The city of St. Cloud, Minn., is installing special LED ceiling lights in municipal buildings that will save money on electricity costs as well as provide another way to get on the Internet and relieve Wi-Fi congestion. The lights, designed by a startup company there, can exchange signals with a special modem attached to a computer by using the visible-light spectrum rather than the radio-wave spectrum. The current system has data speed comparable to DSL, but a follow-on system will have speeds similar to Wi-Fi.