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Name: Monzur M.
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 2000 - Revised Winter 2009-2010 - See below


Question:
Can glass completely prevent ultraviolet ray penetration? Does UV penetration depend on the thickness of glass? As a lab worker I need to check my DNA samples using an UV illuminator. Are my regular eye glasses sufficient to protect my eyes?



Replies:
Glass is not a perfect UV blocker, but it is pretty darn good. The little UV lamp you use will not be enough to injure your eyes if you wear glasses.

And yes, as you expect, the thicker the glass, the more UV it will block. Basically, if thickness x transmits 50% of the UV, thickness 2x will transmit 25%, 3x will transmit 12.5%, and so on. If you have access to a spectrometer, you can check the transmittance of your glasses.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


Hi, Monzur !!

Just to confirm : as you know, there are three types

of Ultra-violet light :

UV-A : With wavelength 400-320 nm is less intense than UV-B, but is more penetrating and can reach and damage easily the retina

UV-B : Wavelength 320-286 burns the skin and damages the cornea and ocular lenses. It is associated with cataracts

UV-C : filtered off by ozone layer

Some sun lenses may not offer adequate UV-protection, just only by darkening the lens to eliminate glare, because the pupil dilates and more harmful UV can penetrate the eyes. Typical sun lenses are simply coated with a bath in material that protects against UV waves. Better lenses contain such material integrated in the composition of the lens. Only with this UV-bath, the protective material can be removed through abrasion and scratches.

Going back to your questions :

1 - Can glass completely prevent ultraviolet ray penetration ? No, not completely but a high percentage can be prevented. The percentage transmitted can be measured in a lab with the help of an instrument. At least 70% of the visible light should be filtered.

2 - Does UV penetration depend on the thickness of glass ?? Yes, for a given concentration of UV-absorbing material, if it makes part of the glass composition. If not, the answer still is "yes" when the glass is tinted. A common substance used is a Poly-Vinyl Alcohol.

3 - You work as a lab to check DNA samples and uses UV light to illuminate them. Your question is if "are my regular glasses sufficient to protect my eyes". Answer : No, they are not.

Alcir Grohmann


Most window glass will absorb a large fraction of the U.V. radiation. You can check your safety glasses (always wear them in any case, of course) by shining the U.V. light through the glasses backed by a sheet of high quality paper (e.g. copier paper). You should not see any fluorescence behind the glasses. The paper will give a strong blue fluorescence if not filtered. Also use side shields on your glasses, because the U.V. light can also hit your eyes indirectly.

Even so, prudence dictates that you not "look into" the U.V. light or shine it directly on your skin. It can give you a "sun burn".

Depending upon the type of lamp, you may produce ozone from the photochemical reaction of short wavelength U.V. and atmospheric O2. If you smell any ozone ( the thundershower odor) use the lamp only in a fume hood. Ozone is very toxic as well as irritating.

Vince Calder

Winter 2009-2010

The first answer in particular is very troublesome because he states that wearing glass spectacles is sufficient for protecting the eyes against UV, and that glass is very good at blocking UV. Glass, in fact, is NOT good at blocking UV, and if the spectacle lenses are made out of glass they do NOT offer enough protection. While some glass, such as that in automobile windows, is very effective at blocking UV (and this is not a property of the glass, but other factors), opthalmic crown glass is not. Also, the second answer incorrectly states that 70% of the visible light should be filtered. The topic is not about sunglasses, and so visible light has nothing to do with it. And assuming this person meant 70% of the UV light should be filtered, this is still incorrect, and now even harmful, as it will lead people (that take this as the meaning) to believe that is enough, when in fact, 99-100% of the UV should be filtered.

Bottom line: GLASS spectacles are NOT adequate for UV protection. Polycarbonate or CR-39 lenses, especially if coated (which most are) are much better, but ultimately, anybody trying to determine if they have proper protection should see how much UV transmittance/absorbance their glasses provide, which can be measured, as mentioned in the first answer, by a spectrometer. If in doubt, they should ask an optometrist or ophthalmologist.

Steven J.



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