A scheme is described for calculating the scattering parameters of patterned conductive films in waveguide. The films can have non-uniform, non-isotropic, and non-local sheet impedances. Once the scattering parameters are known, they can be combined with the scattering parameters of paths, dielectric slabs, and waveguide steps to build up models of complicated components comprising patterned films in profiled lightpipes and cavities. It is then straightforward to calculate the Stokes fields of the total reception pattern, the natural optical modes to which the component is sensitive, the Stokes fields of the individual natural modes, and the spatial state of coherence. The method is demonstrated by modeling an absorbing pixel in a length of shorted multimode waveguide. The natural optical modes change from being those of the waveguide to those of a free-space pixel as the size of the absorber is reduced.