A method of measuring a porosity of a film, by measuring a refractive index of the film in a first environment having a first relative humidity to produce a first refractive index measurement. The refractive index of the film is measured in a second environment having a second relative humidity, where the first relative humidity is different from the second relative humidity, to produce a second refractive index measurement. Multiple gases can be used to create the first and second environments. The first refractive index measurement and the second refractive index measurement are input into a model that correlates refractive index to film porosity, to output the porosity of the film.