Lighting device

1. A lighting device, comprising:

one or more planar light guides, wherein each of the one of more planar light guides includes a respective back face for receiving light and a respective front face for emitting light,

one or more light sources arranged to transmit light into the respective back face of the one or more planar light guides,

one or more semi-transparent masking elements, wherein each of the one or more semi-transparent masking elements having a respective light diffusing surface arranged to partially diffuse light back into the one or more light guides and partially transmit light through a respective semi-transparent masking element so as to mask the one or more light sources, and

one or more light absorbers arranged adjacent to the one or more light sources to absorb at least a fraction of light diffused from the respective diffusing surface of the one or more semi-transparent masking elements, or one or more retro-reflectors arranged adjacent to the one or more light sources to reflect at least a fraction of light diffused from the respective diffusing surface of the one or more semi-transparent masking elements back to an adjacent one of the one or more semi-transparent masking elements back to an adjacent one of the one or more semi-transparent masking elements, wherein the one or more light absorbers or the one or more retro-reflectors are not optically connected to the one or more planar light guides.

2. A lighting device according to claim 1 , wherein the one or more light absorbers or the one or more retro-reflectors are separated from the one or more light guides by air or by a transparent material with a low refractive index.

3. A lighting device according to claim 2 , wherein the one or more light sources are connected to the one or more light guides by a transparent adhesive having a refractive index that is higher than the refractive index of a transparent adhesive connecting the one or more light absorbers or the one or more retro-reflectors.

4. A lighting device according to claim 1 , further comprising out coupling features arranged at a surface of the front face, or a surface of the back face of the one or more light guides, or inside the one or more light guides for coupling light out through the front face at non-masked locations wherein the one or more semi-transparent masking element is not present.

5. A lighting device according to claim 4 , wherein the light out coupling capability of the out coupling features and a transparency of the one or more semi-transparent masking elements is configured to equalize light intensity of light coupled out by the out coupling features and light transmitted by the one or more semi-transparent masking elements.

6. A lighting device according to claim 1 , wherein the one or more light sources are mechanically connected to the one or more light absorbers or the one or more retro-reflectors and wherein the one or more light absorbers, the one or more retro-reflectors, and/or the one or more light sources are fixed to the back face.

7. A lighting device according to claim 1 , further comprising a light diffusing or reflective surface facing the back face of the one or more light guides and arranged next to the one or more light absorbers or, the one or more retro-reflectors, for diffusing or reflecting light transmitted through the back face back into the one or more light guides.

8. A method for illuminating a matrix of image forming transparent pixels of a display, the method comprises providing a lighting device which comprises

a planar light guide with a back face for receiving light and a front face for emitting light,

first and second light sources arranged to transmit light into the back face of the light guide, wherein the first and second light sources are connected to the planar light guide by a transparent adhesive having a refractive index that is higher than the refractive index of a transparent adhesive connecting the first and second light absorbers or the first and second retro-reflectors,

first and second semi-transparent masking elements having light diffusing surfaces, where the masking elements are arranged to partially diffuse light back into the light guide and partially transmit light through the masking elements so as to mask the respective first and second light sources, and

first and second absorbers arranged adjacent to the respective first and second light sources to absorb at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements, or first and second retro-reflectors arranged adjacent to the respective first and second light sources to reflect at least a fraction of light diffused from the diffusing surfaces of the respective first and second masking elements back to the respective first and second masking elements, the method further comprises,

arranging the matrix of image forming transparent pixels in front of the plate shaped light guide.

9. A method according to claim 8 , further comprising out coupling features arranged at a surface of the front face, a surface of the back face of the planar light guide, or inside the planar light guide for coupling light out through the front face at non-masked locations wherein the first and second semitransparent masking element is not present.

10. A method according to claim 9 , wherein the light out coupling capability of the out coupling features and a transparency of the first and second semi-transparent masking elements is configured to equalize light intensity of light coupled out by the out coupling features and light transmitted by the first and second semi-transparent masking elements.

11. A method according to claim 9 , wherein the one or more light absorbers or the one or more retro-reflectors are separated from the one or more light guides by air or by a transparent material with a low refractive index.

12. A method according to claim 8 , wherein the first and second light sources are mechanically connected to the first and second light absorbers or the first and second retro-reflectors, and wherein the first and second light absorbers, the first and second retro-reflectors, and/or the first and second light sources are fixed to the back face.

13. A method according to claim 8 , further comprising:

providing a light diffusing or reflective surface facing the back face of the planar light guide and arranged next to the first and second light absorbers or the first and second retro-reflectors, for diffusing or reflecting light transmitted through the back face back into the planar light guide.

14. A display for showing images, comprising:

a matrix of image forming transparent pixels; and

a lighting device arranged such that a front face of the lighting device faces a face of the matrix of image forming transparent pixels, wherein the lighting device comprises:

one or more planar light guides, wherein each of the one or more planar light guides includes a respective back face for receiving light and a respective front face for emitting light;

one or more light sources arranged to transmit light into the respective back face of the one or more planar light guides;

one or more semi-transparent masking elements, wherein each of the one or more semi-transparent masking elements having a respective light diffusing surface arranged to partially diffuse light back into the one or more light guides and partially transmit light through a respective semi-transparent masking element so as to mask the one or more light sources; and

one or more light absorbers arranged adjacent to the one or more light sources to absorb at least a fraction of light diffused from the respective diffusing surface of the one or more semi-transparent masking elements, or one or more retro-reflectors arranged adjacent to the one or more light sources to reflect at least a fraction of light diffused from the respective diffusing surface of the one or more semi-transparent masking elements back to an adjacent one of the one or more semi-transparent masking elements, wherein the one or more light absorbers or the one or more retro-reflectors are not optically connected to the one or more planar light guides and wherein the one or more light absorbers or the one or more retro-reflectors are separated from the one or more light guides by air or by a transparent material with a low refractive index.

15. A display according to claim 14 , wherein the one or more light sources are connected to the one or more light guides by a transparent adhesive having a refractive index that is higher than the refractive index of a transparent adhesive connecting the one or more light absorbers or the one or more retro-reflectors.

16. A display according to claim 14 , further comprising out coupling features-arranged at a surface of the front face, a surface of the back face of the one or more light guides, or inside the one or more light guides for coupling light out through the front face at non-masked locations wherein the one or more semi-transparent masking element is not present.

17. A display according to claim 14 , wherein the one or more light sources are mechanically connected to the one or more light absorbers or the one or more retro-reflectors, and wherein the one or more light absorbers, the one or more retro-reflectors, and/or the one or more light sources are fixed to the back face.

18. A display according to claim 16 , wherein the light out coupling capability of the out coupling features and a transparency of the one or more semi-transparent masking elements is configured to equalize light intensity of light coupled out by the out coupling features and light transmitted by the one or more semitransparent masking elements.

19. A display according to claim 14 , further comprising a light diffusing or reflective surface facing the back face of the one or more light guides and arranged next to the one or more light absorbers or the one or more retro-reflectors, for diffusing or reflecting light transmitted through the back face back into the one or more light guides.