US 5,550,677 · Granted 1996-08-27

The Auto-Dimming Mirror Patent That Stops Headlight Glare

Imagine your car's rearview mirror automatically darkening when headlights from cars behind you are too bright, then brightening again when they pass. This patent describes the smart sensor and control system that makes that happen—a tiny camera chip that watches the light hitting your mirrors and tells them when to dim or brighten independently.

Curated and verified by the IsItPatented Editorial team

The plain-English version

What it protects

The claim covers a system for automatically controlling the reflectance (how shiny or dark) of multiple mirrors in a car using light sensors and electronic control circuits. What's protected here is the specific method of using a photosensor array—an array of light-detecting pixels—to measure brightness in different zones (rearview, left side, right side), and then sending separate control signals to darken or lighten each mirror independently based on that data. The patent protects both the hardware (the sensor chip and mirror drive circuits) and the logic that decides how bright or dark each mirror should be.

Why it matters

This patent became foundational technology for the auto-dimming mirror feature that's now standard in millions of vehicles worldwide. Donnelly Corporation, a major automotive supplier, used this patent to establish itself as a leader in smart mirror systems. The ability to reduce glare from trailing headlights improves safety by preventing temporary blindness, and the independent control of multiple mirrors meant each surface could respond to its own lighting conditions rather than all dimming together. This kind of adaptive glass technology opened the door to more sophisticated vehicle optics.

Real-world use

Every time you drive at night and your rearview mirror automatically darkens when a car with bright headlights pulls up behind you, you're experiencing this patented system at work. The sensor is constantly watching and adjusting so you can see without being blinded.

Original USPTO abstract

A system apparatus, structure and method for controlling a plurality of variable reflectance mirrors (or mirror segments), including a rearview mirror and side view mirrors, which change their reflectance level in response to a plurality of drive voltages applied thereto, for an automotive vehicle. The system includes a light sensing device and a control circuit formed as a single VLSI CMOS circuit. The light sensing device comprises a photosensor array having a field of view encompassing a rear window area and at least a portion of at least one side window area of the vehicle. The logic and control circuit determines a background light signal from photosensor element signals generated by the photosensor elements in the photosensor array indicative of light levels incident on the photosensor elements. The circuit also determines a peak light signal in three different zones or sub-arrays of the photosensor array. The zones or sub-arrays may correspond to three mirrors or mirror segments. The peak light signals in each of the zones and a common background light signal are used to determine independent and separate control signals, which are then output to separate mirror drive circuits for independently controlling the reflectance level of the rearview mirror and the left and right side view mirrors, or alternatively the segments of a mirror.

Patent details

Publication number: US 5,550,677
Filing date: 1993-02-26
Grant date: 1996-08-27
Assignee: Donnelly Corporation
Inventor(s): SCHOFIELD; KENNETH, LARSON; MARK
CPC class: H04N7/181

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