Electric lamp and discharge devices – With luminescent solid or liquid material – With gaseous discharge medium
Reexamination Certificate
2001-03-19
2004-05-11
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
With gaseous discharge medium
C313S565000, C313S547000, C313S562000, C313S577000
Reexamination Certificate
active
06734616
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel,
which discharge vessel encloses a discharge space provided with a filling of mercury and an inert gas in a gastight manner,
which discharge vessel contains an amalgam which communicates with the discharge space,
and the low-pressure mercury-vapor discharge lamp comprises discharge means for maintaining an electric discharge in the discharge space.
The invention also relates to an amalgam for use in the low-pressure mercury-vapor discharge lamp.
2. Discussion of the Prior Art
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner wall of the discharge vessel may be coated with a luminescent layer comprising a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, such as UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general lighting purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of low-pressure mercury-vapor discharge lamps is generally tubular and circular in section, and comprises both elongated and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of comparatively short, straight parts having a comparatively small diameter, which straight parts are interconnected, on the one hand, by means of bridge parts or, on the other hand, by means of, for example, arc-shaped parts. Compact fluorescent lamps are generally provided with a lamp cap (with integrated electronics).
In the description and the claims of the current invention, the designation “nominal operation” is used to refer to operating conditions where the mercury-vapor pressure is such that the radiation output of the lamp is at least 80% of that during optimum operation, i.e. under operating conditions where the mercury-vapor pressure is optimal. The amalgam limits the mercury-vapor pressure in the discharge vessel with respect to a discharge lamp containing only free mercury. This enables nominal operation of the lamp at comparatively high lamp temperatures, which may occur, for example, when the lamp is subjected to a high load or when the lamp is used in a closed or badly ventilated luminaire. Furthermore, in the description and the claims, the “initial radiation output” is defined as the radiation output of the discharge lamp 1 second after switching on the discharge lamp, and the “run-up time” is defined as the time needed by the discharge lamp to reach a radiation output of 80% of that during optimum operation.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, also referred to as a vapor pressure-controlled lamp, is disclosed in U.S. Pat. No. 4,093,889. The known lamp has a comparatively low mercury-vapor pressure at room temperature. As a result, the known lamp has the disadvantage that also the initial radiation output is comparatively low when a customary power supply is used to operate said lamp. In addition, the run-up time is comparatively long because the mercury-vapor pressure increases only slowly after switching on the lamp.
Apart from the above-described amalgam lamps, low-pressure mercury-vapor discharge lamps are known which comprise both a (main) amalgam and a so-called auxiliary amalgam. If the auxiliary amalgam comprises sufficient mercury, then the lamp has a relatively short run-up time. Immediately after the lamp has been switched on, i.e. during preheating the electrodes, the auxiliary amalgam is heated by the electrode so that it relatively rapidly dispenses a substantial part of the mercury that it contains. In this respect, it is desirable that, prior to being switched on, the lamp has been idle for a sufficiently long time to allow the auxiliary amalgam to take up sufficient mercury. If the lamp has been idle for a comparatively short period of time, the reduction of the run-up time is only small. In addition, in that case the initial radiation output is (even) lower than that of a lamp comprising only a main amalgam, which can be attributed to the fact that a comparatively low mercury-vapor pressure is adjusted in the discharge space by the auxiliary amalgam. An additional problem encountered with comparatively long lamps is that it takes comparatively much time for the mercury liberated by the auxiliary amalgam to spread throughout the discharge vessel, so that after switching on such lamps, they demonstrate a comparatively bright zone near the auxiliary amalgam and a comparatively dark zone at a greater distance from the auxiliary amalgam, which zones disappear after a few minutes.
Furthermore, low-pressure mercury-vapor discharge lamps are known which are not provided with an amalgam and contain only free mercury. These lamps, also referred to as mercury lamps, have the advantage that the mercury-vapor pressure at room temperature and hence the initial radiation output are comparatively high. In addition, the run-up time is comparatively short. After having been switched on, comparatively long lamps of this type also demonstrate a substantially constant brightness over substantially the whole length, which can be attributed to the fact that the vapor pressure (at room temperature) is sufficiently high at the time of switching on these lamps. Nominal operation at comparatively high lamp temperatures can be achieved using a mercury lamp whose discharge space contains (just) enough mercury to bring about a mercury-vapor pressure at the operating temperature which is close to the optimum mercury-vapor pressure. During the service life of the lamp, however, mercury is lost because it is bound, for example, to a wall of the discharge vessel and/or to emitter material. As a result, in practice such a lamp only has a limited service life. Therefore, the mercury dose in mercury lamps is substantially higher, in practice, than the quantity of mercury necessary during nominal operation in the vapor phase. However, this has the disadvantage that the mercury-vapor pressure is equal to the saturation vapor pressure pertaining to the temperature of the coldest spot of the discharge vessel. As the saturation vapor pressure increases exponentially with temperature, temperature variations, occurring for example in a badly ventilated luminaire or when the lamp is subjected to a high load, lead to a reduction of the radiation output. At comparatively low ambient temperatures, the mercury-vapor pressure decreases, which also leads to a reduction of the radiation output.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lamp of the type described in the opening paragraph, which, when it is used regularly, has a comparatively high initial radiation output and a comparatively short run-up time as well as a comparatively high radiation output in a comparatively large ambient-temperature range.
This object is achieved in accordance with the invention in that the amalgam comprises a bismuth-lead compound having a lead content (Pb) in the range between 35≦Pb≦60 at. %, a bismuth content (Bi) in the range between 40≦Bi≦65 at. %, and a mercury content (Hg) in the range between 0.05≦Hg≦1 at. %.
The advantage of using such a Bi—Pb amalgam is that, at room temperature, the mercury-vapor pressure is comparatively close to that of liquid mercury. If the amalgam has the above-mentioned composition, the discharge lamp is nominally operated at a corresponding coldest spot temperature of the discharge vessel which lies in a comparatively wide temperature range from 65 to 165° C. A further advantage of the use of such a Bi—Pb amalgam resides in that the curves, in which the mercury-vapor pressure is plotted as a function of the temperature, can be adjusted via the mercury content. Said properties of the (main) amalgam, i.e. the wide temperature interval and the variable mercury-vapor pressure curves, are obtained by the c
Kaldenhoven Lambert Christiaan Ida
Keur Wilhelmus Cornelis
Lankhorst Martijn Henri Richard
Colón German
Patel Ashok
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