PH I LI PS TECH'NICAL REVIEW

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1 PH I LI PS TECH'NICAL REVIEW VOLUME 29, 1968, No. 12 The iodide discharge lamp L. B. Beijer, C. A. J. Jacobs and T. Tol Gas discharge lamps have the highest luminous efficiency of all radiation sources: the lowpressure sodium lamp excels in this respect with efficiencies of 130 Imf W, and -even up to 175 ImfW when special heat insulation is used. Until recently the only elements that could be usedfor the excitation of light in gas discharge tubes were sodium, mercury and the inert gases. The other elements emitting suitable radiation are either not volatile enough or else they attack the types of glass which can be used for the discharge tube. The artiele below describes a method which gets around these limitations, making it possible to produce radiation sources whose spectral distribution can be freely chosen within certain limits and which give an efficiency only surpassed by that of the sodium lamp. Principle of the iodide lamp and its application in the HPI lamp The first high-pressure mercury-vapour lamp (the HP lampl=i) for street lighting appeared in about Right from the start a great deal of effort was spent in trying to improve the colour rendering of this lamp. In the early attempts at improvement the metals zinc and cadmium, which emit blue and red radiation, were added to the mercury discharge in order to fill up the "holes" in the mercury spectrum. This was not successful mainly because these elements attacked the quartz glass at the temperature required to obtain sufficient vapour pressure. All the work done appeared to indicate that only mercury and the inert gases were suitable for the excitation of light in highpressure discharge lamps (although sodium could be used in low-pressure discharge lamps). In about 1937 a notable success was achieved by an entirely different approach, in the dev~lopment of the HPL lamp. In this lamp the ultra-violet radiation emitted by the mercury is converted into additional visible radiation by means of a fluorescent powder on the outer bulb ui. This lamp quickly superseded the HP lamp. Nevertheless, its colour rendering and also its luminous efficiency still left something to be desired. Ir. L. B. Beijer, C. A. J. Jacobs and Dr. T. Tol are with the Philips Lighting Division, Eindhoven. A few years ago it was found that the earlier conclusions needed to be revised. It was established - or rather rediscovered, since the possibility had been indicated in a patent as long ago as that many elements can be evaporated to give a sufficiently high vapour pressure if they are introduced in the form of special volatile compounds. When this is done it is possible to make the vapour pressure of the element higher at the centre of the discharge than it would be if the element itself were added. The vapour pressure of the element is not increased.in the rest of the tube, but this is not necessary, since the most active part of the gas discharge is found at the centre of the discharge, where most of the radiation is produced. This special method permits the use of elements which are not themselves sufficiently volatile, and it also removes the danger of chemical attack. It was found that iodides are generally very suitable for this purpose. High-pressure mercury-vapour lamps were in fact made in which the desired additional radiation was produced by the addition of an iodide. The development did not rest long at this stage, however, for it [*] In the United Kingdom the lamp types HP, HPL and HPI are designated as MB, MBF and MB[ respectively. ' [I] See J. L. Ouweltjes, W. Elenbaas and K. R. Labberté, Philips tech. ReV. 13, 109, 1951/52, and also W. Elenbaas, Fifty years ofthe high-pressure mercury-vapour lamp, Philips tech.' Rev. 18, , 1956/57, in particular p _~ ~ ~ ~~ ~I

2 354 PHILlPS TECHNICAL REVIEW VOLUME 29 soon became apparent that much greater advantages were to be gained by using the iodides not simply to supplement the mercury spectrum, but as the actual source of the radiation. This results in a considerable gain in efficiency, since it enables elements to be used which, unlike mercury, emit resonance radiation in the visible part of the spectrum. Resonance radiation is radiation which is generated when an electron returns from an excited state to the different function: it takes little or no part in the excitation and ionization processes but helps to maintain the discharge under high pressure and high temperature; it serves as a "buffer gas". A high temperature is needed for sufficient evaporation of the iodides. We first used the principle outlined here in the development of a lamp for road lighting, the HPI lamp, which offers both a better luminous efficiency and a better colour rendering than the HPL Fig. 1. The 400, 2000 and 5000 W H PI lamps in ascending order. The luminous efficiencies are 80, 95 and 110 Im/W respectively, the operating voltages are 125, 245 and 550 V, and the currents are 3.5,9 and 10 A. The lamps have a cylindrical outer bulb of hard glass; the inner bulb is of quartz glass. The outer bulb of the 400 W lamp is evacuated and contains a getter; the outer bulb of the 2000 and 5000 W lamps is filled with nitrogen to improve the heat conduction. ground level. Generally speaking, electron transitions of this nature yield the strongest emission lines. To make use of resonance lines it is necessary to ensure that the absorption of these lines by the gas itself (selfabsorption) is kept to a minimum. In low-pressure discharge lamps, like the low-pressure sodium lamp, the fluorescent lamp, etc. the vapour pressure of the emitting element is kept low for this reason [21. The iodide discharge lamp to be described here is a high-pressure discharge lamp which has elements with a lower excitation level than mercury added to it in the form ofiodides to give resonance radiation. The iodides are added in small enough quantities to ensure low self-absorption. The excitation level of the added elements must be much lower than that of mercury to make sure that the mercury spectrum is not emitted even though the mercury vapour pressure is much higher. In tbese lamps the mercury has an entirely lamp. Three elements were chosen as the radiation emitters for this lamp: Na, TI and In, giving reddishyellow, green and blue light respectively. As in the HP and HPL lamps, argon was adopted as the ignition gas. With careful choice of the quantities of the three iodide additives [31, efficiencies from 80 to l l O lm/w are obtained, compared with 55 to 601m/W from the HP and HPL lamps. The colour-rendering index is about 70 compared with 25 for the HP lamp and 45 for the HPL lamp. HPI lamps of 400, 2000 and 5000 W (jig. 1) are al- [2] A more detailed discussion of low-pressure and high-pressure lamps is given in G. Heller, Comparison between discharge phenomena in sodium and mercury vapour lamps, I and ll, Philips tech. Rev. 1,2-5 and 70-75, [3] For optimization of the quantities to be added (the "dose"), see T. Holmes and J. B. de Boer, Conf. Associated Public Lighting Engineers, Edinburgh 1964, paper No. 2. [4] L. B. Beijer, Proc. 7th Int. Conf. on phenomena ill ionized gases, Belgrade 1965, Vol. ur, p. 182.

3 1968, No. 12 IODIDE DISCHARGE LAMP 355 ready on the market. They are not only used for road lighting but, because of their good colour rendering, for interior lighting as well. An account ofinvestigations which have been carried out on the HPI lamp will be given in the last section of this article. However, one or two of the investigations do help to give a clearer idea ofthe principle underlying the iodide discharge lamp, and are perhaps best dealt with here. perimental HPI lamp was found to be about 1000 "K, and about the same value was measured for the HP lamp. At the axis of the discharge, we measured a temperature of about 4800 "K for the HPI lamp, and about 5700 "K for the HP lamp [4]. The temperature of the are is thus substantially lower in the HPI lamp. Since both lamps were designed to have about the same concentrations of Hg atoms, and since we do not want any mercury radiation from the HPI lamp, a lower are w 3n;:;, 2 IIA......, n IL...r'"l I~OO nm _À w 2mn ~ Q' !!!!2. À nm Fig. 2. Spectral energy distributions of two geometrically identical 500 W discharge lamps whose bulbs are made entirely of quartz glass: a) HP lamp, b) HPI lamp. The visible part of the spectrum is indicated by the bracket. It can be seen that the HPI lamp emits less radiation in the ultra-violet than the HP lamp but rather more radiation in the infra-red. It is also noticeable that the emission in the visible part of the spectrum is more uniform in the HPI lamp than in the HP lamp. ' For comparative experiments we used special HP and HPI lamps, both with an are length of 7 cm and run at 500 W. The production versions have the outer bulb of hard glass and the inner, bulb of quartz glass, but in the speciallamps both bulbswere made of quartz glass to permit measurements over the widest possible range of wavelengths. The wall temperature of this extemperature in the HPI lamp is a necessary condition. The spectral energy distribution of the experimental HP and HPI lamps are shown infigs. 2a and b respectively. It can be seen that the spectrum of the HPI lamp consists mainly of radiation contributions from Na, Tl and In, and not from Hg. This is particularly clear from the low emission in the ultra-violet - the region in

4 :- ----, PHILlPS TECHNICAL REVIEW VOLUME29 which most of the radiation of the HP lamp is produced. The three yellow mercury lines at 577.0, and nm (not separated, in the figure, see arrow) are also very strong in the HP lamp. The only indication of the presence of these emission lines in the spectrum of the HPI lamp is a very small peak, which is only times the' height of the corresponding peak in the HP spectrum. This experimentally determined ratio of radiant intensities was verified by a calculation in the following way. Both the upper and lower energy levels of the lines of interest are well above the ground level, and this means that their occupation densities will decrease very rapidly with temperature. We may therefore assume that the radiation is produced mainly in the hot core of the are and that this radiation is only absorbed to a slight extent as it leaves the are, As a first approximation therefore, the only temperature we need to know is that at the core of the arc. By inserting in Boltzmann's formula the measured temperatures of 4800 ok for the HPI lamp and 5700 ok for the HP lamp, together with the known values of the appropriate excitation levels, we found that the ratio of the radiant intensities was 0.038, which is in good agreement with the experimental value. Further discussion of the principles; other applications Until recently, only a few element's were considered for the excitation of radiation in gas discharge lamps. As we have seen, the reason for this limited choice was that the other elements were either not volatile enough or too chemically active, or both. The first difficulty can be avoided by introducing the element in the form of an iodide - provided the iodide is a) reasonably stable at the temperature ofthe wall, but b) more volatile than the element itself, and c) largely dissociated at the temperature encountered at the axis of the tube. The condition that the iodide should be more volatile arises from the fact that the vapour pressure of the element in equilibrium can never be higher than the saturation vapour pressure of the iodide at the wall temperature. The significanee of the stability of the iodide near to the wall becomes clearer when we realize that, if the iodide is too unstable, the vapour pressure of the element may become higher. than the local saturation pressure, resulting in the partial condensation or deposition of the element. In these circumstances the required pressure cannot be reached at the axis. The second difficulty limiting the choice of elements - the high chemical activity - is also overcome if the iodide is very highly stable at the wall temperature.. For' introduetion in the form of iodides we chose elements with very strong emission lines in the wavelength regions of practical importance. The elements that seem most suitable, partly because of the properties of their iodides, are Li,. Na, Tl, In, Ga and Pb (Table I and jig. 3). The iodides of Tl, In, Ga and Pb are more volatile and the iodides of Li and Na are less active than the respective elements.. The wider choice of elements, with accurate control of the quantity of the iodide, allows us to increase the vapour pressure of the element at the centre of the tube to any value we require, provided it remains below the saturation pressure of the iodide at the wall temperature. This means that we can control the spectral energy distribution of the iodide lamp within certain limits and match the lamp to the needs of a particular application. In the development of the HPI lamp the choice of the iodides and the quantity used was largely determined by the need for the best possible luminous efficiency with reasonably good colour rendering. In the development of a projector lamp (CS!), where the standard of.colour rendering has to be higher, the same three iodides were no longer sufficient and we had to use in addition the red-emitting lithium, as the iodide. The resultant improvement in the colour rendering of the CSI lamp, however, was gained at the expense of its luminous efficiency. We have also made lamps that produce coloured light with a high efficiency (floodlight lamps): a reddish Table I. Some data relating to the elements with the strongest emission lines in the wavelength regions of practical interest, and their iodides. T is the temperature at which the saturation pressure is 1 millibar, and A. is the wavelength of the strongest emission lines. Li Lil Na NaI A. (nm) /589.6 TI TIl 702 In nl 624 Inh 637 InTa 510 Ga Gala 430 Pb Pbh 743 I I Hg /365.5/ ,0/579.0/579.1 Hgh 425

5 1968, No. 12 IODIDE DISCHARGE LAMP 357 yellow lamp with NaI, a green lamp with Til, and a blue lamp with Inl. Other important applications are to be found in the field of photochemical reactions. Lamps we have made for these applications include the types that produce Ga and Pb radiation, mainly in choice of elements that can be used with the iodide technique. Since high pressures are undesirable owing to the line-broadening they cause - and since the efficiency is not important - these lamps are produced as low-pressure types (with no buffer gas) [6]. In practice 12eV Li Na Tl In Ga Pb Hg I 10 8 Fig. 3. The principal energy levels of the elements to be used for producing the light. The metastable levels, which cannot be "emptied" by emission, are indicated with an asterisk. It can be seen that Hg and I (apart from a metastable level at 0.94 ev) are more difficult to excite and to ionize than the other elements. the wavelength region between 340 and 450 nm. Used for the activation of diazo reactions (in photographic reproduction) and for the polymerization of photosensitive lacquer (in photo-etching processes), these lamps give an efficiency 50 % higher than that of the mercury lamps which had to be used before. Another interesting application of iodide discharge lamps is for pumping solid-state lasers: a super-high-pressure lamp [5] containing Til has been used for pumping ruby lasers. One special application is that of the spectral lamps used for calibrating the wavelength adjustment of spectroscopes, spectrophotometers, etc. It is important in this application to minimize the broadening and displacement of the emitted lines. This is done, for example, by keeping the concentrations ofthe emitting elements low. The efficiency of such lamps is clearly not very important. Even so, ill the past the makers have been limited to a few elements for these lamps, while the users on the other hand would like to have a sufficiently wide range of lamps to permit measurements and calibrations to be made at all points in the spectrum. This application has also benefited from thewider an adequate coverage is obtained by adding lamps with Tl, In, Ga and Bi to the existing range. Some of the lamps mentioned above can be seen in fig,», Investigations on the HPI lamp Of the iodide discharge lamps we have developed, the HPI lamp is the one which has been studied in the greatest detail. These studies included comparisons with the HP lamp. The temperature at the core of the two discharges and the spectral distribution have already been mentioned. We shall now discuss in turn the composition of the discharge medium, the division of the input power between radiation and heat (power balance), and the ignition. Composition of the discharge medium It is not at àll easy to give an exact description ofthe chemical equilibrium situations in discharge lamps with (5] The principle of the super-high-pressure lamp has been described by E. G. Dorgelo in Philips tech. Rev. 2, 165, (6] In this case the role ofthe iodidesappears to beratherdifferent from that in the high-pressure lamps - the details are not yet completely explained.

6 358 PHILlPS TECHNICAL REVIEW VOLUME29 such a mixed filling as in the HPI lamp, including as it does Hg, Hgh, 12, I, NaI, Na, Til, Tl, 1nl and In. A good insight can nevertheless be obtained by means of a special thermodynamic approach to the problem. The chief results of these thermodynamic calculations are given below; the actual thermodynamic treatment is explained in the Appendix. Tbe following picture is obtained: In the core of the discharge, that 1S to say in the température zone from about 3500 to about4800 "K, Effective dissociation of NaI, Til and Inl begins in the temperature range above about 2000 "K. Tbe approximate partial pressures are: Hg: Na + Nal: Tl + TIl: In + In1: 4 bars, 1 millibar, 200 millibars, 50 millibars. Since the sum of the partial pressures of the element plus the corresponding iodide is constant over the Fig. 4. Some iodide lamps for various applications. J printing lamp, 2000 W, with Ga and Pb as emitters (principally radiation between 340 and 450 nm) W lamp with the sarne emitters as 1, used for the polymerization of photo-sensitive lacquers. J super-high-pressure lamp, 1000 W, for optical pumping of ruby lasers; Tl is the emitter W projection lamps (CSI) containing LiL,.Nal, Til and 1nl; the right-hand lamp is made without outer bulb for optical reasons. 5 spectral lamp emitting In lines. This lamp differs from the others in that a low pressure discharge takes place and no mercury is used. the three iodides NaI, TIL and Inl are completely dissociated; at these temperatures the iodine is liberated in the atomic state. At the wall temperature of about 1000 "K these three iodides are only very slightly dissociated. Any iodine liberated by the dissociation of these iodides combines to form Hgf, because of the excess of Hg. The HgI2 is not very stable and the result is that free iodine is formed in the lamp at temperatures above 1300 "K. Between 1300 "K and 1500 "K the iodine occurs in the molecular form and above 1500 "K it occurs in the atomic form. The fact that there is only a narrow temperature zone in which molecular h occurs (and then only in a very low concentration) is important since molecular Iz absorbs in the visible region of the spectrum. whole temperature range, this value is also the partial pressure of the element on complete dissociation of the iodide in the central region ofthe discharge. For Tl and In this pressure is substantially higher than the maximum pressure that can be reached with the addition of the element itself. This is not the case with Na; for this element the iodide technique is desirable only because it is so chemically active. With TI and In the partial vapour pressure in the core is so high that one might think that the self-absorption of the Tl and In lines would be too high. Tn the surrounding colder part of the lamp, however, where the self-absorption mainly occurs, the partial pressure of the absorbing element is lower than in the core because the equilibrium has shifted more towards the iodide. This explains why good results are obtained with TI and In in spite of the self-absorption.

7 1968, No. 12 IODIDE DISCHARGE LAMP 359 Power balance As fig. 2 shows, the HPI lamp emits less radiation in the ultra-violet than the HP lamp. In the infra-red region ofthe spectrum the opposite situation holds. However, partly because the HPI lamp converts a greater part of the input power into radiation than the HP lamp does, the overall result is that the HPI lamp radiates considerably more in the visible region of the spectrum. With the two experimental 500 W lamps, the radiated power in the visible region is 120 W for the HPI lamp and 73 W for the HP lamp. In both cases the effective spectral sensitivity of the eye (relative spectral energy distribution of the radiated light) is about the same, so that the improvement in luminous efficiency is about 60%. As we shall explain in the following, this gain in luminous efficiency is even greater at higher powers. From radiation measurements on vertically suspended high-pressure mercury lamps, Elenbaas [7] has calculated that the quantity of radiation 1: produced per unit length of the discharge column depends in a very simple way on the power P dissipated per unit length. Taking Wfcm as the unit, the relation becomes: 1:= O.72x(P-lO). We may interpret this by saying that, whateverthedissipated power may be, there is always a loss of about 10 Wfcm due to heat conduction from the core of the are to the wall of the discharge t~be (the effect of convection is negligible in vertically suspended lamps). Of the remainder of the input power (P - 10) converted into radiation in the core of the are, about 28 % is lost due to absorption by the mercury vapour, the quartz glass of the bulb wall and the surrounding air. We have performed similar measurements on the HPI lamp. Reproducible results are more difficult to obtain here than with the HP lamp, since the iodides show a tendency to distribute themselves non-uniformly over the length of the tube. However, the results of the measurements do indicate that in this case as well, for power inputs between 30 and 75 Wfcm there is a simple relation between the radiation 1: produced per cm and the power P dissipated per cm: 1:= 1X (P - 20). Comparison with equation (1) shows that the HPI lamp loses twice as much heat per cm (20 Wfcm) by conduction as the HP lamp. On the other hand, the radiation produced in the core is not absorbed at all. Further examination of the two equations reveals that whenp is increased, the efficiency ofthe HPI lamp increases more than that of the HP lamp. This indicates that the power supplied to the lamp should be dissipated at the smallest possible electrode spacing. (1) (2) However, the electrodes should not be mounted so close to one another that a lot of the radiation is trapped and absorbed at the ends of the discharge tube. Because of this limitation a relatively low value of P has to be accepted with low-power lamps. Table II shows the power balance of the 500 W lamps used for the comparative experiments. Table II. Distribution of the input power for the experimental 500 W HPI and HP lamps made entirely of quartz glass. Input power Distribution of input power a) not radiated from the discharge comprising: estimated electrode loss b) radiated from discharge comprising: ultra-violet radiation visible radiation infra-red radiation absorbed Effective sensitivity of the eye Luminous efficiency Ignition of the HPI lamp HPI 500W 185 W 45W 315W 44W 120W 151 W m/W HP 500W 110W 40W 390W 112 W 73W 95W 110W m/W There are special problems in the ignition of iodide discharge lamps: they are more difficult to ignite than HP lamps. The difficulty stems from two main causes: the presence of iodine vapour, and the presence of hydrogen. The reason why iodine vapour pushes up the ignition voltage is that iodine is an electronegativeelement, i.e. an iodine atom readily associates with an electron to form a negative ion. Consequently, many of the electrons liberated in an iodide discharge lamp are withdrawn from the ignition process, resulting in a drastic reduction of the electrical conductivity of the gas. Another serious and related problem is that the alkali atoms used in some iodide lamps (Na in the HPI lamp and Na and Li in thecsi lamp)sovery easilydisappear: some of them react with traces of oxygen to form alkaline oxides which are difficult to evaporate, and others diffuse through the quartz wall. In itself, the loss of the alkalis is not so serious, since it can be compensated by providing a large initial surplus, but the loss in alkali corresponds to an increase in the concentration of iodine and therefore to a further increase in the ignition voltage. It was found that the production of hydrogen in HPI lamps is much greater than it usually is in highpressure mercury-vapour lamps. Iodides are hygroscopie, and during the operation of the lamp they give off (7) W. Elenbaas, Physica 4,413,1937. A refinement ofthe equation discussed here is given in W. Elenbaas, Philips Res. Repts. 20, 213, 1965.

8 360 PHILIPS TECHNICAL REVIEW VOLUME 29 water which dissociates. The oxygen then combines with one or more of the metals present in the discharge tube, leaving hydrogen gas behind. At room temperature the gas atmosphere now consists of a mixture of mercury vapour, argon and hydrogen, which is more difficult to ionize than a mixture ofmercury vapour and argon. For argon (without mercury) we know that the presence of 0.01% of hydrogen is sufficient to cause a perceptible increase in the ignition voltage [81. Thus, assuming the usual argon pressure of 30 millibars in. the lamp, a hydrogen pressure of only 3 X 10-3 millibars is enough to cause an increase in the ignition voltage. In the HPI lamp the increase occurs at even lower hydrogen pressures, because the mercury-argon mixture appears to be more sensitive to hydrogen than argon alone. The iodides must therefore be very carefully dried and kept dry before being added to the lamp. Another source of hydrogen is the quartz glass of which the discharge tube is made. Why so much hydrogen can enter the iodide lamp from the quartz glass has not yet been completely explained. It has been found that the evolution of hydrogen can be considerably reduced by prolonged outgassing of the quartz glass at the beginning of the production process. The higher ignition voltage in iodide discharge lamps makes it necessary to use a somewhat more elaborate circuit than that ofthe HP lamp. With reliability particularly in mind, we have chosen the following two methods ofignition from the several methods available. a) For the 400 Wand 2000 W HPI lamps: an electronie ignition unit which applies voltage peaks of 3000 V at each half-cycle until the lamp is ignited, used in conjunction with conventional HP chokes. When existing HP installations have been fitted with this unit they can also be used with HPI lamps. A familiar difficulty with high-pressure mercuryvapour lamps is that it takes a fairly long time to reignite a lamp which has just been switched off and is still hot. This "brute force" ignition method just mentioned is not only very reliable for the ignition of a cold lamp, it also considerably reduces the time needed to reignite a hot lamp. b) For the 5000W HPI lamp: since there are no HP installations for lamps of this rating, a simpler solution is possible in this case, consisting of a transformer with a high open-circuit voltage (860 V r.m.s.). This voltage is high enough to ensure stable operation of the lamp and is suitable for igniting the lamp in conjunction with a glow-discharge starter. Appendix: the gas composition in the HP! lamp The thermodynamic calculations presented here, relating to the HPI lamp, are based on the assumption that the gas mixture in the high-pressure discharge lamp with iodides is to a good approximation in thermal equilibrium everywhere 19J, and that the presence of the radiation field and gas-kinetic effects may therefore be neglected. The initial consideration is that there is a hot central region around the axis of the gas discharge tube and a colder outer zone, and that the two regions can be treated separately. It is known that the iodides of mercury are highly unstable compounds, and we may therefore expect that, in spite of the higher mercury concentration, there will be no iodide of mercury present in the hotter regions ofthe discharge tube. Similarly, there will be virtually no molecular iodine present there. In the colder regions, however, the mercury may be expected to react with iodine and molecular iodine will also be found. We shall now consider first the hot central region of the gas discharge. To illustrate the thermodynamic approach for this region, we take the equilibrium 1nl +t In + I as an example. The constant Kp of this equilibrium is: x; =PlnPI, plni where ps«is the partial vapour pressure of In, etc. The partial vapour pressure of the iodine PI is due 1I0t only to partial dissociation of 1nl, but also to partial dissociation of the other iodides. If 0( is the degree of dissociation of 1nl, we can then also write x; as a Kp = l_api. The vapour pressure of the In iodide no longer occurs in the right-hand side of this expression, and we therefore only need to know PI to be able to calculate the degree of dissociation a as a function ofthe temperature T. The value of PI is for the time being unknown, but we can set the existing partial vapour pressure of iodine between two estimated limit values. We take the upper limit as the value which the partial vapour pressure of iodine would have on complete dissociation aftii, Inl and NaI, rounded off to a power of 10 and expressed in bars. The temperature, which is x at the wall and 4800 "K at the axis, is assumed to have a parabolic distribution. The lower limit is taken to be a factor of 10 lower. We thus arrive at the limit values for PI of 100 and 10 millibars. Using each of these values we have calculated 0( as a function of T and the result is shown in fig. 5. The degrees of dissociation of Nal, Til and 12 were calculated in the same way as a function of T. It can be seen from this figure that at temperatures above 2000 "K the h is already completely dissociated whereas the metallic iodides are hardly dissociated at all. We now take the "hot" region of the gas discharge to refer to those places where the ternperature is greater than 2000 OK. Our tacit assumption thus far that all free iodine is atomic is therefore justified in this temperature range. We can get to know something about the composition of the gas atmosphere in the colder region of the lamp, below 2000 OK, by taking the following approach. Much of the total amount of iodine present in the colder region of the discharge tube is combined with the elements Na, TI and In. The iodine so combined (corresponding to 250 millibars) will not be taken into account since in this temperature range the chemical bonds are so strong that no perceptible changes in the degree of dissociation occur. The rest of the iodine consists of an amount f3 combined in Hgls, an amount y in atomic form, and an amount (1 - f3 - y) in the molecular form. The possibility of mercurous iodide being pres- [8J M. A. Uman, Phys. Rev. 133, A 1266, [DJ See C. F. Galla, Appl. Optics 5, 1285, 1966.

9 1968, No. 12. IODIDE DISCHARGE LAMP 361 Q8 ex: t O Q2 -- to millibar I «Fig. 5. The degree of dissociation lx of 12, Inl, Til and NaI as a function of temperature T, calculated for values of the partial vapour pressure of iodine PI of 10 millibars (solid curves) and 100 millibars (dashed curves) f3 Fig. 6. Distribution of the iodine not combined with Na, TI and In in the temperature range from 1000 to 2000 ok, similarly calculated for PI = 10 millibars (solid lines) and 100 millibars (dashed lines). f3 is the fraction present in the form of Hgls, y is the fraction present in atomic form, and I - f3 - Y is the remaining fraction of molecular h. ent is not taken into account since this iodide is more unstable than mercuric iodide. The values of f3 and y can now be calculated with the aid of two equations, in which the quantity of mercury and the quantity of iodine, whether combined with mercury or not, and both expressed in terms of partial pressures, are used together as parameters. The mercury pressure in the HPI lamp is calculated from the known added quantity and the temperature distribution to be about 4 bars. We have again taken limiting values of 10 and 100 millibars for the iodine vapour pressure. The result is given in fig. 6. Below 2000 "K the formation of Hgls and of 12 must be taken into account. It has in fact been found that the iodine at 1000 ok (i.e. close to the wall) has reacted virtually completely with the mercury. This ties up with theunderlying idea of dividing the thermodynamic treatment into two parts. Fig. 6 also shows that we are justified in not considering Hgls when dealing with the temperature region above 2000 ok. We now have all the information we need: the temperature distribution, the quantities added and the degree of dissociation of the iodides. In principle we can therefore calculate the partial vapour pressures ofthe iodides at every point in the HPllamp.To simplify the work of calculation, it is useful to introduce the further assumption that, for every metal, the sum of its vapour pressure and the vapour pressure of its "iodide is constant throughout the tube, since every metallic iodide molecule that goes to a hotter location and dissociates there will yield one metal atom, and vice versa. Since it is a fairly good approximation to take the total pressure as being constant throughout the discharge tube, this assumption implies that the sum ofthe partial vapourpressure ofthe mercury, the Hgh and the iodine is also constant throughout the tube. We have thus arrived at the results summarized on page 358: the partial vapour pressure of mercury ("'" 4 bars), Na + NaI ("'" I millibar), TI + Til ("'" 200 millibars) and In + Inl ("'" 50 millibars). In addition we have calculated the vapour pressures of the metals separately, and these are shown infig. 7 as a function of the distance to the axis of the discharge, with the partial iodine vapour pressure as a parameter. Since the curves for PI = 100 millibars and PI = 10 millibars lie close together, it is evident that the calculated partial vapour pressures of the metals are not closely dependent on the partial f 1 bar T tooook Jj\ Tl\ Nol n\ \ \! i i Tl 2 Til 1 ",-:\\ \ \ \ In 5 \ i InI Na -... <,, \0\ \ ',\ \ ~~i I~\ \ 'I i i \, \ \\ \ \ \ \ \ \ \, \ \ I i ~ ~ M ~ W W M W W -7R Nol Fig. 7. Partial pressures of Na, TI and In in the HPI lamp as a function of temperature (indicated at the top), and as a function of rlr, i.e. the ratio of the distance to the axis and the radius of the tube (indicated at the bottom). The partial pressures were calculated for iodine vapour pressures of 10 millibars (solid lines) and 100 millibars (dashed lines). The graph also gives curves of the saturation vapour pressures ofthe free elements (chain-dotted lines). If the elements themselves are introduced, i.e. not as iodides, the partial pressure cannot become higher than the saturation vapour pressure corresponding to the lowest temperature of the lamp. Taking this to be the wall temperature of 1000 "K, we see from the figure that Tl and In inot Na) have acquired a considerable higher pressure at the centre after being introduced in the form of iodides. Another important point is to find out whether the curve for the saturation pressure cuts the curve for the pressure of the element introduced as an iodide. Intersection of the curves indicates that the partial pressure of the element introduced as the iodide exceeds the saturation pressure at the temperature corresponding to the point of intersection. In this case a liquid or solid phase will occur and the desired object cannot be achieved. The shape of the curves seems to indicate that this undesired effect will occur with In. In practice, however, this complication has not arisen. In any case, this kind of situation can be prevented by appropriate measures based on these calculations.

10 362 PHILIPS TECHNICAL REVIEW VOLUME29 vapour pressures of the iodine. We thus have a good picture of the composition of the vapour mixture in the HPI lamp. Fig. 7 also shows the curves for the saturation vapour pressures of the free metals, determined by the temperature at each location in the tube. It can be seen that nearly all the calculated vapour pressures of the metals remain below the saturation values and are therefore practicable. Difficulties are onlyto be anticipated with In since the curve for the partial In vapour pressure corresponding to pr = 10 millibars cuts, or perhaps grazes, the curve for the saturation vapour pressure of free In, and this makes the situation rather uncertain. If the curves really did intersect, liquid In would condense in the region to the right of the point of intersection. This would limit the value of the partial In pressure that could be built up in the core. However, practical experience with In has been very reassuring on this point. It does appear, however, that if a higher In vapour pressure should be required, the dissociation equilibrium would have to be shifted more towards the iodide; this can be done by using a higher iodine vapour pressure. For completeness it should be mentioned that the iodine also reacts with the electrodes. The problems connected with these reactions are complicated and have not yet been resolved sufficiently to justify further discussion. We shau just note here that in most types of iodide lamp tungsten electrodes have been found to be resistant to iodine vapour and do not therefore extract much iodine. Summary. A gas discharge lamp is described in which high-efficiency emission is obtained from elements such as Li, Na, TI, In, Ga and Pb which have been introduced in the form of iodides. The lamp is designed as a high-pressure type in which mercury acts as a buffer gas, i.e. the mercury helps to maintain the discharge under high pressure and high temperature, but is hardly at all excited or ionized. The elements introduced as iodides could not be used in the elementary form because they are either not volatile enough or too liable to attack the materials of the lamp. By varying the concentrations of the iodides, the spectral distribution of the emitted radiation can be controlled within certain limits, so as to obtain optimum efficiency for a variety of applications. The lamps made include types for road lighting (HPI lamps), with Na, TI and In as radiation emitters, projector lamps in which the red emission of Li is added to improve the colour rendering (CSI lamps), coloured lamps for floodlighting (reddish yeuow with Na, green with TI and blue with In), radiation sources for photochemical processes (using Ga and Pb) and for the activation oflasers (using TJ), and spectral lamps. FinaUy some investigations on the HPI lamp arediscussed.