Patent Application
20090167179
This application relates to lamps, and particularly a ceramic metal halide (CMH) lamp, and improving the performance of CMH lamps by reducing heat loss associated with the lamp assembly. More particularly, the disclosure is related to controlling heat loss from the arc chamber to the legs with resultant increased efficiency of light radiation.
CMH lamps have become increasingly popular due to significant customer benefits. Traditionally, quartz arc tubes have been commonly used in metal halide arc discharge lamps. More recently, however, there has been a trend toward using CMH lamps that include a ceramic arc tube because of better color uniformity and stability, as well as increased lumens per watt (LPW) and color rendering (Ra) relative to traditional arc discharge lamps. These performance advantages of ceramic arc tubes are enabled by their higher temperature compared with quartz arc tubes and by a reduced rate of sodium loss.
In discharge lamps of this type, efficacy and lamp performance are adversely affected by the loss of energy through thermal conduction along the legs, or ends, of the arc tube. Commonly assigned U.S. Pat. No. 6,621,219 shows and describes one manner of limiting axial heat flux along the arc tube leg by designing the leg structure to have a reduced thermal conductivity, and the details of that disclosure are fully incorporated herein by reference. Particularly, reducing the molybdenum mandrel diameter effectively reduces the thermal conductivity of this component even when the diameter of the overwind wire is increased or when multiple overwinds are used. That is, the overwind provides a distinct reduction in thermal conduction along its length because of the helical conformation when compared to the mandrel portion.
There are three primary areas of thermal conductance in a direction generally parallel to the leg axis. A major thermal conductance is in the leg itself, or the polycrystalline alumina (PCA). The next largest thermal conductance is along the molybdenum mandrel. The third area of thermal conductance, and also the smallest, relates to the molybdenum overwind.
Miniaturization, improved color quality (Ra over 90) and energy efficiency are three major industry trends in the area of CMH technology development. As CMH lamps are made even smaller, it becomes more difficult to achieve a target seal glass temperature, a shorter overall arc tube length that fits inside an outer jacket, and simultaneously achieve targeted photometric performance, i.e., lumens, Ra, etc. Other parties have reduced the length of the leg or the length of the arc chamber, but provide a thicker leg. Consequently, performance of the resultant lamp is not as desired.
A need therefore exists to provide a CMH lamp that avoids an increase in seal glass temperature, has reduced thermal conductivity along the leg and thereby results in more energy distribution into the arc for light, and enables shorter leg length that eases fitting of the arc tube into an outer jacket and provides greater flexibility in arc tube body design to optimize performance.
This disclosure relates to a ceramic metal halide lamp that includes a body having a discharge chamber and first and second hollow legs extending from the discharge chamber. First and second electrode assemblies extend through the legs, respectively and the legs are preferably thin to limit heat flux from the discharge chamber therealong.
The thin legs have a load dissipation factor defined by a cross-sectional area (A) of the leg relative to the power (P) of the lamp (A/P<0.065) or less than 0.065 mm2/watt. Here, the cross-sectional area of the leg is given by A=π*(OD2−ID2)/4, where the OD and ID are the outside and inside diameters of the ceramic leg, if the leg components are cylindrical in shape, or an equivalent area if the components are not substantially cylindrical.
Preferably, the ceramic metal halide lamp is a low watt lamp, more preferably on the order of about 70 watts or less.
The electrode assembly further includes a molybdenum mandrel having a load dissipation factor defined by a cross-sectional area (A) of the mandrel relative to the power (P) of the lamp (A/P<0.0008) less than 0.0008 mm2/watt to reduce the heat flux in the leg.
One or both of these load dissipation factors may be incorporated into a lamp to reduce the heat flux through the CMH lamp leg.
A method of improving performance for a ceramic metal halide lamp includes reducing the heat flux by limiting the leg cross-sectional area-to-power ratio to less than 0.065 mm2/watt and/or limiting a molybdenum mandrel load dissipation factor to less than 0.0008 mm2/watt.
The present disclosure can either advantageously lower the seal glass temperature and thereby improve the reliability of the lamp or provide for a shorter leg while avoiding an increase in seal glass temperature to maintain good reliability.
It is also found that a thin leg enables the use of a smaller seal glass ring and therefore a shorter seal glass wicking length. As a result, the total arc tube length is reduced significantly and eases the fitting of the arc tube inside an outer jacket.
The lamp also achieves superior photometric performance in terms of lumens and Ra by reducing thermal losses in the leg.
Still other benefits and advantages of the disclosure will become apparent from reading and understanding the following detailed description.
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There is a desire to reduce the heat flux from the arc discharge chamber and down the legs in order to protect the seals 34, 36 disposed at the far end of each leg. The seals cannot run too hot or the life of the lamp may be adversely impacted. Moreover, heat transfer down the leg correlates to a corresponding loss of heat from the arc tube. It is desirable to reduce the heat loss from the arc tube in order to keep the intrinsic efficiency of the arc tube high and thus obtain higher lumens per watt and better photometric performance, including high color quality.
One proposed solution is outlined in the commonly-owned U.S. Pat. No. 6,621,219, where the dimension of the molybdenum mandrel was reduced relative to the dimension of the overwind without regard to the lamp wattage. As taught in the present disclosure, it has been determined that a thin leg relative to the lamp wattage is an important feature to limit the heat flux. The thin leg is described by the ratio of the cross-sectional area of the leg (π*(OD2−ID2)/4) compared to the lamp power, and thus is expressed as area/power (A/P) or mm2/W. In higher wattage lamps, this thin leg dissipation factor on the order of less than 0.065 mm2/W may be met because the body is so big and the watts are sufficiently high to keep this ratio low. However, this is much more difficult to achieve in the low wattage lamps. Generally, by low watt is meant approximately 70 watts or less. Other than lamps made by the assignee of the present disclosure, most other lamp manufacturers use “thick” legs when generally shrinking or reducing the overall size of the lamp for low watt applications.
It has been found that the low dissipation factor of the leg has a greater impact with respect to lower wattage lamps. Reducing the transverse lateral dimension of the PCA leg becomes an important consideration in this ratio. As a result, the seal at the far end of the leg is protected and improved photometrics, i.e., higher lumens per watt or better color, i.e., improved Ra, are achieved, because the arc tube can operate more efficiently.
Again, this is particularly important in the lower wattage lamps. The lower wattage lamps are generally smaller and thus the axial lengths of the legs are reduced. Normally, the reduced axial length of the legs would contribute to a higher seal temperature and may adversely impact the lamp life. However, by implementing the thin leg dissipation factor, not only is the leg axially shortened, but the low watt lamp can still achieve an acceptable seal temperature. Moreover, reducing the overall size of the lamp and making it more compact allows the small wattage lamp to be placed within outer jackets or capsules, or placed into smaller reflectors. Therefore, it has been determined that maintaining the PCA leg dissipation factor ratio at less than 0.065 mm2/W allows the lamp to be made smaller, the axial length of the legs shortened, and yet the seals can be moved closer to the arc without being adversely impacted by an increase in heat.
In addition, it has been determined that the electrode molybdenum mandrel load dissipation factor can also be expressed as a cross-sectional area relative to the power. Particularly, the mandrel load dissipation factor or ratio should be less than 0.0008 mm2/W. The smaller diameter molybdenum mandrel is important because the thermal conductance is typically an order of magnitude greater than the thermal conductance of the PCA leg.
So, in order to manage the total thermal flux in the leg, the molybdenum mandrel cross-sectional area relative to the wattage is one factor and, in addition, the PCA leg dissipation factor is another. It has also been advantageously found that a non-linear benefit also results when the dissipation factors of both the molybdenum cross-sectional area relative to the wattage and the PCA leg cross-sectional area relative to the wattage are controlled. These thermal conductances are like electrical resistors in parallel where the net resistance is the same as adding the resistances inversely. The same is true here with the thermal conductances which are inverse of the thermal resistances, i.e., the net thermal conductance is the sum of adding the individual thermal conductances. Consequently, addressing the heat flux by controlling the cross-sectional area of the leg relative to the power reduces the seal glass temperatures and achieves the desired lamp performance. Alternatively, controlling the cross-sectional dimension of the molybdenum mandrel relative to the power also achieves these same objectives. Moreover, when these two thermal conductance features are controlled in combination, the additive effect has an even greater impact.
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The invention has been described with reference to various preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.
