BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a discharge lamp and, in particular, a discharge
lamp in which metal halide is added into mercury vapor for improving color rendering
property.
2. Background Art
[0002] FIG. 3 shows a construction of a metal halide discharge lamp of prior art. At the
time of feeding from the exterior to a discharging electrode 92 disposed in a discharge
room 91a of a bulb 91, in order to maintain the gastightness of the discharge room
91a, a metal foil leading-in wire 93 made of molybdenum etc. is held and sealed by
thermally deforming the bulb 91.
[0003] The use of the metallic foil leading-in wire permits an increase of surface area
as a conductive material and a reduction in pressure of the discharge room 91a per
unit area. However, since the bulb in the softened state is subjected to pressure
welding with the metal foil leading-in wire, both are merely in contact with a flat
surface as shown in FIG. 4. By thermal stresses of temperature rise and fall due to
the repetitive lighting and lighting-off, a contact between the two becomes gradually
loose, leading to leaks.
[0004] Japanese Patent Publication No. Hei. 5-3703 describes a construction where sealing
is performed by deposing a conductive material of round bar on a bulb by using a frit.
However, the use of a round bar shaped conductive material displays a disadvantage
that the ratio of the surface area to the volume, i.e., the ratio of the periphery
to the sectional area, becomes small. Since the sectional area requires a specified
value according to the discharge current value, if the periphery length is small for
the same sectional area, the pressure of a discharge room applied per unit length
is increased. Accordingly, for the whole conductive material, the pressure of the
discharge room per unit area is increased, which is disadvantageous in preventing
leaks.
[0005] As described, although a frit is interposed for increasing the adhesive strength
between a bulb and a conductive material, the frit is melted at the time of sealing
to deposit on the bulb and the conductive material. Therefore, at this melting, impurities
contained in the frit or the composition of the frit evaporate and then enter the
discharge room to damage discharge lamp properties.
SUMMARY OF THE INVENTION
[0006] The above mentioned problems are solved according to the present invention by a metal
halide discharge lamp comprising a metal foil leading-in wire held and sealed by a
thermally deformed glass bulb, characterized in that at least one of surfaces of the
metal foil leading-in wire are furnished with a satin finish.
[0007] The metal halide discharge lamp including the metallic foil leading-in wire with
the satin finished both surfaces that is held and sealed by a bulb is free from the
problem of prior art that the lift time of halide discharge lamps is reduced depending
upon the number of lighting and lighting-off. It is therefore possible to apply to
cases where flickering is made frequently, e.g., head lamps of automobiles, to expand
the use of metal halide discharge lamps of this type, and to improve the reliability.
[0008] In addition, the satin finish by a sand blasting finish using corundum abrasive grains
or an electric field deposition using a melted metallic salt, prevents matters which
will be harmful by heating for sealing with a bulb from entering the discharge room.
This avoids an additional step, e.g., elimination of remaining abrasive grains, and
facilitates to practice the discharge lamp of the present invention, without causing
no cost rise.
[0009] Other features and advantages of the present invention will be apparent from the
following description taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a sectional view of a metal halide discharge lamp according to a preferred
embodiment of the present invention.
[0011] FIG. 2 is a sectional view along line A-A of FIG. 1.
[0012] FIG. 3 is a sectional view of a metal halide discharge lamp of prior art.
[0013] FIG. 4 is a sectional view along line B-B of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] With reference to FIG. 1, reference numeral 1 indicates a metal halide discharge
lamp according to one embodiment of the present invention. In the metal halide discharge
lamp 1, a metal foil leading-in wire 4 made of molybdenum is employed as a conductive
material so as to produce a greater surface area for sectional area, in order that
the gastightness of a discharge room 2a is maintained when performing a feeding from
the exterior to a discharging electrode 3 disposed in the discharge room 2a of a bulb
2.
[0015] Detailed description of the metal foil leading-in wire 4 is given herebelow. Metal
materials as a material of the metal foil leading-in wire 4 are supplied in the market
after they are formed, e.g., by means of rolling, into the form of foil (plate) by
manufacturers so as to have a specified thickness, and further treated to have a fairly
smooth surface, i.e., flaw-free, for increasing their commercial value.
[0016] Therefore, if the metal foil leading-in wire 4 is formed by such a commercially available
metal material, it is impossible to obtain a peripheral length greater than that defined
by its sectional shape. Hence, the present invention is directed to obtain a peripheral
length greater than that defined by the sectional shape of a metal foil leading-in
wire 4 by providing a satin finish 4a on at least one of the surfaces of the aforesaid
metal material to produce fine irregularities thereon, as shown in FIG. 2.
[0017] Detailed description of the satin finish 4a is given herebelow. As a first means,
there is a manner executed by mechanical means, such as sand blasting, in which abrasive
grains that are fine powders of a material having an appropriate hardness are sprayed
and collided on the surface of a metal foil leading-in wire 4 or of a metal material
that is a material to form the metal foil leading-in wire 4.
[0018] As abrasive grains of this type, silicon carbide (SiC), hereinafter referred to as
carborundum, is generally employed because of its outstanding work efficiency in addition
to its hardness. However, from the test and examination conducted by the present inventors,
it has been found out that the abrasive grains of silicon carbide have the following
disadvantages.
[0019] Specifically, since the abrasive grains of silicon carbide have high hardness, they
are strongly cut into the metal foil leading-in wire 4. It is therefore very difficult
to eliminate the remaining abrasive grains on the surface after a sand blasting. The
remaining abrasive grains sublime at the step of sealing, adhere to a discharging
electrode 3 and disappear, which will then make the surface black.
[0020] On the other hand, alumina (Al
2O
3), hereinafter referred to as corundum, is not decomposed at temperatures required
in a sealing step, i.e., about 2000°C, thus causing no influence on the life time
of discharge lamps. Therefore, the present invention employs alumina as an abrasive
grain. This avoids a step of eliminating such remaining abrasive grains as described
above to reduce the number of steps as a whole.
[0021] As a second means for forming a satin finish 4a, there is chemical means, such as
an electric field deposition using a melted metallic salt. The following description
is made taking an example of molybdenum but not limited to this alone. Other metals
or metal mixtures are useable for the metal foil leading-in wire 4.
[0022] Generally, it is impossible to execute an electric field deposition from an aqueous
solution with a metal, e.g., molybdenum, whose ionization tendency is greater than
that of hydrogen (H). Because the melting point of molybdenum is 2893 K, an electric
field deposition in the solution state requires high equipment and technical levels,
making it further difficult to apply it into practice.
[0023] The use of molybdenum chloride (MoCl
5) having a low melting point permits an easy practical application. Specifically,
molybdenum chloride is melted in a crucible under an argon atmosphere and then subjected
to an electric field deposition by using molybdenum as an electrode.
[0024] In this occasion, the molybdenum is deposited in a manner of (Mo
5++5e
-→Mo) on a cathode side, and therefore, by forming a metal foil lead-in wire 4 with
this deposited molybdenum as a material, it is possible to obtain the metal foil leading-in
wire 4 that has an increased surface area due to fine irregularities on the surface,
i.e., a satin finish 4a, as in the case of a sand blasting.
[0025] At the same time, the molybdenum electrode on an anode side is in the state of (Mo→Mo
5++5e
-), i.e., a state where electrical corrosion has been caused, thereby producing fine
irregularities in the same manner. It is therefore possible to use as a material of
a metal foil leading-in wire 4 that has a great surface area as in that of the cathode.
It should be noted that no abrasive grain is required in this manner and thus requires
no step of eliminating remaining abrasive grains.
[0026] The test and examination for the satin finish 4a conducted by the present inventors
show that to improve the adhesive strength with a bulb 2, it is effective to set the
surface roughness in a proper range. Test samples of the satin finish 4a having a
different surface roughness were prepared and their surface roughnesses were measured
according to JIS B0601(1982) standards, in order to obtain an optimum range of surface
roughness.
[0027] In the following description, arithmetical mean deviation of profile is referred
to as "Ra", and ten point height of irregularities is referred to as "Rz". For fifty
samples of satin finish 4a having a surface roughness of Ra=0.2 µm and Rz=1.1 µm,
60% of the samples peeled off a bulb 2. This shows that the surface is too smooth
to improve the adhesive strength. The length measured at that time was 0.8 mm.
[0028] Next, with Ra<0.25µm and 1.1µm<Rz, samples of satin finish 4a were prepared to conduct
the test under the same conditions. The result was 0%, i.e., no sample peeled off
a bulb 2, which satisfied the above object. The mean interval (Sm) of the counted
irregularities not less than ±1 µm in mean line was Sm≤40µm.
[0029] It is extremely difficult to obtain Ra>0.5µm, Rz>4.5µm by means of sand blasting
or electric field deposition as previously described. But even if failed to reach
these values, it is possible to determine whether a satisfactory effect for the object
is obtained or not. Thus, in the present invention, the arithmetical means deviation
of profile is defined as 0.25≤Ra≤0.8(µm) and the ten point height of irregularities
is defined as 1.1<Rz≤4.5(µm).
[0030] Operation and effect of the metal halide discharge lamp 1 with the above mentioned
construction is described herebelow. The increase of surface area of the metal foil
leading-in wire 4 by the presence of the satin finish 4a enables to extend the life
time of the discharge lamp 1 by suppressing that a contact with the bulb 2 is loosen
by thermal stresses due to repetitive lighting and lighting-out to allow the outside
air to enter the discharge room 2a.
[0031] According to the test results, when a conventional molybdenum foil with a smooth
surface is employed as a metal foil leading-in wire 4, on the average 2,000 repetitions
of lighting and lighting-off caused peeling between a bulb 2 and the metal foil leading-in
wire 4 to cause leaks. On the other hand, when used a metal foil leading-in wire 4
of the present invention, on the average 12,000 repetition of lighting and lighting-off
caused a gradual leak. This shows about six times improvement.
[0032] In this manner, the metal halide discharge lamps of the present invention that suppress
a reduction in life time due to repetitive lighting and lighting-off, are extremely
useful because in recent years, metal halide discharge lamps have been finding their
ways into uses where lighting and lighting-off are more frequent than that of conventional
lamps for lighting, e.g., head lamps for automobiles, light sources for projection
type television receiving sets.
[0033] While there has been described what is at present considered to be a preferred embodiment
of the invention, it will be understood that various modifications may be made thereto,
and it is intended that the appended claims cover all such modifications as fall within
the true spirit and scope of the invention.
1. A metal halide discharge lamp comprising a metal foil leading-in wire held and sealed
by a thermally deformed glass bulb, characterized in that at least one of surfaces
of the metallic foil leading-in wire is furnished with a satin finish.
2. The metal halide discharge lamp as set forth in claim 1, wherein both surfaces of
the metallic foil leading-in wire are furnished with a satin finish.
3. The metal halide discharge lamp as set forth in claim 1, wherein the satin finish
is performed by a sand blasting finish using corundum abrasive grains.
4. The metal halide discharge lamp as set forth in claim 1, wherein the satin finish
is performed by an electric field deposition using a melted metallic salt.
5. The metal halide discharge lamp as set forth in claim 2, wherein the satin finish
is performed by a sand blasting finish using corundum abrasive grains.
6. The metal halide discharge lamp as set forth in claim 2, wherein the satin finish
is performed by an electric field deposition using a melted metallic salt.
7. The metal halide discharge lamp as set forth in claim 1, wherein a surface roughness
of the satin finish is defined as 0.25≤Ra≤0.8 (µm), 1.1<Rz≤4.5 (µm), and Sm≤40 (µm)
according to JIS B0601 (1982), where Ra represents arithmetical mean deviation of
profile, Rz represents ten point height of irregularities, and Sm represents mean
interval of irregularities.
8. The metal halide discharge lamp as set forth in claim 3, wherein a surface roughness
of the satin finish is defined as 0.25≤Ra≤0.8 (µm), 1.1<Rz≤4.5 (µm), and Sm≤40 (µm)
according to JIS B0601 (1982), where Ra represents arithmetical mean deviation of
profile, Rz represents ten point height of irregularities, and Sm represents mean
interval of irregularities.
9. The metal halide discharge lamp as set forth in claim 4, wherein a surface roughness
of the satin finish is defined as 0.25≤Ra≤0.8 (µm), 1.1<Rz≤4.5 (µm), and Sm≤40 (µm)
according to JIS B0601 (1982), where Ra represents arithmetical mean deviation of
profile, Rz represents ten point height of irregularities, and Sm represents mean
interval of irregularities.
10. The metal halide discharge lamp as set forth in one of the claims 1 to 9, wherein
the metal foil leading-in wire is a molybdenum foil.