Forbidden pitches are the result of unwanted, non-linear effects that limit yield and not always well understood. Yet, as approximations, they are implicitly deployed through design rules. Many believe they result as a consequence of more complicated light sources. We develop an analytical model of aerial image quality as a function of light source. We show the effect is most pronounced for a point light source, the simplest of all. We develop a method to improve print image quality by illumination source optimization, and show promising first results. Additionally, it is shown how design rules capture forbidden pitches unsatisfactorily.