Matt Slocum / AP

Robotic searchlights illuminate the night sky over the Benjamin Franklin Parkway, Thursday, Sept. 20, 2012, in Philadelphia. The interactive light installation by artist Rafael Lozano-Hemmer, titled "Open Air," was commissioned by the Association for Public Art, in Philadelphia and is scheduled to run till Oct 14 from 8-11 p.m. each night.

See more photos of Philadelphia on PhotoBlog.

Follow @NBCNewsPictures

Sign up for the NBCNews.com Photos Newsletter