What 1975 addition allowed US Open matches to be played at night?

What 1975 addition allowed US Open matches to be played at night? The answer is floodlights.

The installation of floodlights at the West Side Tennis Club enabled the U.S. Open to schedule matches after dark in 1975. Night sessions expanded the tournament's daily timetable and made evening attendance and television coverage practical.

The change came during a transitional period for the event. In the same year, the tournament switched from grass to green Har-Tru clay, partly because of complaints about the grass courts and partly because organizers wanted a surface more suitable for television. The championships remained at Forest Hills through 1977.

Floodlights are sometimes confused with later innovations such as Hawk-Eye or retractable roofs. Hawk-Eye concerns electronic line-calling and replay, while retractable roofs protect matches from rain. Neither created the original night-session capability; artificial lighting did. The modern tournament continues to use illuminated courts at its Flushing Meadows complex.