An automatic headlight dimmer (also called automatic beam control) was a device on automobiles which used a photoresistor to automatically adjust the headlight beams from "high beam" to "low beam" (ie. high and low brightness respectively) when encountering oncoming vehicles during nighttime driving, and to switch back to high beam after the vehicles had passed.
Drivers could also use standard headlight dimmers (such as foot-operated devices) to override the automatic headlight dimmer on cars so equipped.
General Motors "Autronic Eye" / "GuideMatic"
General Motors introduced the first automatic headlight dimmer – called the Autronic Eye – in 1952, on its Cadillac and Oldsmobile models; Buick, Pontiac and Chevrolet models began offering this feature in 1953. Cars with the Autornic Eye were easily identified by a periscope-like phototube that sat on the dashboard's left side, just inside the windshield.
One criticism of early automati headlight dimmers – GM's Autronic Eye in particular – was that the headlights tended to erractically flicker between low- and high-beams in response to minor fluctuations of light, such as street lamps.
GM discarded the troublesome Autronic Eye after 1958 in favor of a revamped automatic headlight dimming system called GuideMatic. Introduced in 1959, the GuideMatic – which had a slimmer appearance than the Autronic Eye and sat at the left side of the dashboard, later moved to the center – had a switch that allowed drivers to adjust when the headlights dimmed. Though the GuideMatic system was an improvement over the Autronic Eye, many GM customers were leery through past experience and fears that the new system was still too erractic. By the mid-1960s, this feature was dropped on all GM models except Cadillac (which continued offering GuideMatic through 1988). In recent years, however, Cadillac once again began offering an automatic headlight dimming system.
- Ford Motor Corporation – An automatic headlight dimmer called AutoDim was offered on several Lincoln models starting in the mid-1950s, and eventually the Ford Thunderbird and some Mercury models had it available as well.
- Chrysler Corporation – Available corporate wide in 1959, top-line Chrysler and Imperial models offered its Automatic Beam Control dimmer throughout the 1960s and early 1970s.