Safe Speed Forums

How about adding polarising filters on all car/bike headlamps and on windscreens (or glasses) such that drivers would no longer be dazzled by oncoming traffic during darkness?

Two problems I foresee:
1) The filters may attenuate too much – the light path from headlamps to eye will pass 2 such filters (the attenuation factor for each will be 50% at least? EDIT, just realised that won't necessarily be valid - the 2nd filter may not have much to attenuate)
2) All the filters will need to be rotatable such that the plane of polarisation of filters on vehicles in opposite directions will have their filters in opposite planes.

Thoughts?


Does anyone know if fog preserves the polarisation of light?

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

On a similar note in terms of what 'we can do' with light - it would be possible to have enormously powerful bulbs, capable of illuminating well over a mile ahead, if we all wore special "sunglasses" at night (in simple terms, imagine that the light was fully red, but reflected as pure green - "sunglasses" that absorbed red would show the green way but prevent dazzle)

These would obviously not be suitable for most routes (where anything [e.g. pedestrians] apart from licenced traffic could be found), but I can't think of a reason that we couldn't use them on motorways - insisting, of course, that drivers wear the appropriate glasses.

Since reflected light is polarised you risk making some things completely invisble.

Puddles spring to mind.

And (topical) ice.

I would be a bit wary of using a system that filters out red light – it would be nice to see brake lights

I’m not sure I understood you correctly: are you thinking that objects illuminated by one colour will reflect as a different colour? There aren’t many materials which will fluoresce at non-UV wavelengths…...

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

I understand what your point, but I’m not so sure it will be so significant. Some surfaces can change or filter out a plane of polarisation but I think this usually occurs when light is reflected forwards (due to refraction); we’re interested only in the light that is reflected back.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Look what I found when researching:

http://www.polarization.com/land/land.html

EDIT: the 45 degree angle idea solves my problem 2)

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

My photography studies tell me all reflected light is polarised.

Which is why I use a polarising filter to shoot through glass, and cut out reflections from pools.

Anyone up on the equations etc to do with fibre optics and joins at an angle - a lot of the equatons etc are the same ---can't find my optics notes

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on

I decided not to post here until I had done some experiments confirming my understanding (I have many polarising filters and sheets at home – I‘m a geek!).

* The plane of polarisation and path of the reflection from pure non-specular (diffuse) surfaces are randomised, irrespective of the angle or polarisation of the incident light beam.
* The reflection of near perpendicular light via pure specular (mirror-like) surfaces have both the incident plane of polarisation and the angle from perpendicular preserved (holds true up to Brewster’s angle ~50 deg).
* The forward reflection of near parallel light via pure specular surfaces will result with the reflected light being polarised (holds true up to Brewster’s angle) and the angle preserved.

Many materials exhibit both specular and non-specular properties; these are surfaces with a thin layer of clear material above the surface of the underlying (specular or non-specular) material. Examples include: glass mirrors, plastics, tiles, anything polished and puddles of water.
In the case of polarised headlamps with puddles of water or ice (near parallel light), a portion of light of a certain polarisation will be bounced forwards off the top of the clear surface; the rest will be refracted towards the lower surface at an angle tending towards perpendicular. When reflected, this portion could have both the plane of polarisation and reflected angle preserved or randomised (depending on whether the underlying material is specular or not) – but not polarised. This portion will propagate out past the clear surface in the same manner (effects of refraction cancelled out).

Therefore, not all reflected light will be polarised and no subject will suddenly become invisible when such a polarised vision system is used, irrespective of emitted polarisation angle (unless the light is passed through a 3rd polarising filter in anti-phase with the other 2 in phase filters). In fact, in this case the only light which does become polarised (which could become invisible) is the light that is reflected forwards away from the car, but the driver can only see the light that is reflected back, so that's irrelevent. Even then, this polarised light will be insignificant when compared to the light emitted directly from the headlamps when illuminating a subject forwards of that surface.

I stand by my previous post.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

If you look through polarised sunglasses at night, it cuts down much of the available light. However your eyes will compensate and open up fully, but maybe not for ALL drivers.
The glare from the road surface of oncoming vehicle lights IS reduced but not eliminated altogether, and the "shades make no difference to your own lights illumination.

My own glasses which I use during daylight hours, are of the polarised & dichroic (interference) type which filter out blue light by reflecting it away from the lens surface, thus allowing the red/yellow component through at maximum (or as close as can be). They appear much darker from outside, but NOT from within - however I would be cautious about wearing them at night, as I'd likely be pulled in the unlikely event of me passing a traffic cop these days!

In my old Peugeot 406 they were fine for ALL instruments, but in my 307, the centre console display is darkened unless you incline your head at 45 degrees - not practicable when driving! Luckily it only affects the clock/ radio display and the fuel/avge.speed/miles elapsed computer display.
I would be against polarised filters because of the darkening, but a polarised light source? That's a different matter!

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

I thought this thread had died!


I appreciate your concerns. Unfortunately there aren’t many practical polarised light sources available; however, use of the brighter HID lamps may be enough to compensate for the outward attenuation. Adding a retractable polarised visor might be a good compromise for the return path.

I’m half tempted to set up a real experiment to see how effective this idea really is – could this be a good enough excuse for buying HID headlamps?

Homer hit the nail on the head. ALL reflected light becomes polarised - so why not simply bounce the light out of the lamp via a highly reflective surface?
(Has to be flat plane, not the lamp reflector)

Obviously it cannot be "simple" or it would have been tried before.... wouldn't it?

The visor idea would need some work too - how many cars have a decent sun visor, let alone a polarising viewing visor!
Different drivers are taller/smaller - Goggles would be better!

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Time to take responsibility for our actions.. and don't be afraid of speaking out!

Hi Ernest.


Sorry, no. A mirror is a great example of an exception (ok, the slight reflection from the glass surface may be polarised but even that would depend on the angle of incidence).


I see where you are going, however, only a portion of the incident light will be reflected forwards as polarised light (look up ‘Brewster’s angle’), so unfortunately we would have the problem of ‘leakage’ as well as attenuation.


I posted a link earlier in the thread describing such an attempt and it’s downfall; those problems won’t apply nowadays.


You might be right. Ultimately I would prefer the return filter to be removed automatically or at a flick of an easy reach switch, preferably on the steering wheel.

Liquid crystal windscreens!! - Polarises at the flick of a switch.
Not sure how clear they are without the polarising filter which is built in though.

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Time to take responsibility for our actions.. and don't be afraid of speaking out!