Photochromic lenses

[Andrew M]

(I am unsure if this should be in private flying or not, but it is a medical related topic - I'm sure if this forum was inappropriate one of our helpful mods could move it - thanks :) !)

I went for my first eye test in ages, before starting a PPL. Luckily, my prescription was only negative 3 diaoptres for both left and right eyes - which to my knowledge is good enough for a CAA class one medical. That is good news

The optician said my colour vision was excellent too. You'll all know the test with the numbers inside the circles with the various red and green colours ? Well, I managed to read the numbers immediately when looking at the book - all I might need to do now I work on my fitness levels :O :D

Now on to the point I was going to make - I understand that glasses are ok, however, a lot of pilots wear sunglasses. (It is part of the stereotypical image after all !). I was just wondering if the photochromic lenses (the kind that adapt to prevent glare when the lens detect UV rays) are certified for corrective vision ?


I don't suspect that there would be any problems, providing the back-up pair of glasses were ordinary single vision lenses ?

(This is initially for a Class 2 medical)

Thanks,
Andrew

[Circuit Basher]

Andrew M - [non-medical opinion to follow]

I'm sure that I've seen negative opinion on photochromatic lenses on PPRuNe and a search reveals that this thread has a posting by Basil on 20 Apr 03 - 2129 (http://www.pprune.org/forums/showthread.php?s=&threadid=41338) which indicates that they are not recommended.

My experience from using them for driving is that they don't react quickly enough to eliminate glare when the sun suddenly appears (from behind cloud, turning, coming out of tunnel, etc) and similarly going the other way. I realise that the argument applies for if you have non-photochromatic lenses (ie) sunglasses / clear lenses, but you tend to expect the lens to react and wait until it starts to change, accepting reduced visibility / eyestrain for a longer period than if you just whip out the spare pair. An optician once said to me that photochromatic lenses interfere with the dilation / contraction of the pupils and could have negative impact in the longer term.

Would recommend either having flip up / down clip-ons or a spearate tinted pair of spectacles.

[buzzc152]

I recall from ATPL exams that photochromtic glasses are not allowed (or maybe not recommended ??) The reason being is that they don't react quickly enough to changing light levels. Up there above the clouds there's a tremendous amount of UV/blue bright light. Best to have something that's going to do a good job of protecting your eyes.

[Crepello]

A slight tangent (diffraction?), but I've an inkling that the max correction is -3 for an initial Class One, -5 for a renewal.

It's a while since I looked into this but if I'm right, you don't want to wait too long before doing the initial Class One.

[Bad medicine]

Sunglasses are a topic dear to the heart of aviators. There are a number of factors which determine the suitability or otherwise of sunglasses for use in the aviation role.

Narrow frames that carry large lenses are the most desirable from a field of view standpoint. The most critical problem with frames arises from the presence of wide sidearms which can significantly impair the peripheral visual field.

Lenses should not be too dark, and should transmit at least 15% of incident light. The tint used should be "neutral density"
(N.D.), that is, a greyish tint that does not distort colour perception. Much has been written about the perceived benefits of various coloured lenses, especially in regard to contrast and perceived brightness. Coloured lenses have been shown to reduce the colour information received, and to have a detrimental effect on both perception and reaction time.

Lenses of polycarbonate material are preferred because of their impact-resistance and ability to absorb ultra-violet and infra-red rays. However, these lenses can scratch easily. Harder materials may be suitable, however these may be more prone to shattering in an impact.

Different jurisdictions and countries have standards to which sunglasses are manufactured. Sunglasses should be chosen which conform to the appropriate standard. For example, the current Australian Standard ensures that sunglasses meet acceptable standards for lens quality, frame strength and lens retention and also ensures that sunglasses provide adequate protection from solar radiation.

Polarising sunglasses should not be used when flying. The polarising filter interacts with the cockpit transparency to produce a distorted and degraded visual image. This effect can also be seen with laminated car windscreens. Better keep the polarised lenses for fishing.

Pilots who wear prescription spectacles can choose from a number
of options for glare protection. Prescription sunglasses with N.D.15 lenses can be obtained, or N.D.15 clip-on or flip-up sunglasses may be worn over prescription spectacles.

Back to the original issue of photochromatic lenses. Photochromatic lenses have several disadvantages that render them unsuitable for use by pilots.

Firstly, their transition times are relatively slow. Photochromatic lenses take about five minutes to increase their density to the level of sunglasses, but more importantly, the bleaching time from maximum to minimum density can be as long as 30 minutes. Although there is a rapid lightening of the lens in the first five minutes, this may be too long when there is a sudden variation in light during a descent into or under cloud, or because of a rapid change in cloud cover.

Their second disadvantage is that, even when fully bleached, photochromic lenses still absorb slightly more light than untinted lenses. Since vision is critically dependent on ambient light levels, even this small decrease of light reaching the eye through photochromatic lenses is undesirable, especially at night or in low light levels. The inherent degradation of these lenses with time causes them to progressively become darker, as they become unable to reach the fully bleached state. This effectively prohibits their use in flying or controlling air traffic.

I hope that "sheds some light" on the subject.

Cheers,

BM

[Andrew M]

Thanks for the advice people - proberly better with the clip on attachments - or even a pair of prescrption sunglasses (which would need to be in addition to the main pair and the back up)

Crepello - I know that our limit is -3 .. let's hope my eyes don't get worse. :)

[Sick Squid]

Andrew, I would recommend going for the class one if you can afford it, if your eventual goal is to become a commercial pilot. I do understand that the CAA are doing a Dick Turpin on it now, and charging something in the order of £400, but as a bullet to be bit (sic) at some stage it is one of the ones to chomp down on early.

Thinking about it, at least Dick Turpin had the decency to wear a mask..... !

Squid

[Andrew M]

My prescription hasn't changed very much over the past 4 years - only an increase of .5d in each lense. The optician told me that growth spurts would generally decrease vision during puberty :)

Considering I'm at the end now (nearly 17 - not far away) then this means that I shouldn't grow any more, therefore vision should remain at it's present level until old age sets in - which is quite a bit away!

Andrew, I would recommend going for the class one if you can afford it, if your eventual goal is to become a commercial pilot. I do understand that the CAA are doing a Dick Turpin on it now, and charging something in the order of £400, but as a bullet to be bit (sic) at some stage it is one of the ones to chomp down on early.

A class one would be great just now - although I don't know if finances could reach just yet - my salary increases with age - so it won't be long until my salary grows a little and this will be possible.

[witchdoctor]

If you're planning the commercial route to ATPL, then £400 for a class 1 will seem like a puff of wind compared to the cost of getting your licence, even if you go the most cost-effective route. If you're struggling for the dosh now, then brace yourself for the next couple of years young fella.;)