This paper proposes rules to size light wells in order to provide the desired quantities of natural light in undergroud spaces. These rules have been determined by processing the results of optical computations using the ray-tracking light simulation program GENELUX. First, the efficiency of light wells alone was computed using the CIE standard overcast sky as a function of light-well dimensions and coefficients of reflexion of the sides (diffuse and specular finishes were also compared). Then, various types of light collectors were tested, and their efficiencies were computed either alone or in combination with the light wells. Recommendations in sizing and spacing of such day-lighting systems were proposed to Light an underground space on the base of specifications regarding the distribution of illuminances on the floor and the required annual duration of lighting with natural means.