View Full Version : Why can't you dim the side-view mirror?

10-28-2002, 04:53 PM
This is something that has bothered me ever since my first time driving, but popped back to the top of the peeve pile as I was driving my new car last night.

You know how you can flip the little button on your car's rear view mirror so the headlights behind you don't shine annoyingly in your eyes, right?

Why don't the side view mirrors have that option?

Option 1) The car companies are cheap-ass bastards who won't put safety features on a car unless the government makes them do it.

I doubt that is the whole story, or car companies would market it as a $100 extra that costs them $2.

Option 2) I am missing something that makes it a harder thing to do than I realize.

Rear view mirrors work like this, I think. It is a triangular wedge of glass with a 'silver' coating on the back. Some incoming light bounces of the front of the glass, and the rest bounces off the silver at a slightly different angle.
Daytime setting is such that light coming in from behind the car bounces off the silver and hits the driver in the eyes. Silver is a good optical reflector, so the image is clear.
At night, with our pupils dilated, this would be too much light from a headlight. So, you flip the mirror so that the silver angles a little bit down and the front of the glass is at the same angle the silver was in daylight mode. So, the only light in the driver's eyes is the reflection off the front of the glass, which is much lower than the amount off the silver.

The only possibility that strikes me as I write this is that in night mode, the line of sight from the driver off the silver backing goes down inside the car. Since the car is dark (usually), that image is hidden by the light from behind.
But, if the side mirror were tilted down, the line of sight would go onto the pavement, which might present confusing double images to the driver.

Did I just answer my own question, or is it something else?

10-28-2002, 05:09 PM
I think it's simpler than that. As I understand it the rear view mirror "dimmer" effect has to do with bouncing the reflection off the roof of the vehicle, instead of directly into your eyes. Since there is no roof to bounce off of, side view mirrors can't use this technique.

Early Out
10-28-2002, 05:24 PM
I think you've got it essentially correct, keeper0. The "dimmed" reflection is just a reflection from the glass face on the rearview mirror - the brighter reflection from the silvered part is angled down, where it can't blind you.

If you want a dimmable outside mirror, just buy yourself an S-class Mercedes. While you're at it, get one for me, too, okay?

10-28-2002, 05:29 PM
Try this. Try flipping your interior mirror to the night setting during the day; you will probably see the reflection of the roof of your car (or possibly your back seat). Since either of these locations will be dark and unlit at night, they will not produce distracting images when you are on the night setting.

Your outside mirror, on the other hand, doesn't have anything dark and neutral to aim at. The side of the car, the road surface, the side of the road or the sky are all likely to have distracting lights shining on them, if not light sources themselves. Apparently the manufacturers have decided that a bright image is preferable to a distracting one.

(I have a convertable and I can tell you that the night setting on my mirror can be very distracting if I am driving in a city with the top down. Seeing street and building lights in your rear view mirror instead of the car behind you is very annoying!)

10-28-2002, 06:55 PM
I can understand where the OP is coming from. Many times I have been near-blinded by even non highbeam headlights reflected off the side mirros into my eyes. Its annoying and dangerous, since my eyes usually tend to get drawn towards some light source and distracts me from the road.

10-28-2002, 07:17 PM
I used to keep my side mirrors tilted inward very slightly, so that when I was sitting in a "normal" position headlights would not blind me at night; I would simply lean forward a bit if I wanted a better side view. (This was when I worked 3rd shift and did most of my driving at night.)

10-28-2002, 10:14 PM
After driving a rented Lincoln Continental that had automatic dimming mirrors I decided to install them on my car as well.

Installing the interior mirror was not too hard with my greatest fear being that I might break the windshield while removing the old and installing the new mirror. I decided to ask the dealership to install the driver's side mirror and wiring harness after learning the hard way that dropping a screw inside the door panel was far too easy. The mechanics searched for the lost screw for 15 minutes before finding it in a spot that will only release it if the car is upside down.

The driver's side mirror receives a signal from the interior mirror to dim once it had reached its full dimming setting. I checked with Porsche and learned that they do not offer an automatic dimming passenger's side mirror yet supposedly Audi offers it on their vehicles.

Oh, and in case you wonder how the automatic dimming mirror works. It has two photo sensors, one aimed toward the front and one aimed toward the rear. When it detects more light coming from the rear than the front it dims the mirror until the difference is no longer present.

10-28-2002, 11:01 PM
How do "night" rear view mirrors work? (https://academicpursuits.us/classics/a4_015b.html)

10-28-2002, 11:51 PM
One of my cars has the auto-darkening feature in the side-view mirrors. I've found that I'm not as distracted by the reflection from the outside mirrors as I am from the inside mirror, possibly because they are situated further outside of my visual field. As such, I don't really feel they are necessary for most nighttime driving.

But lately, a lot of A-holes have taken it upon themselves to upgrade their factory headlights to the newer xenon High-Intensity Discharge headlights, which can be very distracting to other motorists. When one of these guys is behind you, the darkened side-view mirrors significantly reduce the glare.

Joey P
10-29-2002, 01:23 AM
They have auto dimming rear and side mirrors in the new Chevy Tahoes. It's nice to a point, but it makes it difficult to accuratly judge exactly whose behind you, how far etc.... since all you see is blurry headlights. But overall, it's nice that you don't suddenly get blinded since the car has already dimed all your mirrors.

10-29-2002, 08:33 AM
After posting this, I did what I probably should have done in the first place, which is go to How Stuff Works.

Yup, the rear view mirror does "point" to the ceiling and not the back seat in night mode. And it mentions that any light on the ceiling would basically wash out the rear reflection.

A side view mirror doing the same thing would give you a good view of the stars.

So, how do the auto-dimming ones work? Are they kind of like little LCD screens where applying a voltage across it makes the glass more opaque?

10-29-2002, 08:35 AM
it seems the definitive answer to your question is that auto dimming outside mirrors do in fact exist... (my 2000 Audi A6 had them)

You just need to stop buying cheap cars ;)

Having had them though, I can tell you that they're not all they're cracked up to be. They kill depth perception by reducing what you see to just a series of light dots on a black field.

One of the most frequently asked questions at an Audi enthusiast site I frequent is "How do I disable the auto dimming feature for the outside mirrors?"

10-29-2002, 11:09 AM
Originally posted by keeper0

So, how do the auto-dimming ones work? Are they kind of like little LCD screens where applying a voltage across it makes the glass more opaque?

they are electrochromic - essentially a material or plastic that changes colour or shade when charged (much like a rechargeable battery) and then acts as like a pair of sunglasses

10-29-2002, 11:35 AM
I've never tried this but you can probably reduce glare on the sideview mirrors by adding window tinting film. Trouble is it's on all the time, but maybe you could find a happy medium.

10-29-2002, 02:21 PM
Why don't the drivers behind you dim their lights?????:smack: :smack: :smack:

Detroits answer to the age old problem of "over driving" ones headlights:rolleyes: :rolleyes:

Make them brighter.........:wally :wally

Forget about the increased danger of an approaching driver being blinded by the glare......:eek: :eek:

How many crashes have been the result of momentary blindness due to glare???? :confused: :confused:

HAS ANYBODY ASKED???? :mad: :mad:

10-29-2002, 06:59 PM
Originally posted by keeper0

So, how do the auto-dimming ones work? Are they kind of like little LCD screens where applying a voltage across it makes the glass more opaque?
Bingo! My workplace, which was once part of General Motors, developed two kinds of dimming side mirrors. One, which is now used, was an LCD sheet over the mirror. Just like the numbers on a watch, the grid becomes dark when charged, and suddenly you're looking through a window screen. The other method, which I don't think anyone used, uses vacuum to move the mirror back a wee bit from the glass cover. When that happens, tinted liquid surges into the space between. I'm guessing that the second method turned out to be more expensive than the LCD's. Cost is behind most decisions in the auto industry.


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