Optical modules of this type are already known. They are able to emit forward longitudinally a final light beam that is referred to as a “multibeam” or even a “pixel beam”. The final light beam projects forward an image of a matrix array of elementary light sources. By selectively turning-on or turning-off each of the elementary sources, it is possible to create a final light beam that specifically illuminates certain zones of the road in front of the vehicle, while leaving other zones in darkness.
Such an optical module is in particular used in front lighting devices to produce an adaptive lighting function also referred to as an adaptive driving beam or ADB. Such an ADB function is intended to automatically detect a road user liable to be dazzled by a lighting beam emitted in high-beam mode by a headlamp, and to modify the outline of this lighting beam so as to create a zone of shadow in the location of the detected user while continuing to light the road to great distance on either side of the user. The advantages of the ADB function are multiple: user comfort, better visibility with respect to lighting in low-beam mode, greatly decreased risk of dazzle, safer driving, etc.
Such a module generally includes a matrix array of primary light sources, generally formed by light-emitting diodes (LEDs), a first primary optical element including a plurality of light guides and a projecting lens. The light-emitting diodes are arranged on a flat printed circuit board that lies in a plane orthogonal to the direction of projection of the final light beam. The light guides of the primary optical element extend, on the whole, longitudinally from an entrance face for the light to an exit face for the light. The light guides are intended to form the rays emitted by the light-emitting diodes into narrower pencil light beams having the shape of a pixel, which is generally rectangular or square. The exit faces of the light guides form the matrix array of secondary elementary light sources imaged by the projecting lens.
Such a module requires the projected image of the secondary elementary light sources to have a controlled sharpness and light distribution so that the final light beam formed by the combined images of the secondary elementary light sources has a uniform light distribution. This makes it possible to guarantee that the driver of the vehicle will not be annoyed by variations in lighting level due to dispersions in light intensity, for example in zones in which a plurality of secondary elementary sources superpose.
Such an optical module is however liable to be subject to optical aberrations such as spherical aberration, coma, distortion, astigmatism, Petzval field curvature, etc.
The present invention more particularly relates to the solution of problems due to Petzval field curvature. Theoretically, the projecting lens is supposed to have an object focal surface formed by a plane orthogonal to the optical axis of said lens. However, this object focal surface in fact has a concave spherical curvature.
Thus, if the secondary elementary light sources of the matrix array, in our case the exit faces of the light guides, are arranged in a plane orthogonal to the optical axis of the projecting lens, only the secondary elementary light sources that are located on the curved object focal surface will be projected clearly. The projection of the other secondary elementary light sources, which will be located in front or behind the curved object focal surface, will be relatively blurry depending on their longitudinal distance with respect to the object focal surface.
To solve this problem, it has already been proposed to modify the structure of the primary optical element in order to arrange the exit faces of the light guides on a curved surface closely following the curvature of the real object focal surface of the projecting lens. However, since the light-emitting diodes are borne by a flat printed circuit board, the entrance faces of the light guides are arranged in the same plane. Thus, the light guides that are located a distance away from the optical axis of the projecting lens have a larger length than that of the light guides located in proximity to said optical axis.
However, such a primary optical element is not easy to manufacture because of the variable lengths of the light guides.
Furthermore, the length of the light guides located at the ends of the primary optical element is such that the choice of the material used to produce the primary optical element is limited for example to silicone. It is in particular not possible to produce the light guides from polycarbonate or from PMMA.