Headlights -- particularly those coming from thoughtless folks who refuse to dim their brights -- can momentarily blind the on-coming driver, creating a safety hazard.
It's almost as bad when someone shows up behind you, those beams shining directly into your rearview mirror.
The day/night rearview mirror, which first appeared four decades ago on lists of standard or optional equipment for new cars, alleviated reflective glare to some extent. Yet in night mode any mirror objects save for the offending headlamps seem obscure and hard to see.
A more recent spin-off from the day/night mirror uses electrochromic technology to automatically dampen the effect of bright headlights.
Electrochromic technology is a process of darkening glass by applying an electrical current. Such a mirror contains a thin layer of electrochromic gel sandwiched between layers of glass. When sensors detect glaring light, they energize an electronic system that causes the gel to darken and absorb the light's brightness, thus paring the glare's intensity. Once the glare passes, the mirror reverts to normal mode of high reflectivity. Driver never has to lift a finger because the mirror changes automatically, much like photosensitive eyeglasses darken relative to light exposure.
A few pricey automobiles today provide an electrochromic mirror as standard equipment, but now you may add a deluxe electronic mirror to virtually any car -- thanks to a multi-functional design by Gentex. The Night Vision Safety Mirror, which dims automatically, also has a built-in digital electronic compass, a digital thermostat to display outside temperature, plus an ice indicator that pops on for one minute when the temperature dips below 37 degrees. The mirror's wiring package fits most vehicles.