US 4,793,690 · Granted 1988-12-27
The Mirror That Learns How Your Eyes See
This patent covers a rearview mirror that automatically adjusts its darkness to match how bright it is outside—just like your pupils dilate and shrink. Instead of getting blinded by headlights behind you at night, the mirror dims; in daylight, it stays clear. It's basically teaching a mirror to behave like a human eye.
The plain-English version
What it protects
The claim covers an automatic rearview mirror control system that measures light from two directions—forward (ambient light where the driver is looking) and backward (headlights or sunlight hitting the mirror). Based on these measurements, the system electronically adjusts how reflective the mirror is. Critically, the system includes a smart filter that makes the mirror respond faster to brightening light and slower to dimming light, mimicking how the human eye actually adapts. What's protected here is the combination of dual light sensing, adaptive sensitivity based on ambient conditions, and the asymmetric time-filtering algorithm.
Why it matters
Before this patent, rearview mirrors were passive—they reflected whatever light hit them, which meant blinding glare from headlights at night or excessive brightness in daylight. Donnelly Corporation's innovation turned the mirror into an active, responsive device that protects driver vision and safety. This became foundational technology for the anti-glare rearview mirrors that are now standard in most vehicles, representing a shift from purely mechanical car parts to intelligent electro-optical systems that adapt in real time.
Real-world use
Every time you drive at night and the headlights from a car behind you suddenly brighten the interior of your car, an automatic dimming mirror using this technology darkens smoothly to protect your eyes from that glare.
Original USPTO abstract
The specification discloses an automatic rearview mirror control circuit which models the physiological response of the human eye to fluctuating light levels as observed in typical driving conditions. The control circuit calculates an ambient light signal representative of light levels within the driver's field of view and a rear light signal representative of light levels in a direction generally incident to the reflective element. The reflectivity of the reflective element is controlled to optimize rear image information while minimizing on rear image glare at the driver's eyes. The control circuit includes a multiple-rate filter for time-averaging an ambient light signal over a shorter time period during increasing ambient light intensities than during decreasing ambient light intensities to simulate the more rapid adaptation of the human eye to increasing light intensities than to decreasing light intensities. The control circuit automatically alters or adjusts the sensitivity of the mirror to rear light based on the ambient light level. The mirror is more sensitive to rear light in relatively low ambient light, such as experienced on rural highways, than in relatively high ambient light, such as is experienced on urban highways. The control circuit calculates ambient light within the driver' s field of view. The calculation takes into account the forward light, rear light reflected from the interior rearview mirror, and/or rear light reflected from the windshield, pillars, headliner, and exterior rearview mirrors.
Patent details
- Publication number
- US 4,793,690
- Filing date
- 1987-04-27
- Grant date
- 1988-12-27
- Assignee
- Donnelly Corporation
- Inventor(s)
- GAHAN; EDWARD A., MOLYNEUX; KEITH W., SCHOFIELD; KENNETH
- CPC class
- B60R1/088
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