BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to a lighting device comprising at least one light source, an optical beam shaper arranged and designed to redistribute light from the light source and to image the redistributed light to an entrance surface of an optical beam spreader, said optical beam spreader being designed to spread the redistributed light with an angular distribution around a predefined emission direction.

Such a lighting device may be used for automotive signaling applications like stop light, rear light or turn indicator. Today's automotive signaling devices are often used by the car manufacturer additionally for brand signature and recognition. For this purpose the shapes of the signaling illuminating surfaces are often quite complex and unique.

State of the art is to implement these complex shapes by light guide systems. The like guides are formed according to the desired shape of the light emission. These systems however often lack optical efficiency since the light guides are used to generate the complex shapes of the illuminating surface and the legally required beam distribution at the same time.

US 5,791,757 discloses a construction of an automotive rear light which incorporates diffractive optical elements (DOE) together with a thin sheet optical element comprising micro-optical wedges. The light sources are arranged at the side faces of the thin sheet optical element. The diffractive optical elements are used to direct the light from the light sources to predetermined regions of the plurality of micro-optical wedges in the thin sheet optical element. The angle of the wedges varies with each micro-optical wedge depending on beam pattern desired at the emission side of the thin sheet optical element. With such a construction a desired light pattern can be generated at the emission surface of the thin sheet optical element. This system however lacks the possibility to create a predefined, e.g. homogeneous appearance of the light pattern which is independent of the viewing angle within a desired angular space, since every region of the micro-optical wedges is illuminated in-homogeneously and in addition each region emits light in different directions. The light pattern will thus not appear the same from all viewing directions to an observer. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lighting device which allows the generation of a complex light pattern on an emission surface of the device, the appearance of which being independent of the viewing angle within a predefined range of viewing angles.

The object is achieved with the lighting device according to claim 1.

Advantageous embodiments of the lighting device are subject matter of the dependent claims or are described in the subsequent portions of the description and preferred embodiments.

The proposed lighting device comprises one or several light sources, an optical beam shaper arranged and designed to redistribute light from the light sources and to image the redistributed light to an entrance surface of an optical beam spreader. The optical beam spreader is designed to spread the redistributed light with an angular distribution around a predefined emission direction such that all of the redistributed light is spread with the same angular distribution around the predefined emission direction independent of the local angle of incidence on the entrance surface of the optical beam spreader.

The optical light source of the proposed device may be formed for example of a laser or of a light emitting diode (LED). Also several light sources may be used in the proposed device in order to provide the light for generating the desired light pattern or light distribution. The optical beam shaper is arranged and designed to generate the desired light distribution or light pattern at the entrance surface of an optical beam spreader. To this end the optical beam shaper redistributes the light entering from the light source(s) in order to achieve the desired, in particular complex light pattern or light distribution on the entrance surface of the optical beam spreader. The desired light pattern or light distribution is thus generated by the optical beam shaper and not by the design of the light source(s).

The optical beam spreader may be formed of a diffractive optical element (DOE), a holographic optical element or a free-form lens, for example. These optical elements can be fabricated in an appropriate design to generate a desired optical light distribution from the one or several light sources. The required design or form of the optical beam shaper is typically calculated with known computer programs for optical design which are available for simulating and designing such optical elements.

The same applies to the optical beam spreader which is designed to spread the redistributed light entering through its entrance surface with an angular distribution around a predefined emission direction. The predefined emission direction may be the direction perpendicular to the emission surface of the optical beam spreader or of the device but may also be any other direction depending on the desired main emission direction of the device. The optical beam spreader is designed such that all of the redistributed light entering the optical beam spreader through the entrance surface is spread with the same angular distribution around the same predefined emission direction independent of the local angle of incidence of the redistributed light on the entrance surface of the optical beam spreader. To this end, the optical beam spreader is adapted to the different angles of incidence of the redistributed light and thus to the design of the optical beam shaper.

This is in contrast to known beam spreaders or diffusers. The light spreading function of a such a known optical diffuser depends on the angle of incidence of the incoming light on the diffuser element. Therefore, such a diffuser element does not spread two light beams entering the element at different angles of incidence to the same angular distribution around the same emission direction. This leads to a different appearance of the light distribution on the diffuser element for an observer depending on the observing angle. The proposed optical beam spreader however is adapted to the incoming redistributed light, in particular to the different local angles of incidence of this redistributed light, in order to achieve the same angular distribution for all of the redistributed light around the same predefined emission direction. Such an optical beam spreader is preferably formed of a diffractive optical element (DOE) or of a holographic optical element, in particular a holographic optical diffuser. These elements can be tailored by help of computer programs for optical design in order to achieve the desired effect adapted to the incoming redistributed light.

In the proposed lighting device, the optical beam shaper as well as the optical beam spreader may be formed as transmissive or as reflective optics. Furthermore, additional optical elements may be provided in the device, for example in order to at least partly collimate the light emitted by the light source or light sources.

The proposed lighting device can be designed to be used as a signaling device in automotive signaling applications, for example as a stop light, a rear light or a turn indicator. Nevertheless, the lighting device may also be used for other types of lighting applications, for example for interior illumination in the automotive field or in other fields in which such an illumination is desired. BRIEF DESCRIPTION OF THE DRAWINGS

The proposed lighting device is described in the following by way of examples in connection with the accompanying figures. The figures show:

Fig. 1 a schematic view of basic elements of the proposed lighting device;

Fig. 2 an example of a light pattern generated with the beam shaper of the proposed lighting device on the entrance surface of the optical beam spreader; and

Fig. 3 a schematic view of exemplary beam passages of the redistributed light in an example of the proposed lighting device.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a schematic view of basic elements of the proposed lighting device. The device comprises a light source 1, an optical beam shaper 2 and an optical beam spreader 3. As can be seen from the figure, part of the light emitted by the light source 1 passes through the optical beam shaper 2. The optical beam shaper 2 redistributes the light distribution of the light source 1 and images the redistributed light on the entrance surface of the optical beam spreader 3. The optical beam spreader 3 then spreads the incoming light to an angular distribution around a predefined emission direction 8 which is in this example perpendicular to the planar emitting surface of the optical beam spreader 3. The light passage of exemplary light rays 4 emitted by the light source 1 is schematically indicated in the figure. These light rays 4 are deviated by the optical beam shaper 2. The deviated light rays 5 impinge on the entrance surface of the optical beam spreader 3. With these exemplary light rays 4, 5 and other light rays of the light source 1 passing through the optical beam shaper 2 the desired illumination surface distribution is generated at the entrance surface of optical beam spreader 3.

Such an illumination surface distribution or light distribution 7 is exemplary shown in figure 2. In this example, the light distribution 7 comprises dark regions 7a and light regions 7b. Depending on the design of the optical beam shaper 2, any desired light pattern or light distribution 7 can be generated at the entrance surface of the optical beam spreader 3.

Figure 1 also shows an aperture 9 which is used to block disturbing light rays 10 from the light source 1 which do not pass the optical beam shaper 2 or which pass the optical beam shaper 2 in directions different from the directions of the redistributed light without being influenced by the optical beam shaper 2. The optical beam spreader 3 is designed to distribute the incoming light rays 5, i.e. the redistributed light, to an angular distribution around an emission direction 8. The angular distribution is indicated in figure 1 with the light rays 6 which originate from incoming light ray 5. The angular distribution may be symmetric around the emission direction 8 but may also be different, for example different in horizontal and vertical direction. Depending on the design of the optical beam spreader 3, any desired angular distribution can be achieved. In emission direction behind the optical beam spreader 3 an aperture may be arranged which blocks light leaving the optical beam spreader 3 in directions not covered by the angular distribution around the emission direction without being influenced by the optical beam spreader 3. The optical beam spreader 3 is engineered such that each region of the beam spreader spreads the light with the same angular distribution around the desired single emission direction 8, independently of the incidence angle of the light. In this way, the illumination surface, i.e. the light distribution on the emission surface of the optical beam spreader 3, does not change in appearance to an observer independently of the viewing angle if the observer is within the angular range covered by the beam spreader.

Figure 3 shows an example for different angles a, a ^' at which the redistributed light may impinge on the entrance surface of the optical beam spreader 3. The beam cone 11 emitted by the light source 1 is redistributed by the beam shaper 2 in this example to an annular light distribution on the optical beam spreader 3.. The redistributed light 12 impinges with different angles on the entrance surface of the optical beam spreader 3. The beam spreader 3, in this example a diffractive optical element (DOE), is designed such that the redistributed light is spread to an angular distribution around an emission direction 8, which angular distribution and emission direction is identical for all of the redistributed light, i.e. independent on the angle of incidence a, a ^' of the redistributed light 12 on the entrance surface of the optical beam spreader 3. The indicated spread light cone 13 indicates the angular distribution.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. The features of claims 1 to 9 can be freely combined with each other. Any reference signs in the claims should not be construed as limiting the scope of the invention.

List of reference signs

1 light source

2 optical beam shaper

3 optical beam spreader

4 light rays of light source

5 redistributed light rays

6 spread light rays

7 light distribution

7a dark regions

7b light regions

8 emission direction

9 aperture

10 blocked light rays

11 light cone emitted by light source

12 redistributed light

13 scattered light cone