GB2044428A - Rear fog lamp for motor vehicles - Google Patents

Abstract

A rear fog lamp (1) for use in motor vehicles includes a light source (2), a parabolic reflector (3) and a diffuser plate (4) having a prismatic inner surface (5) for diffusion of the beam of parallel rays reflected by the reflector (3). The parabolic reflector (3) subtends an angle (E) around its focus (F) which is less than 2 pi steradians, in comparison with a known lamp whose reflector (6) subtends an angle (G) which is very much greater than 2 pi steradians. The surface 5 comprises a central portion 11 defined by a plurality of spherical lenses 11a arranged side by side and two lateral portions 12 each defined by a plurality of cylindrical lenses 12a arranged side by side, each cylindrical lens having on its convex surface a central longitudinally-extending V-shaped depression. Corner portions 13 defined by groups of spherical lenses 13a may also be provided. <IMAGE>

Description

SPECIFICATION Rear fog lamp for motor vehicles This invention relates to rear flog lamps for motor vehicles, of the type comprising a light source, a parabolic reflector having its focus arranged in correspondence with the light source, in order to reflect the light rays coming from the light source in a direction substantially parallel to the focal axis of the parabolic reflector, and a diffuser crystal, arranged in a plane perpendicular to the focal axis of the parabolic reflector and having a prismatic surface facing the interior of the reflector, for the diffusion of the beam of parallel light rays which is reflected from the parabolic reflector.

Existing European international regu!ations (directive CEE 77/538) restrict the photometric characteristics of rear fog lamps of the type specified above. According to these regulations, the light source of the lamp must comprise an electric-light bulb of the standard filament type having a power of 25 watts (P25-1 bulb according to regulation ECE No. 37). Futhermore, the light intensity of the beam diffused by the lamp must not be less than a fixed level, within a solid angle of predetermined form and dimensions.

In known rear fog lamps, in order to obtain a light beam which satisfies the requirements of the regulations, the parabolic reflector subtends, around its focus, a solid angle appreciably greater than 27r steradians, so as to make it possible to reflect the greater part of the light radiation emitted by the bulb in the space surrounding it.

However, it is not possible to use parabolic reflectors in which the focal distance, that is the distance between the focus and the vertex of the parabolic reflector, is less than 18-20 mm, since for smaller focal distances the characteristics of the light beam diffused by the lamp are too greatly influenced by any manufacturing defects, errors in the positioning of the bulb, and by the fact that the light source is not a point source. Thus, the use of a parabolic reflector which covers a substantial portion of the space surrounding the bulb of the lamp results in rear fog lamps according to the prior art occupying a relatively large amount of space.

It is an object of the present invention to provide a rear fog lamp of the type specified initially, which has an appreciably smaller volume than known rear fog lamps.

According to the present invention there is provided a rear fog lamp for use in a motor vehicle, comprising a light source, a parabolic reflector having its focus arranged in correspondence with the light source whereby to reflect light rays from the light source in a direction substantially parallel to the focal axis of the parabolic reflector, and a diffuser crystal, arranged in a plane perpendicular to the focal axis of the parabolic reflector, and having a prismatic surface facing the interior of the reflector, for the diffusion of the beam of parallel light rays which is reflected by the parabolic reflector, and wherein a parabolic reflector subtends, around its focus, a solid angle not greater than 27r steradians, and the prismatic surface of the diffuser crystal comprises a central portion defined by a plurality of spherical lenses arranged side by side and two lateral portions each of which is defined by a plurality of vertical cylindrical lenses arranged side by side, each cylindrical lens having on its convex surface a central longitudinal depression arranged in a position parallel to the generatrices of the cylindrical lens and defined by two plane faces arranged in a "V" and inclined, so that the light - rays which strike the said plane faces are deflected by the diffuser crystal in two directions arranged, respectively, at one side and at the other side of the vertical plane of symmetry of the lamp, ;and each forming with the said vertical plane an angle of between 80 and 9 .

The use of a parabolic reflector which subtends around its focus a solid angle not greater than 2or steradian makes it possible, once the value of the focal distance of the parabolic reflector is fixed, (a value which, for the reasons stated, may not be less than 18-20 mm), to obtain a lamp of reduced volume both in the radial direction and in the axial direction. On the other hand, the longitudinal Vshaped depressions, with which the cylindrical lenses are provided and which define the said lateral portions of the prismatic surface of the diffuser crystal, make it possible to obtain the required light intensity in the zones of the light beam, diffused by the lamp, which are situated in correspondence with the two directions specified above.In the case of the absence of these depressions, in faet, the light intensity of the beam in correspondence with these zones would be less than the value required by the regulations. This results from the fact that the parabolic reflector of a lamp according to the present invention receives, due to its reduced extent, a smaller portion of the light radiation emitted by the bulb with respect to a rear fog lamp according to the prior art in which the parabolic reflector subtends a much broader solid angle around its focus.

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates diagrammatically the light intensity requirements of the international regulations for the light beam diffused by a rear fog lamp: Fig. 2 is a section along a meridian plane of a rear fog lamp according to an embodiment of the present invention; Fig. 3 is a frontal view of the diffuser crystal of the rear fog lamp of Fig. 2; Figs. 4 to 6 are partial sections, on an enlarged scale, taken along the lines IV--IV, V-V and VI--VI of Fig. 2, and Figs. 7 to 11 illustrate diagrammatically the photometric characteristics of the light beam diffused by the rear fog lamp according to the embodiment of the present invention.

The diagram of Fig. 1 shows the light intensity requirements which must be satisfied, according to the international regulations cited above, by the light beam diffused by a rear fog lamp using a standard bulb of the aforesaid P25-1 type. H and V indicate two lines perpendicular to each other which represent the intersection of a plane, perpendicular to the focal axis of the parabolic reflector of the rear fog lamp, which the horizontal plane of symmetry and with the vertical plane of symmetry respectively, of this lamp.A and C indicate two points arranged on the line V which represent the intersection, with the said plane perpendicular to the focal axis of the parabolic reflector, of two lines which, passing through the focus of the parabolic reflector, are contained in the vertical plane of symmetry of the lamp and are inclined by an angle of 50 with respect to the horizontal plane of symmetry of the lamp at one side, and at the other, of this plane. B and D indicate two points arranged on the line H which represent the intersection with the said plane perpendicular to the focal axis of the parabolic reflector of two lines which, passing through the focus of the parabolic reflector, are arranged in the horizontal plane of symmetry of the lamp at one side, and at the other, of the vertical plane of symmetry and are inclined with respect to this vertical plane by an angle of 100.The international regulations establish that, once a general plane perpendicular to the focal axis of the parabolic reflector of the fog lamp has been considered and once the points A, B, C and D defined above have been determined, the light intensity of the part of the light beam diffused by the lamp which is contained within the rhombus having the said points A, B, C, D as vertices must not be less than 75 watts. Furthermore, according to the regulations, the light intensity in correspondence with the diagonals AC and BD of the rhombus ABCD must not be less than 1 50 watts.

In Fig. 2 is shown a rear fog lamp 1 according to an embodiment of the present invention. The lamp 1 comprises a 25 watt filament bulb 2 of the standard type required by the regulations. The lamp 1 comprises, furthermore, a parabolic reflector 3, having a focus F arranged in correspondence with the filament of the bulb 2, which is designed to reflect in a direction parallel to its focal axis 3a the light rays coming from the bulb 2. A diffuser crystal 4, having a prismatic surface 5 facing the interior of the lamp 1, is adapted to diffuse the beam of parallel light rays coming from the parabolic reflector 3. In the plane of the section of Fig. 2, the parabolic reflector 3 subtends, around its focus F, a plane angle E, less than 7r radians.

In Fig. 2, broken lines show a rear fog lamp according to the prior art, having a parabolic reflector 6 which has the same focal distance as the parabolic reflector 3 of the lamp 1 according to the present invention. The parabolic reflector 6 of the lamp according to the prior art subtends, around the focus F in the meridian plane of the section shown, a plane angle G appreciably greater than 7r radians, The comparison between the lamp according to the present invention and the lamp according to the prior art as shown in Fig. 2 shows clearly the characteristics of reduced volume of space occupied by the lamp according to the present invention.Due to the use of a parabolic reflector of limited extension, the transverse dimension of the lamp is changed from a length R, to a length R2, whilst the axial dimension is changed from a length A1 to a length A2.

On the other hand, on the hypothesis that the diffuser crystal 4 of the lamp according to the present invention may be shaped in an identical manner to the diffuser crystal, indicated by 7, of the lamp according to the prior art, the light intensity characteristics of the light beam diffused by the lamp 1 would be decidedly less than those of the light beam diffused by the lamp according to the prior art. The parabolic reflector of the lamp according to the prior art is, in fact, able to receive a portion of the light radiation emitted by the bulb 2 which is substantially greater than that received by the parabolic reflector 3 of the lamp 1.

This situation is reflected by the diagram of Fig.

7. In this diagram, the values of the light intensity, expressed ln watts, of the part of the diffused light beam which is contained in the horizontal plane of symmetry of the lamp are given as a function of the angle of inclination with respect to the vertical plane of symmetry of the lamp.With reference to the diagram of Fig. 1 , the requirement according to which the light intensity of the beam must not be less than 1 50 watts in correspondence with the diagonal BD of the rhombus ABCD results, in the case of Fig. 7, in the curve of light intensity of the fog lamp being completely greater than the rectangle (drawn with hatching in Fig. 7) defined by the axis of the abscissae, by two lines parallel to the axis of ordinates of abscissa +100 and 100 respectively and by a line parallel to the axis of the abscissae and of an ordinate equal to 1 50 watts.In Fig. 7, 8 indicates the curve relating to the lamp according to the prior art shown in Fig. 2, whereas 9 indicates the curve which would result in the case of the lamp 1 of Fig. 2 according to the hypothesis that the diffuser crystal 4 of this lamp were shaped in similar manner to that of the diffuser crystal 7 of the lamp according to the prior art. As may be seen, the fact that the parabolic reflector 3 "catches" a smaller portion of the light radiation emitted by the bulb 2 with respect to the parabolic reflector 6, would involve a reduction in the light intensity of the light beam emitted by the lamp 1. There would thus be created two "dark" zones 1 0, indicated by double hatching in Fig. 7, in correspondence with which the light intensity would be lower than the value required by the regulations.

In the rear fog lamp according to the present invention, the parabolic reflector 3 of the lamp has a limited extent, so as to make it possible to considerably reduce the volume of space occupied by the lamp, whilst the diffuser crystal 4 is shaped so as to "illuminate" the two "dark" zones 10. In short, a lamp is obtained which has a decidedly reduced volume of occupied space in comparison with the lamps according to the prior art and which is, despite this, able to satisfy the requirements demanded by the international regulations.

It is noted that the light intensity of the light beam emitted by a rear fog lamp does not much vary during movement through the beam in the vertical direction. This means that, if curves are plotted analogous to those shown in Fig. 7, in order to represent the light intensity of the part of the light beam contained in a plane perpendicular to the vertical plane of symmetry inclined with respect to the horizontal plane of symmetry of the lamp, the curves obtained would practically coincide with those of Fig. 7. It is possible therefore to affirm that the type of graphic representation used in the case of Fig. 7 is indicative, in general, of the photometric characteristics of the light beam emitted by the rear fog lamp, with regard to verifying the satisfaction of the requirements demanded by the regulations.

The prismatic surface 5 of the diffuser crystal 4 of the rear fog lamp according to the present invention has a rectangular form (see Fig. 3) and comprises a central portion 11, including a plurality of spherical lenses 1 a side by side with each other, two lateral portions 12, each comprising four cylindrical lenses 1 2a side by side, and four portions 13 arranged in correspondence withthe vertices of the rectangle, each comprising a plurality of spherical lenses 1 3a side by side.

In the particular embodiment of the present invention which is shown in the accompanying drawings, the diffuser crystal 4 comprises polymethyl methacrylate, has a height of 48 millimetres and a width of 73 millimetres. The width of the central portion of the prismatic surface 5 is equal to 33 millimetres, whilst the height of the lateral portions 12 is equal to 36 millimetres. The lateral portions 12, therefore, occupy overall approximately 50% of the total area of the diffuser crystal 4.

Each of the four cylindrical lenses 1 2a which form each lateral portion 1 2 has on its convex surface (see Fig. 4) a central longitudinal depression, arranged in parallel to the generatrices of the cylindrical lens, and defined by two plane surfaces 1 5 inclined in a "V" and forming between them an angle M which is equal, in the example shown, to 1440. It is clear that this angle depends on the particular material which the diffuser crystal comprises and more precisely on the refractive index (in the case of polymethyl methacrylate the index is equal to 1,492) of the material. The aperture L of each depression 16 is equal, in the example shown, to 1.7 millimetres.

Each depression 16, therefore, occupies approximately 34% of the convex surface of each cylindrical lens 12a.

For a particular material of the diffuser crystal 4, the angle of inclination of the plane faces 1 5 of each depression 1 6 is selected so that the light rays coming from the parabolic reflector 3 which strike the said faces are deflected by the diffuser crystal in two directions arranged, respectively, at one side and at the other of the vertical plane of symmetry of the lamp and forming with this vertical plane an angle of between 80 and 90. In this way an increase is obtained in the light intensity of the beam diffused by the lamp in correspondence with the "dark" zones 10 (see Fig.

7). As is apparent from Fig. 7, the "dark" zones 10 correspond in fact to angles of aperture between 70 and 100.

With reference to Fig. 5, the spherical lenses 1 a which define the central portion 11 of the prismatic surface 5 of the diffuser crystal 4 have, in the example shown, a rectangular outline having a width of-3 millimetres, a height of 5 millimetres, and a radius of curvature equal to 8 millimetres.

The spherical lenses 1 3a of the four portions 13, arranged in correspondence with the corners of the rectangle defined by the diffuser crystal 4, have, instead, in the example shown, a width of 5 millimetres, a height of 3 millimetres, and a radius of curvature of 6 millimetres. The four portions 13 are not, however, strictly necessary, as will be apparent from the following description, for the achievement of the necessary requirements of light intensity of the beam diffused by the lamp. In the case, for example, where it might be necessary to provide means for fixing the diffuser crystal 4 to the parabolic reflector 3, these means could be arranged in correspondence with the corners of the rectangle, relinquishing in this way the four prismatic portions 1 3.

Fig. 8 shows the curve of light intensity of the beam diffused solely by the central portion 11 of the diffuser crystal 4. In a manner analogous to what has been done in Fig. 7, on the abscissa the values are given of the angle of inclination with respect to the vertical plane of symmetry of the lamp, whilst on the ordinate the light intensity of the beam is given, expressed in watts. Fig. 8 shows three curves P, Q and R which refer, respectively, to the light intensity of the diffused beam in correspondence with the horizontal plane of symmetry of the lamp, with the plane inclined by 50 below this plane of symmetry and with the plane inclined by 50 above this plane of symmetry.

As may be seen, the three curves P, Q, R, in effect, run together in a single curve. In Fig. 8 there is also given the rectangle already shown in Fig. 7 which defines the requirements demanded by the regulations. It is clear that the light beam diffused by the central portion 11 alone has a light intensity which is sufficient only in a restricted central portion.

In Fig. 9 the three curves P, Q, R are given once more, here again, in effect, coinciding with each other, with regard to the light beam diffused by the two lateral portions 1 2 of the diffuser crystal 4. In this case the light beam has a sufficient light intensity in correspondence with its more external zones, whilst it produces a light intensity much lower than the value required by the regulations in correspondence with its central zone. Fig. 10 shows the three curves P, Q, R with regard to the four prismatic portions 1 3 arranged in correspondence with the vertices of the diffuser crystal 4.

Finally Fig. 11 shows the diagram resulting from the sum of the curves given in Figs. 8 to 10, that is the diagram of the light intensity of the beam diffused by the rear fog lamp according to the embodiment of the present invention. The resulting curves shown in Fig. 11 are all arranged above the hatched rectangle which indicates the requirements demanded by the regulations.

Consequently, due to the particular structure of the diffuser crystal 4, the rear fog lamp according to the present invention, as well as having a much smaller volume of space occupied than the lamps according to the known technology, is able to satisfy the requirements demanded by the international regulations. The diffuser crystal 4 may, of course, be coloured en masse so as to assume the usual red colouring of rear fog lamps.

Naturally, without prejudice to the principle of the invention, the particulars of the construction and the embodiments of the present invention may be extensively varied with respect to what has been described and illustrated purely by way of example without, thereby, departing from the scope of the present invention.

In particular, it is clear that the general idea from which the present invention stems is that of using a parabolic reflector of reduced extent, and of using a diffuser crystal with a prismatic surface shaped so as to increase the light intensity of the beam in correspondence with the "dark" zones, which would otherwise occur through the effect of the use of a parabolic reflector of this reduced extent. The plane faces which define the depressions, described above, with which the said cylindricaPlenses are provided must be, according to the present invention, inclined so as to produce a deflection of the light rays striking them in the direction of these dark zones. If this direction is situated in correspondence with a different angle of aperture from the one indicated in the preceding description, for example, due to a change in the regulations, the spirit of the present invention would not be altered by it.

Claims (9)

1. A rear fog lamp for use in a motor vehicle, comprising a light source, a parabolic reflector having its focus arranged in correspondence with the light source whereby to reflect light rays from the light source in a direction substantially parallel to the focal axis of the parabolic reflector, and a diffuser crystal, arranged in a plane perpendicular to the focal axis of the parabolic reflector, and having a prismatic surface facing the interior of the reflector, for the diffusion of the beam of parallel light rays which is reflected by the parabolic reflector, and wherein the parabolic reflector subtends, around its focus, a solid angle not greater than 2n steradians, and the prismatic surface of the diffuser crystal comprises a central portion defined by a plurality of spherical lenses arranged side by side and two lateral portions each of which is defined by a plurality of vertical cylindrical lenses arranged side by side, each cylindrical lens having on its convex surface a central longitudinal depression arranged in a position parallel to the generatrices of the cylindrical lens and defined by two plane faces arranged in a "V" and inclined, so that the light rays which strike the said plane faces are deflected by the diffuser crystal in two directions arranged, respectively, at one side and at the other side of the vertical plane of symmetry of the lamp, and each forming with the said vertical plane an angle of between 80 and 9".

2. A rear fog lamp as claimed in Claim 1, wherein the two lateral portions of the prismatic surface of the diffuser crystal occupy approximately 50% of the total area of the diffuser crystal.

3. A rear fog lamp as claimed in Claim 2, wherein each lateral portion of the prismatic surface of the diffuser crystal comprises four equal vertical cylindrical lenses.

4. A rear fog lamp as claimed in Claim 3, wherein the central longitudinal depression arranged in correspondence with the convex surface of each cylindrical lens occupies approximately 34% of this convex surface.

5. A rear fog lamp as claimed in Claim 4, wherein the diffuser crystal comprises polymethyl methacrylate.

6. A rear fog lamp as claimed in Claim 5, wherein the angle formed between the two faces of the said V depression is substantially equal to 1440.

7. A rear fog lamp as claimed in Claim 1, wherein the diffuser crystal has a rectangular shape and the prismatic surface of this diffuser crystal has four portions arranged in correspondence with the corners of the rectangle, each defined by a plurality of spherical lenses arranged side by side.

8. A rear fog lamp for use in a motor vehicle as claimed in Claim 1 and substantially as herein described with reference to the accompanying drawings.

9. A motor vehicle including a rear fog lamp as claimed in any one of the preceding claims.