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