Vehicular low beam headlight reflector consisting of upper and lower reflecting sectors

A reflecting surface is divided into first and second reflecting sectors by a plane inclined from the horizontal plane including the optical axis to occupy the upper half and the lower half of the reflecting surface, respectively. A fundamental surface of the first and second reflecting sectors has a reference parabola in the inclined plane, and is a collection of intersecting lines each obtained by cutting an imaginary paraboloid of revolution having an axis extending in a direction taken by a ray after being emitted from a reference point and then reflected at a reflecting point on a parabola that is an orthogonal projection of the reference parabola onto the horizontal plane, passing through the reflecting point, and having a focus at the reference point by a vertical plane including the ray vector. The focus of the reference parabola is set at the center of a filament. The reference point is set in the vicinity of the rear end of the filament for the first reflecting sector, and in the vicinity of the front end of the filament for the second reflecting sector.

BACKGROUND OF THE INVENTION 
The present invention relates to a reflector for a vehicular low beam 
headlight. 
The recent trends of the automobile design have been prompting efforts to 
develop new types of headlights. That is, with the streamlined body shape 
to satisfy various requirements from, for instance, the body design and 
aerodynamic characteristics that are related to the automobile styling, 
headlights need to be constructed so as to accommodate what is called the 
slant nose, i.e., the reduced front portion of a vehicle body. 
However, in forming a light distribution pattern having a cutline specific 
to the low beam with the configuration of conventional headlights, lens 
steps of an outer lens have an important role in the light distribution 
control. Therefore, the outer lens cannot be inclined from the vertical 
axis more than a certain limit. That is, the conventional configuration 
cannot properly accommodate the slant nose. 
In view of the above, various types of headlights have been proposed to 
shift the light distribution control function, which conventionally 
belonged to the lens steps of the outer lens, to the reflector. That is, a 
reflecting surface is divided into a number of light distribution control 
sectors and their shapes are designed so that a combined pattern of 
projection patterns of the respective sectors approximates the standard 
light distribution pattern, to thereby reduce the load in the light 
distribution control imposed on the outer lens. 
However, to produce the light distribution pattern having the cutline 
specific to the low beam by the conventional reflecting surface, the 
number of light distribution control sectors of the reflecting surface 
tends to increase. If the adjacent reflecting sectors are not connected 
smoothly, the light reflected by a step at the boundary goes upward to 
cause glare or becomes undesired light in terms of the light distribution 
control. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a vehicular headlight 
reflector having a reflecting surface of a simplified configuration. 
A vehicular headlight for forming a low beam comprises a reflecting surface 
having an optical axis and represented by a fundamental surface which has 
a reference point on the optical axis and a reference parabola included in 
a first plane inclined from a horizontal plane including the optical axis 
by a first predetermined angle and having a vertex and a focus on the 
optical axis, and which is a collection of intersecting lines each 
obtained by cutting an imaginary paraboloid of revolution having an axis 
extending in a ray vector direction taken by a reflected ray after being 
emitted from the reference point and then reflected at a reflecting point 
on a parabola that is an orthogonal projection of the reference parabola 
onto the horizontal plane, passing through the reflecting point, and 
having a focus at the reference point by a vertical plane including the 
ray vector. According to the invention, the vehicular headlight comprises 
a light source having a central axis extending along the optical axis, and 
first and second reflecting sectors divided by a second plane inclined 
from the horizontal plane by a second predetermined angle to occupy an 
upper half and a lower half of the reflecting surface, respectively, the 
first reflecting sector having the focus of the reference parabola 
approximately at a center of the light source and the reference point in 
the vicinity of a rear end of the light source, and the second reflecting 
sector having the focus of the reference parabola approximately at the 
center of the light source and the reference point in the vicinity of a 
front end of the light source.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Details of a reflector for a vehicular headlight according to an embodiment 
of the present invention is described hereinafter with reference to the 
accompanying drawings. 
FIG. 1 is a front view of a reflector 1. Its reflecting surface 2 is 
divided into two semicircular reflecting sectors 3(1) and 3(2) in terms of 
light distribution control. 
The coordinate system for the reflecting surface 2 is defined as follows. 
The optical axis of the reflecting surface 2 is selected as the x-axis 
(extending perpendicularly to the paper surface of FIG. 1 and having the 
positive direction on the front side). The axis perpendicular to the 
x-axis and extending in the horizontal direction is selected as the y-axis 
(the right-hand side of FIG. 1 is the positive side). The axis 
perpendicular to the x-axis and extending in the vertical direction is 
selected as the z-axis (the upper half of FIG. 1 is the positive half). 
The origin O of the orthogonal coordinate system is located at the center 
of a bulb fixing hole 4 when viewed from the front side. 
A line 5 passing through the origin O corresponds to a plane 6 including 
the x-axis and inclined from the xy-plane by an angle .theta. (.theta.&gt;0 
where the positive direction is the counterclockwise direction about the 
x-axis when viewed from the front side), and conceptually indicates a 
boundary between the reflecting sectors 3(1) and 3(2). The reflecting 
sectors (1) and 3(2) are located on the upper and lower sides of the plane 
6, respectively. That is, the reflecting sector 3(1) exists in the first 
to third quadrants of the yz-plane, and the reflecting sector 3(2) exists 
in the third, fourth and first quadrants of the yz-plane. 
The fundamental surface of each of the sectors 3(1) and 3(2) is of the type 
disclosed in U.S. patent application Ser. No. 07/808,670 filed by the 
present applicant, and is summarized below. 
As shown in FIG. 3, a filament 7 is disposed between point F (hereinafter 
called a "first focus") and point D (hereinafter called a "second focus"), 
with its central axis along the x-axis. Point D is deviated from point F 
by a distance d in the positive direction of the x-axis. 
To clarify the orientation of the filament 7, an assumption "the filament 7 
has a pencil-like form with its one tip on the side of point F having a 
cone-like pointed shape and the other tip on the side of point D being 
flat" is employed just for convenience of description. 
First, a parabola 8 having a focus at point F is assumed on the xy-plane. 
After being emitted from point F (near the rear end of the filament 7) and 
then reflected at point P3 on the parabola 8, a ray 9 travels in parallel 
with the optical axis (i.e., x-axis). On the other hand, after being 
emitted from point D (near the front end of the filament 7) and then 
reflected at point P3, a ray 10 travels toward point RC on a screen SCN 
far from the reflector 1 and crosses the optical axis. That is, the ray 10 
has a vector P3.sub.-- RC as its direction vector. 
Now, another parabola 11 is assumed which has a focus at point D and an 
axis extending parallel to vector P3.sub.-- RC. As shown in FIG. 3, the 
parabola 11 also passes through point P3. 
A paraboloid of revolution is obtained by rotating the parabola 11 about 
its axis, and a parabola 12 is obtained by cutting this paraboloid of 
revolution by a plane including the vector P3.sub.-- RC and perpendicular 
to the xy-plane. 
A curved surface is generated as a collection of the parabolas 12 obtained 
as point P3 is moved along the parabola 8. 
Filament images are projected onto a plane 13 in the following manner in 
the midst of traveling of rays toward the screen SCN. Am image 14 due to 
point P3 is in parallel with the horizontal line. An image 15 due to point 
P5 that is on the parabola 12 and lower than point P3 forms a certain 
angle with the horizontal line. The path taken by a ray 16 after being 
reflected at point P5 is in parallel with the path taken by the ray 10 
after being reflected at point P3 (both of the rays 10 and 16 are emitted 
from point D). 
Since the intersecting line is defined so that the rays relating to the 
formation of the flat ends of the filament images 14 and 15 become in 
parallel with each other, filament images 17 and 18 are formed on the 
screen SCN with point RC as their rotation center (the above parallel rays 
substantially coincide with each other at point RC). 
FIG. 4 schematically shows an arrangement of the filament images due to 
points P3 and P5, and point P4 that is on the parabola 12 and located 
between points P3 and P5. 
In FIG. 4, J(X) indicates a filament image corresponding to each 
representative point X. Filament images J(P3), J(P4) and J(P5) due to 
points P3, P4 and P5 are arranged with point RC on the horizontal line 
H--H as their rotation center. That is, as indicated by arrow M, the 
filament image rotates counterclockwise about point RC as the reflection 
point goes down (P3.fwdarw.P4.fwdarw.P5). The filament images are located 
under the horizontal line H--H while their flat ends are always directed 
to point RC. 
FIG. 5 shows how the reflecting surface 2 is generated. In FIG. 5, point P 
is an arbitrary point located on the parabola 8 that is included in the 
xy-plane. (By introducing a parameter q, coordinates of point P are 
expressed as (q.sup.2 /f, -2q, 0).) After being emitted from point F and 
then reflected at point P, a ray 19 travels in parallel with the x-axis as 
indicated by a vector PS. 
On the other hand, after being emitted from point D and then reflected at 
point P with a reflection angle smaller than that of the ray 19 according 
to the law of reflection, a ray 20 travels straight (indicated by a vector 
PM) forming a certain angle .alpha. with the ray 19. 
Now, an imaginary paraboloid of revolution 21 (indicated by a two-dot chain 
line) is assumed which has a focus at point D and an axis passing through 
point P and extending along the ray vector PM. A cross-sectional curve is 
obtained by cutting the paraboloid of revolution 21 by a plane .pi.1 
including the ray vector PM and parallel with the z-axis. (An intersecting 
line 22 of the paraboloid of revolution 21 and the plane .pi.1.) 
It is apparent that the above cross-sectional curve (indicated by a dashed 
line) is a parabola. The fact that rays emitted from point D and then 
reflected at arbitrary points on the intersecting line 22 travel in 
parallel with each other conform to the situation described in connection 
with FIG. 3. 
In this manner, the fundamental surface is obtained as a collection of 
intersecting lines of the imaginary paraboloids of revolution 
corresponding to points P on the parabola 8 and the planes including the 
respective axes of the imaginary paraboloids of revolution and parallel 
with the z-axis. 
This curved surface is expressed by Eq. 1 with the use of parameters shown 
in Table 1. 
TABLE 1 
______________________________________ 
Parameter Definition 
______________________________________ 
f Focal length of parabola 8 (OF) 
d Interval between points F and D (FD) 
q Specifying a point on parabola 8 
h Height in z-direction from plane z = 0 
Q = (f.sup.2 + q.sup.2)/f 
______________________________________ 
##STR1## (1) 
##STR2## 
z = h 
##STR3## 
The process of deriving Eq. 1 is not described here because doing so may 
unduly complicate the description of the invention. But it is noted that 
Eq. 1 can be obtained based on only the above description and knowledge 
of elementary algebraic geometry. Further, it is understood that Eq. 1 
Equation 1 is generalized into Eq. 2 in which a parabola on a plane 
inclined from the xy-plane by an angle .theta. is employed instead of the 
parabola 8. 
##EQU1## 
By substituting .theta.=0 into Eq. 2, it is easily verified that Eq. 2 
includes Eq. 1. 
FIG. 2 shows how the filament 7 and foci are located with respect to the 
reflecting surface 2. The central axis of the filament 7 extends along the 
x-axis. 
Point F is the first focus common to the reflecting sectors 3(1) and 3(2) 
and is located on the x-axis at the center of the filament 7 that is away 
from the origin O by a distance f. Point G1 is the second focus of the 
reflecting sector 3(1), and is located in the vicinity of the rear end of 
the filament 7, i.e., located on the positive side of the x-axis at a 
position away from the origin O by a distance g1. If parameter du is 
defined as g1-f, a relationship du&lt;0 holds. Point G2 is the second focus 
of the reflecting sector 3(2), and is located in the vicinity of the front 
end of the filament 7, i.e., located on the positive side of the x-axis at 
a position away from the origin O by a distance g2. If parameter dd is 
defined as g2-f, a relationship dd&gt;0 holds. 
Therefore, the reflecting sector 3(1) has a reflecting surface according to 
Eq. 2 in which the first and second foci are located at points F and G1, 
respectively. More specifically, equations for the reflecting surface of 
the sector 3(1) is obtained by substituting d=du and .theta.=.theta..sub.0 
(.theta..sub.0 corresponds to the cutline angle) into Eq. 2. On the other 
hand, the reflecting sector 3(2) has a reflecting surface according to Eq. 
2 in which the first and second foci are located at points F and G2, 
respectively. More specifically, equations for the reflecting surface of 
the sector 3(2) is obtained by substituting d=dd and .theta.=.theta..sub.0 
into Eq. 2. 
Table 2 shows the definitions of the above parameters. 
TABLE 2 
______________________________________ 
Distance from 
Distance from 
origin to 1st 
1st focus to 
Angular 
Sector 
focus 2nd focus parameter .theta. 
______________________________________ 
3(1) f .vertline.du.vertline. 
.theta..sub.0 
3(2) f .vertline.dd.vertline. 
.theta..sub.0 
______________________________________ 
FIGS. 6-8 schematically show projection patterns 23(1) and 23(2) produced 
by the reflecting sectors 3(1) and 3(2) and a combined projection pattern 
26 thereof. In those figures, H--H and V--V denote the horizontal line and 
the vertical line, respectively, and point o is an intersecting point 
thereof. 
FIG. 6 shows the projection pattern 23(1) by the reflecting sector 3(1). In 
FIG. 6, symbols 24N, 24M and 24F denote patterns (schematically drawn) 
produced as combinations of filament images reflected at points located at 
circles having different distances from the x-axis when viewed from the 
front side. The pattern 24N corresponds to the circle closest to the 
x-axis, and the pattern 24F corresponds to the circle most distant from 
the x-axis. The pattern 24M corresponds to the circle located at the 
middle of the circles of the patterns 24N and 24F. 
As shown in FIG. 6, most of each of the patterns is located below the 
horizontal line H--H; only an upper edge portion is located above the 
horizontal line H--H. The vertical width decreases in the order of 24N, 
24M and 24F. 
FIG. 7 shows the projection pattern 23(2) by the reflecting sector 3(2). In 
FIG. 7, symbols 25N, 25M and 25F denote patterns (schematically drawn) 
produced as combinations of filament images reflected at points located at 
circles having different distances from the x-axis when viewed from the 
front side. The pattern 25N corresponds to the circle closest to the 
x-axis, and the pattern 25F corresponds to the circle most distant from 
the x-axis. The pattern 25M corresponds to the circle located at the 
middle of the circles of the patterns 25N and 25F. 
As in the case of the patterns shown in FIG. 6, most of each of the 
patterns is located below the horizontal line H--H; only an upper edge 
portion is located above the horizontal line H--H. The vertical width 
decreases in the order of 25N, 25M and 25F. On the other hand, on the 
whole the horizontal widths are somewhat smaller than those of the 
patterns of FIG. 6. 
FIG. 8 schematically shows the pattern 26 that is a combination of the 
projection patterns of FIGS. 6 and 7. The portion located below the 
horizontal line H--H is bowl-shaped, and the portion located above the 
horizontal line H--H has the upper edge that is inclined downward toward 
the right. 
The projection pattern 26 is the basis of the light distribution pattern to 
be obtained finally, and it is necessary to horizontally diffuse the 
pattern 26 and form the cutline by certain measures. 
In conventional headlights, lens steps having diffusive action are formed 
on an outer lens disposed in front of the reflector 1. However, it becomes 
difficult to form lens steps having strong horizontal diffusive action as 
the inclination of the outer lens is increased. In such a case, it is 
necessary to shift the diffusive action to the reflector. 
The present invention employs a method of diffusing light only by the 
reflector 1 by smoothly undulating the reflecting surface 2. More 
specifically, a set of equations representing a wave-like pattern are 
combined with the abovedescribed equations representing the reflecting 
surface 2. 
The following function is introduced for that purpose: 
##EQU2## 
In the normal distribution type (or Gaussian) function Aten(s, W) using 
parameters s and W, the parameter W specifies the degree of attenuation. 
FIG. 9 shows the shape of the function Aten(s, W). 
Further, a periodic function WAVE(s, .lambda.) using a parameter .lambda. 
is introduced: 
##EQU3## 
The parameter .lambda. specifies the wavelength, i.e., pitch of the cosine 
wave. FIG. 10 shows the shape of the function WAVE(s, W). While in this 
embodiment the cosine function is employed as the periodic function, other 
various periodic functions may be used when necessary. 
A damped periodic function Damp shown in FIG. 11 is obtained as a product 
of the above two kinds of functions. The reflecting surface 2 can be 
undulated by applying to it a function produced from the basic function 
Damp. 
FIG. 12 shows an example of undulations applied to the reflecting surface 
2. In FIG. 12, among protrusions and dents formed on the reflecting 
surface 2, the protrusions are schematically indicated by lines. 
As shown in FIG. 12, regions for the application of undulations do not 
coincide with the sectors of the reflecting surface 2. Circular waves 
having the center at the origin O are applied in a fan-shaped region 27(1) 
of the reflecting sector 3(1) close to the xy-plane. Plane waves 
developing in the horizontal direction are applied in the remaining 
region. 
FIG. 13 schematically shows a projection pattern 28 produced by the 
reflecting pattern 2 as modified by application of the undulations 
described above. FIG. 13 shows that the pattern approximating the standard 
light distribution pattern can be obtained only by the action of the 
reflecting surface. 
As described above, according to the invention, the number of light 
distribution control sectors can be reduced. Since the reflecting sectors 
can be connected smoothly at the boundary, the light reflected at the 
boundary neither causes conspicuous glare nor becomes undesired light in 
forming the light distribution pattern. 
Further, by introducing the undulations in the manner as described above, 
the dependence on the outer lens in the light distribution control can be 
reduced to enable construction of reflectors suitable for the slant-type 
vehicle body shape.