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Kiss my glass.

April 14, 2007

Matthew Vasey, MD



Remember the pneumonic R-O-Y-G-B-V (infrared, red, orange, yellow, green, blue, violet, ultraviolet) to represent the order of decreasing wavelengths of the electromagnetic spectrum of visible light from back in the day. Well, whether you do or do not, that's the package light has chosen for entry into our atmosphere. While the wavelengths ROYGBV pose a minimal threat to us, the threat from wavelengths beyond red and violet can be quite significant.

Three types of ultraviolet light exist: UVA, UVB, and UVC. These rays posses a wavelength beyond the spectrum of visible violet. Fortunately, we have the ozone to shield us from the most harmful UVC rays. The remainder of ultraviolet light makes its way to our face and body, 3.5% in the form of UVB and 96.5% UVA. (1)

Given this information, an appreciation of glass would be warranted, as glass offers us an important layer of transparent protection aside from Earth's set of high-fashion designer ozones. Glass in the form of window, automobile and sunglasses have variable levels of protection from the elements of solar radiation i.e. heat, visible light and the UVA's and UVB's.

Seven types of window glass exist today, and they include (2):
1. Clear glass - existing as a barrier alone
2. Tinted or heat-absorbing glass - modest reduction of heat, visible and UV light
3. Reflective glass - significant protection of elements by way of, go figure, reflection
4. Low-emissivity glass - common upgrade in residential housing, effective for all forms except UVA
5. Laminated glass - protective and more importantly, for your windshield sake, shatter-proof by way of an undetectable layer of film between two layers of glass
6. UV-blocking coated glass - very protective glass
7. Spectrally selective and UV-blocking insulating glass - blocks 99% of UV light, 70% of solar heat gain and 70% of visible light.

Moehrle et al. felt it necessary to study the glass of the Mercedes-Benz. They concluded windshields offer the best protection with side and rear windows offering comparable protection to each other, albeit less than from the windshield. (3) Mind you, this was a Benz tested, not a jalopy. Each state has laws about tinting automobile windows that should be consulted before deciding to alter a vehicles window tint.

Australia is leading the world with their implementation of sunglasses standards.(4) America has set a standard with voluntary adherence for manufactures (5). Doing your own internet research with the consultation of an ophthalmologist or eye-care specialist is the way to go. Unfortunately, as with many products marketed in the United States, you don't always get what you pay for.


References:

1. Diffey, BL. What is light? Photodermatol Photoimmunol Photomed 2002;18:68-74.
2. Tuchinda, C, Srivannaboon, S, Lim, H. Photoprotection by window glass, automobile glass and sunglasses. J Am Acad Dermatol 2006;54(5)845-53.
3. Moehrle M, Soballa M, Korn. UV exposure in cars. Photodermatol Photoimmunol Photomed 2003;19:175-81.
4. Cole BL. Should sunglasses be required to comply with the sunglass standard? Clin Exp Optom 2003;86:74-6.
5. Davis JK. The sunglass standard and its rationale. Optom Vis Sci 1990;67:414-30.
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