The present invention relates to a discharge lamp for a headlight, more particularly to the shapes of light-intercepting films included in a lamp.
When a discharge lamp is used as a light source for a vehicle headlight or for a liquid crystal projector, it is combined with a reflecting mirror. In recent years, to prevent a reflecting mirror from being deteriorated by ultraviolet rays radiated from a discharge lamp, a discharge lamp including ultraviolet-cutting glass as an outer tube has been used widely.
In general, to realize a proper light distribution by combining a reflecting mirror and a discharge lamp, it is required to control the position of a luminous portion, namely, an arc, with respect to the reflecting mirror with extremely high precision.
However, because the light distribution of an arc, which is a luminous portion of a discharge lamp, is influenced by such factors as the shape of an arc tube, internal pressure, lamp voltage, tube current, etc., it is difficult to control it mechanically in the same way as a filament used in a bulb, etc. Thus, it has been proposed to obtain an accurate light distribution by intercepting optically a part of the arc, which is difficult to control for its position, by forming a light-intercepting film on an outer tube.
JP-9-500489A proposes a discharge lamp including an arc tube enclosed by an outer tube. In the discharge lamp, external lead wires extend from respective electrodes to respective contact points to a base. A neck portion of the arc tube is fixed into the base, and a power supply line for one external lead wire extends along the external surface of the outer tube.
On the outer tube, a light-intercepting film extends on the side close to the base, from the position making an angle xcex1 of 50 degrees with a line perpendicular to the outer tube at the center region between the electrodes, to the position making an angle xcex2 of 65 degrees with the same line. Furthermore, on the outer tube, two band-shaped light-intercepting films extend in parallel with the outer tube, and the two band-shaped light-intercepting films face apart from each other. The two band-shaped light-intercepting films have respective edges making an angle xcex3 of 165 degrees with respect to the circumference of the outer tube, and respective edges facing with each other and making an angle xcex4 of between 85 degrees and 145 degrees.
FIG. 11A illustrates a configuration of a headlight using the conventional discharge lamp. A discharge lamp 40 includes an arc tube 42 enclosed by an outer tube 41, and is arranged within a mirror 45 having a front glass 44 mounted at its opening. A light-intercepting film 46a extends on the side close to a base 43 of the outer tube 41, and two band-shaped light-intercepting films 46b (only a film on one side is shown) extend in parallel with the axial direction of the outer tube 41.
As shown by the arrows xe2x80x9caxe2x80x9d, light radiated from the arc tube 42 is reflected by the mirror 45, and passes through the front glass 44 to illuminate forward. FIG. 11B shows a light distribution pattern. A region 48 is a region illuminated by light passing through the front glass 44.
A region 50 indicated by a dot pattern shows a region where light passing through the front glass 44 does not reach. The boundary between the regions 48 and 50 is a cutline 49. To form such a light distribution pattern, unwanted light is cut from light radiated from,the arc tube 42 by the light-intercepting films 46a and 46b and sections 47a, 47b of a light-intercepting plate 47.
Viewing the light distribution pattern in the up-and-down direction, the illuminated region indicated by 48a in FIG. 11B is wider than the illuminated region indicated by 48b. When a discharge lamp forming such a light distribution pattern is used for a vehicle, both sides of driving lane and opposing lane can be illuminated. For example, in the case of left-hand traffic, because the region 48b on the side of opposing lane is cut for its upper illuminated region compared to the region 48a on the side of driving lane, blinding of oncoming vehicles can be prevented.
In such a conventional discharge lamp, although generation of glare resulting from unwanted light in the direction to pass through these light-intercepting films can be inhibited by the light-intercepting film 46a on the side close to the base 43 and the two band-shaped light-intercepting films 46b, it has not been able to avoid glare resulting from unwanted light passing through a portion where no light-intercepting film is formed, particularly a portion far from the base 43.
For example, unwanted light (arrows xe2x80x9cbxe2x80x9d and xe2x80x9ccxe2x80x9d) radiated from the arc tube 42 and reflected by the end on the side far from the base 43 (particularly, at the corners of the end of the outer tube 41) is reflected by the mirror 45, and passes through the front glass 44 to illuminate forward. Such light is unwanted to form the light distribution pattern as shown in FIG. 11B, and it will illuminate the region 50 that does not need to be illuminated, or will illuminate the region 48 that has been illuminated by the necessary light indicated by the arrows xe2x80x9caxe2x80x9d over again. Thus, unevenness is generated in the intensity distribution of the light distribution pattern, resulting in the generation of glare.
To prevent such generation of glare, it is necessary to intercept unwanted light trying to enter the front glass 44 by providing a light-intercepting plate separately. Thus, increased structural complexity and increased weight of a lighting fixture have not been able to be avoided.
The present invention solves the above-mentioned conventional problem. It is an object of the present invention to provide a discharge lamp further including a light-intercepting film extending in a portion far from the base, so that glare can be reduced, and a lighting fixture can be simplified and reduced in weight.
In order to accomplish the above object, the present invention provides a first discharge lamp including: an arc tube having a discharge space in which a pair of electrodes having tips facing with each other are arranged; an outer tube enclosing the arc tube; a base that fixes one side of the outer tube; and two band-shaped light-intercepting films extending on a surface of the outer tube in the direction of the axis of the arc tube and in parallel with each other, the two band-shaped light-intercepting films having at least a portion overlapping both ends of the discharge space when viewing the outer tube in the direction perpendicular to the axis of the arc tube, wherein:
when the tip within the discharge space of the electrode on the side of the top of the outer tube between the pair of the electrodes is determined as a basis position, in at least one of the two band-shaped light-intercepting films, an extending portion is formed that is on the side of the top of the outer tube with respect to the basis position and extends in the circumferential direction of the outer tube. According to such a discharge lamp, reflected light from the end of the outer tube on the side far from the base can be cut, and unwanted light can be cut more reliably. Thus, unevenness in the intensity distribution of the light distribution pattern can be reduced, and generation of glare can be prevented. Furthermore, because glare can be reduced by the lamp itself, it is not necessary to intercept unwanted light by providing a light-intercepting plate separately, and it is not necessary to add a further mechanism for reducing glare such as a light-intercepting plate, so that a lighting fixture can be simplified and reduced in weight.
It is preferable that a power supply line further is arranged so as to face the side face of the outer tube, and the extending portion is formed so as to extend toward the side opposite to the side on which the power supply line and the outer tube face each other.
It is preferable that when under the condition in which the outer tube is cut in a plane including the center point between the tips of the pair of the electrodes in the direction perpendicular to the axis of the arc tube, the angle, taken with respect to the side opposite the extending portion, between the two lines connecting the center point and respective edges of the two light-intercepting films in the direction of the axis of the arc tube on the side of the extending portion is determined as an angle xcex3, and
under the condition in which the outer tube is cut in the direction perpendicular to the axis of the arc tube in a portion where the extending portion is formed, the angle, taken with respect to the side opposite the extending portion between the two lines connecting the point on the axis of the arc tube and respective edges of the two light-intercepting films in the direction of the axis of the arc tube on the side of the extending portion is determined as an angle xcex5, the maximum value of the angle xcex5 is at least (xcex3+10) degrees.
It is preferable that respective edges on one side of the two band-shaped light-intercepting films are connected through the extending portion.
It is preferable that the light-intercepting films are formed by uniting a heat-resistant light-intercepting sheet material with the surface of the outer tube. According to such a discharge lamp, the precision of the location of the light-intercepting films can be enhanced.
It is preferable that the heat-resistant light-intercepting sheet material is a greensheet containing an inorganic material and an inorganic matrix component, and the greensheet is united with the surface of the outer tube by calcining the greensheet adhered on the surface of the outer tube.
Next, the present invention provides a second discharge lamp including: an arc tube having a discharge space in which a pair of electrodes having tips facing with each other are arranged; an outer tube enclosing the arc tube; a base that fixes one side of the outer tube; and first two band-shaped light-intercepting films extending on a surface of the outer tube in the direction of the axis of the arc tube and in parallel with each other, the first light-intercepting films having at least a portion overlapping both ends of the discharge space when viewing the outer tube in the direction perpendicular to the axis of the arc tube,
further including a second light-intercepting film covering the outer tube in the circumferential direction, wherein when the tip within the discharge space of the electrode on the side of the top of the outer tube between the pair of the electrodes is determined as a basis position, both edges of the second light-intercepting film in the direction of the axis of the arc tube are located on the side of the top of the outer tube with respect to the basis position. According to such a discharge lamp, reflected light from the end of the outer tube on the side far from the base can be cut, and unwanted light can be cut more reliably. Thus, unevenness in the intensity distribution of the light distribution pattern can be reduced, and generation of glare can be prevented. Furthermore, because glare are can be reduced by the lamp itself, it is not necessary to intercept unwanted light by providing a light-intercepting plate separately, and it is not necessary to add a further mechanism for reducing glare such as a light-intercepting plate, so that a lighting fixture can be simplified and reduced in weight.
In the second discharge lamp, it is preferable that the angle made by the line passing through the center point between the tips of the pair of the electrodes and perpendicular to the axis of the arc tube and the line connecting the center point and the edge of the second light-intercepting film closer to the center point is at least 40 degrees.
In the second discharge lamp, it is preferable that the angle made by the line perpendicular to the axis of the arc tube and the line connecting the center point and the edge of the second light-intercepting film farther from the center point is at least 70 degrees.
It is preferable that the second light-intercepting film covers the outer tube in a ring form.
It is preferable that the edge of the second light-intercepting film farther from the center point between the tips of the pair of the electrodes is located at the top of the outer tube, and the second light-intercepting film covers the outer tube in the circumferential direction and covers the top of the outer tube. According to such a discharge lamp, light trying to pass through the top or the vicinity of the top of the outer tube can be cut. Thus, it is not necessary to set a cap for intercepting light at an end of the discharge lamp or to provide a light-intercepting plate separately in front of the discharge lamp so as to cut such light.
It is preferable that the light-intercepting films are formed by uniting a heat-resistant light-intercepting sheet material with the surface of the outer tube. According to such a discharge lamp, precision of the location of the light-intercepting films can be enhanced.
It is preferable that the heat-resistant light-intercepting sheet material is a greensheet containing an inorganic material and an inorganic matrix component, and the greensheet is united with the surface of the outer tube by calcining the greensheet adhered on the surface of the outer tube.