Photoreversible gratings in photochromic organic guest/host systems are written using uv- radiation (363.8 nm) or in the visible (514.5 nm) with diffraction efficiencies up to 7%. Poly(Methyl MethAcrylate) or polyimide are used as guest polymers, doped with the fulgides Aberchrome 540 or 670. The thickness of the films is between 50 and 100 micrometers , the grating constant is 2 - 3 micrometers . By adjusting an additional light source the diffraction efficiency rises (here 17%) and it can be tuned optically. With uv-radiation a permanent grating is written in a crosslinkable polyimide. Due to this crosslinking a selective in-diffusion of an active dye into not illuminated areas of the polyimide (grating constant: 2 micrometers ) takes place. This latent grating of the distributed fulgide molecules can be optically activated additionally to the permanent grating resulting from the photoreaction. The diffraction efficiency can be tuned by a homogeneous illumination.