Ellipsometry Porosimetry (EP)  measures the change of the optical properties and thickness of the materials during adsorption and desorption of a volatile specie either at atmospheric pressure (EPA) or under reduced pressure depending on the application (EP). The EP technic is unique by its ability to measure porosity of very thin films down to 10nm, reproducibility and its speeds of measurement. Compares to traditionnal Porosimeter, Ellipsometer porosimeters is well suited for very thin film pore size and pore size distribution measurement. Film porosity is a key factor in Silicoin based technology using low k materials, organic industry (encapsulated OLED\'s) as well as in the coating industry using SolGel technics.