Tungsten oxide thin film gaschromic theory can be concluded as the reduction of hydrogen being input in the surface of thin film and then becomes hydrogen atom, spread in nanopore and then enter tungsten oxide molecules, the transparent WO3 molecule turns in to blue tungsten bronze HxWO3.Chemical equation is:
WO3（colorless）＋xH→ HxWO3 （blue）
For fading, input air or oxygen and hydrogen-oxygen reaction happens, water comes into being, hydrogen atom fading from tungsten bronze, color turns from blue into colorless. The most widely applied theory is brought up by Faughnan, it says the process of WO3 coloring state is caused by change from W6+ to W5+ and results in light absorption, the input of electron and ion into crystalline interval causes coloring state, which the hydrogen atom diffuses into tungsten oxide molecule, one of the electron of hydrogen atom is transit to wolfram atom, the equation is as below: W6+＋e- →W5+
During which the depth of color is related to the diffusion of hydrogen atom amount. The more it is diffused, the deeper it will be. Besides that, other theory models are came into being, like color center model, transmission model and dipole model.