Ultraviolet light and vision

There is increasing evidence that both the light we see ( visible light) and the light we don't see (UV light) may have potentially damaging effects on the eye and ocular adnexa. This is particularly true for the ultraviolet (UV) wavelengths.Clearly, viewing a very intense light source is potentially hazardous to the eye and intense UV can be hazardous but questions remain as to the precise conditions in our natural outdoor environment or the workplace environments, which can pose hazards to the eye. Certainly, acute eye injuries are known to occur from unprotected viewing of welding arcs and midday sunlight reflected off a glacier. However, there have been honest differences of opinion regarding the potential hazards from chronic exposure.

The well known acute effect of excessive UV exposure to the eye (photokeratitis) is familiar to the public as `welder's flash' or `snow blindness'. Solar retinitis with an accompanying blind spot resulting from staring at the sun was once referred to as `eclipse blindness' and associated `retinal burn'. Only in recent decades, did it become clear that solar retinitis (or photoretinitis) was the result of a photochemical injury mechanism following exposure of the retina to shorter wavelengths in the visible spectrum, i.e., violet and blue light, and not due to thermal burn as was previously speculated.

The eyes exposure outdoors to UV light depends on many factors. When the sun is overhead at noon, the level of UV exposure is 10 times greater than that at either 3 h before or 3 h after noon. An untanned person with fair skin would receive a mild sunburn in 25 minutes at noon, but would have to lie in the sun for at least 2 hours to receive the same dose after 3:00 PM. The greatest retinal exposure to blue light and to UV (either indoors or outdoors) is most likely to occur when one is outside standing or walking in a field of snow without eye protection. Under these conditions, one can readily be exposed to levels exceeding occupational exposure limits. As most skiers know, failure to wear ski goggles can result in several effects upon the eye, sometimes loosely referred to as `snow-blindness'. This can result from the unusually intense reflection of light and UVR from the snow. Fresh snow reflects 85% of the UVR compared to only 1 or 2% from grass. Unless one employs a goggle geometry with side-shields, or a closely fitting wrap-around design, the specification of 99% or 100% UV blocking become misleading because UV light can enter the eye from the sides and also by reflecting off the edges of the sunglasses. Therefore, side shields and overhead protection are critical and can be provided with wrap-around sunglasses or goggles. Polarising sunglasses protect against excessive glare.

Seasonal and weather variations, cause different amounts of UV radiation to reach the Earth at any given time. Developed by the National Weather Service (NWS) and EPA, the UV Index predicts the next day's ultraviolet radiation levels on a 0-10+ scale, helping people determine appropriate sun-protective behaviors. Click to calculate the UV Index.

Cataract formation because of ultraviolet light damage to the lens has been addressed by a number of epidemiological studies. The strongest association links UV-B exposure and cortical cataract formation. This association was borne out by both the Beaver Dam Eye Study and a population-based survey of Maryland watermen.

In contrast to cataract (where UV light exposure may be causative), Macular Degeneration does not seem to be related to UV exposure. However, macular degeneration seems to be related with "visible light" exposure (visible light forms the color spectrum). In the Maryland watermen study, advanced macular degeneration was more common in men exposed to increased levels of blue light, but not in those with increased levels of ultraviolet exposure. Similarly, the Beaver Dam Eye Study found that exposure to visible light was associated with ARMD in men. No association between sunlight and macular degeneration was found in women in that study, but the authors proposed that the women studied may have had less sunlight exposure.

Prevention of potential ocular and adnexal damage from ultraviolet radiation can be addressed in a number of ways. First, and most obvious, is the simple avoidance of excessive exposure to UV radiation.

When exposure cannot be avoided, then protection is the next order of business. The use of a broad-brimmed hat or visor can help shield the eyes from both visible and invisible light. The eyelids, the same as the skin elsewhere on the body, should be protected by use of a sunscreen. Since these lotions and creams are not ophthalmic formulations, care must be taken in applying them on or near the eyelids to avoid accidental contact with the ocular surface, which could result in ocular irritation.

The most effective method of shielding the eyes from UV radiation is the use of UV-absorbing spectacles. Large frames and wraparound designs offer the greatest protection to both the eyes and the eyelids. UV-absorbing contact lenses are available from a number of lens manufacturers and are preferred to non-UVabsorbing materials in individuals who spend a lot of time outdoors. Since contact lenses may shield the cornea from UV light but offer no protection to the conjunctiva, sclera, or eyelids, they do not provide the same level of ocular protection against UV radiation exposure that spectacles do.

Most individuals equate sunglasses or photochromic lenses with UV radiation protection. However, lens tint or color is not indicative of the UV blocking ability of a lens. Most modern clear spectacle lenses are made of plastic (CR-39) or polycarbonate, and these materials absorb a substantial portion of UV-A radiation and almost all UV-B. Polycarbonate lenses have inherent UV blocking property. In addition they are thinner, lighter and shatter-resistant. UV transmittance can be further decreased by the use of chromophores that can be incorporated into the lens material without affecting the transmission of visible light. The so-called photochromic or variable-tint lenses provide increasing protection against UV radiation as exposure increases, becoming darker with the stimulus of ultraviolet light and lightening when the light stimulus is removed. They represent the most physiologic modality for UV radiation protection based on level of exposure. Manufacturers have developed new sunglasses designed to protect eyes from the sun's harmful effects. They promise protection from ultraviolet light and other kinds of natural radiation. Click on the link to help you select the right sunglasses.

On average, older people need about three times as much light as younger people. People with glaucoma often require much higher light levels, while there are conditions such as central cataract where lower light may be better. The quality and quantity of the light needed for comfortable, useful vision becomes particularly important when vision is impaired.

Glare results when light (natural or artificial) shines directly into your eyes or reflects off a shiny surface. When arranging a room to be used for reading or working, do not face the windows. Seat yourself so that windows are behind you or to your side. If that is not possible, use blinds or shades to control the light during daylight hours. Light from lamps or fixtures should be positioned directly onto the task (book, newspaper, playing cards, hobby/craft, etc.). Gooseneck, adjustable arm, and clip-on lamps offer good flexibility. In dimly lit restaurants, use a pocket flashlight. When reading or working outside, use a visor to shut out sky light glare.

Incandescent light is the thermal radiation emitted by a hot object such as the filament of a light bulb or the surface of the sun. Actually daylight is a form of incandescent light. In a common incandescent light bulb, an electric current flows through a tungsten wire, which gets heated and emits thermal radiation. Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot tungsten wire.

The Chromalux Bulb is made of special composition glass that is not colored or coated, but instead contains Neodymium, a rare earth element. This element is able to absorb yellow - as a result, the light appears whiter (in contrast to yellowish hue of the common bulbs). Contrary to claims, these bulbs do not even come close to being full spectrum, or duplicating sunlight. The only difference between these bulbs and normal bulbs is in the amount of yellow light emission.

Fluorescent tubes use a non-thermal radiation process to produce light. This is the only type of lighting that can come close to being 'Full Spectrum'. A type of fluorescent bulb is now available as a direct, screw-in replacement for some incandescent bulbs. They consume from one-sixth to one-tenth as much electricity, radiate very little heat, and, although expensive, last about ten times as long as the conventional incandescent bulbs. In addition, these modern tri-phosphor compact fluorescent bulbs provide light very close to the quality of incandescent. They provide excellent color rendition--food looks natural, colors look normal and offer a “comfortable” atmosphere for most people. There are considerable differences in the light spectrum emitted by the sun and artificial light sources.