The utilization of energy from classical natural sources has some limitations therefore new possibilities for its enhancement are sought. Tubular light guides (TLG) are simple devices that allow the transmission of daylight into interiors of domestic or commercial buildings. The light transmission efficiency (LTE) is important indicator of the light guide performance. It depends on the geometry and optical parameters of the light guide but it seems also on the sky conditions. LTE can be investigated by measurements or calculations. Measurements under a real sky meet with difficulties of sky luminance distribution stability because sun is continually moving and properties of atmosphere are changing. Measurements of the LTE made under the artificial sky with the precisely defined sky luminance distribution can give answer to the question: "Depends the light transmission efficiency of tubular light guides on the sky type determined by the luminance distribution?" It was found that theoretical prediction methods combined with numerical methods could be used to answer this question. In this paper are presented results of theoretical calculations of the tubular light guide transmission efficiency using ray-tracing prediction method compared with measurement results obtained under the artificial sky installed at the Institute of Construction and Architecture of Slovak Academy of Sciences in Bratislava. The main aim of this paper is to offer a method for determination of tubular light guide inner reflectance and mathematically prove that light guide transmission efficiency is different for various standardized overcast sky types. (C) 2011 Elsevier Ltd. All rights reserved.