Natural light and Gothic architecture represent two inseparable concepts. There are many theories around the gothic light and its symbolic and theological values, but there isn’t any deep study of this light as a physical quantifiable and qualifiedly reality and therefore, classifiable. The present work presents some of the results of the investigation trying to reach a methodology to understand the gothic light, showing unpublished new factors which define all its qualities. The investigation has been developed through a deep analysis of six representative gothic spaces and proposing a new classification according to “types of light quality” that is intended to be a new conditioning factor to keep in mind in the future, helping to understand and respect, in possible interventions on the architectural heritage, what was the main engine to start the architectural project and which today is not valued enough due to the lack knowledge: the light .