Claims
- 1. A high pressure discharge lamp construction comprising:
- an outer bulb;
- first and second high pressure sodium discharge tubes within the outer bulb; and
- means for electrically connecting the discharge tubes in series, one of said arc tubes having a sodium and mercury content such that the one arc tube operates in the unsaturated mode, the other of said arc tubes having a sufficient sodium and mercury content to operate in the saturated mode.
- 2. The construction of claim 1 wherein the first and second arc tubes are arranged side-by-side in spaced relationship.
Parent Case Info
This is a continuation of application Ser. No. 455,739, filed Dec. 21, 1989, now abandoned.
This invention relates to a high pressure sodium discharge lamp construction employing series connected discharge tubes operating at high wattage.
Of interest is copending application Ser. No. 07/455,728 filed concurrently herewith, now U.S. Pat. No. 5,028,845 (case 3804-0332) entitled High Power High Wattage Series Arc Discharge Lamp Construction in the name of Aelterman et al. and U.S. Pat. No. 4,751,432 issued in the name of Van Delm, both of which are assigned to the assignee of the present invention.
High Power Series Discharge Lamps are described in the above-mentioned U.S. Pat. No. 4,751,432 and in British Patent Specification No. 1,332,852. In such lamps, two discharge tubes are connected in series and located within an evacuated outer bulb. The series connected arc tubes are employed to correct the color of the light emitted by the combined discharge lamps by using, for example, different types of discharge tubes in the series combination. Typically, a high voltage starting pulse is required to start high power lamps at high pressure. For example, a high pressure sodium discharge lamp contains a fill gas Xenin, of 10 kPa, typically. The typical high pressure sodium arc tube contains some tens of milligrams of sodium-mercury amalgam which is partially vaporized when the tube attains its operating temperature. The sodium content of the amalgam is about 20%. The non-vaporized amalgam collects in the coolest part of the discharge tube of a given lamp. However, other lamps may have the amalgam reservoir outside the discharge tube. By changing the color characteristics of each of the series connected discharge tubes, a more uniform or desirable color spectrum may be attained by the combination then otherwise would be possible from a single lamp.
For discharge tubes operating at relatively high power, for example, about 400 watts each, a relatively large starting voltage pulse is required, typically 1.5 to 5 kV. Once the lamps are ignited, they operate to produce a combined luminance output.
A problem recognized by the present inventors is that such sodium discharge lamps typically are of the saturated type. In a saturated lamp, liquid sodium-mercury amalgam is always present in the lamp structure. As a result, the lamp does not have constant lamp voltage characteristics such that the voltage will vary with a number of parameters during the life of the lamp. As the lamp ages, for example, its voltage may increase. Also, if a portion of the discharge tube blackens or deteriorates such that its light and heat transmission characteristics decreases, the voltage of the lamp will also increase.
In operating such a lamp, a ballast is used in combination with the lamp circuit to provide a stable lamp current. Such a ballast has parameters that are matched with the combined power of the two series connected discharge tubes such that for two 500 watt arc tubes connected in series the ballast is one which will be designed to operate with a 1000 watt lamp. In a high pressure sodium lamp operating in the saturated mode, the excess amalgam present in the discharge results in the lamp voltage increasing as the lamp wattage varies such that too high a lamp voltage causes the lamp to extinguish, and too low a lamp voltage results in a lamp not reaching its specified lamp output. The actual operating point of the given high pressure sodium lamp is influenced by electrical and non-electrical factors including the voltage level of the power supply, ballast characteristics and operating temperature. The influence of the voltage supply variations on both lamp voltage and lamp power is usually indicated by a lamp line curve. The present inventors recognize a problem with such series operated saturated high pressure, sodium discharge tubes in that such series connected tubes tend to prematurely fail. The reason for the failure is that when one saturated arc tube increases in voltage the other decreases due to the voltage-current characteristic of an arc discharge so that the increase in voltage in the one lamp eventually is significantly higher than normally expected by conventional knowledge.
A high pressure discharge lamp construction in accordance with the present invention comprises an outer bulb and first and second high pressure sodium discharge arc tubes within the outer bulb. Means are provided for electrically connecting the discharge tubes in series. At least one of the arc tubes has a sodium content such that the at least one arc tube operates in the unsaturated mode. An unsaturated arc tube operates at a constant voltage. When a saturated arc tube connected in series with the unsaturated arc tube shifts in voltage as that lamp ages, the voltage level of the unsaturated arc tube tends to remain constant.
Therefore, the voltage of the saturated arc tube, when subjected to conditions tending to cause the voltage to increase, will increase a magnitude significantly less than might otherwise occur if both tubes were saturated.
US Referenced Citations (5)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 1332852 |
Oct 1973 |
GBX |
Continuations (1)
|
Number |
Date |
Country |
| Parent |
455729 |
Dec 1989 |
|
Patent Grant
5142188
