Fo­cus on

Auto-dim­ming mir­rors

Leisure Wheels (South Africa) - - CONTENTS - Text: GG van Rooyen

Ac­cord­ing to leg­end, NASA spent a for­tune dur­ing the early days of the space race to cre­ate a pen that could op­er­ate nor­mally in the ze­ro­grav­ity of space. The Rus­sians, mean­while, sim­ply used a pen­cil. This story, sadly, is apoc­ryphal: while there is un­doubt­edly wis­dom to be gleaned from the story’s mes­sage of sim­plic­ity and fru­gal­ity, the tale it­self is pure fic­tion. NASA’s rocket sci­en­tists didn’t over­look the ex­is­tence of the pen­cil. In fact, early NASA as­tro­nauts did use pen­cils, but th­ese were even­tu­ally deemed too un­safe. They flaked and broke (cre­at­ing float­ing de­bris), and were flammable. So, NASA opted for a pen; a pen that al­ready ex­isted and didn’t need R&D fund­ing.

Now, like the space pen, an auto-dim­ming rearview mir­ror can seem like an an­swer to a ques­tion that no one both­ered to ask. Af­ter all, a cheaper and ar­guably more el­e­gant so­lu­tion al­ready ex­isted. Even the cheap­est rearview mir­ror on the mar­ket can sim­ply be flipped up to re­duce glare. But, like the space pen, the auto-dim­ming mir­ror de­serves more credit. The auto-dim­ming (or elec­trochromic mir­ror) is truly safer than a stan­dard one.


The prob­lem with a stan­dard rearview mir­ror, of course, is that, while it can re­duce glare by be­ing flipped up and pro­vid­ing a ‘re­flec­tion’ of what’s hap­pen­ing be­hind you, this has the ef­fect of dim­ming ev­ery­thing sub­stan­tially. So, while it’s great when you’re deal­ing with an in­con­sid­er­ate driver who’s sit­ting right be­hind you with his brights on, it can make things hard to see when there’s an ab­sence of light.

An auto-dim­ming mir­ror doesn’t do this. Not only does this kind of mir­ror dim au­to­mat­i­cally when there’s a bright light be­hind you, it also dims in pro­por­tion to the light source it’s deal­ing with, mean­ing the dim­ming ef­fect will be far less pro­nounced when there’s a rel­a­tively faint light in your mir­ror.

An­other im­por­tant ben­e­fit of an au­todim­ming rearview mir­ror is that it pre­vents

some­thing called the Trox­ler ef­fect. This is an op­ti­cal il­lu­sion that re­moves un­chang­ing stim­u­lus from your field of vi­sion when you fix­ate on a spe­cific point. Be­cause of this phe­nom­e­non, when you’re driv­ing at night, the glare from bright lights is not only blind­ing while they are present, but it cre­ates a blindspot in your vi­sion even af­ter the ve­hi­cle has turned off. Ac­cord­ing to Gen­tex, a com­pany that man­u­fac­tures auto-dim­ming rearview mir­rors, the Trox­ler ef­fect can re­duce a driver’s re­ac­tion time by as much as 1.4 sec­onds. A car driv­ing at 100km/h will travel 37.5m in that time.


So, how does an au­todim­ming rearview mir­ror ac­tu­ally work? As the size and heft of one of th­ese mir­rors tells you, quite a lot is hap­pen­ing be­hind the re­flec­tive sur­face. The mir­ror it­self darkens when a light source hits it, thanks to elec­trochromism (which is why auto-dim­ming mir­rors are also called elec­trochromic mir­rors). An elec­trochromic ma­te­rial changes colour when charged by an elec­tri­cal cur­rent. Send volt­age through it, and it darkens. Re­move the volt­age, and it light­ens. This is largely a chem­i­cal re­ac­tion that is kicked off by adding elec­tric­ity. With the elec­trochromic ma­te­rial added to an au­todim­ming mir­ror, elec­tric volt­age changes the way in which it ab­sorbs and re­flects light. An auto-dim­ming mir­ror usu­ally has a set of cam­eras or sen­sors (pho­to­di­ode­based pho­tode­tec­tors) that are light-sen­si­tive semi­con­duc­tors, which turn light into cur­rent. Th­ese sen­sors are at­tached to a mi­cro­pro­ces­sor that can de­tect glare from head­lights and send a charge through the elec­trochromic ma­te­rial to re­spond to this in­put.

As men­tioned, the sys­tem re­sponds to the amount of light present. The more in­tense the glare, the more the mir­ror will darken. The mir­ror it­self con­sists of two lay­ers of glass with a layer of gel in be­tween, which is where the elec­trochromic ma­te­rial resides. Add light (and con­se­quently volt­age), and the gel darkens. Take it away, and it light­ens.

Above: An auto-dim­ming mir­ror can make it eas­ier to drive at night. Be­low: Dif­fer­ent man­u­fac­tur­ers use slightly dif­fer­ent ver­sions but the auto-dim­ming rearview mir­rors all work the same way, mak­ing use of an elec­trochromic ma­te­rial that darkens thanks...

Above: The Nis­san Smart Mir­ror is a rearview mir­ror with a built-in video screen. The driver has the op­tion of switch­ing to the video view, which makes use of a cam­era in the rear of the ve­hi­cle. Left: In­side a mod­ern rearview mir­ror.

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