Patent Description:
The term automotive light is understood to mean indifferently a rear automotive light or a front automotive light, the latter also known as a headlight.

As is known, a vehicle light is a lighting and/or signalling device of a vehicle comprising at least one external light of the vehicle having a lighting and/or signalling function toward the outside of a vehicle such as, for example, a position light, a direction indicator light, a brake light, a rear fog light, a reversing light, a low beam headlight, a high beam headlight, and the like.

The automotive light, in its simplest form comprises a container body, a lenticular body and at least one light source.

The lenticular body is placed so as to close a mouth of the container body so as to form a housing chamber. The light source is arranged inside the housing chamber, which may be directed so as to emit light towards the lenticular body, when powered with electricity.

The method of manufacture of an automotive light, once assembled the various components, must provide for the attachment and hermetic sealing of the lenticular body to the container body.

Such sealing and attachment is usually performed by laser welding.

The laser welding techniques of the prior solutions are not however free from drawbacks, since the laser welding processes of the lenticular bodies to the container body are rather complex, slow and therefore expensive.

It is known of to perform simultaneous laser welding in which, initially, the respective edges or perimeter profiles of the lenticular body and the container body which are counter-shaped so as to interface, are placed in mutual contact in the assembly configuration of the vehicle light, at a weld interface. Simultaneous welding is then carried out simultaneously on the entire weld interface.

During the welding step, the container body acts as an absorbing member in relation to the light beam emitted by the laser source while the lenticular body acts as a transmissive member of said light beam.

The lenticular body substantially transmits, without absorbing it, the laser beam incident thereon which reaches the weld interface. At this point, the laser beam is absorbed by the perimeter edge of the container body which warms up to softening point.

The softening, accompanied by a reciprocal pressure of approach between the bodies, determines the partial interpenetration between the profiles and therefore the realization of a weld bead on said weld interface.

As may be imagined, for the quality and strength of the weld bead the incident energy on the entire interface must be evenly measured.

In this regard, simultaneous laser welding techniques are known in which a laser source, for example a laser diode is connected to a bundle of optical fibres which have the function of spreading the laser beam generated and directing it towards a plurality of points on the weld interface.

The optical fibres end in a light guide that distributes the light beams on said weld interface.

This solution is not without disadvantages.

In fact, the light guide is not able to distribute the energy of the light beams uniformly throughout the weld interface, completely nullifying the effect of the discrete distribution of the optical fibres. In fact, despite the presence of the light guide, peaks of energy interspersed with minimum energy are always observable and such a non-optimal wave distribution of energy has negative repercussions on the quality of the weld. Solutions according to prior art are disclosed, for example, in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>.

The object of the present invention is therefore to perform the welding of vehicle lights using equipment and a related simultaneous laser welding method that can overcome the technical drawbacks related to the solutions of the prior art.

This purpose is achieved by a welding equipment of a vehicle light according to claim <NUM>, as well as by a welding method of a vehicle light according to claim <NUM>.

Other embodiments of the present invention are described in the dependent claims.

Further characteristics and advantages of the present invention will be more clearly comprehensible from the description given below of its preferred and non-limiting embodiments, wherein:.

The elements or parts of elements common to the embodiments described below will be indicated using the same reference numerals.

With reference to the aforementioned figures, reference numeral <NUM> globally denotes an automotive light, which the description which follows refers to without by so doing losing its general application.

As mentioned above, the term vehicle light is understood to mean indifferently a rear vehicle light or a front vehicle light, the latter also known as a headlight.

As is known, the vehicle light comprises at least one light outside of the vehicle having a lighting and/or signalling function, as for example a position light, which may be a front, back, side position light, a direction indicator light, a brake light, a rear fog light, a reversing light, a low beam headlight, a high beam headlight, and the like.

The automotive light <NUM> comprises a container body <NUM>, usually of polymeric material, which typically permits the attachment of the automotive light <NUM> to the relative vehicle.

For the purposes of the present invention, the container body <NUM> may be any shape, size and assume any position: for example, the container body may not be directly joined to the body or other fixtures of the associable vehicle.

According to one embodiment, the container body <NUM> delimits a containment housing <NUM> which houses at least one light source (not shown) electrically connected to electric connection means for supplying power to the same, and adapted to emit a light beam to propagate outside the automotive light. For the purposes of the present invention the type of light sources used is irrelevant; for example, the light source is a light source of light emitting diodes (LED).

The container body <NUM> is delimited by a first perimetral profile <NUM>.

A lenticular body <NUM> in turn delimited by a second perimetral profile <NUM> is joined to the container body <NUM>.

The lenticular body <NUM> is applied to the container body <NUM> so as to close said containment seat <NUM> housing at least one light source.

For the purposes of the present invention the lenticular body <NUM> may be external to the vehicle light <NUM>, so as to define at least one outer wall of the vehicle light directly subject to the atmosphere.

The lenticular body <NUM> closes the containment seat <NUM> and is suitable to be crossed by the beam of light produced by the light source which is transmitted to the outside of the containment seat <NUM>.

To such purpose, the lenticular body <NUM> is made of at least partially transparent or semi-transparent or translucent material, and may also comprise one or more opaque portions, so as to allow in any case the at least partial crossing of the light beam produced by the light source.

According to possible embodiments, the material of the lenticular body <NUM> is a resin such as PMMA, PC and the like.

The first and second perimetral profile <NUM>,<NUM> of the container body <NUM> and of the lenticular body <NUM> are at least partially counter-shaped to each other so as to interface, in an assembled configuration of the vehicle light <NUM>, at a weld interface <NUM>.

As better explained below, following the laser welding at the weld interface <NUM>, the welding bead is formed and the partial interpenetration between the perimetral profiles <NUM>,<NUM> is realized.

The assembly of the automotive light <NUM> comprises the step of joining at least partially to each other the respective first and second perimetral profiles <NUM>,<NUM>. For example, the step is provided for of arranging the lenticular body <NUM> to close the containment housing <NUM> of the container body <NUM> so as to join the respective first and second perimetral profiles <NUM>,<NUM>.

The production method of the vehicle light provides for joining to each other the lenticular body <NUM> and the container body <NUM>, at said perimetral profiles <NUM>,<NUM>, by means of simultaneous laser welding.

The welding of the lenticular body <NUM> to the container body <NUM> is a simultaneous laser welding, wherein the light beam emitted by at least one laser source <NUM> is routed towards the perimetral profiles <NUM>,<NUM> so as to reach the first perimetral profile <NUM> of the container body <NUM> after passing through the lenticular body <NUM>.

During the simultaneous laser welding step, the container body <NUM> acts as an absorbing member in relation to the light beam emitted by the laser source <NUM> and the lenticular body <NUM> acts as a transmissive member of said light beam.

Welding takes place by using special simultaneous laser welding equipment <NUM>, particularly suitable for welding a vehicle light <NUM> in which the simultaneous welding of the lenticular body <NUM> and the container body <NUM> is provided for.

Such simultaneous laser welding equipment <NUM> of a vehicle light <NUM> according to the present invention comprises a placement support (not shown in the drawings) for supporting the container body <NUM> and the lenticular body <NUM> of the vehicle light <NUM> to be welded together at the respective first and second perimetral profiles <NUM>,<NUM> joined at the weld interface <NUM>. Without detracting from its general application, the equipment <NUM> may be that described in the <CIT> on behalf of the same applicant.

The equipment <NUM> thus comprises a plurality of laser sources <NUM>, preferably a plurality of laser diodes <NUM>, suitable to emit light beams.

According to possible further embodiments, the laser source <NUM> is not limited to a laser diode but may comprise a CO2 laser source, wherein the laser beam is produced by a mixture of gases comprising CO2, or a YAG laser, in which the laser beam is produced by a solid-state crystal, or further types of laser sources.

The welding equipment <NUM> further comprises a plurality of optical fibres <NUM> each extending from an input end <NUM> to an output end <NUM>. In particular, the optical fibres <NUM> are associated with the laser sources <NUM> at said input end <NUM>, through which the light beam is channelled inside the fibre <NUM> and internally transmitted by subsequent reflections until it comes out through the output end <NUM>.

According to a possible embodiment, a single optical fibre <NUM> is associated at its input end <NUM> to each laser source <NUM>, <NUM> so as to receive, channel and transmit towards the weld interface <NUM>, the light beam produced by said corresponding laser source <NUM>,<NUM>.

In other words, each optical fibre <NUM> is associated at its input end <NUM> to a single respective laser source <NUM>, <NUM> so as to receive, channel and transmit towards the weld interface <NUM>, the light beam produced by said corresponding laser source <NUM>, <NUM>.

Thus, only one corresponding optical fibre <NUM> may be associated with each light source <NUM>,<NUM>.

Of course, in a different embodiment not illustrated, the simultaneous laser welding equipment <NUM> may comprise at least one laser source <NUM> associated with a bundle of optical fibres <NUM> which then branch along the weld interface <NUM>.

Optical fibres <NUM>, in a known manner, transmit by total internal reflection the light beams introduced through their input ends <NUM> to their output ends <NUM>.

Preferably, the output ends <NUM> of the optical fibres <NUM> are spaced apart by a pitch of between <NUM> and <NUM>.

The equipment <NUM> comprises a fibre-holder support device <NUM> for supporting the output ends <NUM> of the optical fibres <NUM>, suitable for locking at predetermined positions said output ends <NUM> of said optical fibres <NUM>, to arrange the latter so as to appropriately radiate the entire weld interface <NUM>, as better described below.

Furthermore, the equipment <NUM> comprises a light guide <NUM> associated with the fibre-holder support device <NUM>.

In addition, the light guide <NUM> is shaped to receive, at a light input wall <NUM>, the light beams coming out of the output ends <NUM> of the optical fibres <NUM> and to convey them through a light output wall <NUM> thereof to the weld interface <NUM>.

The light guide <NUM> is provided with at least one hollow seat <NUM> extending between the light input wall <NUM> and the light output wall <NUM>.

The light guide <NUM> is connected in cascade to the fibre-holder support device <NUM>, wherein each output end <NUM> of each optical fibre <NUM> blocked in said fibre-holder support device <NUM> is placed adjacent to the light input wall <NUM> of the light guide <NUM> so as to be overlooking said hollow seat <NUM> of the light guide <NUM>.

Preferably the seat <NUM> of the light guide <NUM> delimits a continuous perimetral contour <NUM>, counter-shaped to said weld interface <NUM>.

The seat <NUM> of the light guide <NUM> is delimited by side walls <NUM>,<NUM> preferably flat and parallel to each other with respect to a cross-section plane perpendicular to a curved abscissa S-S defining said perimetral contour <NUM>.

According to a further embodiment, the seat <NUM> of the light guide <NUM> is delimited by side walls <NUM>,<NUM> which are flat and divergent from each other at the output end <NUM> of each optical fibre <NUM> with respect to a cross-section plane perpendicular to a curved abscissa S-S defining said perimetral contour <NUM>. In other words, the side walls <NUM>,<NUM> may diverge by moving from the laser sources <NUM> to the weld interface <NUM>.

Therefore, the perimetral contour <NUM> is a closed contour, which may extend along a closed polyline, which follows the profile of the weld interface <NUM>.

The closed contour may comprise curved segments, or may comprise a combination of straight and/or curved segments connected together seamlessly to form a closed profile.

Advantageously, the light guide <NUM> comprises diffusion means <NUM> that pass through said perimetral contour <NUM> of the seat <NUM> of the light guide <NUM> internally, so as to intercept and influence the light beams that propagate inside said seat <NUM>.

The diffusion means <NUM> are shaped to expand the light beams along a direction substantially tangent to a curvilinear abscissa S defining said perimetral contour <NUM>, before it comes out of the light output wall <NUM> and/or at the latter.

In other words, the diffusion means <NUM> can be arranged both inside the light guide <NUM> and at its light input wall <NUM> or its light output wall <NUM>.

According to a possible embodiment, (<FIG>, <FIG>), said diffusion means <NUM> comprise refractive diffusion means <NUM>, made of a material having a different refractive index with respect to a filler medium that fills the perimetral contour <NUM> of the seat <NUM> and suitable to be crossed by the light beams incident on it.

Typically, the filling medium comprises air and therefore the light guide <NUM> is a so-called negative light guide.

It is also possible to provide that said filler medium comprises, for example, of a polymeric material such as PMMA, which is obviously transparent to the light beam. In this case a positive light guide is spoken of.

In addition, the light guide <NUM> may also be partially hollow and partially filled with a filler medium.

According to one embodiment, said refractive diffusion means <NUM> comprise a film <NUM> of polymer material transparent to the light beam and having a different refraction index to the refraction index of the filler medium of the perimetral contour <NUM> of the seat <NUM>.

According to a possible embodiment, (<FIG>), said film <NUM> comprises a plurality of optics <NUM> configured to expand the light beams incident thereon.

More specifically, the light guide <NUM> has a pocket <NUM> extending inside the light guide <NUM> in order to trace the perimeter of the weld interface <NUM>. The film <NUM> is placed inside said pocket <NUM> so to cross the entire hollow seat <NUM> of the light guide <NUM>. In fact, the hollow seat <NUM> is entirely interrupted by the film <NUM>, so that a ray of the light beam propagated inside the hollow seat <NUM> of the light guide <NUM> is influenced by said film <NUM>. In a preferred embodiment, the film <NUM> is anisotropic, i.e., provided with micro-optics <NUM> oriented so as to expand the light beams in a direction T substantially tangent to said curvilinear abscissa S along the entire perimetral contour <NUM>.

An anisotropic film is in fact more efficient for the purpose than an isotropic one. However, since the geometry of the weld interface <NUM> is complex, it is necessary to break the anisotropic film <NUM> into segments so that the micro-optics <NUM> are always oriented so as to longitudinally expand the light beams along the perimetral contour <NUM> and not transversely. In other words, the direction of diffusion of the film <NUM> must always be tangent to the weld interface <NUM>. The segments of the film <NUM> are then laid inside the pocket <NUM> seamlessly along the perimetral contour <NUM> until it is entirely traced. Preferably the film <NUM> is located at a distance greater than <NUM> from the outlet end <NUM> of the optical fibres <NUM>, for the laser powers normally used in headlights. Advantageously, the film <NUM> constitutes a barrier for unwanted dirt fragments, which may accumulate over time to the detriment of the functioning of the simultaneous laser welding equipment <NUM>.

The diffusion effect obtained on the light beam is schematized in <FIG> in which the opening that the light beam undergoes is evidenced, in said tangent direction T-T to the main extension direction S-S of the perimetral contour <NUM>, after having crossed the refractive diffusion means <NUM>,<NUM> such as the film <NUM>.

According to a possible embodiment, with particular reference to <FIG>, said diffusion means <NUM> comprise reflective diffusion means <NUM> that reflect and expand the light beams incident thereon.

For example, said reflective diffusion means <NUM> comprise at least one pin <NUM> placed facing the output end <NUM> of each corresponding optical fibre <NUM>.

The pin <NUM> is positioned so that the output end <NUM> is interposed between the pin <NUM> and the weld interface <NUM> so as to reflect towards the weld interface <NUM> the light beam incident on it.

According to one embodiment, the pin <NUM> is oriented so that a perpendicular N to the pin identifies, with an optical axis F of the corresponding optical fibre, an angle of incidence γ of between <NUM> and <NUM> degrees. The term optical axis means a main axis of propagation and symmetry of the light beam emitted by the laser source <NUM>.

More specifically, in an embodiment illustrated in <FIG>, the light guide <NUM> comprises at least one side portion <NUM> defining the side wall <NUM> and a central portion <NUM> defining the side wall <NUM>. These portions <NUM>, <NUM> are therefore intended to form a shaped coupling to define the hollow seat <NUM>. The side portion <NUM> of the light guide <NUM> is provided with first through holes <NUM> which open onto the hollow seat <NUM>. These first through holes <NUM> of the side portion <NUM> of the light guide <NUM> are intended to house the output ends <NUM> of the optical fibres <NUM> facing the hollow seat <NUM>. As illustrated in <FIG>, the output ends <NUM> of the optical fibres <NUM> can be fastened to the side portion <NUM> with a head <NUM> of a screw inserted into a blind hole <NUM> in the first side portion <NUM> of the light guide <NUM>. The head <NUM> of the screw then engages a protrusion <NUM> of the output end <NUM> of a greater diameter greater than that of the first through holes <NUM> of the side portion <NUM> of the light guide <NUM>. The side portion <NUM> thus acts as a fibre-holder support device <NUM>. This side portion <NUM> is further provided with second through holes <NUM> that open onto the hollow seat <NUM> and are intended to house the pins <NUM>. The side wall <NUM> of the central portion <NUM> of the light guide <NUM> has a step <NUM>, on which one end of the pin <NUM> extends, once the pin <NUM> is inserted into a respective second through hole <NUM> of the side portion <NUM>. The pins <NUM> fitted by interference in the second through holes <NUM> of the side portion <NUM> thus cross the entire hollow seat <NUM> of the light guide <NUM> remaining facing the respective output ends <NUM> of the optical fibres <NUM>.

Advantageously, the height of the light guide <NUM> is lower than a conventional light guide, for the same pitch P for positioning the output ends <NUM> of the optical fibres <NUM>, to the benefit of smaller encumbrance and weight. In fact, by increasing the opening angle β of the light beam reflected by the pins <NUM>, the output ends <NUM> of the optical fibres <NUM> can be placed closer to the weld interface <NUM> thereby decreasing the height of the light guide <NUM>.

The pin <NUM> may have different geometries; preferably, the pin <NUM> has a circular or elliptical cylindrical section geometry. In other words, it is a cylindrical body with a main extension axis P-P and has, with respect to a cross-section plane perpendicular to said main extension axis P-P, a circular or elliptical geometry.

According to one embodiment, the pin <NUM> has a semicylindrical, geometry, with a semi-circular or semi-elliptical cross-section having convexity facing said output end <NUM> of the optical fibre <NUM>.

In other words, the convex portion of the pin faces the output end <NUM> of the optical fibre <NUM> so as to reflect the light beam towards the weld interface, expanding it.

According to one embodiment, said pin <NUM> is coated with a gold layer, since the gold has an excellent reflectance; this coating amplifies the expansion effect along the direction substantially tangent to the curvilinear abscissa S defining the perimetral contour <NUM>.

Preferably, the pin <NUM> has a diameter <NUM> greater than or equal to a diameter <NUM> of the output end <NUM> of the corresponding optical fibre <NUM>.

Preferably the diameter <NUM> of the pin <NUM> is greater than a width <NUM> of the seat <NUM> of the light guide <NUM>, said width <NUM> being the distance between the side walls <NUM>,<NUM> of the light guide <NUM> which delimit the perimetral contour <NUM> in a radial direction R-R perpendicular to said curvilinear abscissa S-S.

The technical diffusion effect obtained by the pin <NUM> is schematised in <FIG>. In particular, it may be noted how the beam emitted from the output end <NUM> of the optical fibre <NUM> is incident on the pin <NUM> and is reflected by it towards the weld interface <NUM> with an opening angle β much greater than α which would be obtained with the optical fibre <NUM> only (<FIG>).

According to a possible embodiment (<FIG>), the automotive headlight <NUM> simultaneously comprises both refractive diffusion means <NUM> and reflexive diffusion means <NUM>.

Preferably, the reflective diffusion means <NUM> are positioned upstream of the refractive diffusion means <NUM>, relative to the weld interface <NUM>, and the laser sources <NUM> are arranged in proximity to the reflective diffusion means <NUM>.

In this way, the light beam produced by the laser sources <NUM> first encounters the reflective diffusion means <NUM>, is reflected and expanded by the latter towards the refractive diffusion means <NUM> that influence the beam incident thereon further expanding it. The doubly expanded beam is therefore free to be incident on the weld interface <NUM>.

This hybrid system, i.e. comprising both refractive diffusion means <NUM> and reflexive diffusion means <NUM>, ensures that no energy accumulations are formed within the container body <NUM> (absorbent material), under the weld interface <NUM>. In fact, at a level below the weld interface <NUM>, light beams could overlap giving rise to excessive heating of the resin and therefore to imperfect welding. Thus, it is possible to observe a synergistic effect due to the simultaneous use of reflective diffusion means <NUM>, such as the pin <NUM>, and refractive diffusion means <NUM>, such as the film <NUM>, wherein the reflective diffusion means <NUM> contribute to widening the light beam in a direction tangential to the S-S curvilinear abscissa, while simultaneously the refractive diffusion means <NUM> homogenize the laser radiation in depth.

Obviously, as seen, it is possible to provide embodiments comprising, as diffusion means <NUM>, only refractive diffusion media (<FIG>) or exclusively reflexive diffusion means <NUM> (<FIG>).

The welding method of a vehicle light according to the present invention will now be described.

In particular, the simultaneous laser welding method of a vehicle light according to the present invention comprises the steps of:.

As seen, the diffusion means <NUM> may comprise refractive diffusion means <NUM> and/or reflexive diffusion means <NUM>.

The method may comprise the steps of diversifying the energy produced by the laser sources <NUM>,<NUM> located along the weld interface <NUM>. In this way, for example, the laser welding can be adapted and optimized to specific and complex three-dimensional geometries of the headlight and, in particular, of the weld interface <NUM>.

It is also possible to provide for steps of staggering the turning on of at least two laser sources <NUM>,<NUM> spread along the weld interface <NUM>. Again, in this case too the welding can be optimized with respect to specific and complex three-dimensional geometries of the headlight and, in particular, of the weld interface <NUM>.

This method may also comprise the steps of diversifying the energy produced by the laser sources <NUM> spread along the weld interface <NUM>.

It is also possible to stagger the turning on of at least two laser sources <NUM> spread along the weld interface <NUM>.

As may be appreciated from the description, the present invention makes it possible to overcome the drawbacks mentioned of the prior art.

In particular, it is possible to apply the simultaneous laser welding method quickly and inexpensively also to vehicle lights having any type of complex geometry, even with curvatures and thicknesses highly variable along the perimeter of the light.

In fact, the light guide of the invention is able to distribute the energy of the light beams evenly throughout the weld interface, completely nullifying the discrete distribution effect of the optical fibres, generating an energy distribution with a substantially straight and constant trend. As explained, the presence of refractive diffusion means makes it possible to obtain the thermal peak precisely at the weld interface.

Moreover, the presence of reflective diffusion means makes it possible to reduce overall the height and therefore the overall dimensions of the light guide compared to the solutions of the prior art.

Advantageously, the optical fibres are considerably spaced apart compared to an equivalent beam architecture of the prior art, and therefore are fewer in number to cover the same perimeter of weld interface.

This is made possible thanks to the provision of diffusion means, whether reflective and/or refractive, as described.

In particular, the diffusion means are able to widen the light beam coming from the fibres so as to cover a greater extension: therefore, it is possible to influence, that is, radiate, the entire welding perimeter with a smaller total number of laser sources and therefore of optical fibres.

In other words, it is possible to increase the pitch between adjacent fibres compared to the solutions of the prior art and thus reduce the overall costs of the welding equipment.

As seen, in fact, it is possible to switch from the current pitch or distance between optical fibres of <NUM>-<NUM>, up to a pitch of <NUM> (correct?): in other words, the pitch is substantially quadrupled compared to the prior solutions with optical fibre bundles. This means that, for the same extension of the weld interface, the total number of optical fibres to be used in the equipment can be reduced to at least a quarter.

In addition, the reduction in the number of optical fibres allows a significant reduction in power losses since the total number of optical fibres is reduced.

Claim 1:
Simultaneous laser welding equipment (<NUM>) of a vehicle light (<NUM>) comprising:
- a placement support for supporting a container body (<NUM>) and a lenticular body (<NUM>) of a vehicle light (<NUM>) to be welded together at reciprocal perimetral profiles (<NUM>,<NUM>) associated at a weld interface (<NUM>),
- at least one laser source (<NUM>,<NUM>) suitable for emitting light beams,
- a plurality of optical fibres (<NUM>), each extending from an input end (<NUM>) to an output end (<NUM>), the optical fibres (<NUM>) being associated, at said input ends (<NUM>) to said at least one laser source (<NUM>,<NUM>) and being suitable to transmit said light beams,
- a light guide (<NUM>) provided with at least one hollow seat (<NUM>) having at least one light input wall (<NUM>), which receives the light beams coming from the output ends (<NUM>) of the optical fibres (<NUM>) and a light output wall (<NUM>) which sends the light beams towards the weld interface (<NUM>),
wherein the seat (<NUM>) of the light guide (<NUM>) delimits a continuous perimetral contour (<NUM>), counter-shaped to said weld interface (<NUM>),
characterised in that
the light guide (<NUM>) comprises diffusion means (<NUM>) that pass through said perimetral contour (<NUM>) of the seat (<NUM>) of the light guide (<NUM>)internally , so as to intercept and influence the light beams that propagate inside said seat (<NUM>), the diffusion means (<NUM>) being shaped to expand the light beams along a direction substantially tangent to the curvilinear abscissa (S-S) defining said perimetral contour (<NUM>), before it projects from the light output wall (<NUM>) and/or at the latter.