The photochromic properties of transparent=opaque transition and the change of the refractive index (Dn) have been exploited to develop devices for astronomical spectrographs, both as focal plane masks and as dispersive elements (volume phase holographic gratings, VPHGs), respectively. In both cases the possibility to write and erase with suitable irradiation has revealed a new perspective for non-disposable and fully customisable items for spectroscopy. High contrast focal plane masks based on the photochromism of poly-1,2-bis(2-methyl-3-thienyl)per- fluorocyclopentene have been obtained. Concerning the use of photochromic molecules as dispersing elements, low molecular weight diarylethenes and diarylethenes polymers have been considered and ellipsometric measurements were carried out to determine the refractive index change which accompanies the photochromic process.