Disclosed is a lightguide in the form of a plate (1), a first surface of which is provided with first straight parallel grooves (8) (diffusing the light circulating in the guide). The inventive lightguide is characterized by the fact that it is provided with second straight parallel grooves (9) (diffusing the light circulating in the guide) which extend in an oblique direction, particularly in an orthogonal direction, relative to the first grooves such that the guide diffuses in a uniform manner the light that is introduced into the guide via three or four edges thereof.