Mercury-free metal halide lamp

Details Mercury-free metal halide lamp Mercury-free metal halide lamp

1999-09-16

2001-07-24

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With gas or vapor

With particular gas or vapor

C313S570000, C313S643000

Reexamination Certificate

active

06265827

ABSTRACT:

TECHNICAL FIELD
This invention relates to a mercury-free metal halide lamp usable for general luminaries, and motor vehicle headlights constructed with reflectors and the like.
BACKGROUND ART
Conventionally, metal halide lamps have been employed for such uses as a light source for motor vehicle headlights and so forth. Conventional metal halide lamps typically have such a construction in which three types of materials, rare gas (gaseous matter), mercury (liquid matter), and halide of metal (solid matter), are enclosed in an arc tube. More specifically, an example of such lamps is as follows:
As shown in
FIG. 12
, an approximately spherical-shaped arc tube
101
encloses a fill material
102
. The arc tube
101
is composed of a light-transmissive vessel made of quartz. Each of the ends of the arc tube
101
is sealed at a seal portion
103
. A pair of tungsten electrodes
104
is provided in the arc tube
101
. Each of the electrodes
104
is connected with a lead wire
106
via a molybdenum foil
105
hermetically sealed in the seal portion
103
. The dimensions of this metal halide lamp are as follows:
Arc tube internal volume: 1.7 cc
Distance between the electrodes
104
: approx. 16 mm
The contents in the fill material
107
are as follows:
Hg (mercury): 21.5 mg (12.6 mg/cc)
TlI (thallium iodide): 0.27 mg (0.16 mg/cc)
InI (indium iodide): 0.04 mg (0.021 mg/cc)
NaI (sodium iodide): 1.9 mg (1.14 mg/cc)
Xe (xenon): 12 kPa (at room temperature)
When the lamp according to the above construction is operated under the condition where the electric current is controlled in order for the lamp power to be maintained at 100 W, a luminous flux of approximately 6200 lm is emitted by the electric discharge between the electrodes
104
. In this operation, all of the mercury and a portion of the metal halides such as TlI etc. are evaporated, and a voltage (operating voltage) drop of 100 V is caused between the foremost ends of the electrodes
104
.
The above-mentioned rare gas (Xe) is enclosed in order to facilitate a starting (start of discharge) and to increase the light output immediately after the starting. The metal halides (such as TlI) are enclosed in order to obtain an appropriate light output during a stable operation.
Mercury is enclosed in order to obtain a high voltage between the electrodes (operating voltage), which is required for the stable operation of the lamp. A voltage increasing effect of mercury is, more specifically, represented by the following equation as disclosed in, for example, Japanese Unexamined Patent Publication No. 06-13047 etc.
Vla=
20
+k
(proportional constant)×
n
Hg
0.56
×L,
In the equation, Vla is an operating voltage (V), nHg is an amount of mercury per unit arc tube internal volume (mg/cc), and L is a distance between electrodes (mm).
From the equation, it is understood that the operating voltage is proportional to the product of the distance between electrodes and the approximately ½ power of an atomic density of mercury. In the above equation, the constant ‘20’ is the sum of the voltage approximately at the electrodes and a voltage by the effects of the rare gas and metal halides. According to this equation, if mercury is not added, the operating voltage is greatly dropped (nHg=0, and thereby the operating voltage is approximately 20 V.). Therefore, the electric current is required to be increased in order to operate the lamp with the same power (in comparison with the case of the operating voltage being approximately 100 V, the required current is approximately SA, which is 5 times as large.). Hence, electrode losses are increased, and a conspicuous blackening of the arc tube is caused by a sputtered matter of the electrodes, thus deteriorating the luminous flux. Specifically, the arc tube is blackened in as short as several tens of hours and reaches the end of its lamp life.
In view of the above problem, in conventional lamps, the operating voltage is increased to be approximately 70 to 100 V by adjusting the amount of mercury, and thereby the lamp current is suppressed and the electrode losses (Joule loss) are also reduced. A long lamp life up to several thousand hours (for example, approximately 6000 hours) is thus achieved
However, while mercury brings about such a desirable effect that the operating voltage can be increased as above, it incurs such drawbacks as follows.
Firstly, since mercury causes a deterioration of luminous efficacy, attaining a bright lamp becomes difficult. That is because mercury has the second highest excitation potential in all the elements, next to rare gases, and therefore the light emission is little when compared with other metallic elements employed as metal halides. This fact is also seen from the spectral distribution of the above-described metal halide lamp, as shown in FIG.
13
. Specifically, the emitted light of the lamp retains a plurality of line spectra, and the major wavelengths are 410.01 nm and 451.1 nm by In, 535.0 nm by Ti, and 589.0 nm and 589.6 nm by Na. Since mercury contributes little to the light emission, very little light emission by mercury is observed. On the other hand, in the case where no mercury is added in the above lamp, a high luminous efficacy of approximately 70 lm/W (the whole luminous flux is approximately 7000 lm) is obtained.
Secondly, a step of enclosing mercury, being a liquid matter, is necessary in the manufacturing steps of such a lamp, which tends to increase the manufacturing cost.
In addition to the above drawbacks, in recent years, metal halide lamps containing no mercury have been increasingly desired since a global environmental concern has been growing.
In view of these problems and perspectives, in order to raise an operating voltage without adding mercury, Japanese Unexamined Patent Publication No. 06-84496 etc. discloses an example of a technique in which the fill pressure of Xe is increased. More specifically, according to the description, in a metal halide lamp in which only a rare gas and metal iodides such as ScI
3
and NaI are enclosed in the arc tube and no mercury is contained, an operating voltage of 50 V or higher can be achieved by satisfying the equation,
P×L≧
40,
where the distance between electrodes in the lamp is L (mm), and in the case of the rare gas to be enclosed being Xe, the fill pressure of Xe at room temperature is P (atm).
In accordance with the above teachings, the present applicants prepared a lamp that has the same shape as the one illustrated in the aforementioned
FIG. 12
, with the major dimensions and the fill material being as follows, and the operating voltage of the lamp was measured using the lamp thus prepared.
Arc tube internal volume: 0.025 cc
Distance between electrodes: approx. 4 mm
The fill material 107 contained the following.
ScI
3
(scandium iodide): 0.04 mg
NaI (sodium iodide): 0.21 mg
(The total weight of ScI
3
and NaI is 0.25 mg)
Xe (xenon): 10 atm (at room temperature)
In this lamp, P×L becomes 40, and therefore this lamp satisfies the condition of the above-described lamp. However, when this lamp was operated with a lamp power of 35 W, the operating voltage resulted in 35 V, falling short of 50 V described in the publication. As a result, electrode sputtering was caused by the large lamp current, which led to blackening of the arc tube wall by the sputtered electrode material attached to the arc tube inner wall, and consequently the emitted luminous flux was reduced in an early stage. It is considered from the above result, that, in order to obtain an operating voltage of 50 V or higher, the minimum Xe pressure (10 atm) which satisfies the condition of P×L≧40 is insufficient, and according to the assumption made by the present inventors, it is necessary that the Xe pressure be controlled at a pressure of approximately 25 atm, which is far higher than 10 atm as set forth in the description.
However, controlling the fill pressure of Xe at such a high level incurs other drawbacks as described in the following.
Firstly, Xe shows a high ionization potenti

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