An apparatus for applying a viscous modification material includes a material source of the modification material and a needle connected to the source. A material for selectively modifying the optical properties of cells comprises a basic material and a modification substance distributed in the basic material at a prescribed concentration, wherein the basic material comprises a viscous fluid, a gel, a two-dimensional or three-dimensional porous substrate or a material which hardens in situ. A diagnostic apparatus for determining the optical properties of cells which have been exposed to such a material and an apparatus for irradiating cells whose optical properties have been differently modified with such a material are also provided.