What system automatically turns on headlights when it gets dark?

Automatic headlights are a system that turns a car's headlamps on when exterior light levels become low.

A light sensor, often mounted near the top of the dashboard or behind the windshield, measures ambient brightness. The vehicle's control electronics compare that reading with a threshold and activate the appropriate lamps in darkness, at dusk, in tunnels or during heavy weather. Some systems add a short delay to prevent momentary shadows from switching the lights repeatedly.

Automatic headlamps have roots in mid-20th-century automotive electronics. General Motors introduced the Autronic Eye in 1952 as an automatic high-beam dimming system. In 1964, GM's Twilight Sentinel used a photoelectric cell to respond to changing outdoor light and became an important predecessor to modern light-sensing controls.

Automatic headlights are not the same as high-beam assist. Automatic headlights respond mainly to ambient light; high-beam assist switches between low and high beam after considering traffic and road conditions. Daytime running lights also do not necessarily provide nighttime illumination.