1. Field of the Invention
The present invention relates to a projector-type vehicle headlamp. More specifically, the present invention relates to a vehicle headlamp which is configured so as to illuminate an overhead sign.
2. Description of the Related Art
Generally, a projector-type vehicle headlamp has a light source that is placed on an optical axis extending in a front and rear direction of a vehicle, a reflector that reflects light from the light source in a forward direction and toward the optical axis, and a projection lens that is disposed in front of the reflector and through which the light reflected from the reflector is irradiated in a forward direction of the headlamp.
When such a projector-type vehicle headlamp is configured so as to provide a low-beam irradiation, a shade is disposed between the projection lens and the reflector. The shade shields part of the light reflected from the reflector so as to eliminate upward irradiation light, thereby providing a beam irradiation with a low-beam light distribution pattern having a predetermined cut-off line.
In such a projector-type vehicle headlamp, upward irradiation light is almost completely eliminated by the shade, and therefore, an overhead sign disposed above a road surface in front of the vehicle cannot be clearly seen.
In order to ensure light irradiation that illuminates an overhead sign, a rear-face frosted lens has been proposed to be employed as a projection lens. The rear-face frosted lens is a lens having a rear side surface on which a frosting process is performed.
However, the rear-face frosted lens is expensive, thereby leading to a high cost of the vehicle headlamp.
Therefore, there has been proposed a projector-type vehicle headlamp in which irradiation light for illuminating an overhead sign can be obtained at the time of low-beam irradiation without using the expensive rear-face frosted lens (see, e.g., JP-A-2003-297117).
In a vehicle headlamp 10 shown in FIG. 11, a projector-type lamp unit 11 is disposed in a lamp chamber formed by a lamp body and a cover which are not shown.
The lamp unit 11 includes: a projection lens 18 which is placed on an optical axis Ax extending in a front and rear direction of a vehicle; a light source 12a which is placed behind a rear focal point F2 of the projection lens 18; a reflector 14 which reflects light emanated directly from the light source 12a in the forward direction and toward the optical axis Ax; a shade 20 which is disposed between the light source 12a and the projection lens 18; and a light shielding plate 22 which is integrated with the shade 20. The shade 20 shields part of the light reflected from the reflector 14 so as to eliminate upward irradiation light
The light source 12a is a light emitting part (filament) of a light-source bulb 12. The light-source bulb 12 is a so-called H7 halogen bulb, and is attached to the reflector 14 so as to be coaxial with the optical axis Ax extending in the front and rear direction of the vehicle.
The reflector 14 has a reflecting surface 14a. The reflecting surface 14a has substantially an ellipsoid spherical shape having a center axis which coincides with the optical axis Ax.
The reflecting surface 14a includes a general reflecting area 14aA and a lower reflecting area 14aB. The general reflecting area 14aA is formed in a shape of an ellipsoid. In a vertical cross section including the optical axis Ax, a first focal point F1 of the ellipsoid is set at a position of the light source 12a, and a second focal point of the ellipsoid is set at a rear focal point F2 of the projection lens 18. According to such a configuration, the reflecting surface 14a reflects light from the light source 12a in a forward direction and toward the optical axis Ax such that the light is substantially converged at the rear focal point F2, which is the second focal point of the ellipsoid, in the vertical cross section including the optical axis Ax.
The projection lens 18 is coupled to a front end of the reflector 14 via a holder 16. The holder 16 is formed into a cylindrical shape which extends forward from a front end opening of the reflector 14, and a rear end portion of the holder is screw-fastened to the reflector 14 at several points.
The projection lens 18 is fixedly supported on the front end portion of the holder 16. The projection lens 18 is a plano-convex lens in which a front surface thereof is convex and the rear surface thereof is flat, and is placed such that the rear focal point F2 thereof coincides with the second focal point of the reflecting surface 14a of the reflector 14.
According to such a configuration, the projection lens 18 allows the light reflected from the reflecting surface 14a of the reflector 14 to pass therethrough so as to be collected toward the optical axis Ax.
The shade 20 is formed from a metal plate. As shown in FIG. 11, an upper edge 20a which forms a cut-off line of a low-beam light distribution pattern is vertically disposed so as to pass through the rear focal point (the second focal point of the reflecting surface 14a) F2 of the projection lens 18, thereby shielding part of the light reflected from the reflecting surface 14a so as to eliminate upward irradiation light from the projection lens 18. Therefore, low-beam irradiation light which irradiates downward with respect to the optical axis Ax is obtained.
As shown in FIG. 12, the shade 20 is positioned and fixed by holding an annular outer circumferential portion 20e thereof at a fastening portion between the holder 16 and the reflector 14. On the outer circumferential portion 20e, as also shown in FIG. 12, a plurality of screw insertion holes 20c through which each screw for fastening the holder 16 to the reflector 14 is inserted, and a positioning pin insertion hole 20d into which a positioning pin 14b of the reflector 14 is fitted are formed. An optical axis alignment between the shade 20 and the reflector 14 is adjusted by fixing the shade 20 to the reflector 14 together with the holder 16.
As shown in FIGS. 12 and 13, the shade 20 has an opening 20b which penetrates through the shade 20 in the vicinity of the upper edge 20a in order to form an overhead sign illuminating light. The opening 20b is laterally elongated and is substantially in a rectangular shape. The opening 20b allows the light reflected from the lower reflecting area 14aB of the reflecting surface 14a to transmit therethrough, thereby forming the overhead sign illuminating light B.
As shown in FIG. 12, the light shielding plate 22 includes: a main portion 22A which extends rearward and obliquely downward from a portion between the upper edge 20a of the shade 20 and the opening 20b; and a supporting portion 22B which extends downward along the rear face of the shade 20. The light shielding plate is formed by applying a punching process and a bending process on a metal plate, and is integrated with the shade 20 by spot welding or the like.
The light shielding plate 22 shields the light reflected from the general reflecting area 14aA of the reflecting surface 14a to be directed toward the opening 20b with the main portion 22A, thereby adjusting the amount of light to be irradiated from the opening 20b as the overhead sign illuminating light. Therefore, the light flux of the overhead sign illuminating light B is not excessively increased, and glare to a driver of an oncoming vehicle can be prevented from being generated.
As shown in FIG. 11, in a thus configured vehicle headlamp 10, part of the light reflected by the general reflecting area 14aA of the reflector 14 to be directed toward the rear focal point F2 is shielded by the shade 20, thereby adjusting the irradiation light to be a low-beam light distribution. As a result, the low-beam light distribution pattern P shown in FIG. 14 is formed.
On the other hand, the light reflected by the lower reflecting area 14aB of the reflector 14 to be directed toward the opening 20b of the shade 20 transmits through the opening 20b, and thereafter, it is irradiated in a forward direction by the projection lens 18 so as to form the overhead sign illuminating light B. Therefore, the light distribution pattern (overhead sign pattern) Pohs for overhead sign illumination is formed as shown in FIG. 14.
Therefore, even without using an expensive rear-face frosted lens, overhead sign illuminating light can be obtained, and the visibility of an overhead sign can be improved. FIG. 14 shows light distribution patterns which are formed by beams that are irradiated forward from the vehicle headlamp 10, on a virtual vertical screen located at a position 25 m in front of the lamp.
The low-beam light distribution pattern P is a low-beam light distribution pattern for left-hand traffic, and has a horizontal cut-off line CL having a level difference on the left and right sides on the upper edge.
The light distribution pattern Pohs is formed so as to extend in the left and right direction which is homothetic to the shape of the opening 20b of the shade 20.
However, with the above-described vehicle headlamp 10, as shown in FIG. 14, dark and non light distributed region Q is formed so as to extend in the left and right direction in a strip-like manner between the low-beam light distribution pattern P and the light distribution pattern Pohs. This non light distributed region Q is formed to have a shape that is widely expanded in the vertical direction. Therefore, due to the darkness of the non light distributed region Q being conspicuous, there is a problem that the driver may experience visual discomfort. Further, there is another problem that a regulation of an adequate illuminance in such a region (for example, the European regulation) cannot be observed.
The non light distributed region Q is formed by shielding a light with a strip-like portion 20f (see FIG. 13) between the upper edge 20a of the shade 20 and the opening 20b. Therefore, it is theoretically possible to reduce the vertical width of the non light distributed region Q by bringing the position of the opening 20b close to the upper edge 20a so as to narrow the width of the strip-like portion 20f, thereby making the existence of the non light distributed region Q inconspicuous.
However, when the width of the strip-like portion 20f is narrowed, the strength of the upper edge 20a is lowered, thereby causing problems such as a distortion of the upper edge 20a while the opening 20b is formed. Consequently, in practice, the width of the strip-like portion 20f cannot be narrowed. Therefore, it has been impossible to eliminate the discomfort produced by the non light distributed region Q, and to adhere to the regulations.