A light diffuser (10), which is particularly suitable for introducing diffuse light into a tissue, is produced by interpenetration of a diffuser material in a liquid state into a boundary layer (4) of a porous shaping material, by which process a diffuser surface is formed having a surface structure which represents essentially a negative of the pore structure of the shaping material and includes undercut structures induced by a surface tension. The light diffuser (10) is e.g. produced by introducing a diffuser blank (1) including material which is liquefiable through mechanical vibration into the shaping material and simultaneously stimulating it with mechanical vibrations, such that the liquefiable material liquefies at least there where it is in contact with the shaping material and is pressed into the shaping material. An in situ production of the diffuser is particularly advantageous for photodynamic therapy in bone tissue (20).