THE DEFINITION AND USE OF OPTICAL INVARIANTS FOR THIN ISLAND FILMS

In the context of a description of the optical properties of thin film s in terms of the excess polarization it is shown that four complex co nstitutive coefficients are needed to describe both the coverage and t he location of the film material relative to the surface of the substr ate. Expressions are given relating these coefficients for different c hoices of the position of the dividing surface (with respect to which the excess polarizations are calculated). This makes it possible to co nstruct seven real invariant combinations which are independent of thi s choice. Formulae for the reflectances and transmittances for s- and p-polarized light in terms of these invariants are given for the case that both the substrate and the ambient are non-absorbing. For the spe cial case of spherical islands, explicit formulae are given for the co nstitutive coefficients. Finally the theory is applied to interpret ex perimental results for a thin latex film.