A passive collimating tubular skylight consisting of a radiant energy-collecting aperture, a radiant energy-delivering aperture, and a radiant energy passageway between these two apertures, the passageway having a specularly reflective interior surface and a configuration to improve the collimation of the radiant energy passing therethrough. The skylight can be configured with the radiant energy-collecting aperture located above the roof of a building, oriented to collect sunlight; and equipped with a sealed weatherproof glazing, with the radiant energy-delivering aperture, or luminaire, located at ceiling level within the building, and equipped with a diffusing glazing; and with the reflective tubular light passageway constructed with a larger cross sectional area near the radiant energy-delivering aperture than near the radiant energy-collecting aperture. In complete accord with the second law of thermodynamics, and as proven by experimental results, the new passive collimating tubular skylight provides significant advantages over the prior art, including better solar energy collection, higher throughput optical efficiency, improved radiant energy collimation, enhanced interior illumination levels, and more precise positional control of the interior illumination.