Foglight having a transverse filament for a motor vehicle

A motor vehicle foglight of the type comprising: a lamp having a transverse horizontal filament (10); a reflector (20) whose axis (Ox) passes in the same vertical plane as the center of the filament (F.sub.O); and a closure glass (30). According to the invention the surface of the reflector is a surface without discontinuity forming images of the filament in which all the points of the images are situated below a horizontal cutoff and in a horizontal strip of substantially constant height situated beneath said cutoff. The invention is applicable to foglights which do not include a masking cup.

The present invention relates to a motor vehicle light suitable for 
providing illumination in fog. 
BACKGROUND OF THE INVENTION 
The beam corresponding to such illumination is a short range beam which is 
limited in the upwards direction by a substantially horizontal cutoff 
plane, which is spread very wide in the sideways direction, and which does 
not include any rising rays which could give rise to undesirable optical 
phenomena in conjunction with the droplets of water in suspension in a 
fog. 
Prior art foglights generally include an axially oriented filament, i.e. a 
filament lying on the axis of the foglight. 
Several solutions have been proposed for the associated reflector. The 
first solution consists in providing a reflector in the form of a 
paraboloid of revolution whose focus is situated at the front end of the 
filament (in the light emission direction). This provides a slightly 
divergent beam and the rays in its top half are deflected downwardly by 
deflector elements provided in the closure glass. 
However, this gives rise to considerable excess thickness in the glass 
which makes it difficult to mold, particularly when the glass is literally 
made of glass. 
Proposals have also been made to form a reflector using two axially offset 
half-paraboloids. The top half-paraboloid is focused on the rear end of 
the filaments so as to form a conventional cutoff beam while the bottom 
half-paraboloid is focused on the front end of the filament so that all 
the images of the filament are situated beneath the cutoff. 
Such a foglight suffers from two major drawbacks. Firstly the reflector has 
a surface discontinuity where the two half-parabolids meet: since the 
paraboloids are focused on different points, they necessarily have 
different apexes, or else they have different focal lengths, and in either 
case they have different profiles along a connection plane; as a result of 
this, a reflector manufactured in accordance with this teaching is never 
perfect, in practice, at the connection, thereby giving rise to parasitic 
light rays being emitted above the cutoff. 
Secondly, and above all, the beam generated by the bottom half-paraboloid 
is spread out sideways in a satisfactory manner only in a zone situated 
immediately below the cutoff. This non-uniform spread of the beam goes 
against the desired aim of a fog lamp beam which is to obtain relatively 
uniform sideways spread. 
Finally, proposals have been made in French published patent application 
No. 2 536 503 in the name of the Applicant, for a foglight in which the 
filament remains axially oriented and the reflector comprises a composite 
surface without discontinuity suitable for forming images of the filament 
in which nearly all the points of the images are situated beneath the 
cutoff. Such a solution makes it possible to obtain a completely uniform 
sideways spread of the beam together with a well-defined cutoff. 
However, using an axial filament with a reflector of this type necessary 
gives rise to filament images which are large in size and which are 
vertically oriented or at a small angle to the vertical. Such images need 
spreading sideways by a considerable amount, and to do this it is 
necessary to provide appropriate prisms or ribs on the closure glass. A 
second drawback of this type of fog lamp, likewise due to the existence of 
such vertical images, is that the beam is thicker than required in 
practice, with the bottom portion of the beam not contributing to useful 
illumination and possibly even constituting a hinderance by illuminating 
the road too close to the vehicle. 
Finally, prior art foglights exist which include a lamp having a horizontal 
filament extending across the foglight axis, and associated with a 
reflector in the form of a paraboloid of revolution whose focus is 
situated at the center of the filament. In this way, the beam does not 
include large filament images, however it remains necessary to deflect the 
images in a vertical direction using prisms formed in the closure glass 
and with the consequent risk of emitting parasitic rays in an upwards 
direction from the edges of the prisms. 
The present invention seeks to mitigate the drawbacks of the prior art and 
to provide a foglight in which the closure glass needs to provide 
substantially no vertical deflection of the light rays, and in which the 
beam obtained is properly uniform in width as well as in height. 
SUMMARY OF THE INVENTION 
To this end the invention provides a motor vehicle foglight of the type 
comprising: a lamp having a transverse horizontal filament; a reflector 
whose axis passes in the same vertical plane as the center of the 
filament; and a closure glass; the foglight including the improvement 
whereby the surface of the reflector is a surface without discontinuity 
forming images of the filament in which all the points of the images are 
situated below a horizontal cutoff and in a horizontal strip of 
substantially constant height situated beneath said cutoff.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows the images of the filament as projected onto a projection 
screen by the reflector of a foglight in accordance with French published 
patent application No. 2 536 503 in the name of the Applicant. As 
mentioned above, there are several large images which are oriented in 
directions that are close to the vertical. These images give rise to 
excessive beam thickness in the middle of the beam, thereby hindering the 
driver's view by illuminting the road too close to the vehicle, and thus 
requiring means on the closure glass for providing a relatively high 
degree of sideways spreading. 
The foglight in accordance with the invention as shown diagrammatically in 
FIGS. 2 and 3 comprises a lamp (not shown) having a filament 10, a 
reflector 20, and a spreading glass 30 which closes the foglight. 
The filament is disposed in a horizontal plane and is oriented transversely 
to the axis Ox of the reflector. More precisely, in the present example, 
the filament which is assumed to be a cylinder of length 2l and of radius 
r, is offset upwardly from the horizontal plane xOy by a distance equal to 
its radius, such that its light-emitting surface is tangential to said 
plane. In addition, the filament is disposed in the direction y'Oy so that 
its center is vertically above a point F.sub.0 lying on the axis Ox. The 
distance of the apex of the reflector from the point F.sub.0 is marked 
f.sub.0. Naturally, the position of the filament could vary a little 
relative to the above-mentioned position without thereby going beyond the 
scope of the invention. 
The surface of the reflector is a surface without discontinuity, designed 
to form images of the filament in which all the points of the images are 
situated beneath a horizontal cutoff passing through the axis of the 
foglight (referenced h'Hh in FIGS. 4a, 4b and 5), and lie in a horizontal 
strip of substantially constant height which is delimited along the top by 
said cutoff. 
Advantageously, all of these images have their highest points situated on 
the cutoff or very close thereto. 
The term "absence of discontinuity" is used to mean that first order 
continuity is sensured at all points on the surface of the reflector, and 
that second order continuity is ensured at all points of the surface 
except for two localized defects which, as explained below, appear in the 
form of very small kinks in curvature. It is recalled that second order 
continuity means that the tangential planes at any point of a line drawn 
on the surface are of the same on both sides of the line. 
In practice, such a disposition makes it possible to make real surfaces by 
stamping or by injection molding which are very close to the theoretical 
design surfaces, thereby avoiding the defects specific to the 
above-described system having two offset half-paraboloids. 
Theoretical calculation shows that the surface defined by the following 
equation in an orthogonal frame of reference (O,x,y,z) as shown in FIGS. 2 
and 3 has the specified properties: 
EQU x=y.sup.2 /4f.sub.0 +z.sup.2 /4f.sub.0 Q (1) 
where: 
##EQU1## 
and: r is the radius of the filament; 
l is the half-length of the filament; and 
f.sub.0 is the distance between the center of the filament and the plane 
yOz (i.e. the X co-ordinate of the point F.sub.0). 
Further, if the radius r is assumed to be very small, the above equation 
becomes, to a first approximation: 
EQU x=y.sup.2 /4f.sub.0 +z.sup.2 /4f.sub.0 S (2) 
where: 
##EQU2## 
and also has the specified properties, although the quality of the result 
obtained is slightly less good. 
These surfaces intersect the plane xOy with a parabola of focal length 
F.sub.0, and define complex surfaces which act on the images of the 
filament in a manner described in greater detail below. 
Also, it can be shown that the surfaces defined mathematically above are 
second order continuous except for two localized defects in the vertical 
plane xOz, where continuity is assured only to the first order. Thus, a 
very slight kink is to be found in these regions, but in practice the kink 
is eliminated during the polishing stages that are conventionally included 
in the reflector manufacturing process. Further, these localized defects 
give rise to substantially no anomalies in the beam obtained. 
FIGS. 4a and 4b show the images of the filament as projected onto a 
standardized screen at 25 meters (m) from the foglight by reflection at 
points situated on a common horizontal plane of the reflector, 
respectively at positions z=-40 mm (FIG. 4a) and z=-20 mm (FIG. 4b). These 
two figures should be compared with FIG. 1 which shows the image 
distribution applicable to a foglight having a composite surface without 
discontinuity and associated with an axial filament. Unlike the FIG. 1 
situation, the distribution of images beneath the cutoff, and in 
particular in a horizontal direction, is much more uniform in this case, 
and more precisely it can be seen that the lengths of the filament images 
shorten progressively as the images rotate about their centers from the 
horizontal towards the vertical. Thus, not only is the highest point of 
each image situated very close to the cutoff h'Hh, but also the lowest 
points of these images project very little beyond a bottom cutoff 
referenced b'b in FIGS. 4a, 4b, and 5, thereby obtaining a beam whose 
thickness is approximately constant over a considerable width. This 
avoids, in particular, illuminating the road too close to the vehicle, as 
occurs with the large vertical images shown in FIG. 1, and which should be 
avoided in practice. 
FIG. 5 is a plot of isocandela curves C of decreasing value when going 
outwardly from the middle, showing the illumination provided by the 
reflector as a whole, and it can be seen that the top horizontal cutoff 
h'Hh is sharp, that the beam is very wide and of practically constant 
thickness, and the concentration point P is situated beneath the cutoff 
h'Hh and is centered on the vertical v'Hv. 
The closure glass may optionally provide additional sideways spreading of 
the beam. However, such spreading is obtained by the very nature of the 
reflector and there is no need to provide large excess thicknesses in the 
closure glass: the closure glass is thus easy to mold and may be made of 
plastic material or of the glass per se. 
Further, when the reflector is truncated top and bottom by a pair of 
horizontal plane cheeks 21 and 22 (see FIG. 3), it may be advantageous to 
ensure that the cheeks are non-reflecting so as to avoid a considerable 
quantity of light from being diffused above the cutoff. 
The reflector may be designed in various different shapes. Thus, FIG. 6a is 
a diagrammatic front view of a foglight without its closure glass, and 
foglight having a reflector whose width y.sub.1 is approximately twice its 
height z.sub.1. In FIG. 6b, these proportions are interchanged, with the 
height z.sub.2 being approximately twice the width y.sub.2. These values 
are determined so that the total effective areas of the two reflectors are 
approximately identical. The influence of the shape of the reflector 
outline on the characteristics of the beam obtained can be determined. In 
particular, the light flux (in lumens) has been measured for both of these 
foglights in a longitudinal vertical plane xOz as a function of the 
inclination (in degrees) below the cutoff. FIG. 7 shows two curves 
.phi..sub.1 and .phi..sub.2 obtained using the foglights of FIGS. 6a and 
6b, respectively. 
As can be seen, the FIG. 6b foglight gives more light flux between about 
0.degree. and about 2.degree. beneath the cutoff, such that the yield un 
this region is slightly increased. In contrast, this configuration gives 
rise to residual light flux of very low value above the cutoff, and in 
practice the residual light flux will not hinder the driver's view. 
With the FIG. 6b, reflector shape, tests have also been performed to see if 
varying the position of the filament in a vertical direction relative to 
said shape (and not relative to the surface of the reflector which is 
always determined relative to the position of the filament) would lead to 
modifications in the illumination characteristics of the beam. FIG. 8a 
shows the filament situated at a distance z.sub.3 beneath the top edge of 
the reflector, which distance is about one-tenth of the total height 
z.sub.2 of the reflector, and FIG. 8b shows the filament at a distance 
z.sub.4 which is substantially equal to z.sub.3 above the bottom edge of 
the reflector. Here again the areas of the two reflectors are equal. 
The variations in light flux (in lumens) as a function of inclination (in 
degrees) relative to the cutoff for these two foglights are shown by 
curves .phi..sub.3 and .phi..sub.4 in FIG. 9. As can be seen, when the 
lamp is upwardly offset, not only is there more light flux between about 
0.4.degree. and 2.degree. beneath the horizontal, but also the cutoff at 
the horizontal is much sharper, which cutoff is illustrated by the steep 
slope of the curve. 
Thus, although the invention may be implemented with any reflector outline 
and with any relative disposition of the filament with respect to a given 
reflector outline, it is advantageous to increase the height of the 
foglight as much as possible (to the detriment of its width) and to offset 
the lamp upwardly therein. 
Naturally, the present invention is not limited to the various embodiments 
described above, and the person skilled in the art can readily make any 
variation or modification thereto without going beyond the scope of the 
invention.