SHADES OF CONCERN

Ian Fraser takes a polarised position on pilot sunglasses…

That time has come again when we prepare ourselves for that ace of all fly-ins, the LAA Rally. Clean and polish the aircraft, put on that best flying suit but, most important of all, think ‘good weather’. It’s going to be bright and sunny, isn’t it, so make sure we all remember our sunglasses.

Not just any shades will do at this trend-setting event, you must have the very latest ‘fashion shades’ – it just wouldn’t do to be seen wearing ‘last year’s model’. The sartorial woke police could be there and, if you’re not careful, that ever-damming picture will go viral online: Last year’s shades, a sad case…

But why am I, of all people, writing about sunglasses? My wife will tell you that fashion passed me by half a century ago. Well, there is an interesting technical issue here. Do you mount your tablet or phone in your cockpit in portrait or landscape mode? “Has he gone completely mad,” I hear you mutter, “First shades now tablets – perhaps he has been on the wrong ones!”

But no, it’s actually a serious matter. Picture 1, a state-of-the-art phone in portrait orientation and viewed through a sunglass lens shows a dimmed (they are sunglasses after all) but perfectly readable display. Picture 2 of the same device in landscape mode, and seen through the same sunglass lenses, renders the display completely unreadable. What is happening?

Well, many state-of-the-art sunglasses and tablets, phones, navigators or even glass cockpits, use polarised glass in their displays and lenses, and this is the unfortunate consequence.

Light naturally ‘vibrates’ and polarised lenses work because they only pass light vibrating in one direction, rejecting the rest. This reduces brightness and glare (glare is often polarised light) and once light has been ‘polarised’ by passing through a screen or lens, it will only pass through material with the same relative polarisation.

You may recall that school science test where you place one polarised sunglass lens over another. In one position whatever is behind will be perfectly visible but rotate one lens and the pair will eliminate light transmission altogether.

As many modern electronic devices use a polarised display, what you are expecting to see is already polarised, so viewing the display through misaligned polarised glasses, you may see exactly this zero-transmission effect. If both the polarised lenses of the glasses and display are not in the same orientation, the display may be invisible while the rest of the cockpit (not polarised) gadgetry will look normal. Rotate one (glasses or device) to realign them and everything comes back.

Picture 1

Picture 2

Picture 3 Picture 4

Picture 3 and picture 4 illustrate this with cockpit display instruments observed through a polarised sunglass lens. Picture 3 shows all to be perfectly readable but in picture 4, after rotation of the sunglass lens the Autopilot display (but nothing else) vanishes.

Similarly, pictures 5 and 6 show the effect on engine instruments and the radio.

Although the glasses used for the photographs showed a dramatic effect between portrait and landscape of the map device, as far as I am aware there is no standard for orientation of polarisation in any of the optometry, fashion, phone, or avionics worlds.

You take pot luck that the relative polarisation alignments will not obscure your instrument displays viewed through the glasses you buy. Nor can I find any formal regulation or advice from the CAA relating to this phenomenon; while they do discourage the use of polarised glasses (see medical standards) it’s not due to their impact on readability of instruments. So, it’s up to you to be aware of and to avoid this problem.

When you visit the pilot supply stands expected to be at the event, before you buy, just check. Do the tablet, phone instruments or other displays you use in the aircraft have a polarised screen, and if so, will those latest optical fashion statements work with it? I wouldn’t like you to get lost on the way home! ■