Optics of hemispherical top dome and its effect on tubular light guide efficiency: diffuse light case.

To predict the energetic effectiveness of a tubular light guide accurately, a theoretically founded approach has to be used rather than any empirical approximation. The computed illuminance below a light guide can become inaccurate if neither Fresnel's equations nor realistic optical path lengths in a cupola are taken into consideration. It is shown that incorporation of both of them into a theoretical model results in lowered luminous flux below the light guide. Assumption of directionally independent transmission coefficient leads to average errors in luminous fluxes of about 10%. The peak errors are typically higher and correspond to lightbeams crossing a hemispherical top dome near its circular base. The solution concept used in this paper can improve predictions of energetic effectiveness for tubular light guides under diffuse daylight conditions.