Patent Application
20090174327
This invention relates to ceramic discharge metal halide (CDM) lamps, and more particularly relates to CDM lamps with a significantly reduced hot re-strike time.
CDM lamps typically require ten to fifteen minutes after a momentary power outage to cool sufficiently to reach a breakdown voltage allowing re-strike to occur. By comparison, quartz metal halide lamps typically exhibit re-strike times in the range of from about six to ten minutes, and high pressure sodium (HPS) lamps typically exhibit re-strike times in the range of from about one to two minutes. In addition, HPS lamps can exhibit essentially instant re-strike times when employing a second, inactive discharge tube in parallel with the first, which strikes as soon as power is restored. This approach has proven unworkable in CDM lamps, particularly the high wattage versions, because the much higher vapor pressures in the CDM lamps.
In accordance with the present invention, it has been discovered that by increasing the size of the outer bulb relative to the ceramic discharge vessel of a high wattage (150 W or greater) CDM lamp, the hot re-strike time is reduced. This size difference is represented herein by the ratio A, which is the ratio of the diameter D of the outer bulb to the inside diameter ID of the ceramic discharge vessel. This ratio must be greater than about 5.8, and is preferably at least about 8.7.
It has been further discovered that by filling the outer bulb of such a lamp with an inactive gas such as one or more of nitrogen, helium, neon, argon, krypton or xenon, the hot re-strike time is also reduced.
It has been further discovered that in such a lamp in which the hot re-strike time has been reduced by one or both of the above means, the hot re-strike time is further reduced by the addition to the discharge tube a metal having a gettering capacity for iodine, such as Sc, Ce or Na.
In summary, the hot re-strike time of a high wattage (150 W or greater) ceramic discharge metal halide (CDM) lamp is reduced by: (a) increasing the ratio A of the diameter D of the outer bulb to the inner diameter ID of the discharge vessel; or (b) filling the outer bulb with an inactive gas such as one or more of nitrogen, helium, neon, argon, krypton or xenon; or by implementing both (a) and (b). The hot re-strike time can be further reduced by combining (a) and/or (b) with (c), the addition of a getter metal for iodine, such as Sc, Ce or Na, to the discharge vessel.
In accordance with a preferred embodiment of the invention, (a), (b) and (c) are combined to result in a high wattage (150 W or more) CDM lamp wherein the ratio A is chosen to be at least 12; nitrogen gas is chosen to be present in the outer bulb in an amount to result in a pressure of from about 100 to 500 Torr; and Sc metal is added to the salts of the discharge tube in the amount of from about 3.75 to 6.25 wt. %.
The ionizable filling of the discharge vessel 3 typically includes an ignition gas such as Xe, Ar or Kr. The ionizable filling also includes Hg and iodides of Na, Ca, Tl and rare earths, such as Dy, Ho and Tm.
Such a prior art CDM lamp is described in more detail in U.S. Pat. Nos. 6,555,962; 6,031,332; and 5,973,453, the entire specifications of which are incorporated herein by reference. Typical hot re-strike times for these lamps are from about ten to fifteen minutes.
The discharge vessels were PCA arc tubes with standard dimensions of 9.8 mm×38 mm (ID×IL), and sealed to the PCA with a high temperature glass. The discharge vessels were charged with a salt mixture containing NaI, CaI2, TlI and rare earth iodides. Xe with a small addition of Kr as a starting aid was used as the ignition gas. Hg was dosed at 4.6 mg, except for the lamps whose outer bulbs were gas-filled. These lamps were dosed with from 5 to 13 mg of Hg in order to obtain operation to within 10% of 400 W. The discharge vessels were seasoned for fifteen minutes before testing.
Variables in the series of seven lamp designs (designated 1-7) include two different outer bulb sizes, the first representing the prior art lamp and designated ED18, having a diameter of about 2¼ inch, and the second, designated ED37, having a diameter of about 4 ⅝ inch, approximately 105% of the diameter of the ED18 bulb. Some outer bulbs were maintained in vacuum, while others were filled with nitrogen to a pressure of 300 Torr. Vacuum-containing lamps had barium ring getters, while gas filled lamps had solid state getters. Some discharge vessels were given a dose of 2 mg of scandium metal, which corresponded to 5 wt. % of the salts.
These results demonstrate that the two design features of larger size of the outer envelope, and gas fill of the outer envelope each results in a decrease of the hot re-strike time (to 11.7 and 8.2 minutes, respectively), while the combination of these two features results in a further decrease (to 7.4 minutes), and the combination of either of these features with the addition of Sc to the discharge tube results in further decreases (to 6.7 and 6.4 minutes, respectively), and the combination of all three features results in the greatest decrease (to 4.2 minutes).
It can also be seen that the gas fill alone has a somewhat larger effect than an increase in the bulb size alone, resulting in a decrease in the hot re-strike time from 12.2 to 8.2 minutes, or 32%, for lamp 3, versus a decrease from 12.2 to 11.7 minutes, or 4% for lamp 2. This effect can also be seen by comparing the hot re-strike times for lamps 5 and 7, having both a larger outer bulb and Sc. The addition of the gas fill results in a decrease of the hot re-strike time from 6.7 minutes for lamp 5 to 4.2 minutes for lamp 7, a decrease of approximately 37%.
These results can be compared with the hot re-strike time of 7.4 minutes for lamp 4, having a gas filled ED37 envelope, but no Sc. While lamp 8 showed about 4% improvement over lamp 4, further tests showed inconsistent results. Lamp 9 showed a much larger improvement of about 43% over lamp 4, and lamp 10 showed a further improvement over lamp 9 of about 10%. However, the larger amount of Sc in lamp 10 reduced the lamp voltage by a factor of 2 thus requiring more than 10 mg of Hg to be added.
Based on these and other considerations, it is preferred to add the Sc in an amount of at least about 1.5 mg., below which improvements in hot re-strike time tend to be slight or inconsistent, and no more than about 2.5 mg., above which further improvements are obtainable, but may be accompanied by significant drops in lamp voltage.
It is important that the iodine getter be added as the metal in order to take up the excess iodine while the lamp is cooling, thus reducing the time to reach a low enough breakdown voltage for re-strike to occur.
Although not relied upon to define the invention, theory suggests that during lamp cooling, the highly electronegative I− ion forms, depleting the discharge space of the free electrons needed for the lamp to re-strike. If excess Hg is present, it can getter the excess iodide by forming HgI2. However, HgI2 forms and condenses out of the hot discharge gas at a relatively low temperature. The addition of a getter metal such as Sc results in the preferential formation of ScI3, which removes excess iodide ions more quickly because it forms and condenses out of the hot discharge gas at a higher temperature than HgI2. The embodiments and examples set forth herein are presented to explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other embodiments, variations of embodiments, and equivalents, as well as other aspect, objects, and advantages of the invention, will be apparent to those skilled in the art. Thus, the principles of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB05/53817 | 11/18/2005 | WO | 00 | 5/18/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60629797 | Nov 2004 | US |
