In thin film of chalcogenide glass $As_2 Se_3$, holographic grating is recorded at the wavelength(633nm) of He-Ne laser, and its diffraction efficiency is measured in real time.
The decreasing transmittance of thin film is also measured in time, as it is being exposed by a given intensity of light, and an experimental formula is suggested.
By using this formula, the temporal behavior of diffraction efficiency for holographic grating is qualitatively explained.
In the appendix, $Ag-As_2S_3$ thin film, which has a diffraction efficiency much higher than $As_2Se_3$ thin film, is also used to demonstrate a real time holography.
