This paper provides a procedure of easy approximation to obtain the direction of the illumination vector for a window under an overcast sky. When working with uniform skies, it is simple to demonstrate that the direction of the illumination vector is determined by the centre of the window and the illuminated point. However, when a luminance distribution is produced such as that specified by the Moon-Spencer law, the origin of the vector is no longer the centre of the window, but some point within a fragment of the window. In this paper, this fragment of window is delimited. (c) 2007 Elsevier Ltd. All rights reserved.