UV is generally divided into four ranges. We are actually only interested in three of them.

1. VACUUM UV

Rays in the range of about 100 nm to 200 nm exist only in a vacuum, and are not important to us. This is why most discussions about UV only include the next three types.
2. FAR UV or UV-C

The rays in this range, from 200 nm to 290 nm, are generally absorbed by the ozone layer of the atmosphere, and therefore are not interesting to us. As we damage the ozone layer of the atmosphere with pollution, (and no, I am not going to get into a discussion with anyone on whether this is actually happening, whether it is the result of man-made pollution, or whether it is worth worrying about) UV-C may become more important to us.
3. UV-B

The rays in this range, from 290 nm to 320 nm, are almost entirely absorbed by the tear film, the cornea, and the conjunctiva. Thus they do not reach the retina, which is the light-sensitive portion of the eye.

But 'absorption' does not mean 'no worries.' When biological tissues absorb any portion of the electromagnetic spectrum a change occurs. In this case, the changes are slow and reversible; but long intense exposure will result in permanent changes. An example of a temporary change is 'photokeratitis' or snow blindness, which is simply a sunburned cornea. This is uncomfortable and results in red-eye, dryness, scratchiness, and discomfort. Continued exposure results in permanent changes, for example, pterygium and pinguecula, which you will learn about in anatomy, and which are permanent thickening in the clear covering of the eye and are unsightly at best. Welders flash is another condition caused by UV-B exposure of the cornea.

The portion of UV-B that does reach the lens of the eye is thought to be a factor in the development of 'brown' or 'sunshine' cataracts, one of the forms that cataracts can take.

4. NEAR UV or UV-A

Near UV, 320 nm to 380 nm, is transmitted by the cornea and is partially absorbed by the lens. In a very young child the lens transmits most of the UV-A. As we age the lens begins to turn mildly yellowish, and it absorbs progressively more UV-A. This yellowing of the lens is NOT the same as cataract formation. However, one form of cataract, the 'brown' or 'sunshine' cataract, may possibly be caused by UV-A as well as UV-B.

Many medications increase the body's reaction to UV exposure. Examples are analgesics, antibacterial agents, tranquillizers, diuretics, antifungal agents, and contraceptives.

ANSI standards set the upper limit of the UV range as 380 nm. All other limits indicated here are approximate; there is no set wavelength where one attribute stops abruptly and another begins. We frequently use 400 nm for a UV cut-off because that is the wavelength used by UV absorption dye manufacturers to classify their product.

Crown glass absorbs UV below 300 nm, but transmits 90% of the UV over 300 nm.

Untreated CR39 absorbs UV below 360 nm, and transmits 90% of UV-A above 360 nm. We routinely use dyes to increase CR39 absorption of UV to 400 nm.

Clear coated polycarbonate absorbs all UV below 380 nm. The absorptive pigments are in the clear scratch resistant coating.

Photochromatic glass lenses such as Corning's PGX absorb all UV below 315 nm, and transmit only 4% of the UV-A up to 380 nm in the full darkened state.

UV absorption is the mechanism used by both glass and plastic photochromatic materials to cause their color change. Thus, plastic photochromatic lenses also absorb UV. I have not seen the absorptive ratings for plastic photochromatics published in textbook form yet, so I am not sure of the transmission and cut-off ranges.
 
 

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