During `type III' edge-localized modes (ELMs) on Alcator C-Mod, electroncyclotron emission (ECE) diagnostics show brief signal drops of secondharmonic X-mode and signal increases of fundamental harmonic O-mode. Theseare explained in terms of refraction effects and are found to be useful toinfer the associated ELM geometrical dimensions. A new ray tracing code,which can accommodate poloidal variations, has been developed for thisinvestigation. The ELMs are modelled satisfactorily as a density loss froma poloidally elongated region.