Energy audits of existing buildings require efficient devices and procedures to assess the solar potential through windows and building openings. The use of insolation is related to solar rights, solar gain, daylight potential and solar control. The methods described in the literature to assess insolation on windows require the geometric characterization of solar obstructions. In this paper we present a simple optical device which permits sky vaults that can be viewed from a central point of building openings to be characterized by means of photographs and image analysis. When these images are processed, it is possible to determine hours of solar insolation and characterize the obstruction angles, the sky opening indicator and the view factor indicator. The results of the proposed model are shown graphically in the form of stereographic, orthogonal, gnomonic, isoaire [9] and cylindrical sun path projection diagrams. (C) 2011 Elsevier Ltd. All rights reserved.