light from free space to the sample and from the sample to free space in the case of light transmission along the surface normal they equate to t 1 2 1 n u2062 u2062 and u2062 u2062 t 2 2 u00b7 n 1 n a similar model for the transfer function can be obtained for the case of a reflection sensor geometry the complex index of refraction n n ik can be separated into two parts a real part n and an imaginary part k the index of refraction is related to the complex permittivity as follows the transfer function model can include a model of the complex permittivity in the case of free water the simplest model that can be used is the debye relaxation model which is further described in deybe p polar molecules new york chemical catalog 1929 where u2208 is the material permittivity u2208 0 is the static permittivity of water u2208 u221e is the water permittivity at high frequency f is the measurement frequency and f 0 is the relaxation frequency of water both u2208 0 and f 0 are very temperature dependent in the case of free water in a low dielectric medium like paper eq 2 can still apply but a constant term characterizing the dielectric constant of the medium must be added in low moisture application lt 10 a sizeable amount of water in paper is not free but bound to the cellulose fibers if the bound water is modeled name a similar debye relaxation model as free water a more.