. 43%, which is lower than theca. 74% for the fluorescent lamp, see Section 2.1. Theworking point of the sodium discharge is so adjusted thatthe thermal loss in the positive column is that which isneeded to keep the discharge tube at the correct workingtemperature. For the fluorescent lamp, about 25% of theinput electrical energy is converted into visible radiation. Inthe low pressure sodium lamp no phosphor layer is necess-ary, and about 35% of the input electrical energy is con-verted into visible radiation, resulting in a lamp luminousefficacy in excess of ISOlmW"1. Reviews of present daylow-pressure sodium lamps have been given by Beijer etal.42 andKoedamefa/.43

3.2 Recen t developmen ts

3.2.1 Understanding of the discharge process: An extensivetheoretical and experimental study of the low pressuresodium discharge has been carried out by v. Tongeren.44 Inthis study, special attention was paid to the control of thetube wall temperature and thus the sodium vapour pressure.The experimental data was compared with a numericalmodel. This model, which included depletion of the sodiumground state, gave a fairly adequate description of the dis-charge parameters with the exception of the electric field.Very recently Vriens45 has considered in detail the ionisationprocesses in these discharges. Multistep electron impactionisation via many higher excited states is found to be adominant process, and this will lead to a decrease in thecalculated electric field.

3.2.2 Arc voltage control: With the low pressure sodiumdischarge lamp, one problem which tended to arise was anincrease in the lamp voltage during the life of the lamp.This is due to the presence of a cooler region in the dischargetube wall. During the life of the lamp, sodium will migratebecause of thermal gradients to that colder region. This will

130

120-

110

100

without cap ..

100 2000 5000 10000t.h

20000

Fig. 16 Relative rise of lamp voltage of a 90 watt low-pressuresodium (SOX) lamp as a function of burning time

The insert shows schematically the construction of the isolating cap


eventually lead to a lowering in the sodium vapour pressure,an increase in the degree of depletion and hence an increasein the lamp voltage, see Section 3.1.3. Techniques, bothdirect46 and indirect,47 have been developed in order todetermine the local sodium vapour pressure. Jacobs era/.48

describes the measures which have been taken to improvethe lamp voltage control during life. Fig. 16 shows a thermalisolating metal cap which eliminates the colder region at thebend, and the relative rise in lamp voltage of a 90 W lowpressure sodium (SOX) lamp as a function of burning time,with and without the cap.

3.2.3 Modified current waveform: Experimental studieshave shown that if the usual sinusoidal current waveform isreplaced by one which has a rectangular form, then theluminous efficacy of the lamp can, in certain cases, beimproved.49* 43 This is due to a diminished degree of de-pletion during a half-cycle, and the consequent rise in dis-charge efficiency.

Another aspect of the low pressure sodium dischargelamp is its fairly high ignition and re-ignition voltage at thebeginning of each half-cycle, immediately after currentreversal during the warm up of the lamp. Especially for thelarger lamp types, this necessitates a large open voltagewhich is supplied by a leakage transformer. However, sincethe open voltage is large compared with the lamp r.m.s.voltage, the result is a large lamp ballast with fairly highlosses. If a rectangular-form current were used, then the re-ignition peak would be almost eliminated, since the plasmawould have no time to decay at current reversal. In practice,such a current waveform is not a viable proposition. How-ever, if the rate of change of current close to current reversalis increased, then the plasma retains a higher degree ofionisation than would be the case for a sinusoidal current,and thus the re-ignition voltage is lowered.

The final result of these considerations is a new type ofballast; the hybrid ballast. The circuit diagram for this newballast is shown in Fig. 17, while the lamp voltage andcurrent for the leakage transformer ballast and the newhybrid are given in Fig. 18. The hybrid ballast consists of anautotransformer with saturating and nonsaturating coilstogether with a series capacitor and a starter. The design issuch that the secondary side of the ballast produces acurrent waveform with a high third harmonic content. Ascan be seen in Fig. 186, the resultant current waveform isbetween a sinusoidal one and a rectangular form. Table 1summarises the properties of the new hybrid ballast com-pared with the leakage transformer ballast.

3.2.4 Miniaturisation: Up until recently, the smallest lowpressure sodium lamp was the SOX 35 W lamp with a lumi-nous flux of 46501m.42 For specific applications thereis a need for a lamp with a still lower luminous flux which

mains

lamp starter

F'9- 17 Circuit diagram for a new hybrid ballast

155

retains the excellent property of the low pressure sodiumlamp, namely its high luminous efficacy. This has led to thedevelopment of the SOX 18 W lamp with a luminous fluxof 18001m, and a luminous efficacy of lOOlmW"1.47 Inthis small lamp it has been possible to use one metal plate,located at the bend in the discharge tube, to act both as athermal insulation, see Section 3.2.2, and as a mountingspring.

lamp voltage(V) and current(l)

lamp voltage (V) and current (I)

Fig. 18 Measured low-pressure sodium lamp voltage and currentwithin a cycle

a with leakage transformer ballast (sinusoidal current)b with the new hybrid ballast (modified current waveform)

Table 1: Data for 90 W version

property

weight (kg)power losses (W)instant reignition3rd harmonics in ,%.mains currentsystem efficacy (ImW"1)no load current

leakagetransformer

7-735no

40

1073 X Ijamp

hybridballast

3-321yes

7-5

118< l lamp

3.3 Possible future developments

It is expected that possible future developments for the lowpressure sodium lamp will follow somewhat similar lines asthose for the fluorescent lamp discussed in Section 2.3. Forexample, the use of recombination structure, as described inSection 2.3.2, has also been applied in laboratory tests tothe low pressure sodium lamp.so There will be a continuingsearch for increases in the luminous efficacy of the lamp,

further advances in the lamp control gear, probably due toa further introduction of electronics, and also a furtherminiaturisation of the lamp.

4 Concluding remarks

Although both the fluorescent lamp and the low-pressuresodium lamp have been produced commercially for anumber of decades, it is seen that progress is still beingmade. This is true both as far as an understanding of thephysics of the devices is concerned, and in lamp perform-ance. As for all lamps there is a continuous effort directedtowards the improvement of the luminous efficacy of exist-ing lamp types. At the present time there is also a search formore compact low pressure discharges which could replacethe tungsten incandescent lamp in certain applications. Thisexpands the range of interest for both the physics of dis-charge, and lamp technology.

5 References

1 ELENBAAS, W., ed.: 'Fluorescent lamps and lighting' (PhilipsTechnical Library, 1959)

2 HENDERSON, S.T., and MARSDEN, A.M., eds.: 'Lamps andlighting' (Edward Arnold, 1972)

3 WAYMOUTH, J.F.: 'Electric discharge lamps', (M.I.T. Press,1971)

4 VRIENS, L., and LIGTHART, F.A.S.: 'Energy balance andcoulomb relaxation in low-pressure gas discharges', Philips Res.Rep. 1977,32, pp. 1-7

5 FRANCK, G., and LUDEMANN, H.: 'Massenspektrometrischeanalysis der Hgt-Bildung in der positiven Saule der Hg-Ar-Niederdruckentladung', Z. Naturforsch. 1972, 27a, pp. 1278-86 '

6 FRANCK, G.: 'ArHg+-ions in the mercury-argon low pressure'discharge mass spectrometric analysis of the formation process',ibid., 1973, 28a, pp. 1481-88

7 VRIENS, L., KEIJSER, R.A.J., and LIGTHART, F.A.S.: 'Ion-isation processes in the positive column of the low-pressureHg-Ar discharge',/. Appl. Phys. 1978,49, pp. 3807-13

8 LIGTHART, F.A.S.: 'Model calculations on the positive columnof Hg-noble gas discharges', paper GB-3, 31st Gaseous Elec-tronics Conference, 1978

9 ROCHE, W.J., and MILKE, H.W.: 'Fluorescent lamp startingaids - how and why they work', /. Ilium. Eng. Soc. 1974, 4, pp.29-36

10 BERGMAN, R.S., HAMMER, E.E., and ROBERTS, V.D.: 'Start-ing behaviour of rapid start fluorescent lamp with internal start-ing aids', ibid. 1976, 5, pp. 131-36

11 EVANS, G.S.: 'Fluorescent lamps, krypton and the conservationof energy', Light Des. & Appl. 197'4,4, (7), pp. 10-14

12 COHEN, S., and SADOSKI, T.T.: 'Impedance modified fluor-escent lamp',/. Ilium. Eng. Soc. 1979, 8, pp. 179-85

13 KOEDAM, M., and OPSTELTEN, J.J.: 'Measurement andcomputer-aided optimisation of spectral power distributions',Lighting Res. & Technol. 1971, 3, pp. 205-210

14 THORNTON, W.A.: 'Luminosity and colour-rendering capabilityof white light',/. Opt. Soc. Am. 1971,61.. pp. 1155-63

15 VERSTEGEN, J.M.P.J., RADIELOVIC, D., and VRENKEN,L.E.: 'A new generation deluxe fluorescent lamp - combining anefficacy greater than 80 lm/W with a colour rendering index ofabout 85', /. Ilium. Eng. Soc. 1975, 4, pp. 90-98

16 VRENKEN, L.E.: 'New fluorescent lamps for interior lighting',Lighting Res. and Technol. 1976, 8, pp. 211-18

17 HAFT, H.H., and THORNTON, W.A.: 'High performancefluorescent lamps',/. Ilium. Eng. Soc. 1972, 2, pp. 29-35

18TERVRUGT, J.W., MANDERS, L.W.J., and WANMAKER,W.L.: Fluorescent lamp coatings with layers of different compo-sitions', Lighting Res. & Technol. 1975, 7, pp. 23-29

19 PIPER, W.W.: The use of Eu2+ and Mn2+ to improve luminousefficacy in cool-white fluorescent lamps', ElectrochemicalSociety spring meeting Seattle, 1978, Abstract 338A, pp.842-44

20 VRENKEN, L.E.: 'Real energy saving fluorescent lamps', /.Ilium. Eng. Soc. 1978, 7, pp. 154-60

21 VRENKEN, L.E.: 'Fluorescent lamps with very high luminousefficacy', Lighting Res. & Technol. 1978, 10, pp. 161-63

156 IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980

22 DENNEMAN, J.W., DE GROOT, J.J., JACK, A.G., andLIGTHART, F.A.S.: 'Insights into the 26 mm. diameter fluor-escent lamp', paper for 1979 IES Conference, Atlantic City,USA

23 ECKHARDT, K., and KUHL, B.: 'Neue leuchtstofflampen furdie innenraumbeleuchtung', Lichttechnik, 1970, 22, pp. 389-93

24 BLOEM, J., BOUWKNEGT, A., and WESSELINK, G.A.: 'Somenew mercury alloys for use in fluorescent lamps', /. Ilium. Eng.Soc. 1977,6, pp. 141-47

25 HIROTA, T., OHNO, H., WATANBE, H., and MORITO, H.:'Fluorescent lamps using a mercury dispenser', /. Light. & Vis.Env. 1977, 1, pp. 1-6

26 SMULDERS, H.A.G.S., 'Electronics in lamp circuits', Light. Res.& Technology 1977, 9, pp. 65-72

27 KANEDA, I.: 'An investigation of minimised ballast upon every-half-cycle ignited discharge lamp operating system with analoguecomputer simulation',/. Light. & Vis. Env. 1977, 1, 29-39

28 KOEDAM, M., and VERWEIJ, W.: 'The influence of the supplyfrequency on the luminous efficacy of fluorescent lamps', Proc. •7th Int. Conf. on Phenomena in Ionised Gases, Belgrade, 1965,3, 392-95

29 DROP, P.C., and POLMAN, J.: 'Calculations on the effect ofsupply frequency on the positive column of a low-pressure Hg-Ara.c. discharge',/. Phys. D: 1972, AP-5, pp. 562-68

30 CAMPBELL, J.H.: 'New parameters for high frequency lightingsystems', Ilium. Eng. 1960, 55, pp. 247-56

31 VERDERBER, R., SELKOWITZ, S., and BERMAN, S.: 'Energyefficiency and performance of solid-state ballasts', Light. Des. &Appl. 1979, 9, (4), pp. 23-28

32 HASKER, J.: 'A new concept for fluorescent lamps', /. Ilium.Eng. Soc. 1976, 6, pp. 29-34

33 ANDERSON, J.M.: 'Electrodeless fluorescent lamps excited bysolenoidal electric fields', Ilium. Eng. 1969, 64, pp. 236-44

34 ANDERSON, J.M.: U.S. Patents 3.521.120, 3.987.335 and4.017.764

35 HOLLISTER, D.D.: U.S. Patent 4.010.40036 HAUGSJAA, P.O.: U.S. Patent 4.070.60337 ADAMS, O.: 'Use of glass in electric lamps', Light. Res. &

Technol. 1978, 10, 83-93

38 JOORMANN, H.J.M., and WESSELINK, G.A.: U.S. Patent3.563.772

39 ELENBAAS, W., v. BOORT, H.J.J., and SPIESSENS, R.:'Improvements in low-pressure sodium vapour lamps', Ilium.Eng. 1969,64, pp. 94-98

40 v. BOORT, H.J.J., and GROTH, R.: 'Low-pressure sodiumlamps with indium oxide filters', Philips Techn. Rev. 1968, 29,pp. 17-20

41 WESTON, R.F.: 'High output sodium lamps', Electrical Times1959, 135, pp. 719-22

42 BEIJER, L.B., V. BOORT, H.J.J., and KOEDAM, M.: 'Thesodium discharge lamp', Light. Des. and Appl. 1974, 4, (7), pp.15-24

43 KOEDAM, M., DE VAAN, R.L.C., and VERBEEK, T.G.:'Further improvement of the l.p.s. lamp', ibid. 1975, 5, (9), pp.3.5-45

44 V. TONGEREN, H.F.J.J.: 'Positive column of cesium- andsodium-noble-gas discharges', Philips Res. Repts. Sup. No. 3,1975

45 VRIENS, L.: 'Multistep ionisation in the positive column of low-pressure Na-Ne and Ne discharges', /. Appl. Phys. 1978, 49, pp.3814-20

46 VRIENS, L.: 'Near-resonant dye laser scattering and two-photonabsorption in a sodium neon plasma', ibid. 1977, 48, pp.653-61

47 JACK, A.G.: 'Studies concerned with improvement of the lowpressure sodium discharge lamp', Light. Res. and Technol. 1978,10, pp. 150-55

48 JACOBS, C.A.J., SPRENGERS, L., and DE VAAN, R.L.C.: 'Arcvoltage control in low and high pressure sodium lamps', /. Ilium.Eng. Soc. 1978, 7, pp. 125-31

49 POLMAN, J. v. TONGEREN, H., and VERBEEK, T.G.: 'Lowpressure gas discharges', Philips Techn. Rev. 1975, 35, pp.321-30

50 HASKAR, J., and v. IJZENDOORN, P.R.: Three differentrecombination structures : application and effects ingas-discharge lamps', Paper for the 1979 IES Conference,Atlantic City, USA

IEEPROC, Vol. 127, Pt. A, No. 3, APRIL 1980 157

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Fluorescent lamps and low pressure sodium lamps