US Patent 7655366 Inkless reimageable printing paper and method

An image forming medium includes a substrate and an imaging layer coated on or impregnated into a substrate, where the imaging layer is formed of an imaging composition that includes a solvent or a polymeric binder, and a photochromic material selected from substituted fulgides and substituted dithienylethenes, dissolved or dispersed in the solvent or polymeric binder, and where the imaging composition exhibits a reversible homogeneous-heterogeneous transition between a colorless state, an intermediate colorless or colored erasable state, and a final colored stable state, the photochromic material converting from the colorless state to the colored stable state upon irradiation with light of a first wavelength and converting from the colored stable state to the colorless state upon exposure to heat and light.