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).
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.
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.