The ability of a typical daylighting fenestration product, like a Tubular Daylighting Device, to capture low-angled winter light may differ significantly from its ability to capture high-angled summer light. The ratio of summer to winter light output can be an indicator of the product’s effectiveness in delivering (or not delivering) consistent light throughout the course of a year. Ideally, a daylighting product will produce uniform levels of output regardless of season (or climatic conditions), resulting in steady and reliable interior illumination despite variations in exterior conditions or seasonal changes in the daylighting resource.