Continuous observation of the time-varying glottal area lacks a direct, quantitative, non-invasive measurement method despite its relevance to study breathing, speech production, swallowing, etc. External photoglottography (ePGG) relies on external glottal transillumination and sensing, it is therefore suitable for non-invasive and continuous observation. Nevertheless, a formalized relationship between ePGG signal and glottal area is lacking. The current paper proposes a Multi-Signal-ePGG (MSePGG) algorithm approach based on characterization of ePGG measurements under controlled conditions using mechanical glottal replicas. MSePGG accounts for main parameters affecting the ePGG signal: glottal area to be quantified and measurement conditions such as tissue properties and signal amplification. It is shown that MSePGG enables quantitative and continuous measurement of the time-varying glottal area on mechanical replicas. Application to a human subject is illustrated and discussed.