United States Patent (19)
`Reiss et al.
`
`USOO6O17138A
`Patent Number:
`11
`(45) Date of Patent:
`
`6,017,138
`Jan. 25, 2000
`
`54 MOTOR VEHICLE HEADLAMP HAVING A
`DISCHARGE LAMP WITH MASKS AND A
`MULTI-ZONE REFLECTOR
`M
`75 Inventors: Benoit Reiss, Paris; Jean-Pierre Aynie,
`Le Pre Saint Gervais, both of France
`73 Assignee: Valeo Vision, Bobigny, France
`
`21 Appl. No.: 08/965,740
`22 Filed:
`Nov. 7, 1997
`30
`Foreign Application Priority Data
`Nov. 8, 1996 |FR)
`France ................................... 96.13667
`(51) Int. Cl. .................................................. F21V 7/00
`52 U.S. Cl. .......................... 362/297; 362/516; 362/517;
`362/518; 362/539; 362/303; 362/305; 362/348;
`362/255; 362/256
`58 Field of Search ..................................... 362/516, 517,
`362/518, 539, 303, 305,348, 297, 255,
`256
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,100,594 7/1978 Gould ...................................... 362/308
`5,086,376 2/1992 Blusseau ......
`... 362/518
`5,299,101
`3/1994 Serizawa ......
`... 362/510
`5,567,044 10/1996 Lopez ...................................... 362/517
`
`5,671,995 9/1997 Serizawa et al. ....................... 362/510
`FOREIGN PATENT DOCUMENTS
`O 466 605 1/1992 European Pat. Off..
`O 645 578 3/1995 European Pat. Off..
`195 43 008 5/1996 Germany.
`2 271 629 4/1994 United Kingdom.
`2 296 559 7/1996 United Kingdom.
`OTHER PUBLICATIONS
`French Search Report dated Aug. 8, 1997.
`Primary Examiner Sandra O'Shea
`ASSistant Examiner Ronald E. DelGizzi
`Attorney, Agent, or Firm Morgan & Finnegan, L.L.P.
`57
`ABSTRACT
`A motor Vehicle headlamp has a discharge lamp, a reflector
`and a Smooth or nearly Smooth cover lens, the reflector being
`adapted to generate a beam below a cut-off line. At least two
`masks are associated with the discharge lamp, and define
`two shadow Zones on the reflector; the reflector includes at
`least two reflective Zones for forming portions of the beam
`below the cut-off line, with different amounts of sideways
`Spreading of the light; and these Zones of the reflector are
`Separated by generally radial transition planes, at least Some
`of which extend into the Shadow Zones. A particular appli
`cation is to enable wide cruising beams to be produced to
`European Standards without any optical anomalies.
`
`27 Claims, 8 Drawing Sheets
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`1
`MOTOR VEHICLE HEADLAMP HAVING A
`DISCHARGE LAMP WITH MASKS AND A
`MULTI-ZONE REFLECTOR
`
`FIELD OF THE INVENTION
`The present invention relates in general terms to motor
`vehicle headlamps. More particularly, it relates to a head
`lamp of the kind in which the reflector has a surface which
`is preferably defined mathematically, and which is adapted
`in particular to form by itself a European Standard cruising
`beam with a light Source consisting of a lamp having a
`certain type of mask or occulting element.
`
`BACKGROUND OF THE INVENTION
`The Company Valeo Vision is the proprietor of numerous
`patents relating to reflective Surfaces which are capable of
`producing by themselves beams of given configurations, and
`in particular European Standard type cruising beams which
`are of Substantial width without any intervention by the
`cover lens of the headlamp. Such Surfaces are entirely
`effective when they are used in cooperation with light
`Sources that consist of incandescent filaments.
`However, there is today a tendency to use, instead of
`filament lamps, discharge lamps which are known to pro
`duce a particularly highlight output. Nevertheless, this type
`of light Source has the disadvantage that the geometry of the
`pattern of emission of light by the Source is not well
`controlled, so that when the above mentioned carefully
`defined reflective Surfaces are used in conjunction with
`discharge lamps, there is a high degree of degradation of the
`photometry of the resulting light beam, in particular as
`regards the generation of a clean cut-off.
`In addition, any cut-out, interruption or discontinuity in
`the surface of the reflector, which may be desirable in the
`case where it is required to generate portions of the beam
`having different positions within the final beam, and having
`different degrees of widthwise spread, will run the risk of
`giving rise, having regard to the high energy of the light
`Source, to parasitic radiation which tends to dazzle the
`drivers of vehicles travelling in the opposite direction.
`Thus, with a discharge lamp the designer has a natural
`tendency to continue to use entirely Smooth reflective
`Surfaces, which may be striated, and this imposes constraints
`on the technology.
`This is why it has been proposed, in particular in United
`Kingdom patent specification GB 2 296 559, to place any
`cut-outs, interruptions or discontinuities, which may be
`present for example in the transition region between two
`Zones of the reflector, in correspondence with Shadows
`defined by masks associated with the light Source. The
`arrangements described in that patent are however limited to
`the case where the reflective Surfaces are parabolic. The
`Zone of the reflector which forms the main part of the beam
`(i.e. an upper Zone) has a Surface which corresponds to the
`same paraboloid over its whole extent. The reflector is then
`unable to generate by itself a directly usable wide beam, So
`that the use of a striated cover lens is necessary, with the
`usual disadvantages of this type of lens, Such as loSS of light
`flux in the lens, limitations in the degree of Spread that can
`be obtained using striations, and So on.
`DISCUSSION OF THE INVENTION
`An object of the present invention is to overcome the
`above mentioned drawbacks in the current State of the art,
`and to propose a headlamp which has a reflector which,
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`although it may have discontinuities and although the light
`Source is a discharge lamp, will at the same time generate a
`cut-off beam of Satisfactory photometry, Substantially with
`out any parasitic light.
`According to the invention, a motor Vehicle headlamp,
`comprising a discharge lamp, a reflector, and a Smooth or
`very slightly deviating cover lens, the reflector being
`adapted to generate a beam Situated below a cut-off line, is
`characterised in that at least two masks, defining two shadow
`Zones on the reflector, are associated with the lamp, and in
`that the reflector comprises at least two reflective Zones for
`forming portions of the beam situated below the Said pre
`determined cut-off line with degrees of spread in different
`widths, and in that the Said Zones are Separated by generally
`radial transition planes, at least Some of which extend into
`the Said Shadow Zones defined by the Said maskS.
`In a headlamp according to the invention, any cut-outs,
`interruptions or discontinuities between the various Zones of
`the reflector, which are present due to the fact that the Zones
`of the reflector spread the light sideways by different
`amounts, will not cause any prejudicial anomalies in the
`beam.
`The reflector preferably includes a first Zone extending
`essentially Sideways with respect to the lamp.
`According to a preferred feature of the invention, the Said
`first Zone extends between a first half plane and a Second half
`plane, the Said half plane being inclined above and below the
`horizontal respectively, and the half plane inclined below the
`horizontal extends into one of the two shadow Zones.
`Preferably, the said first Zone comprises a first sub-zone
`close to the lamp and having a horizontal generatrix based
`on a parabola So as to cause the reflected radiation to be
`convergent, and a second sub-zone spaced away from the
`lamp and having an essentially parabolic horizontal genera
`trix.
`The generatrices of the first and Second Sub-Zones are
`preferably joined together with continuity and without any
`interruption of slope.
`According to another preferred feature of the invention,
`the Second Sub-Zone includes a plurality of striations for
`Spreading the light, the Said striations being carried on a base
`Surface generated with the aid of the Said generatrix.
`Preferably, these spreading striations are distributed in at
`least two groups of Striations, with at least a first group
`providing horizontal spreading of the light and at least one
`Second group providing spreading of the light in an oblique
`direction which is essentially parallel to an inclined portion
`of the said cut-off line.
`The spreading striations preferably include a third group
`of striations defining a curve guide which ensures continu
`ouS transition from one striation to another, as between
`striations of the first group and striations of the Second
`grOup.
`Preferably, the said first Zone extends between a radial
`half plane inclined at about 25 degrees above the horizontal
`and a radial half plane inclined at about 30 degrees below the
`horizontal.
`According to a further preferred feature of the invention,
`the reflector includes a Second Zone extending both above
`the first Zone and into a lateral portion of the reflector
`opposed to the first Zone.
`This second Zone is preferably delimited by the said first
`half plane and by a third half plane situated substantially in
`alignment with the first half plane and extending into the
`other shadow Zone. The third half plane is preferably a radial
`half plane inclined at about 20 degrees below the horizontal.
`
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`Preferably, the Second Zone has a base Surface, the hori
`Zontal generatrix of which has a profile giving a horizontal
`deviation which, going from the center towards the side
`edges of the Zone, varies between two nil values, passing
`locally through maximum values of convergence and of
`divergence. The Said horizontal generatrix of the Second
`Zone is preferably continuous and without interruption of
`Slope. Alternatively or in addition, the Said horizontal gen
`eratrix is preferably of parabolic form in two lateral Sub
`Zones. In that case, Striations, for horizontal spreading of the
`light, are preferably formed on the base Surfaces of the two
`lateral Sub-Zones.
`According to yet another preferred feature of the
`invention, the reflector includes a third Zone extending into
`its lower part, between the first and Second Zones.
`Preferably, the said third Zone has a surface, the horizontal
`generatrix of which has a profile giving a horizontal devia
`tion which, going from the center towards the lateral edges
`of the Zone, Varies between two nil values, passing locally
`through maximum values of convergence and of divergence.
`The Said horizontal generatrix of the Second Zone is prefer
`ably continuous and without any interruption of slope.
`Alternatively or in addition, the Said horizontal generatrix is
`preferably of parabolic form in two lateral Sub-zones.
`The surface of the said third Zone is preferably entirely
`Smooth and without any interruption of Slope.
`In appropriate embodiments of the invention, the genera
`trices of the base Surface of the Second Zone and of the
`Surface of the third Zone are preferably not coincident, and
`the Said generatrices may have different base focal lengths.
`In this latter case, the focal length of the generatrix of the
`surface of the third Zone is preferably shorter than the focal
`length of the generatrix of the Surface of the Second Zone.
`According to a still further preferred feature of the
`invention, the geometric axis of the Said third Zone is offset
`downwardly with respect to the geometric axis common to
`the first and Second Zones, and extends parallel to the Said
`common axis. This downward offset may for example be
`about 1 to 2 millimeters.
`According to another preferred feature of the invention,
`one of the Zones of the reflector has striations applied by
`geometric projection on a Smooth base Surface, and the
`striations, before projection, have different curvatures on
`either Side of a Summit.
`Further features and advantages of the invention will
`appear more clearly on a reading of the following detailed
`description of a preferred embodiment of the invention,
`which is given by way of non-limiting example only and
`with reference to the accompanying drawings.
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a front view of a reflector for a motor vehicle
`headlamp in accordance with the invention.
`FIG. 2 is a front view of the associated discharge lamp and
`its maskS.
`FIG. 3 shows diagrammatically the profile of a striation
`used in a first Zone of the reflector of FIG. 1.
`FIG. 4 shows the horizontal generatrix of the said first
`Zone of the reflector.
`FIG. 5 is a detailed view, in front elevation, of the said
`first Zone of the reflector.
`FIGS. 6a and 6b show the photometric distribution of the
`beam which is obtained, respectively, with a first Sub-Zone
`of the first Zone of the reflector and with the whole of the first
`Zone, in the absence of the cover lens.
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`FIG. 7 shows the horizontal generatrix of a second Zone
`of the reflector of FIG. 1.
`FIG. 8 is a detailed view, in front elevation, of the said
`Second Zone of the reflector.
`FIGS. 9a and 9b show the photometric distribution of the
`beam obtained, respectively, with the Second Zone of the
`reflector and with the whole of the first and second Zones, in
`the absence of the cover lens.
`FIG. 10 shows the horizontal generatrix of a third Zone of
`the reflector of FIG. 1.
`FIG. 11 is a view in vertical axial cross section of the said
`third Zone and of the arc generated by the discharge lamp.
`FIG. 12 shows the photometric distribution of the beam
`which is obtained with the third Zone of the reflector, in the
`absence of the cover lens.
`
`DESCRIPTION OF A PREFERRED
`EMBODIMENT OF THE INVENTION
`In the description that follows, the various features of the
`reflector will be described with the aid of an orthogonal or
`Cartesian frame of reference (O, x, y, z), where O is a
`reference point Situated in the base region of the reflector, OZ
`is the axis of the reflector, Oy is vertical, and Ox is
`perpendicular to Oy and OZ. In the event that certain
`documents cited in the following description refer, for the
`definition of Surface equations, to frames of reference with
`different orientations, a person familiar with this technical
`field will of course be able to make the necessary adaptations
`by Simple permutation of variables in the equations.
`In addition, the description will be given in respect of a
`headlamp which is adapted for driving on the right, that is
`to Say the nearSide of the vehicle is on the right, the
`headlamp having a normalised cut-off which rises on the
`right. The person skilled in the art will be able to make the
`necessary adaptations for driving on the left, for example by
`Symmetrical transformation with respect to a vertical plane
`passing through the axis of the reflector.
`With reference to FIG. 1, this shows a reflector for a
`headlamp (or headlight) in accordance with the invention.
`This reflector is adapted to give by itself the desired light
`distribution for a normal cruising beam of the European
`type, that is to say the front lens (not shown) of the headlight
`is Smooth or has no more than a very slight deviating
`capability.
`The reflector is indicated by the general reference numeral
`1, and is Sub-divided into three distinct reflective Zones 11,
`12 and 13 which are joined to each other in a manner that is
`either continuous or discontinuous. The reflector is adapted
`to cooperate with a discharge lamp 2 which is shown
`diagrammatically in FIG. 2 and which incorporates two
`masks or occulting elements, which in this example consist
`of bands 21 and 22 of opaque paint applied to the globe 20
`of the discharge lamp.
`These opaque bands 21 and 22 are configured in Such a
`way as to mask the radiation issuing from the arc of the
`discharge lamp in two well-defined angular Sectors, each of
`which has an extent of 25 degrees. These Sectors consist of
`a first sector S1, and a second sector S2. The sector S1
`extends between two half planes P11 and P12, one of which
`is horizontal, the other being inclined at 25 degrees below
`the half plane P11, and the two half planes passing through
`the axis of the discharge lamp. The Second Sector S2 extends
`between two half planes P21 and P22 inclined respectively
`at 15 degrees and 40 degrees below the horizontal, these two
`half planes also passing through the axis of the discharge
`lamp.
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`These masked Zones S1 and S2, or Shadow Zones, are
`conventionally arranged in cooperation with the discharge
`lamp in order to facilitate formation by the reflector of a
`normalised European cut-off beam, which is delimited by
`one horizontal half plane on the offside and one half plane
`rising at 15 degrees above the horizontal on the nearSide. It
`will of course be understood that these numerical values are
`in no way limiting.
`In addition, the discharge lamp may, in another version,
`include masks or occulters which are separate components
`fitted around the globe of the discharge lamp, and which
`consist for example of metallic bands for absorbing light.
`Two of the three radial half planes of transition between
`the three reflective Zones 11, 12 and 13 of the reflector lie in
`the sectors S1 and S2 which are masked by the opaque bands
`21 and 22. In particular, the transition half plane PT12
`between the Zone 11 and the Zone 12 of the reflector is in this
`example inclined at +25° above the horizonal, while the
`transition half plane PT23 between the Zones 12 and 13 is
`inclined by 20 below the horizontal on the lateral side
`opposed to the discharge lamp. The third transition half
`plane, PT31, is inclined at -30 below the horizontal on the
`same side as the half plane PT12.
`Again, these values are of course in no way limiting, in
`particular with a view to increasing the Symmetry of the
`Surface with respect to the vertical axial plane yOZ, and
`facilitating adaptation of the headlight for driving on the left
`hand side of the road.
`The Structure and optical function of each of the Zones 11,
`12 and 13 of the reflector 1 will now be described.
`Zone 11 (FIGS. 4 and 5)
`The essential purpose of the Zone 11 is to ensure that the
`beam is brought into a region situated below and close to the
`inclined right hand half cut-off line, with a view to giving
`Satisfactory illumination of the nearside (right hand) verge
`of the road at distances of the order of 40 meters and more.
`This Zone 11 is defined by a base surface SB1, on which
`striations are applied for spreading the light. The method of
`applying these striations will be preferably that which is
`described in European patent specification No. EP 0 645
`578A in the name of the company Valeo Vision.
`The base Surface is preferably a portion of a paraboloid
`with a modified base, Such as is described in detail in French
`patent specification No. FR 2609 148A in the name of the
`company Valeo Vision, to which reference should be made
`for more detail.
`However, it will be recalled here that these Surfaces
`include Vertical Sections which are parabolas. The focal
`length f1 of these parabolas is preferably of the order of 27
`mm, while their focus F1 is situated on the axis Oz and offset
`towards the rear with respect to the center, along the axis OZ
`of the arc produced by the lamp. This axial offset is
`preferably in the range between about 2 and 6 mm.
`FIG. 4 shows the horizontal generatrix of such a base
`Surface, this base Surface being a parabola of the same focal
`length (27 mm) and having the same focus (offset as
`indicated above), but having a central portion, or base
`portion, which is modified in order to ensure convergence of
`the reflected rays. The dimensioning of this base portion is
`determined in Such a way that the half angle of this
`convergence, and therefore that of the divergence which
`follows in the distance, is preferably of the order of 20
`degrees. In addition, as is shown in FIG. 4, the transition
`between the modified portion and the non-modified portion
`preferably lies at a lateral distance of the order of 50 mm
`with respect to the optical axis.
`The modified central Sub-Zone 111 of the Zone 11 is used
`just as it is, that is to say it is a Smooth, continuous reflecting
`Surface, without either cut-outs or sharp changes of contour.
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`The outer sub-zone 112, which is purely parabolic, of the
`Zone 11 extends between the central Sub-zone 111 and the
`Side edge of the reflector, and receives a certain number of
`striations, which are not shown on the generatrix of FIG. 4
`but which are arranged to ensure well-defined spreading of
`the light, as will be described below.
`This outer Sub-Zone 112 preferably has a eight regions of
`striations which are defined as follows.
`A first region 1125 lies above a horizontal separation
`plane P156 which is offset upwardly with respect to the axial
`plane XOZ, and extends as far as the transition half plane
`PT12 between the Zones 11 and 12. The region 1125
`comprises vertical striations S15, the profile of which is
`preferably Symmetrical with respect to a vertical central
`plane, So that these Striations may for example be cylindri
`cal. The radius of these striations is determined in Such a
`way that they produce a spreading of the light which is
`homogeneous with the Spreading obtained by the adjacent
`portion, lying above the half plane PT12, of the Zone 12
`which will be described later on in this description.
`A region 1126 of striations is situated between the hori
`Zontal plane P156 and a further horizontal plane P164 which
`lies below the plane 156 but above the axial plane XOy. The
`region 1126 consists of further vertical striations S16, which
`are more compacted, i.e. they are narrower and more
`numerous, but which provide a spreading effect Similar to
`that obtained from the striations S15.
`A region 11242 of striations lies between the plane P164
`and a plane P1442 which is inclined at 7.5 degrees down
`wardly with respect to the plane P164 about a point C41
`lying in the plane P164 slightly into the sub-zone 112 of the
`Zone 11. This region 11242 consists of striations S142 which
`are curved with respect to a center of curvature at the point
`C41.
`A region 1124 of striations lies between the plane P1442
`and a plane P1441 which is parallel to, and lies below, the
`plane P1442. The region 1124 consists of straight striations
`S14, the directions of which are inclined at 7.5 degrees with
`respect to the vertical plane yOZ.
`A region 11241 of striations lies between the plane P1441
`and a plane P1443 which is inclined at 7.5 degrees down
`wardly with respect to the plane P1441 about a point C42.
`The point C42 is situated in the plane 1441, slightly into the
`sub-zone 112. The region 11241 consists of striations S141
`which are curved with respect to a center of curvature at the
`point C42.
`A region 1123 of striations lies between the inclined plane
`P143 and a plane P132 which is parallel to the plane P143
`and offset downwardly from it. The region 1123 consists of
`straight striations S13 the directions of which are inclined at
`15 degrees with respect to the Vertical.
`A region 1122 of striations lies between the plane P132
`and a plane P121 which is parallel to the plane P132 and
`offset downwardly from it. The region 1122 consists of
`striations S12 which are aligned with the striations S13.
`Finally, a region 1121 of striations lies between the plane
`P121 and the transition half plane PT31 at which the Zone 11
`joins the Zone 13. The region 1121 consists of striations S11
`which are aligned with the striations S12 and S13.
`The profiles of the striations S14 inclined at 7.5 degrees,
`and those of the striations S13, S12 and S11 which are
`inclined at 15 degrees, are determined in Such a way as to
`provide spreading, below and along the inclined half cut-off
`line of the normalised cut off, and to favour illumination in
`the vicinity of the normalised measuring points 75R and
`5OR of the projection screen.
`In this regard, the striations S11 to S14 preferably have an
`asymmetrical profile, Such as is shown diagrammatically in
`
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`projection in FIG. 3, with a portion ST" and a portion ST".
`The portion ST" has a reduced radius of curvature, such as
`to provide a high degree of Spreading of the light outwardly,
`that is to Say towards the right of the projection Screen. The
`portion ST" has a greater radius of curvature and is adapted
`for only slight inward spreading of the light, this being in
`order to preserve a major quantity of light in the central
`region of the beam, above and in the vicinity of the central
`bend of the cut-off line.
`In addition, the circular guide striations S142 and S141
`have downwardly evolving profiles, Such as to join, with
`continuity and without any cut-outs or breaks, on the one
`hand the striations S16 and S14 (for the striations S142), and
`on the other hand, the striations S14 and S13 (for the
`striations S141).
`Reference is now made to FIG. 6a, which shows a set of
`isoluX curves on a Screen which is graduated in percentages.
`These curves represent the photometry of that part of the
`beam which is generated by the Smooth sub-zone 111. This
`part of the beam rises slightly above the horizontal axis, So
`as to fill the Zone situated below the normalised inclined half
`cut-off line HC.
`By contrast, FIG. 6b, to which reference is also made,
`shows the light distribution obtained with the whole of the
`Zone 11. It will be observed that the maximum light con
`centration defining the Scope of this part of the beam lies in
`alignment with the bend in the cut-off line and immediately
`above the inclined half cut-off line, which is entirely appro
`priate for good illumination of the nearSide edge of the road,
`and contributes to Visual comfort.
`Zone 12 (FIGS. 7 to 9)
`This Zone is designed to give increased horizontal Spread
`ing of the light, So as to give the beam the Substantial width
`that is a factor in Visual comfort.
`The Zone 12 preferably includes a base surface SB2 (on
`which striations are applied as will be seen later herein),
`which is constituted, either by a portion of a Surface which
`automatically generates horizontal cut off, as is defined in
`particular in French patent specification No. FR 2536 503A,
`or by a portion of a Surface for automatically generating
`horizontal cut-off with a modified base in the manner defined
`in French patent specification No. FR 2609 148A, or again
`by a portion of a Surface for automatically generating
`horizontal cut-off with modified intermediate Zones, in the
`manner defined in either one of French patent Specifications
`Nos. FR 2 639888A and FR 2 664 677A. Reference should
`be made to the descriptions in these various cited Specifi
`cations for more detail as to the construction of Such
`Surfaces.
`In the first case, the vertical croSS Sections of the Surface
`are portions of parabolas with evolving foci, Such as to put
`all the images of the light Source below and level with a
`horizontal cut-off line defined by the horizontal axis hh of
`the projecting Screen.
`In the Second and third cases, the vertical croSS Sections
`of the Surface are parabolas. Their focal length is preferably
`about 24 mm, while their focus is preferably offset towards
`the rear with respect to the arc generated by the discharge
`lamp. AS regards the horizontal generatrix of Such a Surface,
`in the first case this consists of a parabola, while in the
`Second case it is again a parabola, but the base Zone of this
`parabola is modified So as to give convergence of the
`reflected light, and therefore spreading of the light in the far
`field. In the third case, the croSS Section is a parabola which
`is modified in its central Zone by a group of Zones which are
`Successively divergent and/or convergent and/or parabolic.
`In the present example the base Surface is configured in
`accordance with the dimensioning illustrated in the FIG. 7b
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`6,017,138
`
`8
`of French patent specification No. FR 2 664 677A. In other
`words, the base Surface SB2 of the Zone 12 includes, in its
`left hand part, and reading from left to right in FIG. 7 (that
`is to say in its right hand part and from right to left in FIG.
`8), sub-zones 121, 122 and 123. The sub-zone 121 is situated
`between the planes X1 and X2, and it has a horizontal
`generatrix. In the Sub-Zone 122, the horizontal generatrix is
`modified So as to start (going from the outside towards the
`inside) from a nil horizontal deviation at the point X2,
`passing through a maximum horizontal convergence point at
`point X3 and reverting to nil horizontal deviation at point
`X4. In the sub-zone 123, the horizontal generatrix is modi
`fied So that, starting with nil horizontal deviation at the point
`X4 and going from the outside towards the inside, it passes
`through a point of maximum horizontal divergence at point
`X5, and reverts to nil horizontal deviation at a point lying in
`the plane of the vertical axis XOz.
`These various profiles of the horizontal generatrix of the
`base Surface together constitute a continuous and derivable
`line (that is to say without any Sudden interruption in its
`slope).
`The right hand part of the generatrix of the Said base
`Surface may or may not be Symmetrical with the left hand
`part, but it follows the same evolutions in terms of horizontal
`deviation. This right hand part consists of Sub-Zones 124,
`125 and 126 corresponding to the Subzones 123, 122 and
`121 respectively, with characteristic points X6 to X10 cor
`responding to the points X5, X4, X3, X2 and X1 respec
`tively.
`In addition, the quantification of the equation of this base
`Surface is preferably Such that the maximum horizontal
`convergence given by the sub-zones 122 and 125 is about 25
`to 30 degrees, and the maximum horizontal divergence
`given by the sub-zones 123 and 124 is about 20 degrees.
`The Zone 12 has the specific striation pattern which will
`be described below. First of all, the Sub-zones 122 to 125,
`with modified horizontal generatrix, are left as they are: they
`are smooth (continous and derivable) over their whole
`extent. By contrast, each of the two endmost sub-zones 121
`and 126, with parabolic base Surfaces, includes a set of
`Vertical guide Striations which are designed to give a Sub
`Stantial degree of horizontal spreading of the light. These
`striations are indicated in FIG. 8 at S121 and S126
`respectively, and preferably have a profile, before being
`projected on the Surface, in which the radius of the profile is
`in the range between 20 and 40 mm, the width of the profile
`preferably being in the range between 6 and 8 mm.
`In addition, and in the manner described in the above
`mentioned European patent specification No. EP 0 645
`578A, the striations S121 and S126 may have evolving
`profiles Such that striations aligned vertically with each other
`can join each other without any discontinuity, which enables
`optical anomalies Such as dazzling parasitic radiation to be
`avoided.
`In a modified version, not only the Sub-zones 121 and 126
`are given Striations, but So also are parts of the Sub-Zones
`122 and 125 which are adjacent to the sub-zones 121 and
`126, and which extend between the horizontal planes in the
`positions X2