Buildings, May 2014.
http://www.buildings.com/article-details/articleid/17254/title/the-impact-of-tubular-daylighting-on-energy-and-leed.aspx

"Daylighting is a popular energy-saving element because of its simple application, significant cost savings, and ability to create healthy, enjoyable spaces. But don’t remain in the dark about which daylighting methods are most effective." "One of the most efficient types of top lighting comes from tubular devices."