Dimming Mirrors Explained

Things change dramatically when other motorists join the same stretch of tarmac. We are sure most of you drivers out there have experienced the transition from enjoying the drive in the dark to the panic caused by near blindness. When an incoming vehicle approaches, its headlights turn into your eyes' worst enemies, no matter the direction they're pointing at. When said car comes from the front, for a brief, terrifying few seconds, everything turns black. When it comes from the rear, your field of vision retracts, desperately trying to battle the glare.

Of course, dimming mirrors are useful in all night driving conditions. Even in places illuminated by some sort of street hardware, incoming traffic may turn driving at night into a tricky business, especially if you are short-sighted or suffer from some other visual disorder. And here's where dimming mirrors come in, those wonderful bits of tech. In this piece, we will try to explain this technology, a sometimes life-saving and always thrill-free innovation.

But first, why are such devices needed? Well, the answer is simple. Because of something called the Troxler effect, or fading. Named so after the one who first discovered it, Swiss physician Ignaz Paul Troxler, it is a phenomenon that occurs when you try to focus your vision onto a single point in space for more than 20 seconds, ignoring everything else. This will make a stimulus outside the range of sight fade away - to see how that works, try focusing on the point in the photo below for 10-15 seconds, and you'll notice how the circle around it disappears (or check the video below the text for more details).

That could all seem like fun and games and a neat party trick, but for drivers it could also be very dangerous. When behind the wheel, the Troxler fading occurs after the source of some glare (like, say, incoming headlights) has left the field of vision. That's right, it's not the presence of light on dark roads that causes the most problems for drivers, but the fact it disappears all of a sudden, leaving utter darkness in its wake, and a touch of momentary blindness.

Studies conducted to reveal glare effects on the reaction times of drivers affected by it have shown that the Troxler effect increases critical response time by up to 1.4 seconds. It may not seem all that much, but here are some numbers that'll make things a lot clearer: when traveling at a speed of 60 mph (100 kph), it would take a driver 123 feet (38 meters) to see and react to road hazards in post-glare environments. Even simpler, we're talking about 123 feet of driving blind as a bat.

So, solutions had to be devised to mitigate the dangers that arise because of this. The cheapest, and possibly most effective solution carmakers came up with is the dimming mirror.

That would be the piece of reflective stuff fitted inside the car, but with some interesting characteristics, of particular concern for us being its ability to dim. Depending on who and how they make it, dimming glass comes with different names: smart glass, SPD glass, photochromatic, liquid crystal, thermotropic or electrochromic. Yet, regardless of the names use for it, the glass' main feature is the ability to turn from clear to tinted, colored, or opaque when subjected to light.

A Michigan based company called Gentex is considered to be responsible for the rise of the electrochromic mirror. It came up with such a solution in 1982 (although crude ideas on how to do this were around since the 1940s), but it would take another five years for the electrochromic-based dimming technology to be developed. At first, these mirrors were only for interior use, but exterior dimming mirrors came about not long after that, in 1991.

In the automotive industry, the technology used for the creation of dimming mirrors is called electrochromism and the resulting glass electrochromic. And it works in an extremely simple and effective way.

To do that, the dimming mirror must first know when it is time to act. The ones used in the automotive industry are fitted with sensors to detect the intensity of the light hitting their surface. Usually, there are two sensors on the mirror, one pointed to the front and the other to the rear, and they control the dimming of both interior and exterior mirrors.

When active, the sensors are constantly on the lookout for low ambient light. When they detect poor lighting they instantly know you're driving at night, so they start the second phase of their operation, scanning for sources of glare. They detect it as a change in light intensity, and they trigger an electrical charge.

This charge is applied to the glass through a low-voltage power supply. Electricity travels through an electrochromic gel captive between the mirror's two electrically conductive-coated pieces of glass. As a result of this charge, the mirror darkens proportionally to the light detected by the sensors. When the glare is no longer detected, it reverts to its normal condition.

And that's it. Simple, effective, and not so blind at night now.