Is photochromic redundant for UV protection with polycarb or Trivex lens?

[Pete Hanlin]

UV by definition is electromagnetic energy that is shorter than visible light- 390nm is visible light.

Concerning the damage potential of various wavelengths (regardless of how we label them)...
It is well-established that the energy of light decreases as wavelength increases. Therefore without a doubt light in the 380-415nm wavelength range has more energy than light in the 415-455nm wavelength range. The chart I posted came from a paper I read years ago (should have copied the source into the graphic), and I originally pasted it into a CE presentation I give on UV and Infrared just because it demonstrates a general point. Namely, UVB has a greater impact on the body than UVA, even though exposure to UVA is far greater. This is seen in the effects UVB and UVA have on the human body. If you're exposed to UVB for just a few hours, erythema and photokeratitis will occur a few hours after exposure, whereas UVA exposure results in increased skin pigmentation the next day, skin thickening over the next few weeks, and over the course of years it contributes to the formation of cataracts in the eye. When this discussion began, I remembered the graph and posted it to make a point regarding 380-415nm light.

So, back to the "how can 415-455nm light be more phototoxic- or damaging to the eye- than 380-415nm" question. There are a couple of factors at play. First, far less 380-415nm light reaches the retina compared to 415-455nm, because the former is largely absorbed by the anterior structures of the eye (especially as we get older). Second (and perhaps more importantly), if you look at the pathology of AMD, it appears a key role is played by A2E (a photosensitizer found in lipofuscin). The wavelength that triggers oxidation (or the production of reactive oxygen species) in A2E is centered right around 435nm (which is probably why study after study has landed on 435-440nm light to be particularly damaging to the eye). There are a quite a few papers on the subject, one I would recommend is: Neelam K. et al., The role of blue light in the pathogenesis of Age-Related Macular Degeneration, Points de Vue, International Review of Ophthalmic Optics, N71, Autumn 2014. (You can find the paper at this link: http://www.pointsdevue.com/article/r...r-degeneration).

Sorry for the length, but to summarize 380-415nm...

• 380-415nm light is mostly absorbed by the cornea and crystalline lens, where it causes less damage than the 295-380nm range of UVB and UVA to which we are commonly exposed. Therefore, I have no problem with ANSI setting 380nm as the upper limit that must be filtered for lenses claiming to block UV, if for no other reason that setting this limit allows a lens to claim its UV filtering feature without requiring the lens to be visibly tinted.
• While light in the 380-415nm does reach the retina (after all, we can see this range, so it must be reaching our retina to some degree), at least one of the mechanisms related to the progression of AMD is associated with a longer wavelength (435nm).

I'm not saying there is anything wrong or undesirable with filtering 380-415nm light (other than the cosmetic consideration- because the lens will have a yellow tinge). However, from a UV perspective I don't think we should be requiring a lens to filter this range to claim UV filtering (because of the cosmetic issue and the fact that UVB and UVA is much more important to filter). Also, from a Blue perspective it may be a bit misleading to claim a "blue blocking" feature for a lens which primarily blocks only this range (with little filtering of 415-455nm).

[NoUV]

Hello again.

I've been doing some research to understand what I've been told in this thread. I've started by looking for two references that have been cited, which are ISO and ANSI standards. One potential source of misunderstanding has arisen in the ISO document. However, I have been unable to get a copy of the relevant ANSI material.

Spectral ranges are defined in ISO 21348. A copy is available at:
http://www.spacewx.com/pdf/SET_21348_2004.pdf
Its definitions of UV and visible light overlap. UV, specifically UVA, extends up to 400 nm. Visible extends down to 380. Purple extends down to 360. There is no color defined for 360 to 380.

These are not errors. Since visibility of light varies by species and further by individual, the information in the table of ISO 21348 is purely definitional, not phenomenological. Both the overlap of UV with visible and the absence of a color for low-wavelength visible emphasize that the boundaries of these terms are matters of subjective agreement, not objective facts. And it does seem to lead to some confusion in the thread above, where some people talk about UV ending at 380 and others at 400. Of course, the important issue is not what name is give to a particular range of wavelengths, but whether exposure to it is harmful.

So the next important reference I looked for is the relevant ANSI standard. I found that the ANSI standard for eyeglasses is Z80.1. There is a Quick Reference summary of it at:
https://www.thevisioncouncil.org/sit...rence%20v2.pdf
This does not say anything about UV or other light hazards. The ANSI standard for sunglasses is Z80.3. There is no online summary available of Z80.3. The Z80 standards are available for sale from ANSI at:
http://webstore.ansi.org/FindStandar...NSI+Z80%7cnull
Z80.1 costs $75, while .3 is $50. I did a search for these document in libraries and did not find any in a library near Los Angeles or any library that would send them by inter-library loan.

The Z80 standards are written by the Vision Council, which is also the source of the Z80.1 Quick Reference linked above. I wrote to them to ask about these standards. What I got was:
* Z80.1 does not cover light hazard protection, but defers to Z80.3 on that subject.
* There is no Quick Reference of Z80.3.
* Z80.3 defines limits for UV, visible, IR, and solar blue.
* The Vision Council will not send me a copy of even just the specific pages of Z80.3 relevant to light hazard protection.
* They did send me section 3.8.3, of Z80.3, which is the definition of UV mean transmittance. It shows that ANSI uses a different definition of UV than ISO does, ending at 380 nm.
* They also sent me part of section 4.10.3, which states the actual standard for UV mean transmittance. However, the part they sent refers to other parts not included and is undecipherable on its own.
* They refused to send me any more than that.

This leaves me really curious to know what the standard actually does say. Not only Z80.3, but to also find out if it's true that Z80.1 completely defers to Z80.3 on anything about light hazard protection. I could find out all of that by sending ANSI $125. I fully understand ANSI's need to support its work with the sale of publications, but I'm not in a position to be providing that support. Is there someone here that has access to those standard who would be willing to send page scans of just the relevant pages on this subject? If you feel this is something that can be legitimately posted here, you could attach them to a reply. Otherwise, you could send me a private message here and I'll give you my e-mail address to send them to privately. I promise to keep that correspondence confidential and not to post them on the Internet.

Thank you.

[Steve Machol]

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