The subject of the present paper is a fixed daylighting system known as the LIF light guide. The system consists of a fixed arrangement for capturing sunlight incident at various angles and re-directing it by means of an optical wave-guide which provides diffuse ceiling illumination. The light guide is generally sandwiched between two glass panes and mounted above the windows to avoid obstructing visibility. Its intended function is to furnish offices and other working areas with a uniform level of natural illumination while avoiding glare.