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Abstract: Tubular light guides represent a convenient concept for illumination of windowless parts of buildings by natural light. The transmittance of this optical device generally varies with geometrical characteristics as diameter and length and also with its properties. A high light transmittance of the roof dome and diffuser as well as high quality of the inner reflective surface facing of the light guiding central duct are required. External daylight conditions such as sky cloudiness or sky luminance distribution have undoubtedly a vital influence on the tubular light guide efficiency. A daylight computer simulation study focused on an evaluation of the tube light efficiency is presented in the article. The efficiencies of straight tubular light guides are computed for the standard overcast and clear sky luminance distributions. The results obtained can be useful for predetermination of a luminous flux emanating a light guide, for evaluation of indoor daylighting, and for elaboration of various daylight strategies. [Copyright &y& Elsevier]

A comprehensive study of straight light pipe efficiency and illuminance distribution below ceiling level is performed using HOLIGILM tool. The recommendations for light guide systems can be made for different latitudes and sky luminance patterns using the data computed here. The computational results show that the light pipe transmittance is a monotonic function of solar altitude, except for low-sun angles. Redistribution of photons at the light pipe base can be characterized by a scalar value called “asymmetry parameter”, g . A use of g is an efficient attempt to the classification of a large number of luminous intensity distributions. The minimum value of g is at low solar elevations (between 10° and 30°) because of balanced contribution of diffuse light of a sky and the direct sunbeams. The low values of g indicate the redistribution of light is more uniform. Luminous intensity solid determined by means of HOLIGILM tool is used to compute illuminance distribution at a working plane. The peak intensity and intensity gradient of a bright ring projected onto a working plane can be used to characterize the optical properties of different light pipes in different climatic zones. [ABSTRACT FROM AUTHOR]