BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a double end type metal halide bulb or lamp with
low power consumption for use in automotive lighting, and more particularly to a composition
of the metal halide bulb or lamp capable of providing sufficient light emitting efficiency
in spite of a small input power of less than 35 W. The shape or diameter of the electrodes
are adjusted in order to keep a temperature within a discharge chamber in a rated
power range of 20-30 W, thereby obviating or preventing the evaporation of any amount
of the metal halide and also preventing the strength of the metal spectrum from decreasing.
Discussion of the Related Art
[0002] Metal halide bulbs or lamps are used in various fields including illumination in
sports facilities, because of their characteristics such as high color rendering property
and high efficiency. In recent years, energy saving is becoming more important, and
it is expected to further improve the efficiency of the metal halide lamps. Specifically,
low power consumption and size reduction are major subjects when developing new models.
[0003] One of the most popular usages of a metal halide bulb or lamp is in endoscopes. The
metal halide lamp in an endoscope operates with 21 W having arc length 1.2 mm, resulting
in high incidental efficiency to an optical fiber. In the automotive lighting industry,
35 W metal halide bulbs have started to prevail, and are used in some automobile models
in Europe and Japan. In Europe, standards for 35 W metal halide bulbs for use in automobiles
are on the way to be established, and in Japan discussions for establishment of standards
will start in the near future.
[0004] Fig. 4 illustrates the spectrum of the metal halide composition included in these
low wattage metal halide bulbs or lamps. The major element of the metal halide is
Scl
3-Nal. The composition having Scl
3-Nal as a major element of the metal halide composition enables the metal halide lamp
to provide high radiation efficiency in visible wavelengths and also high efficiency,
as compared with the metal halide composition of Na-Tl-ln, or Dy-Tl-(ln).
[0005] In recent years, a metal halide bulb or lamp having a Scl
3-Nal composition with an efficiency of 901 m/W has been developed as a result of the
study of the shape of a glass envelope, the structure of the sealed end of the glass
envelope, and the composition ratio of the metal halide.
[0006] This double end type metal halide lamp with low power consumption has a relatively
short arc length, is substantially a dot light source, and a large amount of light
is obtained. Specifically, the 35 W metal halide bulb for use in automobiles is required
to have instant lumen output, and a rare gas is sealed in said bulb by applying predetermined
pressure for enabling a very high, i. e. excessive current flow at start-up of the
metal halide lamp.
[0007] Fig. 5 illustrates such a conventional double end type metal halide lamp with low
power consumption which is optimally designed in terms of thermal capacity for obtaining
a sufficient temperature within a glass envelope in order to start and keep the evaporation
of metal halide therein. The dimensions of the glass envelope are as follows. The
thickness t of the glass envelope at a maximum external diameter portion 62 is more
than 1.5 mm (t > 1.5 mm); the diameter ⌀ a of an arc chamber 66 is more than 2.6 mm
(⌀a > 2.6 mm); in the arc chamber 66, a distance d between a tip portion 65 of an
electrode 69 and a wall 67 of the arc chamber 66 is equal to or more than around 1.0
mm (d ≥ 1.0 mm).
[0008] Fig. 6(a) illustrates an enlarged cross sectional view Sg of the first neck portion
63 and the second neck portion 64 of the glass envelope made of quartz glass. Fig.
6(b) illustrates an enlarged cross sectional view S of the maximum external diameter
portion 62 along a surface perpendicular to the longitudinal axis of the glass envelope.
The enlargement ratio between the Sg and S is around 0.26

.
[0009] The volume Vg of the glass envelope at the arc chamber portion 61 having a length
l between the first neck portion 63 and the second neck portion 64, the volume Vs
1 of the glass envelope at an adjacent portion extending the length l from the first
neck portion 63 toward a nearer end of the glass envelope, and the volume Vs
2 of the glass envelope extending the length l from the second neck portion 64 toward
the nearer end of the glass envelope, have the following relationship:

and Vg=95.8 mm
3.
[0010] The pair of electrodes 69 is substantially a cylinder, respectively, and its diameter
⌀ is equal to or more than 0.25 mm. The electrode tip portion 65 has substantially
the same diameter as the remaining portion of the electrode 69. The pair of electrodes
69, respectively, have electrical connections to a molybdenum foil, whose end has
a shape like a knife blade or a wedge, for obtaining predetermined air-tightness and
avoiding excessive stress concentration; said foil having the following dimensions:
thickness 20-28 µm, width 1.5-2.0 mm, and length 6-8 mm.
[0011] As described above, on designing of the 35 W automotive metal halide lamp, it is
sufficient to consider just the entire shape and end portion structure of the glass
envelope for obtaining sufficient temperature to start and keep the evaporation of
metal halide within the glass envelope, and it is not required to determine in detail
the entire shape or diameter of the electrode. However, when designing the Scl
3-Nal metal halide lamp with a power consumption of less than 35 W, it is required
to determine more specifically the electrode structure, because light color shifts
to blue due to low light emitting efficiency. As the input power is small, the evaporation
amount of metal halide is also small.
[0012] The conventional metal halide lamp with low power consumption has the following problems.
On designing a metal halide lamp with a power consumption less than 35 W, it is impossible
to achieve sufficient a temperature in an arc chamber 66 by downsizing the scale of
designing parameters of the parameters for 35 W bulbs. When each of the designing
parameters is just downsized, the evaporation amount of the metal halide is insufficient
such that the light emitting efficiency decreases and the light color shifts to blue.
The aforementioned metal halide lamp with an operating power 21 W has overcome the
light emitting efficiency problem to the extent that it can be used as an endoscope.
However, designing parameters are different for uses between endoscope and automobile.
Since the endoscope is not required to have instant lumen output property, the metal
halide lamp in 21 W has not yet overcome the standards of start-up properties for
use in automobiles.
SUMMARY OF THE INVENTION
[0013] The present invention is directed to an automobile headlight that substantially obviates
one or more of the above problems due to the limitations and disadvantages of the
related art.
[0014] It is an object of the invention to provide a double end type Scl
3-Nal metal halide lamp for use in automobile with rated power consumption smaller
than 35 W, more specifically 20-30 W, being capable of an instant lumen output.
[0015] The above object is achieved by sealing Xenon gas under high pressure and applying
excessive current at start-up of the metal halide lamp, and by providing an electrode
structure capable of enduring excessive current at start-up of the metal halide lamp.
The electrode structure is achieved by adjusting the designing parameters of electrode
such as the diameter or entire shape, for mitigating thermal emission from electrode
tip portions such that the temperature in the glass envelope is maintained. Thereby
low light-emitting efficiency due to low power input and light color shift to blue
are prevented.
[0016] The double end type Scl
3-Nal metal halide lamp with rated power consumption smaller than 35W, more specifically
20-30 W, of the present invention comprises a pair of electrodes whose diameter ⌀n
is equal to or less than 0.25 mm (⌀n ≤ 0.25 mm), and the diameter ⌀ P of the electrode
tip portion is equal to or larger than the diameter ⌀ S of the remaining electrode
portion (⌀P ≥ ⌀S). The electrode tip portion is spherical or cylindrical, and the
cross section area of the electrode increases as a cross section moves toward the
tip portion for mitigating thermal emission from the electrode tip portion and preventing
low light emission efficiency due to small input power. The arc chamber is substantially
a sphere, ellipsoid, or any similar shape, and comprises the pair of electrodes, mercury,
rare gas, and at least one kind of metal halide sealed therein. Since a rare gas,
more specifically Xenon gas, is sealed within an arc chamber under high pressure,
when excessive current, is applied instant lumen output is achieved.
[0017] Additional objects and advantages of the invention will be set forth in part in the
description which follows, and in part will be obvious from the description, or may
be learned by practice of the invention. The objects and advantages of the invention
will be realized and attained by means of the elements and combinations particularly
pointed out in the appended claims.
[0018] It is to be understood that both the foregoing general description and the following
detailed description are exemplary and explanatory and are intended to provide further
explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated in and which constitute a part
of this specification, illustrate several embodiments of the invention and together
with the description, serve to explain the principles of the invention.
Fig. 1 illustrates a front view and cross sectional views of a first preferred embodiment
of the present invention.
Fig. 2 (a) illustrates an electrode structure of the first preferred embodiment of
the present invention.
Fig. 2 (b) illustrates an electrode structure of the second preferred embodiment of
the present invention.
Fig. 2 (c) illustrates an electrode structure of the third preferred embodiment of
the present invention.
Fig. 2 (d) illustrates an electrode structure of the fourth preferred embodiment of
the present invention.
Fig. 3 is a graph showing light emitting efficiency as a function of power in comparison
between the double end type metal halide lamp with low power consumption of the present
invention and a conventional one.
Fig. 4 illustrates spectrum distribution of a Scl3-Nal metal halide lamp.
Fig. 5 illustrates a front view of a conventional double end type metal halide lamp
with low power consumption.
Fig. 6 (a) illustrates an enlarged cross sectional view of the first and second neck
portions of the glass envelope along the A-A line in Fig. 5.
Fig. 6 (b) illustrates an enlarged cross sectional view along the B-B line in Fig.
5 which is perpendicular to the longitudinal axis of the glass envelope and passes
through the glass envelope at a portion having the maximum external diameter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] Reference will now be made in detail to the preferred embodiments of the present
invention. Whenever possible, the same references numbers will be used throughout
the drawings to refer to the same or like parts.
[0021] Fig. 1 illustrates a front view and cross sectional views of the first preferred
embodiment of the present invention. The double end type metal halide bulb or lamp
with low power consumption being operated with a power of less than 35 W comprises
a glass envelope 100, an arc chamber 6, an electrode structure comprising molybdenum
foils 21 and a pair of electrodes 20. The glass envelope 100 comprises an arc chamber
portion 1 surrounding the arc chamber 6 and at least one sealed portion 8 adjacent
to the arc chamber portion 1. The length q of the sealed portion is shown in Fig.
1. The entire shape and end structure of the glass envelope is adjusted as described
in Japanese patent application No. HEI 10-195647. Detailed description about how to
determine dimensions of the glass envelope will be provided later.
[0022] Figs. 2 (a)-(d) illustrate electrode structures of the first to fourth preferred
embodiments of the present invention. The electrode material of the present invention
is tungsten (including W-1.7% ThO
2). The first electrode structure in Fig .2(a) comprises a molybdenum foil 21 and a
first or second electrode 20 which is electrically connected to the molybdenum foil
21 and is projected within arc chamber 6. A diameter ⌀ n of the first or second electrode
20 is equal to or less than 0.25 mm (⌀n ≤ 0.25), thereby mitigating thermal emission
from the electrode 20 and maintaining the temperature in the glass envelope. Tip portions
23 of the electrodes 20 have substantially the same diameter as the remaining portions
22 of the electrodes 20.
[0023] The second electrode structure in Fig. 2(b) comprises a molybdenum foil 21 and a
first or second electrode 20 which is electrically connected to the molybdenum foil
21 and is projected within an arc chamber 6. The electrode 20 comprises a spherical
electrode tip portion 23 and remaining electrode portion 22. A diameter ⌀ S of the
remaining electrode portion 22 is equal to or smaller than the diameter ⌀ P of the
spherical electrode tip portion 23, and the diameter ⌀ P of the spherical electrode
tip portion 23 is equal to or less than 0.25 mm (⌀S ≤ ⌀P ≤ 0.25 mm). Since the cross
section area of the spherical electrode tip portion 23 is enlarged enabling the mitigation
of thermal emission from the electrode tip portion 23 to the extent of maintaining
evaporation of metal halide, sufficient light-emitting efficiency is achieved in spite
of small input power.
[0024] The third electrode structure in Fig. 2(c) comprises a molybdenum foil 21 and a first
or second electrode 20 which is electrically connected to the molybdenum foil 21 and
is projected within an arc chamber 6. In this embodiment, the shape of the electrode
tip portion 23 is a cylinder. The electrode 20 comprises said cylindrical electrode
tip portion 23 and remaining electrode portion 22. A diameter ⌀ Y of the remaining
electrode portion 22 is equal to or smaller than the diameter ⌀ X of the cylindrical
electrode tip portion 23, and the diameter ⌀ X of the cylindrical electrode tip portion
23 is equal to or smaller than 0.25 mm (⌀Y ≤ ⌀X ≤ 0.25 mm). The cross section area
of the cylindrical electrode tip portion 23 is further enlarged as compared with the
second preferred embodiment. The third electrode structure is also able to mitigate
thermal emission from the electrode 20 such that evaporation of metal halide is maintained.
Accordingly, it is prevented to decrease light-emitting efficiency even though input
power decreases.
[0025] The fourth electrode structure in Fig. 2(d) comprises a molybdenum foil 21 and a
first or second electrode 20 which is electrically connected to the molybdenum foil
21 and is projected within an arc chamber 6. In this embodiment, the cross section
area of the electrode 20 increases as a cross section moves toward the projecting
end of the electrode 20. A diameter ⌀ K of the projecting end of the electrode 20
is equal to or smaller than 0.25 mm (⌀K ≤ 0.25 mm). The fourth electrode structure
is also able to mitigate thermal emission from the electrode 20, and it is prevented
from decreasing the light-emitting efficiency even though input power decreases.
[0026] In all the preferred embodiments described above, the entire shape and end structure
of the glass envelope is adjusted as described in the Japanese patent Application
No. 10-195647. Such preferred dimensions of the glass envelope are briefly explained
as follows based on the metal halide lamp in Fig. 1.
[0027] The thickness t at a portion 2 having the maximum external diameter of the glass
envelope is equal to or smaller than 1.5 mm (t ≤ 1.5 mm). The maximum internal diameter
of the arc chamber ⌀ a is equal to or smaller than 2.6 mm (⌀a ≤ 2.6 mm). The thickness
t and the maximum internal diameter ⌀ a are determined for the purpose of maintaining
the temperature within the glass envelope for achieving sufficient light emitting
efficiency in spite of small input power.
[0028] The distance d between an electrode tip portion 5 and a wall 7 of the arc chamber
6 is 0.6-1.3 mm (0.6 mm ≤ d ≤ 1.3 mm). The maximum value 1.3 mm is determined for
the purpose of maintaining temperature in the glass envelope and achieving sufficient
light emitting efficiency in spite of small input power. The minimum value 0.6 mm
is determined for preventing occurrence of a non-stabilized arc which may cause a
sudden turn-off of the metal halide lamp.
[0029] The ratio of the cross section area is adjusted as follows: the cross section area
Sg of the first neck portion 3 or the second neck portion 4 is equal to or less than
the cross section area S which is perpendicular to the longitudinal axis of the glass
envelope and passes the maximum external diameter portion 2

; preferable Sg/S values are within a range from equal to or more than 0.15 up to
equal to or less than 0.25

, the optimized Sg/S value is around 0.2

. These values are determined such that thermal emission from the arc chamber portion
1 of the glass envelope 100 is mitigated and the temperature in arc chamber portion
1 is maintained, thereby it is able to achieve sufficient light emitting efficiency
in spite of small input power.
[0030] The volumes of the glass envelope 100 at the arc chamber portion 1 surrounding the
arc chamber 6, and sealed portions 8 are determined as follows. The volume Vg of the
arc chamber portion 1 having a length l between the first neck portion 3 and the second
neck portion 4, the volume Vs
1 of the sealed portion 8 extending the length l from the first neck portion 3 toward
a nearer end of the glass envelope 100, and the volume Vs
2 of the sealed portion 8 extending the length l from the second neck portion 4 toward
a nearer end of the glass envelope 100, have the following relationship: 0.4Vg<Vs
1, and Vs
2<0.9Vg. These values are determined such that thermal emission from the arc chamber
portion 1 of the glass envelope is mitigated and the temperature within the arc chamber
portion 1 is maintained, thereby sufficient light emitting efficiency is achieved
in spite of small input power. The value of 0.4Vg is determined for preventing excessive
thermal emission from the arc chamber portion 1 of the glass envelope.
[0031] The above-identified double end type metal halide lamp with low power consumption
comprises a spherical or elliptic arc chamber 6 which includes a pair of electrodes
20, mercury, rare gas, and at least one kind of metal halide. The metal halides Scl
3 and Nal are sealed within the arc chamber 6 with a ratio from 3:1 to 1:8 (Scl
3:Nal = 3:1-1:8) in weight. The rare gas, Xenon gas, is also sealed in the arc chamber
6 under high pressure enabling an excessive current flow at the start-up of the bulb
or lamp. The excessive current flow is required for instant lumen output which is
essential for use in automobiles.
[0032] As described in the specification of the Japanese Patent Application No. 10-195647,
the adjustments of the glass envelope described above are sufficiently effective when
each adjustment is individually made. However, combination of each adjustment makes
it more effective.
[0033] The conventional 35 W metal halide lamp is able to provide high temperatures to the
extent of enabling sufficient evaporation of the metal halide. However, on designing
a metal halide lamp with a smaller input power consumption than 35 W, it is essential
to adjust e. g. the entire shape or diameter of the electrode structure because, without
such adjustments, light emitting efficiency decreases when the input power is smaller
than 35 W, and the light color may shift to blue.
[0034] By adjusting not only the structure of the glass envelope, such as the entire shape,
or the shape of the end portions, but also the electrode structure, thermal emission
from the electrode tip portion is mitigated, and the temperature of the glass envelope
is maintained. Thereby sufficient light emitting efficiency is achieved in spite of
small input power. This adjustment is highly effective for metal halide lamps whose
power consumption is smaller than 35 W, specifically 20-30 W.
[0035] Fig. 3 is a graph showing light-emitting efficiency of the metal halide lamp with
low power consumption as a comparison between the preferred embodiment of the present
invention in a solid line and a conventional 35 W metal halide lamp in a broken line.
In the preferred embodiment, an electrode 20 has a diameter smaller than 0.25 mm,
and the electrode tip portion 23 has larger diameter than the remaining electrode
portions 22. Additionally, the arc chamber portion 1 and sealed end of the arc chamber
portion 1 are adjusted as described in the Japanese patent application No. 10-195647.
As shown in this graph, the preferred embodiment of the present invention achieved
light emitting efficiency of 60-90 lm/W in spite of small input power. The operational
advantages of the metal halide lamp according to the preferred embodiment of the present
invention will now be described. First, even though the power consumption of the metal
halide lamp is less than 35 W, the arc chamber portion is maintained in a sufficiently
high temperature for metal halide evaporation. Therefore, although the power consumption
decreases, light emitting efficiency does not decrease very much and light color shift
to blue is prevented. Second, instant lumen output is achieved in spite of a small
input power of less than 35 W, specifically in the range of 20-30 W. This is achieved
by adjusting the electrode shape and structure, by sealing Scl
3 and Nal in the arc chamber, and by applying an excessive current at start-up of the
lamp. The combination of these parameters is able to provide more effective metal
halide lamps.
[0036] It will be apparent to those skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope thereof. Thus, it
is intended that the present invention cover the modifications and variations of this
invention.
1. A double end type metal halide bulb or lamp being operated at a rated power less than
35 W comprising a glass envelope having an arc chamber (6), at least one kind of metal
halide included in the arc chamber (6), an arc chamber portion (1) surrounding the
arc chamber (6), and sealed portions (8) adjacent to the arc chamber portion (1),
a first electrode (20) partly projecting into the arc chamber (6), a second electrode
(20) partly projecting into the arc chamber (6), characterized in that a diameter
⌀ n of the first and second electrodes (20) is equal to or smaller than 0.25 mm.
2. A double end type metal halide lamp according to claim 1,
characterized in that a diameter ⌀ P of the electrode tip portion (5, 23) projecting
into the arc chamber (6) is equal to or larger than a diameter ⌀ S of the remaining
electrode portion (22).
3. A double end type metal halide lamp according to claim 1,
characterized in that the electrode tip portion (5, 23) projecting into the arc chamber
(6) is spherical.
4. A double end type metal halide lamp according to claim 1,
characterized in that the electrode tip portion (5, 23) projecting into the arc chamber
(6) is a cylinder.
5. A double end type metal halide lamp according to claim 1,
characterized in that the cross section areas of the first and second electrodes (20)
increase as the cross section moves toward a projecting end (5, 23) of the electrode
(20).
6. A double end type metal halide lamp according to claim 1,
characterized in that the metal halide included in the arc chamber (6) comprises Scl3 and Nal.
7. A double end type metal halide lamp according to claim 1,
characterized in that a rated power range is 20-30 W.
8. A double end type metal halide lamp according to claim 6 and 7,
characterized in that instant lumen output is achieved by sealing rare gas at high
pressure and applying excessive current at start-up of the lamp.