Sunglasses
Shades of concern
Headset review
Ian Fraser takes a polarised position on pilot sunglasses…
T
hat 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
20 | LIGHT AVIATION | September 2021
