Aha.
I think I've got it.
This has been bugging the hell out of me for the last day or so so have been doing alot of reading and think I've figured it out.
It takes the human eye a short time to get accustomed to low light levels. The receptors in the human eye that are used in the dark are highly sensitive to light - have you ever crawled out of bed in the middle of the night because you're dying for the loo and without a thought flicked the bathroom light on? Yep - squinting and blinking and lots of AARGGHing - then you flick the light back off. But by then you've lost your 'night vision' and you're almost blind in the dark.
Same thing happens when some muppet leaves their main beam on and it flashes across your face when night driving - you can't see diddly squat afterwards.
Well imagine you're on your own driving on a motorway and need to check your map to see what junction you're coming off at. Turning on a white courtesy/map reading light and then looking at the map in the light will result in you losing a substantial portion of your ability to see in the dark, as the white light is reflected off the map. However if the light you use is red this doesn't badly compromise your night vision (it's why armed forces use red lights at night as a) they can still see in the dark afterwards, and b) the light isn't easily visible to possible enemy forces). So using a red light to read pace notes is fine, as these are written in dark ink on white paper (and the red light won't dazzle the driver next to the navigator or prevent the navigator from then being able to see outside the car) BUT.... a map has markings in many colours, some of which would not be clearly visible in pure red light. Now, here you have a red light with green mixed in, which produces a yellow light with a red tinge. Still easy on the eye but you'll be able to see red map markings too in that light.
Right. Now we move onto polarised light. This is light where the light waves vibrate in one plane only as opposed to the multi-directional vibrations of normal rays. Polarisation is a natural effect produced by some reflecting surfaces, such as glass, water, polished wood, etc., but can also be simulated by placing a special screen in front of the light source. When a polarising filter is placed in front of a light source it can eliminate undesirable reflections from a subject such as water, glass, or other objects with shiny surfaces. Whether it's the coloured filters or the mirrors inside it, it's a pretty safe bet that the light emitted into the cabin is polarised.
Now look at the direction of the light. It's not aimed in that particular direction in order to shine on the screen or dash per se. I think it's directed as it is so that there is a beam of light that passes alongside the side of the driver's head - so whenever they pick up a map and hold it at head height so that they can glance at it without really taking their eyes off the road then the light is in exactly the right position, illuminating the map.
Because it's polarised, when there's no map in the beam of light it won't reflect off the screen or dash and dazzle the driver. Because of it's colour it won't mean that after a glance at a map illuminated by the light the driver won't be able to see clearly in the dark. Because it's angled as it is the light won't blind the driver in the rear view mirror, not really be visible to other drivers.
There ya go. That's my two penn'orth
