Sunglasses provide comfort by reducing the amount of visible light reaching the eyes. Most also absorb invisible ultraviolet light, and a few absorb infrared light.
Long-term exposure to ultraviolet light is thought to increase the risk of cataracts and damage to the retina. Wearing a pair of sunglasses that absorbs visible light but not ultraviolet is probably more damaging than not wearing sunglasses at all, because it leaves the wearer less conscious of the dose of ultraviolet he is receiving.
The ultraviolet spectrum is divided into bands. Ultraviolet in the UV-B band is more harmful than light in the UV-A band, because at shorter wavelengths light has more energy.
Various standards have been set for how much of the ultraviolet light a pair of sunglasses has to block, and most standards describe different classes of sunglass. The British standard of 1987¹, for example, required General Purpose sunglasses to block 97.1% of UV-B and 92.75% of UV-A, while Special Purpose sunglasses had to block 99.82% of UV-B and 95.5% of UV-A. The British also used shade numbers: cosmetic glasses having 1.1 to 2, general purpose glasses from 2.5 to 3.1, and special purpose glasses from 3.1 to 4.1.
The American National Standards Institute also defined three classes.²
For Cosmetic and General Purpose sunglasses, but not Special Purpose, ANSI included provisions for letting through enough red, yellow and green light that traffic signals could be read.
In 1978, the U. S. Food and Drug Administration published a monograph on sun protection in the Federal Register. The Sunglass Association of America voluntarily adopted the standards in the FDA monograph, asking members to label all sunglasses as “Cosmetic,” “General Purpose,” or “Special Purpose,” with transmittance limits. Unfortunately, many sunglasses sold in the United States, particularly cheap imports, are outside the Sunglass Association’s program.
On the basis of animal tests some researchers believe blue light has harmful long-term effects, but this remains controversial. The British standard provides that no particular wavelength in the blue-green may exceed overall transmittance by more than 20%.
In June 1992 the FDA proposed a revised voluntary standard, by which all sunglasses would block 99% of the UV-B. Sunglasses advertised as UV-blocking sunglasses would have to block 99% of UV-A and UV-B, and 60–90% of the visible light. Sunglasses sold for extreme conditions, like skiing, would block 92–97% of the visible light and have side shields. The ANSI standard for UV-A transmittance is also being reconsidered and it is expected that by 1994 it will call for all types of sunglasses to block 99% of the UVB.
1. British standard BS2724:1987.
2. ANSI Z80.3-1986.
|home|||||people index|||||search|||||to contact Sizes|||||acknowledgements|||||help||||
Copyright © 2000 Sizes, Inc. All rights reserved.
Last revised: 20 August 2004.