The invention relates to a electrodeless high-pressure discharge lamp comprising:
Such an electrodeless high-pressure discharge lamp is known from EP-A-0,440,381.
Heat generated by current passage through the electric leads or through the coil and heat radiated by the discharge vessel towards the coil render it necessary to cool the leads so as to prevent the supply assuming a too high temperature. The coil should also be prevented from assuming a comparatively high temperature, and thus a comparatively high electrical resistance. A higher electrical resistance would cause the ohmic losses to increase, and thus also the temperature.
In the known lamp, cooling is realised in that a heat sink is fastened against each of the two electric leads. Heat is thus removed from these leads through the heat sinks to the surroundings.
Metals are highly suitable, being good thermal conductors, for forming heat sinks and any fins thereof. Since only one of the two electric leads can be connected to earth, while the other one is live, the heat sink of this other electric lead is live. This is a safety hazard of the known lamp.
In the known lamp, a dielectric is accommodated between the electric leads so as to form a capacitance Cp in parallel to the coil. Dielectrics which can be used are polytetrafluoroethene, mica and polyimide. The said capacitance Cp renders it possible in conjunction with another capacitance Cs connected in series with the coil elsewhere to adapt the impedance of the load, consisting of the coil and the discharge, to the electric supply.
A similar electrodeless high-pressure discharge lamp is also known from US-A-5,030,889.
US-A-5,039,903 and US-A-5,042,139 disclose electrodeless high-pressure discharge lamps in which each of the ends of the electric coil is connected to a ballast via a respective heat sink, and the electric leads extend at a distance from these heat sinks. It is also possible to circulate water through the coils in these lamps in order to cool the coils. A drawback of this is that lines, a drive and energy consumption are necessary for this purpose.
US-A-4,910,439 discloses an electrodeless high-pressure discharge lamp in which a forced air current cools the electric coil. This lamp has the drawback that a motor and lines are necessary for cooling, and also that energy is required for this purpose.
It is an object of the invention to provide an electrodeless high-pressure discharge lamp of the kind described in the opening paragraph which has an improved safety as regards inadvertent touching of electrical parts and which comprises reliable means for cooling the electric coil and the electric leads.
According to the invention, this object is achieved in that the live electric lead is electrically insulated from the heat sink by aluminium nitride.
Aluminium nitride combines a comparatively high electrical resistivity of approximately 1012 Ωm with a comparatively high thermal conductivity of approximately 150 Wm-1K-1. This means that it is highly suitable for connecting a live electric lead thermally, but not electrically, to an electrically conducting body.
In an embodiment of the electrodeless high-pressure discharge lamp according to the invention, the live lead is in physical, and thus in thermal contact with the neutral lead through interposition of aluminium nitride, while the neutral lead is in contact with the heat sink. The live lead then transfers heat through the aluminium nitride and the neutral lead transfers heat to the heat sink and thus to the surroundings. The heat sink may in this case surround the two leads entirely or substantially, however, surrounding the live lead at a distance, possibly with the interposition of an electric insulator such as, for example, aluminium nitride.
An advantage of this embodiment is that a parallel capacitance Cp is provided across the coil with the aluminium nitride as a dielectric. The value of this capacitance to be set, however, imposes limitations on the geometry of the leads and of the interposed aluminium nitride.
In alternative embodiments, a parallel capacitance Cp across the coil may be incorporated at a greater distance from the coil, for example, in the electric supply. In such an embodiment, each of the electric leads may have its own heat sink, the live lead being connected to its heat sink via aluminium nitride, for example, in that the two portions of a two-part heat sink are pulled towards one another with the interposition of a respective body made of aluminium nitride between a heat sink portion and the live lead. In a modification thereof, however, the heat sink of the live lead is integral with the heat sink of the neutral lead.
In an embodiment which is convenient as regards construction and dimensioning, the two electric leads are together surrounded by a heat sink which is in contact both with the live lead and with the neutral lead via aluminium nitride. In this case portions of the - for example two-part - heat sink may be pulled both against the neutral lead and against the live lead with the interposition of aluminium nitride each time.
The electrodeless high-pressure discharge lamp according to the invention renders possible a compact shape in which the use of a fluid such as air or water, and of circulating means for this purpose is dispensed with. Ohmic losses and damage to the electric supply to be used are nevertheless effectively counteracted.
Embodiments of the electrodeless high-pressure discharge lamp according to the invention are shown in the drawing, in which
In Fig. 1, the electrodeless high-pressure discharge lamp has a light-transmitting lamp vessel 1, made of quartz glass in the Figure and having a volume of 2 cm3, which is closed in a vacuumtight manner and which has a ionizable filling of 2.5 mg NaI, 1.5 mg CeI3 and 125 mbar Xe. Alternatively, however, the lamp vessel may be made of ceramic material, for example, monocrystalline or polycrystalline ceramic material such as sapphire or sintered aluminium oxide. An electric coil 2, for example made of copper, is present around the lamp vessel, with three turns in the Figure, which coil has end portions 3, 4 which are electrically connected to a neutral electric lead 5 which extends away from the coil and to a live electric lead 6, respectively. These leads, made of copper in the Figure, are to be connected to an electric supply. The electric leads 5, 6 are connected to a heat sink 7.
The live lead 6 (see Fig. 2) is electrically insulated from the heat sink 7 by aluminium nitride 8. The live lead 6 is in contact with the neutral lead 5 through the interposed aluminium nitride 8, and the neutral lead in its turn is in contact with the heat sink 7.
The lamp shown has a luminous efficacy of 100 lm/W at a consumed power of 160 W. The lamp vessel 1 is accommodated in a reflector 9 which is closed off by a grid 10 of metal wire. The heat sink 7 has fins 11 for an improved heat transfer to the surroundings.
The two leads 5, 6 together with the aluminium nitride 8 form a capacitance Cp which is in parallel to the coil 2 and which together with a capacitance Cs connected in series elsewhere adapts the coil and the discharge to their supply unit.
In Fig. 3 and subsequent Figures, corresponding parts are always given the same reference numerals.
In Fig. 3, each of the electric leads 5, 6 has its own heat sink 7, 17, and the live lead 6 is in physical contact with aluminium nitride 8 which is in physical contact with the heat sink 17 of the live lead. A capacitance Cp is connected in parallel to the discharge and the coil elsewhere.
In Fig. 4, the heat sink of the live lead 6 is integral with the heat sink 7 of the neutral lead 5.
The two electric leads 5, 6 in the embodiment of Fig. 5 are together surrounded by a heat sink 7 which is in contact both with the live lead 6 and with the neutral lead 5 via aluminium nitride 8. The heat sink 7 is mounted on a metal support plate 12.