WO2010001316A1

WO2010001316A1 - Mercury-free and zinc-free high intensity gas-discharge lamp

Info

Publication number: WO2010001316A1
Application number: PCT/IB2009/052762
Authority: WO
Inventors: Klaus Schoeller
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2008-07-04
Filing date: 2009-06-26
Publication date: 2010-01-07

Abstract

The invention describes a mercury-free high intensity gas-discharge lamp (1) comprising a discharge vessel (5) enclosing a fill gas in a discharge chamber (2) and comprising a pair of electrodes (3, 4) extending into the discharge chamber (2), for which lamp (1) the fill gas in the discharge chamber (2) is free of zinc iodide, and the fill gas includes a halide composition comprising sodium iodide and scandium iodide to a combined proportion of at least 30 wt% and at most 80 wt%, and thulium iodide, to a proportion of at least 20 wt% and at most 70 wt%.

Description

MERCURY-FREE AND ZINC-FREE HIGH INTENSITY GAS-DISCHARGE LAMP

FIELD OF THE INVENTION

The invention describes a mercury-free high intensity discharge lamp.

BACKGROUND OF THE INVENTION

In a high-intensity discharge lamp, an electric arc established between two electrodes produces an intensely bright light. Such a lamp is often simply referred to as a 'HID' lamp. In prior art HID lamps, a discharge chamber contains a fill gas comprising mostly Xenon and a combination of halides - usually sodium iodide and scandium iodide - and one or more other metal salts that vaporise during operation of the lamp. Older HID lamps included mercury in the fill gas, since mercury has a high vapour pressure. For obvious health and environmental considerations, the use of mercury in such lamps is being phased out. When used in automotive headlamp applications, HID lamps have a number of advantages over other types of lamp. For instance, the light output of a metal halide xenon lamp is greater than that of a comparable tungsten-halogen lamp. Also, HID lamps have a significantly longer lifetime than filament lamps, and the halogen cycle ensures that these lamps are not subject to blackening. These and other advantages make HID lamps particularly suited for automotive headlamp applications.

Along with the colour temperature, other characteristics of such lamps, for example operational voltage, lamp driver characteristics, dimensions, etc., are specified in different countries by the appropriate regulations, for example by ECE-R99 in Europe, where 'ECE' stands for 'Economic Commission for Europe'. Often, the lamps specified in these regulations are simply referred to by their designation, e.g. 'Dl', 'D4', etc.

The colour point, or colour temperature, of an automotive lamp is crucial for safety. Firstly, the headlamps of a vehicle must sufficiently illuminate the road for the driver of that vehicle, and secondly, other drivers should not be subject to potentially dangerous glare from the headlamps of that vehicle. Prior art xenon HID lamps used in headlights normally provide a light output with a colour temperature up to 4000K.

The colour of an automotive headlight must comply with certain standards in order to ensure uniformity and therefore also to promote safety for drivers. One such standard is the SAE system, which was developed by the Society of

Automotive Engineers in the USA to define the colours for automotive headlights, and which will be known to a person skilled in the art. Studies have shown that the colour temperature of an automotive lamp should be considerably higher than 4000 K, and the X and Y co-ordinates of the corresponding colour point, as graphed using the SAE system, should lie on or close to the blackbody line (a locus of points corresponding to an ideal black body radiator). Such colour temperature characteristics of automotive headlights improve recognition in the dark, therefore increasing safety in nighttime driving. This is because, even at the same intensity, light with a higher colour temperature - for example bluish- white light - is perceived by the human eye to be brighter than light with a lower colour temperature, for example light with a yellow hue. These requirements are leading to an increased demand on the part of customers for xenon HID lamps with high efficiency mentioned, but also with a higher colour temperature.

However, designing lamps to produce a bluish light is not necessarily a straightforward process, since, under equal conditions, the luminous flux output by a lamp producing blue light is lower than that of a lamp producing yellow light. For this reason, it is difficult to obtain a lamp that delivers a blue light with a colour temperature greater than 4500 K with an acceptable level of luminous flux. Attempts to raise the colour temperature in prior-art lamps usually involve adding indium iodide (InI₃) and zinc iodide (ZnI₂) to a sodium iodide (NaI) and scandium iodide (SCI₃) mixture. In state of the art lamps, for example in Dl or D2 lamps (containing mercury), a loss of light output up to 30% is observed, so that the efficiency of these lamps is unsatisfactory. Other D3 and D4 lamps (mercury-free) achieve a light output only marginally satisfying the regulation requirements, for example a light output of only 3200 +/- 450 Im. In brief, none of the previous approaches have been able to effectively raise the colour temperature without suffering from a loss in light output. For example, EP 1 768 165 A2 describes a mercury- free high-pressure discharge lamp with a fill gas comprising, for example, sodium iodide and thulium iodide. As another necessary constituent of the fill gas, zinc iodide is used. However, a satisfactory lumen output is only delivered by these fill gas compositions when included in high-power lamps such as 10OW, which need to be driven at correspondingly high voltages. This document also mentions one other example as a comparison, in which the fill gas only comprises sodium iodide and thulium iodide. In this example, the lamp voltage obtained is too low (27V) and the luminous efficiency is unsatisfactory (only 45 lm/W) for use in an automotive application. For this reason, in most state of the art lamps, zinc iodide was included in the fill gas in order to be able to achieve a higher operation voltage. However, the use of zinc iodide results in a loss in light output of up to 10%. Furthermore, the necessity of zinc iodide as an additional constituent in prior art lamps raises the cost of each lamp accordingly.

Therefore, it is an object of the invention to provide a more economic mercury- free high-intensity xenon discharge lamp that satisfies the criteria for a Dl - D4 automotive headlamp, while having a high colour temperature as well as a high luminous flux.

SUMMARY OF THE INVENTION

To this end, the present invention describes a mercury- free high intensity gas-discharge lamp comprising a discharge vessel enclosing a fill gas in a discharge chamber and comprising a pair of electrodes extending into the discharge chamber, for which lamp the fill gas in the discharge chamber is free of zinc iodide, and the fill gas includes a halide composition comprising sodium iodide (NaI) and scandium iodide (SCI₃) to a combined proportion of at least 30 wt% and at most 80 wt%, and thulium iodide (TmI₃), to a proportion of at least 20 wt% and at most 70 wt%.

Experiments with the lamp according to the invention, which does not include zinc in the fill gas composition, have - surprisingly - shown that the absence of zinc does not have a noticeable effect on the lamp voltage. At the same time, a significantly higher light output can be achieved with at least 30 wt% combined sodium iodide and scandium iodide in the fill gas. Therefore, by omitting zinc iodide and compiling the fill gas to include this minimum combined amount of sodium iodide and scandium iodide, the lamp according to the invention allows a higher light output to be achieved, without the lamp voltage being adversely affected in any way. In a simple and economic solution, therefore, the lamp according to the invention provides a particularly high light output while being more cost-effective to manufacture than prior art lamps. Another obvious advantage of the lamp according to the invention is that, with the fill gas described, a very high level of light output (lumens) per Watt, i.e. a high level of efficiency, can be reached with a colour temperature well placed in the blue region required for automotive applications. The addition of thulium iodide (TmI₃) results in a significant increase in the colour temperature that can be reached at this high level of lamp efficiency.

Advantageously, the lamp according to the invention can be used in place of a prior art Dl - D4 headlamp without having to replace any existing electronics or fittings, so that the customer requirements mentioned in the introduction can be met.

The dependent claims and the subsequent description disclose particularly advantageous embodiments and features of the invention.

In the following, pertinent initial lamp parameters such as colour temperature, operating voltage, lumen output etc., apply for a lamp age of 15 hours according to ECE regulations. This is because these parameters are obtained after the first fifteen hours of operation of such a lamp, which is regarded as the 'ageing' time. As mentioned above, it is highly desirable in automotive applications for the colour temperature of a headlight to lie close to the blackbody line in an SAE representation, as will be known to a person skilled in the art. Therefore, in a particularly preferred embodiment of the invention, the halide composition of the lamp also comprises indium iodide (InI₃) to a proportion of at least 0 wt% and at most 50 wt%. The addition of indium iodide serves to lower the Y co-ordinate of the colour point. By appropriate choice of the proportions of the metal salts in the fill gas, a colour temperature can be obtained whose colour point has X and Y co-ordinates that lie on, or at least very close to, the blackbody line.

The combined amount of sodium iodide and scandium iodide in the fill gas, as already indicated, serves to yield a high efficiency of the lamp. Evidently, the relative proportions of these metal salts can be adjusted as required. With approximately equal levels of sodium iodide and scandium iodide, i.e. 50:50, the lumen output of the lamp is only subject to minor alteration, while allowing the X co-ordinate of the colour point to be positioned closer to the blackbody line. On the other hand, increasing the relative proportion of sodium iodide while decreasing that of scandium iodide serves to prolong the lifetime maintenance of the lamp, i.e. the lamp can provide relatively constant lumen output over a longer lifetime. Therefore, in a further preferred embodiment of the invention, the proportion of sodium iodide in the halide composition is at least 15 wt% and at most 40 wt%, and the proportion of scandium iodide in the halide composition is at least 10 wt% and at most 40 wt%. Further adjustments of the colour temperature can be obtained with the addition of small amounts of other rare earth compounds in the fill gas. By adding a small amount of one ore more additional rare earth metal salts, the X and/or Y coordinate of the colour point can be adjusted so that a desired colour temperature can be obtained precisely. Therefore, in a further embodiment of the invention, the halide composition preferably comprises one or more additives of a group of rare earth halides comprising dysprosium iodide (DyI₃), thallium iodide (TlI), neodymium iodide (NdI₃), and holmium iodide (HoB), to a proportion of at most 10%.

The lumen output and the colour point of an HID lamp are governed by many factors, such as fill gas composition, dimensions of the discharge chamber, size and position of the electrodes, etc. Furthermore, the physical construction of the lamp, the conditions under which it is operated, and the pressure of the fill gas in the lamp all serve to influence its light output. Therefore, in a further preferred embodiment of the invention, the construction parameters of the lamp and the composition of the fill gas are chosen such that a colour temperature in the range of 4000 K to 7000 K in the SAE field is attained by the lamp when operated with an initial operating voltage of at least 38 V and at most 55 V. The fill gas in the lamp preferably comprises xenon gas under a pressure of at least 12 bar and at most 17 bar in a non-operational state. This is referred to as the 'cold pressure' of the lamp.

For automotive headlight applications to date, lamps rated at 35 W are generally used. Therefore, the lamp according to the invention preferably has a rated or nominal power of 35 W. The physical construction characteristics of the lamp are preferably such that the capacity of the discharge chamber of the lamp is at least 15μl and at most 30μl, while the inner diameter of the discharge chamber can be between 2.2 mm and 2.6mm, preferably 2.4 mm, and the outer diameter of the discharge chamber can be between 5.9 mm and 6.3 mm, preferably 6.1 mm. In such a lamp, the halide composition in the fill gas of the lamp preferably has a combined weight of at least lOOμg and at most 400μg.

However, the lamp according to the invention is not limited to a 35 W realisation. With appropriate choice of construction parameters, the lamp can also be realised, for example, as a 25 W lamp. In such a lamp, the capacity of the discharge chamber is at least 10 μl and at most 25 μl, having an inner diameter measuring between 2.0 mm and 2.4 mm , preferably 2.2 mm, and an outer diameter measuring between 4,5 mm and 6,1 mm, preferably 5.5 mm. In this lower-power realisation, the halide composition in the fill gas preferably has a combined weight of at least 50 μg and at most 300 μg. The electrodes of HID lamps are generally made of tungsten, since tungsten has a very high melting point, as will be known to the skilled person. A tungsten electrode that contains thorium (called a thoriated tungsten electrode) operates at a temperature below its melting temperature compared to a pure tungsten electrode, so that the electrode is not so prone to deformation during operation. However, like mercury, thorium poses health and environmental risks. Thorium is a low-level radioactive material requiring precautions in handling so as to avoid inhalation or ingestion, and its use is also undesirable from an environmental point of view. Therefore, the electrodes of the lamp according to the invention are preferably thorium- free tungsten electrodes, i.e. tungsten electrodes that do not comprise a thorium additive. To obtain a stable arc using such an electrode, experiments pertaining to the lamp according to the invention have shown that the dimensions of the electrode can play an important role. Maintenance of a stable arc depends to a large extent on the geometry of the electrodes, in particular their diameter, since the thickness of the electrodes governs the electrode temperature that is reached during operation, which in turn determines the commutation behaviour and the burn-back of the electrodes according to the ballast parameters. The diameter of the electrode within a pinch region of the lamp is therefore preferably at least 200μm and at most 320μm, and the diameter at the tip of the electrode is preferably at least 200μm and at most 360μm. The electrode can be realised as a simple rod shape of uniform diameter from tip to pinch, or can be realised to be wider at the tip that at the pinch. Evidently, these dimensions apply to the initial dimensions of the electrodes before burning.

As is known to a person skilled in the art, the electrodes in a HID lamp of the type described here protrude from opposite sides into the discharge chamber, so that the tips of the electrodes are separated by a small gap. In the lamp according to the invention, the electrode tips are preferably separated by a real distance of at least 3 mm and at most 5 mm, preferably 3.6mm. The optical separation between the electrode tips, i.e. the separation as seen through the glass of the inner chamber, will appear larger than the actual separation. A electrode separation of 3.6 mm corresponds to an optical separation of 4.2 mm.

A lamp of the type described here preferably comprises an outer chamber within which the discharge chamber is disposed. This outer chamber can be transparent quartz glass, or it can be treated to influence the colour of the emitted light. Therefore, in a further preferred embodiment of the invention, the discharge chamber of the lamp is disposed within a quartz glass outer chamber, which outer chamber is treated with a compound of neodymium, for example neodymium oxide (Nd₂O₃) and/or a compound of cobalt, for example cobalt aluminate CoAl₂O₄. The effect of these compounds is to absorb yellow light emitted by the lamp during operation. For example, neodymium oxide has a strong absorption band centred at a wavelength of 580nm so that this yellow light does not pass through the outer chamber wall. The treatment of the outer chamber can therefore comprise, as appropriate, an actual doping of the quartz glass from which the outer chamber is made, or a coating applied to a surface of the outer chamber.

Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. Fig. 1 shows a cross section of a gas-discharge lamp according to an embodiment of the invention; Fig. 2 shows a cross section of a gas-discharge lamp according to a further embodiment of the invention; Fig. 3 shows a table of experimental results using a number of embodiments of the lamp according to the invention. Fig. 4 shows an SAE chart of the colour point of a D4S lamp after 15h burning;

In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.

In Fig. 1 , a cross section of a quartz glass gas-discharge lamp 1 is shown according to an embodiment of the invention. Essentially, the lamp 1 comprises a quartz glass discharge vessel 5 enclosing a discharge chamber 2 containing a fill gas. Two electrodes 3, 4 protrude into the discharge chamber 2 from opposite ends of the lamp 1. During manufacturing, the quartz glass of the discharge vessel 5 is pinched on both sides around the electrodes 3, 4 to seal the fill gas in the discharge chamber 2. The capacity (or volume) of the discharge chamber 2 is governed by the inner diameter D₁ and outer diameter D₀ of the discharge vessel 5. The inner and outer diameters D₁, D₀ are measured at the widest point.

The electrodes 3, 4 are essentially thorium- free tungsten rods that protrude into the discharge chamber 2 and are optically separated from each other by a distance of 4.2mm according to the R99 regulation. The electrodes of a lamp according to the invention can be realised as simple rods of uniform thickness from base to tip. However, the thickness of the electrodes can equally well vary over different stages of the electrodes, so that, for example, an electrode is thicker at its tip and narrower at the base. In the embodiment described in the diagram, electrodes 3, 4 are shown with an outer diameter of up to 300μm (this value of diameter is the initial value before burning). For the sake of clarity, the diagram shows only the parts that are pertinent to the invention. Not shown is the base and the ballast that is required by the lamp for control of the voltage across the electrodes. When the lamp 1 is switched on, the ballast's igniter rapidly pulses an ignition voltage at several thousand volts across the electrodes 3, 4 to initiate a discharge arc. The heat of the arc vaporises the metal salts in the fill gas. Once the arc of high luminous intensity is established, the ballast regulates the power, so that the voltage across the electrodes 3, 4 accordingly drops to the operational level, in this example, to a level between 38V and 55V.

Since potentially damaging ultraviolet light is generated by the arc in the HID lamp 1 , the quartz discharge vessel 5 may be enclosed by a doped quartz glass shield or envelope to absorb this radiation. Such an outer chamber 6 is shown in Fig. 2. This outer chamber 6 can be treated by doping the glass itself, for example with neodymium oxide (Nd2θs), or by applying a coating of, for example, cobalt aluminate CoAl₂O₄ to an inner or outer surface of the outer chamber 6, using techniques that are known to the skilled person. This treatment ensures that yellow light is absorbed, allowing a further improvement of the 'blueness' of the light emitted by the lamp 1. The light that is passed through is then collected and distributed using HID-specifϊc optics, not shown in the diagram, such as reflectors and collimators in headlamp construction for ensuring that as much as possible of the light output is put to use. Since these and other additional components will be known to a person skilled in the art, they will not be explained in more detail. In tests with lamps containing zinc iodide, a satisfactory luminous flux was not obtained for a desired blue light in the region of 5000K. For example, a lamp with a fill gas comprising sodium iodide (8 wt%), scandium iodide (7 wt%), thulium iodide (72 wt%) and zinc iodide (13 wt%), a luminous flux of only 2400 Im was obtained, with a lamp voltage of 44V. In another test, a lamp with a fill gas comprising sodium iodide (10 wt%), scandium iodide (10 wt%) and thulium iodide (80 wt%) was tested. Even though zinc iodide was omitted from the fill gas composition, a lamp voltage of 44V was reached, with an increase in luminous flux at 2600 Im. This shows that the inclusion of zinc iodide is not necessary for maintaining a desired lamp voltage, and omitting the zinc iodide even has a positive effect on the light output. However, the perceived brightness of these lamps can still be improved in view of the automotive requirements outlined in the introduction. Better results were obtained using a fill gas composition according to the invention, as will be demonstrated with the aid of Fig. 3, which shows a table of results obtained in a series of experiments with a D4S lamp according to the invention with a nominal power of 35 W, with measurements taken after 15 hours of burning. The first column lists the batch number of the experimental results for the corresponding row. The next four columns list the percentages of a number of metal halides in the fill gas composition. As can be seen from the table, the first batch, with a fill gas composition with 45 wt% NaI, 30 wt% ScB and 25 wt% TmB, achieves an operating voltage 40V, a light output of 3,300 Im, and a colour temperature of 4100 K. A higher colour temperature and a higher operating voltage are obtained by the second batch, which has a fill gas composition with 38 wt% NaI, 23 wt% ScB, 38 wt% TmB and 1% InB. Here, the addition of indium iodide gives a better colour temperature, further into the blue range. Similarly, the fifth batch, with a fill gas composition with 18 wt% NaI, 14 wt% ScB, 30 wt% TmB and 38% InD gives a very high colour temperature. Even though the lumen output of the lamp is somewhat lower at 2500 Im, this is still exceptionally high for a lamp with such this colour point. . The colour points of the lamps in batches 1, 2 and 4 were experimentally observed to lie close to or on the blackbody line.

In Fig. 4, a graphical realisation of the experimental results of Fig. 3 is shown in an SAE graph, which plots the X and Y co-ordinates of the observed colour point. The solid black lines indicate the regulations, or the limits for a permissible range in colour temperature while the broken line represents the blackbody line. Three relevant colour temperature curves are given by the dotted lines Tl, T2, T3 which correspond to colour temperatures of 5000K, 4444K, and 4000K respectively. The colour point CP_ref corresponds to a reference lamp with the usual addition of zinc iodide in the fill gas. As can be seen from the diagram, the colour point achieved by this lamp is close to the regulation boundary, and is therefore unsatisfactory. The colour point CPl corresponds to lamp from the 1st batch, with 25% TmI₃ in the fill gas. This lamp yields a satisfactory colour temperature with a colour point on the blackbody line. CP2 and CP4 are also on or near the blackbody line, and therefore deliver satisfactory values for colour temperature and luminous flux. CP3 is within an acceptable distance from the blackbody line. CP5 is below the blackbody line, but remains within the permissible range. Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. For the sake of clarity, it is also to be understood that the use of "a" or "an" throughout this application does not exclude a plurality, and "comprising" does not exclude other steps or elements.

Claims

CLAIMS:

1. A mercury- free high intensity gas-discharge lamp (1) comprising a discharge vessel (5) enclosing a fill gas in a discharge chamber (2) and comprising a pair of electrodes (3, 4) extending into the discharge chamber (2), for which lamp (1) - the fill gas in the discharge chamber (2) is free of zinc iodide , and

- the fill gas includes a halide composition comprising sodium iodide and scandium iodide to a combined proportion of at least 30 wt% and at most 80 wt%, and thulium iodide, to a proportion of at least 20 wt% and at most 70 wt%.

2. A lamp (1) according to claim 1, wherein the halide composition comprises indium iodide to a proportion of at least 0 wt% and at most 50 wt%.

3. A lamp (1) according to claim 1 or claim 2, wherein the construction parameters of the lamp (1) and the composition of the fill gas are chosen such that a colour temperature in the range of 4000 K to 7000 K is attained by the lamp (1) when operated with an initial operating voltage of at least 38 V and at most 55 V.

4. A lamp (1) according to any of the preceding claims, wherein the fill gas comprises xenon gas under a pressure of at least 12 bar and at most 17 bar in a non- operational state.

5. A lamp (1) according to any of the preceding claims, wherein the proportion of sodium iodide in the fill gas is at least 15 wt% and at most 40 wt%, and the proportion of scandium iodide in the fill gas is at least 10 wt% and at most 40 wt%.

6. A lamp (1) according to any of the preceding claims with a nominal power of 35W, and for which lamp (1)

- the capacity of the discharge chamber (2) is greater than or equal to 15μl and less than or equal to 30μl;

- the inner diameter of the discharge chamber (2) comprises at least 2.2 mm and at most 2.6mm;

- the outer diameter of the discharge chamber (2) comprises at least 5.9 mm and at most 6.3 mm; and

- the halide composition in the fill gas of the lamp (1) has a combined weight of at least lOOμg and at most 400μg.

7. A lamp (1) according to any of claims 1 to 5 with a nominal power of 25 W, and for which lamp (1)

- the capacity of the discharge chamber (2) is greater than or equal to 10 μl and less than or equal to 25 μl; - the inner diameter of the discharge chamber (2) comprises at least 2.0 mm and at most 2.4 mm;

- the outer diameter of the discharge chamber (2) comprises at least 4.5 mm and at most 6,1 mm; and

- the halide composition in the fill gas of the lamp (1) has a combined weight of at least 50 μg and at most 300 μg.

8. A lamp (1) according to any of the preceding claims, wherein the halide composition comprises one or more additives of a group of rare earth halides comprising dysprosium iodide , thallium iodide and holmium iodide , to a proportion of at most 10%.

9. A lamp (1) according to any of the preceding claims, wherein the electrodes (3,4) are arranged at opposing ends of the discharge chamber (2) and wherein an electrode (3, 4) of the lamp (1) is a thorium- free tungsten electrode (3, 4), for which electrode (3, 4) - the diameter of the electrode (3, 4) within a pinch region of the lamp (1) is at least 200 μm and at most 320 μm;

- and the diameter at the tip of the electrode (3, 4) is at least 200 μm and at most 360 μm.

10. A lamp (1) according to any of the preceding claims, wherein the tips of the electrodes (3, 4) are separated by a distance of at least 3 mm and at most 5 mm.

11. A lamp (1) according to any of the preceding claims, wherein the discharge vessel (5) of the lamp (1) is disposed within a quartz glass outer chamber (6), which outer chamber (6) is treated with a compound of neodymium and/or a compound of cobalt.