An optical device for modifying a light distribution pattern of a light source radiating first light beams to a first geometric quarter-space and second light beams to a second geometric quarter-space, the first and second geometric quarter-spaces being defined by mutually perpendicular first and second geometric planes (205, 206) so that the first geometric plane constitutes a boundary between the first and second geometric quarter-spaces, the optical device comprising: - a lens-section for acting as a lens for modifying a light distribution pattern of the first light beams, - a first reflector surface for reflecting a first part of the second light beams from the second geometric quarter-space to the first geometric quarter-space, and - a second reflector surface for reflecting a second part of the second light beams from the second geometric quarter-space to the first geometric quarter-space, wherein the first and second reflector surfaces are successively in a direction perpendicular to the first geometric plane so that the first reflector surface is between the first geometric plane and the second reflector surface, and wherein the first reflector surface is farther from the second geometric plane than is the second reflector surface so that an area (219, 319) between the first reflector surface and the second geometric plane constitutes a propagation passage for the second part of the second light beams on a route to the second reflector surface, characterized in that the optical device is a transparent piece comprising: - a first cavity for the light source, - a second cavity whose surface constitutes the first reflector surface so that total internal reflection takes place when the first part of the second light beams arrive, from inside the transparent piece, at the surface of the second cavity, and - a third cavity whose surface constitutes the second reflector surface so that total internal reflection takes place when the second part of the second light beams arrive, from inside the transparent piece, at the surface of the third cavity, a wall between the first and second cavities being farther from the second geometric plane than is a wall between the second and third cavities, and a part of the wall between the second and third cavities constitutes a part of the route of the second part of the second light beams to the second reflector surface. - a lens-section for acting as a lens for modifying a light distribution pattern of the first light beams, - a first reflector surface for reflecting a first part of the second light beams from the second geometric quarter-space to the first geometric quarter-space, and - a second reflector surface for reflecting a second part of the second light beams from the second geometric quarter-space to the first geometric quarter-space, wherein the first and second reflector surfaces are successively in a direction perpendicular to the first geometric plane so that the first reflector surface is between the first geometric plane and the second reflector surface, and wherein the first reflector surface is farther from the second geometric plane than is the second reflector surface so that an area (219, 319) between the first reflector surface and the second geometric plane constitutes a propagation passage for the second part of the second light beams on a route to the second reflector surface, characterized in that the optical device is a transparent piece comprising: - a first cavity for the light source, - a second cavity whose surface constitutes the first reflector surface so that total internal reflection takes place when the first part of the second light beams arrive, from inside the transparent piece, at the surface of the second cavity, and - a third cavity whose surface constitutes the second reflector surface so that total internal reflection takes place when the second part of the second light beams arrive, from inside the transparent piece, at the surface of the third cavity, a wall between the first and second cavities being farther from the second geometric plane than is a wall between the second and third cavities, and a part of the wall between the second and third cavities constitutes a part of the route of the second part of the second light beams to the second reflector surface.