Contradiction on Glasses and Canopies

[apatti]

All,
I was just re-reading a thread about eyeglasses and came across a contradiction that bugs me. An eye doc posted that photochromatic lens won't work under RV canopies beacuse the canopies block the UV rays that activate them. Other reputable posters said unequivocally that their photochromatic lens did darken under the canopy. The confusion makes me wonder if:

1) photochromatic lens are not activated by UV rays but rather some other mechanism, or
2) our canopies do not block UV rays (something I was told they do by a reputable source).

Any thoughts on the apparent contradiction?

Thanks,

[hevansrv7a]

My wife is very sensitive to the sun. She burns through the canopy. I had to install stick-on sun shades over the entire top of the canopy and carry additional spot ones which we move as our relative bearing to the sun changes.

Not all UV is the same but that's all I can say for sure.

[RVadmirer]

What could be in the canopy to block UV? Good idea if it really is there...

[Dean Pichon]

Our canopies do indeed pass UV radiation (light). We can debate the wavelength and the attenuation, but I know it is enough to burn or tan my skin and it is enough to darken photochromatic glasses. Polycarbonate (our canopies are acrylic) in contrast is a very good UV blocker.

[Norman CYYJ]

I would get sunburned in the RV-6 that I built a few years ago and my friends glasses would darken. In my RV-7 I don't get sun-burnt and his glasses won't go dark. So I would say not all canopies are made with the same formula.

[flyeyes]

Originally Posted by apatti

All,
I was just re-reading a thread about eyeglasses and came across a contradiction that bugs me. An eye doc posted that photochromatic lens won't work under RV canopies beacuse the canopies block the UV rays that activate them. Other reputable posters said unequivocally that their photochromatic lens did darken under the canopy. The confusion makes me wonder if:

1) photochromatic lens are not activated by UV rays but rather some other mechanism, or
2) our canopies do not block UV rays (something I was told they do by a reputable source).

Any thoughts on the apparent contradiction?

Thanks,

The acrylic material used to make our canopies (polymethylmethacrylate) lets a fair amount of of the near UV through, unless it is specifically modified to block UV.

This is important because many early intraocular lenses (for cataract surgery) were made from polymethylmethacrylate(PMMA) and did not have UV protection.

As an interesting (to me, anyway) historical note, the first intraocular lens was designed by a British ophthalmologist, sir Harold Ridley, who was caring for wounded RAF pilots during the battle of Britain. He noted that canopy fragments blasted into the eye were well tolerated as long as there was no infection.

He inquired from Supermarine what material the canopy was made from (Perspex a.k.a. PMMA) and commisioned the Rayner lens company of England to make a tiny lens out of this material to be used to replace the natural human lens after cataract surgery. The rest is (literally) history.

PMMA can have chromophores mixed into it or coatings on the surface to black all UV, but normally it is opaque only in the shorter wavelengths.

[WillyEyeBall]

I think Norman CYYJ has the correct answer that some RV canopies have, and others do not, but don't know for sure. My RV-4, and my friends RV-7 and 8 have significant UV blocking components in their canopies. All military windscreens/canopies whether acrilic or polycarbonate have UV blocking by regulation, and I tested Army aircraft years ago. However, Van may have several canopy makers that could be using different amounts of UV blockage. Anyone concerned about this could send me approximately a 1 inch square of their canopy if they still have any left, and I can test it in 5 seconds with the percent visual, UVA and UVB transmission with a spectral plot. If there is a difference in canopy vendors, we might be able to make that determination if you supply info on when you purchased the aircraft, kit number, and make. Use the lab mailing address: USAARL, ATTN: McLean, Bldg 6901, PO Box 620577, Fort Rucker, AL 36362-0577. Don't send FED EX, otherwise you'll need street adress, telephone numbers, etc.
Bill McLean
RV-4
willyeyeball@juno.com

[Martin Sutter]

My wife and I both wear Transition lenses. While these rapidly turn dark outside, they do not darken in our RV7A or in the car and we have to use sun glasses. Something is blocking the rays!

Martin Sutter
Building and flying RV's since 1988