We present measurements of the optical constants of a series of spin cast conjugated polymer and thermally evaporated small molecule films on silicon substrates. The optical constants are measured, using spectroscopic ellipsometry, from 0.75eV up to just below the band-gap of each material. The optical constants are then obtained by fitting a homogeneous Cauchy model to the ellipsometry data. Using this data the waveguiding behaviour of a double hetero-structure device with edge emission is modeled using the well-established Beam Propagation Method (BPM). The waveguide loss of double hetero-structure devices is modeled for different device geometries and across the emission band of an experimentally prepared light emitting, waveguiding double hetero-structure organic device. The emission characteristics of the experimental device compare favourably with results of the simulation based on the measured optical constants. These results have important implications for the design of future electrically pumped organic laser devices.