The diffuse radiation is one component of the global radiation affecting the output of photovoltaic collectors. This radiation depends on the view factor between the collector and the sky. The view factor is 1 for a horizontal collector, and there is a simple expression for the view factor for a single inclined collector on a horizontal plane. However, for a photovoltaic system with a number of rows deployed on a horizontal or on an inclined plane, the view factor is more complicated to calculate because one collector row may obscure part of the sky for the other row. More complication is added when the azimuth of the collector rows do not coincides with the azimuth of the inclined plane. The present article deals with view factors of photovoltaic collectors deployed on inclined planes and oriented in any direction. A general mathematical expression for this view factor was developed and may be used to calculate the diffuse radiation reaching photovoltaic systems. (C) 2012 Elsevier Ltd. All rights reserved.