EP2988318A1 - Metal halide lamp with high colour rendering - Google Patents

Abstract

This invention relates to metal halide lamp comprising a discharge vessel, shape of which is optimized to obtain a high colour rendering with a low vessel wall temperature.

Thus, the lower burner temperature allows for a long life lamp that also has a very good colour rendering. The shape of the discharge vessel is optimized by shape factor s_l and aspect ratio f₀ and the size for a given shape is determined by the wall loading P_D.
Wherein

wherein
D outer diameter
R_I center part radius
P_la lamp power
L_body length center part.

Description

FIELD OF INVENTION
• The present invention relates to metal halide lamps. Particularly, present invention relates to metal halide lamp comprising a ceramic discharge vessel, shape of which is optimized in order to obtain a high colour rendering in combination with a low vessel wall temperature.
DESCRIPTION OF THE RELATED ART
• Metal halide lamps are high intensity discharge lamps which generate light from a so-called burner or arc tube filled with an ionisable gas and a mixture of metal halides. The envelope of this burner is usually made from quartz or from a ceramic material like polycrystalline alumina (PCA). Translucent ceramic is better in heat resistance than quartz glass and usually better suited for metal vapour discharge lamps.
• These so-called ceramic metal halide lamps typically consist of a centre bulb that defines a discharge space and a pair of side tubes extending from both ends of the centre bulb. The side tubes are usually cylindrical with a diameter smaller than the centre bulb. The central bulb can have shapes like e.g. cylindrical, spherical, ellipsoidal or a more general spheroid shape. The boundary portion between the centre bulb and the side tubes is usually defined by a curvature radius. The current suppliers mount through a bore of the side tubes and extend some portion inside the centre bulb. In the discharge space ionisable materials are provided comprising a starting gas like e.g. argon or xenon, mercury and a mixture of metal halides. Metal halides commonly used are e.g. Sodium Iodide (NaI), Thallium Iodide (TlI), Calcium Iodide (CaI2) and rare earth iodides like e.g. Dysprosium Iodide (DyI3), Holmium Iodide (HoI3), Thullium Iodide ((TmI3), Cerium Iodide (CeI3), Lanthanum Iodide (LaI3), Neodymium Iodide (NdI3) or Praesodinium Iodide (PrI3). The centre part of early discharge tubes was cylindrically shaped, e.g. EP 0587238 . Most present-day discharge burners have spheroid shapes, as described e.g. in EP 2140479 or EP 1465239 . They can also have a central cylindrical part with spherical end pieces, as in US 5,936,351 .
• An important characteristic of a metal halide lamp is its ability to render the colours of the objects in lights in a natural way. This characteristic is usually expressed with a measure (factor) calculated from the lamp spectrum: the colour rendering index or CRI. It is an index that describes the rendering of a set of reference colours under the lamp under study and at the given correlated colour temperature of the lamp. The CRI is a function of the mix of metal halides used. By selecting an optimal mix, the CRI can be improved for any given colour temperature.
• Reference may also be made to the following-
• Publication No. US2012280616 (A1 ) relates to a lamp which includes a discharge vessel having sealed therein an ionizable fill including at least an inert gas, mercury, and a halide component having less than 1% thallium and, optionally, less than 1% indium, based on the entire halide mole % present therein, the remaining halide component including an alkali metal halide, an alkaline earth metal halide, and a rare earth halide. For example, the halide component may include less than 1 mol % thallium halide, optionally less than 1 mol % indium halide, sodium halide, at least one of calcium or strontium halide, and at least one of cerium or lanthanum halide.
• Publication No. US4808876 (A ) is directed to metal halide discharge lamp including an arc tube design having a particular end shape which allows control of the temperature distribution within the arc tube, so that sensitivity of the arc tube performance to orientation is reduced and concentration of the metallic halide components in the arc region is maintained relatively constant.
• Increasing the colour rendering of a metal halide lamp can be done by reducing the size of the discharge vessel thereby increasing its temperature (for a given wattage). While this is very beneficial for increasing the colour rendering, the increased temperature of the discharge vessel also decreases the life of the lamp because the chemical processes that finally limit the life proceed faster at higher temperature.
• Another important parameter for increasing the CRI is the temperature of the pool of metal halides in an operating lamp. The CRI increases strongly with this temperature.
• Reference may also be made to the following-
• Publication No. US6774566(B2 ) is related to a high pressure discharge lamp characterized by that the minimum wall-thickness Tmin of the light-transmissive ceramic discharge vessel is equal to or more than 0.1 mm, and the inner diameter D of the swollen portion and the curvature radius R of the concave outer surface around the boundary portion between the swollen portion and the slender cylindrical portion satisfies an equation 0.1≦R/D≦1.5. Light-transmissive ceramic discharge vessel 1: Made of light-transmissive alumina ceramics; 23 mm in overall length. Swollen portion 1a: 6.0 mm in the maximum outer diameter; 5.0 mm in the maximum inner diameter. Thin cylindrical portion 1b: 1.7 mm in outer diameter; 0.7 mm in the maximum inner diameter D.
• Boundary portion between Swollen portion 1a and Thin cylindrical portion 1b: 4 mm in curvature radius R of the concave outer surface CF.
• Publication No. JP2003272560 (A ) discloses a metal halide lamp with a ceramic tube such as polycrystalline alumina ceramic arc tube as material and composed by filling the inside of the arc tube 1 with, as filling substances, metal halides including at least one kind out of dysprosium halide, thulium halide, holmium halide, and cerium halide, and sodium halide. When it is assumed that an inter-electrode distance in the arc tube and the tube inside diameter at the center part of the arc tube are Le (mm) and [phi]i (mm), respectively, the value of an arc tube shape parameter Le/[phi]i satisfies a range of 0.45-0.65.
• Publication No. US8390196 (B2 ) provides a metal halide lamp comprising a ceramic discharge vessel, wherein the discharge vessel has a wall enclosing a discharge space with an ionizable filling, the discharge space further enclosing electrodes having electrode tips arranged opposite each other and arranged to define a discharge arc between the electrode tips during operation of the lamp, the discharge vessel having a spheroid-like shape with a major axis and a length L1 (outer length), a largest inner diameter d1 and a largest outer diameter d2 and further having curved extremities with a curvature with radius r3, wherein an aspect ratio L1/d2 is 1.1<=L1/d2<=2.2 and a first shape parameter r3/d2 is 0.7<=r3/d2<=1.1.
• Publication No. US2003080681(A1 ) relates to a metal halide lamp equipped with a ceramic arc tube, which has a high luminous efficacy and a color-rendering characteristic. With arc tube vessel, on an external surface, has two transitional areas each having a minimum curvature radius of 0.3 mm and maximum value for the minimum curvature radius at each of the transitional areas is 7 mm.
• Publication No. JP2005293939(A ) relates to a ceramic metal halide lamp which has free lighting direction by effectively restraining the change of the color of emitted light in lighting direction. The tube shape parameter expressed by the ratio of the inner diameter of the main tube part [phi]i to the distance Le between the top end part of the electrodes is 0.45 to 0.65. The light emitting substance is composed of a basic component and an auxiliary component. The basic component is composed of halide of rare earth metal and sodium, and the auxiliary component is composed of a metal halide composed of halide of mercury.
• Publication No. US2008224615(A1 ) relates to a metal halide lamp, and a luminaire using the metal halide lamp. The metal halide lamp comprises an arc tube that includes: a translucent ceramic envelop having a central tube having an inner diameter of 5.5 mm or more. In addition, a curvature radius of the inner surface of each boundary region between the central tube and the joining portions is in the range of 0.5 mm to 2.5 mm.
• However, when increasing the temperature of the metal halides, the temperature of the whole burner is increased and as discussed above high envelope temperature reduces the lamp life. It is therefore advantageous to limit the temperature of the burner wall as much as possible.
• Therefore in view of the above prior arts there is a need to provide an alternative metal halide lamp which preferably obviates one or more of the drawbacks described above.
• Thus, in the present invention, the vessel shape is optimized to obtain a high colour rendering in combination with a low vessel wall temperature.
OBJECTS OF THE INVENTION
• A primary object of the present invention is to provide a metal halide lamp, ceramic discharge vessel shape of which is optimized. Another object of the present invention is to provide a metal halide lamp of a high colour rendering in combination with a low vessel wall temperature.
• Yet another object of the present invention is to provide a metal halide lamp with high colour rendering having long life span.
• Further object of the present invention is to provide a metal halide lamp with high luminous efficacy up to 1141m/W.
• Still further object of the present invention is to provide a metal halide lamp which is efficient and reliable.
• Still another object of the present invention is to provide a metal halide lamp with very fast start up time.
STATEMENT OF THE INVENTION
• According to this invention there is provided a metal halide lamp comprising a discharge vessel, shape of which is optimized to obtain a high colour rendering with a low vessel wall temperature by shape factor s₁ and aspect ratio f₀ and the size for a given shape is determined by means of wall loading P_D wherein $${\mathbf{<figure-callout\; id="1"\; label="s"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}={\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathrm{1}}/\mathrm{<figure-callout\; id="0"\; label="D\; f"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">D</figure-callout>}$$

  

  $${\mathbf{f}}_{}$$ $${\mathbf{}}_0={\mathrm{L}}_{\mathrm{body}}/\mathrm{D},$$

  

  and $${\mathbf{P}}_{\mathrm{D}}={\mathrm{P}}_{\mathrm{la}}/{\mathrm{D}}^2\mathrm{.}$$

  
SUMMARY OF THE INVENTION
• Accordingly the present invention discloses a metal halide lamp by optimizing the vessel shape to obtain a high colour rendering in combination with a low vessel wall temperature. The burner shape is defined by a plurality of parameters namely, Outer diameter (D), Center part radius (R₁), Neck radius (R₂), Center cylindrical part length (L_cyl), Total length (L), Wall thickness (t_w), Capillary outer diameter (D₁₎, Capillary bore (J) and other parameters like A, L_cav, L_body etc. are calculated employing these defining parameters.
• In a preferred embodiment of the present invention, the shape of the discharge vessel is optimized by factor s₁ and f₀ and the size for a given shape is determined by the wall loading P_D wherein $${\mathbf{<figure-callout\; id="1"\; label="s"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}={\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathrm{1}}/\mathrm{D}\left(\mathrm{the\; shape\; factor},\mathrm{sometimes\; also\; called\; the\; first\; <figure-callout\; id="0"\; label="shape\; factor\; f"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">shape\; factor</figure-callout>},\right)$$

  

  $${\mathbf{f}}_{}$$ $${\mathbf{}}_0={\mathrm{L}}_{\mathrm{body}}/\mathrm{D}\left(\mathrm{the\; aspect\; ratio}\right),$$

  

  and $${\mathbf{P}}_{\mathrm{D}}={\mathrm{P}}_{\mathrm{la}}/{\mathrm{D}}^2\left(\mathrm{the\; wall\; loading}\right)\mathrm{.}$$

  
• In another embodiment of the present invention, the scaling factors allow reduction in the number of independent variables and are chosen in such a way that the polycrystalline alumina (PCA) and electrode temperature profile do not change as long as these factors are kept constant.
• In yet another embodiment of the present invention, the shape of the discharge vessel has two identical end pieces and a cylindrical part in the centre with a length larger or equal than 0 (L_cyl ≥ 0).
• In still another embodiment of the present invention, the ionizable filling comprises mixes of NaI-LaI₃-CaI₂-TlI or NaI-DyI₃-TmI₃-HoI₃-TlI.
• In another embodiment of the present invention, LaI₃ may be replaced by CeI₃, PrI₃, SmI₃ or NdI₃ or a mix thereof. Similarly, the mix of DyI₃-TmI₃-HoI₃ can be replaced by a mix of any number of the salts DyI₃, HoI₃, TmI₃, ErY₃, YbI₃ or LuI₃.
BRIEF DESCRIPTION OF DRAWINGS
• Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:

FIG. 1 shows:

Parameterization of the burner shape of metal halide lamp according to the present invention;

FIG. 2 shows:

Electrode diameter d_el and electrode insertion length I_ins inside the lamp of present invention;

Figure 3 shows:

Array of shapes as function of shape factor s₁ and aspect ratio f₀;

Figure 4 shows:

An example of a wall temperature profile for a lamp burning vertically.

Figure 5a shows:

maximum wall temperature, colour rendering index, luminous efficacy and correlated colour temperature plotted as function of shape parameters s₁ and f₀ for wall loading P _D =0.70, 0.95 and 1.20 respectively (from left to right) for a metal halide mix NaI-CaI2-LaI3-TlI.

Figure 5b shows:

maximum wall temperature, colour rendering index, luminous efficacy and correlated colour temperature plotted as function of shape parameters s₁ and f₀ for wall loading P _D =0.70, 0.95 and 1.20 respectively (from left to right) for a metal halide mix DyI3-HoI3-TmI3-NaI-TlI.

Figure 6 shows:

CRI and luminous efficacies for a set of constant wall temperatures and metal halide mixes NaI-CaI2-LaI3-TlI (a) and DyI3-HoI3-TmI3-NaI-TlI (b), constructed by varying P _D (varying the vessel diameter for a given lamp power) and for the cases of L_cyl=0.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
• The invention discloses a metal halide lamp. Figure 1 and 2 illustrate the parameters that fully define the metal halide burners according to the present invention. Further, it is customary to use scaling factors for the lamp power to PCA size, the lamp current to electrode diameter and the lamp current to electrode (insertion) length. The burner shape can be completely defined by 8 parameters:
• Outer diameter D
• Center part radius R₁
• Neck radius R₂
• Center cylindrical part length L_cyl
• Total length L
• Wall thickness t_w
• Capillary outer diameter D₁
• Capillary bore J
• Other parameters like A, L_cav, L_body, ... can be calculated with the help of said eight defining parameters
• These scaling factors offer reduction in the number of independent variables and are chosen in such a way that the polycrystalline alumina (PCA) to electrode temperature profile does not change as long as these factors are kept constant. The selection of convenient scaling factors is done partly on empirical and partly on theoretical grounds.
• The traditional scaling factor used for power scaling is P_la/S_int, where S_int is the internal PCA bulb surface, but also P_la/S_out, where S_out is the outer bulb surface, is sometimes used. However, it was found that scaling with the surface of the PCA bulb is inconsistent for different shapes. Therefore lamp power is scaled to the square of the PCA outer diameter with the factor $$P_D=\frac{P_{la}}{D^2}$$

  
• The dependence of temperature and photometric variables on P_D can be different for different values of the shape factors s₁ and f₀. As shown in fig. 2, electrodes are inserted into the burner e.g. according to EP 0587238 . The diameter of the rod that protrudes into the vessel is called electrode diameter and is denominated by d_el. The length that protrudes inside the vessel starting from the onset of the radius R2 is called insertion length and is denominated by I_ins.
• For electrode scaling a simple one-dimensional electrode model is used, according to which the electrode temperature profile is maintained if d_el and l_ins are varied keeping $${\mathrm{I}}_{\mathrm{del}}={\mathrm{del}}^{\mathrm{3}/\mathrm{2}}/{\mathrm{I}}_{\mathrm{la}}$$

  

  and $${\mathrm{I}}_{\mathrm{ins}}={{\mathrm{<figure-callout\; id="3"\; label="l\; ins"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">l</figure-callout>}}_{\mathrm{ins}}}^{\mathrm{3}}/{\mathrm{I}}_{\mathrm{la}}$$

  

  constant.
• In the present invention, the metal halide burners are parameterized by 5 parameters:
• PCA shape by $${\mathbf{<figure-callout\; id="1"\; label="s"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">s</figure-callout>}}_{\mathrm{1}}={\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">R</figure-callout>}}_{\mathrm{1}}/\mathrm{D}\left(\mathrm{the\; shape\; factor},\mathrm{sometimes\; also\; called\; the\; first\; <figure-callout\; id="0"\; label="shape\; factor\; f"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">shape\; factor</figure-callout>},\right)$$
  
  $${\mathbf{f}}_{}$$ $${\mathbf{}}_0={\mathrm{L}}_{\mathrm{body}}/\mathrm{D}\left(\mathrm{the\; aspect\; ratio}\right),$$
  
  Power loading by P _D=P_la/D² (the wall loading) and
• Electrode loading by $${\mathrm{I}}_{\mathrm{ins}}={{\mathrm{<figure-callout\; id="3"\; label="l\; ins"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">l</figure-callout>}}_{\mathrm{ins}}}^{\mathrm{3}}/{\mathrm{I}}_{\mathrm{la}}$$
  
  $${\mathrm{I}}_{\mathrm{del}}={\mathrm{del}}^{\mathrm{3}/\mathrm{2}}/{\mathrm{I}}_{\mathrm{la}}\mathrm{.}$$
  
• The present invention focuses on the role of the shape of the discharge vessel, given by s₁ and f₀. The size for a given shape is determined by the wall loading P_D.
• Figure 3 gives an indication of how the shape of the vessel changes with s₁ and f₀. An array that provides possible shapes that can be made with varying s₁ and f₀. Combinations with small f₀ and large s₁ are not considered (crossed out in red) because the joint of the two body halves in the centre is not at right angles. This constitutes a difficulty in the manufacture of these vessels and creates unwanted stress and temperature problems in an operating lamp. In the present invention, preferably shapes with two identical end pieces and a cylindrical part in the centre with a length larger or equal than 0 (L_cyl ≥ 0) are considered. The shapes in Figure 3 at the lower right corner are crossed out because they do not comply with this criterion. In these shapes the end pieces join under an angle and this leads in practice to difficulties to produce.
• By making a very large number of experiments in which all the above parameters are systematically varied, an empirical model for the photometric and vessel wall temperature characteristics could be developed. Hereto the photometric quantities (such as luminous efficacy, correlated colour temperature, chromaticity coordinates and colour rendering index) and the full vessel wall temperature profile, are measured for each set of parameters. The vessel wall temperature profile is defined by 7 parameters: 5 temperatures at specific positions on the vessel surface and the temperature gradients at T_lo and T_hi, as shown in Figure 4. All the measurements are done in vertical burning position. The dots indicate specific positions on the surface of the vessel, wherein T_frlo is the temperature at the lower position corresponding to 0.8 times the length of the arc tube chamber L_body, T_lo is the temperature at the lower transition area between the extending side tubes and the centre bulb, T_max is the maximum temperature, T_hi is the temperature at the upper transition area between the extending side tubes and the centre bulb, and T_frhi is the temperature at the upper position corresponding to 0.8 times the length of the arc tube chamber L_body. The experiments are made with 2 types of metal halide mixes: NaI-LaI₃-CaI₂-TlI and NaI-DyI₃-TmI₃-HoI₃-TlI. Using this model with electrode diameter and electrode insertion length corresponding with I_del=0.15, I_ins=18, and NaI-LaI₃-CaI2-TlI chemistry the graphs of Figure 5 are obtained. These graphs show the maximum vessel wall temperature (T_max), the luminous efficacy (LumEff), the colour rendering index (CRI) and the correlated colour temperature (CCT) as a function of the shape factor s₁ and the aspect ratio f₀ for several values of the loading P_D.
  The dots on the curves of the graphs of Figure 5 indicate the point where the length of the middle cylindrical part is zero. The curves end here because the shapes with L_cyl < 0 are not considered.
• From the curves it is clear that the highest CRI, for any given P_D, is reached with small s₁ and small f₀. Interestingly, these configurations also correspond with the lowest T_max. However, this combination of high CRI with low T_max has the drawback of lower luminous efficacy. For any given s₁ the highest CRI is reached at the L_cyl=0 point. The temperature gradient ΔT=T_max-T_lo is to be kept as small as possible in order to limit chemical corrosion of the vessel wall. It is indeed known that metal halide salts in the liquid state that are present inside the vessel at a location where there is a large temperature gradient corrode the wall at that location thereby shortening the life of the lamp. From the graphs of Figures 5(a) and 5(b) it is seen that ΔT increases with s₁ and with f₀. This aspect hence also favours arc tube vessel design with small shape factor s1 and small L_cyl.
• Figure 6 depicts the CRI and luminous efficacies for a set of constant wall temperatures constructed by varying P_D (vary the vessel diameter for a given lamp power) and for the cases of L_cyl=0. It is clear that the highest CRI is reached at the smaller s₁. Even at low maximum wall temperature high CRI, in excess of 90, can still be obtained.
• All the above findings are also valid for different electrode diameters and insertion length. It is also known that similar results are obtained if the LaI₃ from the experiments is replaced by CeI₃, PrI₃, SmI₃ or NdI₃ or a mix thereof. Similarly, the mix of DyI₃-TmI₃-HoI₃ can be replaced by a mix of any number of the salts DyI₃, HoI₃, TmI₃, ErY₃, YbI₃ or LuI₃. Experiments done with varying compositions of NaI-LaI₃-CaI₂-TlI also demonstrate that within the experimental parameter range (CaI₂ from 10 to 50 wt.%, LaI₃ from 0 to 20 wt.% and TlI from 0 to 10 wt.%, NaI concentration is the balance) the claimed results remain valid. In various experiments for example the fill comprises of NaI, CaI₂ (10 to 50 wt%), a mix of LaI₃, CeI₃, PrI₃, SmI₃ or NdI₃ (0 to 30 wt.%) and TlI (0 to 15 wt.%) and for second combination the fill comprises of NaI, a rare earth mix of the compounds DyI₃, HoI₃, TmI₃, ErY₃, YbI₃ or LuI₃ (0 to 30 wt.%) and TlI (0 to 15 wt.%).
Features
• High luminous efficacy up to 114 lm/W
• Improved Colour Rendering Index - particularly with reds!
• Best in class life of 20,000 hours
• Excellent lumen maintenance ∼90%
• Very fast start up time
• It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

Claims (9)

1. Metal halide lamp comprising a discharge vessel, shape of which is optimized to obtain a high colour rendering with a low vessel wall temperature by shape factor s₁ and aspect ratio f₀ and the size for a given shape is determined by means of wall loading P_D wherein $${\mathbf{s}}_{\mathrm{1}}={\mathrm{R}}_{\mathrm{1}}/\mathrm{D}$$

   

   $${\mathbf{f}}_0={\mathrm{L}}_{\mathrm{body}}/\mathrm{D},$$

   

   
   and $${\mathbf{P}}_{\mathrm{D}}={\mathrm{P}}_{\mathrm{la}}/{\mathrm{D}}^2\mathrm{.}$$

   
2. The metal halide lamp as claimed in claim 1, wherein the discharge vessel comprises at least two identical end pieces with a cylindrical part therebetween having a length larger or equal than 0 (L_cyl ≥ 0).
3. The metal halide lamp as claimed in claim 1 or 2, wherein the shape factor (s₁), aspect ratio (f₀) and wall loading (P_D) ranges between 0.3 - 0.5. 0.9 -2.2 and 0.7 - 1.2 respectively.
4. The metal halide lamp as claimed in any of the preceding claims, wherein the ionizable filling in the burner of lamp comprises metal halide mixes of NaI-LaI₃-CaI₂-TlI and NaI-DyI₃-TmI₃-HoI₃-TlI.
5. The metal halide lamp as claimed in claim 4, wherein LaI₃ may be replaced by CeI₃, PrI₃, SmI₃ or NdI₃ or a mixture thereof.
6. The metal halide lamp as claimed in any of the preceding claims, wherein the mix of DyI₃-TmI₃-HoI₃ can be replaced by a mix of any number of the salts DyI₃, HoI₃, TmI₃, ErY₃, YbI₃ or LuI₃.
7. The metal halide lamp as claimed in any of the preceding claims, wherein the fill comprises of NaI, CaI₂, a mix of LaI₃, CeI₃, PrI₃, SmI₃ or NdI₃ and TlI, weight of which is 10-50%, 0-30% and 0-15% respectively.
8. The metal halide lamp as claimed in any of the preceding claims, wherein the fill comprises of NaI, a rare earth mix of the compounds DyI₃, HoI₃, TmI₃, ErY₃, YbI₃ or LuI₃ which is 0 to 30 % by weight and TlI which is 0 to 15% by weight.
9. The metal halide lamp substantially as herein described with reference to the accompanying drawings.

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