Light guide (2; 102) made from transparent material, comprising: - an entry face (6; 106) for the light rays emitted by a light source (10; 110); - an exit face (32; 132) for the light rays; - refraction means (14, 16, 18; 114) situated on the optical path of a proportion of the light rays propagating between the entry face (6; 106) and the exit face (32; 132), and configured so as to collimate said portion of light rays passing through said means; - means (8, 12; 108; 112) for coupling with the light source (10; 110) able to collimate, preferentially exclusively, in the thickness of the guide, the rays entering through the entry face (6; 106); characterised in that the coupling means comprise a cavity (8; 108) on the entry face (6; 106), the cavity (8, 108) comprising a first surface of revolution about an axis perpendicular to the midplane of the guide and lying in a plane of the entry face (6, 106), the cavity (8, 108) being able to refract the rays coming from the light source (10; 110) situated in the cavity and encountering the first surface of revolution, so as to collimate said rays in the thickness of the guide in planes parallel to the midplane of the guide; and in that the coupling means also comprise a reflection surface (12; 112) on one or each of the faces of the guide forming its thickness, the reflection surface being a second surface of revolution about the axis and having a cone shape, so that the reflection surface (12, 112) is able to reflect the rays coming from the light source (10, 110) situated in the cavity and encountering the second surface of revolution, so as to collimate said rays in the thickness of the guide in directions parallel to the midplane of the guide. - an entry face (6; 106) for the light rays emitted by a light source (10; 110); - an exit face (32; 132) for the light rays; - refraction means (14, 16, 18; 114) situated on the optical path of a proportion of the light rays propagating between the entry face (6; 106) and the exit face (32; 132), and configured so as to collimate said portion of light rays passing through said means; - means (8, 12; 108; 112) for coupling with the light source (10; 110) able to collimate, preferentially exclusively, in the thickness of the guide, the rays entering through the entry face (6; 106); characterised in that the coupling means comprise a cavity (8; 108) on the entry face (6; 106), the cavity (8, 108) comprising a first surface of revolution about an axis perpendicular to the midplane of the guide and lying in a plane of the entry face (6, 106), the cavity (8, 108) being able to refract the rays coming from the light source (10; 110) situated in the cavity and encountering the first surface of revolution, so as to collimate said rays in the thickness of the guide in planes parallel to the midplane of the guide; and in that the coupling means also comprise a reflection surface (12; 112) on one or each of the faces of the guide forming its thickness, the reflection surface being a second surface of revolution about the axis and having a cone shape, so that the reflection surface (12, 112) is able to reflect the rays coming from the light source (10, 110) situated in the cavity and encountering the second surface of revolution, so as to collimate said rays in the thickness of the guide in directions parallel to the midplane of the guide.