A forward model for motional Stark effect spectra is developed. The forward model includes beam attenuation and considers the detection of partially polarized light. The forward model is applied to simulate spectra for Wendelstein 7-X. A fit version is used to analyze data from experimental spectra from ASDEX Upgrade. The fit validates the model and enhances the confidence in the simulation. The structure and the content of the model are described to allow modular implementations to guide physics-based diagnostic designs.