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Whoops, no head-up display!

September 2, 2017

In an idle moment recently (I don’t get many of those at the moment) I was scrolling through Honest John’s car advice in the Daily Telegraph. I found this one:

This struck a chord with me, because I’ve just bought a pair of Polarised sunglasses to eliminate reflected glare from the inside of my car windscreen – which can be a major problem if the sun’s in front of me and shining directly onto the top of the dashboard.

So, some basic physics. When light is reflected off a surface, most of the light that’s vibrating parallel to the surface is reflected, while most of the light that’s vibrating at other angles is absorbed or diffused. This means the reflected light is mostly vibrating in one direction – this is what “polarised” light means. Reflections from the inside of a car windscreen will be horizontally polarised.

Polarised glasses are designed to eliminate horizontally polarised light because it’s horizontally polarised light which reflects from surfaces such as roads, puddles and lakes, tables and so on. That’s why I bought my new glasses – to eliminate the reflection from the inside of the windscreen when driving towards the sun.

Head-up displays work by projecting the display upwards so that it reflects off the inside of the windscreen – as shown above.

If you wear Polarised glasses, these will cut out any light reflected from the inside of the windscreen, however it got there, so you will not be able to see a head-up display. Reactolite glasses aren’t polarised, they just darken the lenses, so the display will still be visible.

Obvious really, Honest John!

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