Light is a form of energy that is produced naturally by the sun and artificially by incandescence, fluorescence, lasing and other means. Visible light is necessary for sight. The electromagnetic spectrum of light determines how visible light is perceived as colour by the eye. The spectrum is measured by the wavelength of each hue in the visible range (400 to 700 nanometres [nm]). Invisible light, not perceived by the human eye, also contains energy from ultraviolet (UV) and infrared light. Invisible light is, however, absorbed by the eye and its supporting structures.

Light energy is contained in little "packets" or photons that, when absorbed, can cause heat or chemical reactions to occur. This thermal or photochemical response can be toxic and damaging to the eye if the amount of response is beyond the natural reparative processes. UV light has more energy per photon than visible light. The potential for damage is therefore greater with UV light absorption. The thinning of the ozone layer and our longer life span have increased the risk with both high-intensity short-term exposure and low-intensity long-term exposure. Environment Canada provides a daily UV index on a scale of 0 to 10, with a value of greater than 7 causing the average person to burn in 20 minutes. This index is provided to heighten awareness and promote the use of sunscreens, sunglasses and appropriate clothing.

UV phototoxicity has been implicated in causing or accelerating the progression of several diseases of the eye or its supporting structures, including the eyelids. The following is a partial list of some of the more common diseases involved.

• Basal cell and squamous cell carcinoma, melanoma: skin cancer of the eyelids.
• Pingueculae, pterygia: conjunctival growths between the eyelids that may encroach on the visual axis and obstruct or distort vision.
• UV photokeratitis, snow blindness, arc welder's burn: acute superficial "burn" of the cornea from high-intensity short-term exposure to UV radiation.
• Spheroidal degeneration, Labrador keratopathy: loss of corneal clarity from long-term exposure to UV radiation.
• Cataracts: opacification of the crystalline lens of the eye.
• Eclipse or solar retinopathy: damage to the retina from acute exposure during an eclipse or while gazing directly at the sun.
• Macular degeneration: central retinal damage in the part of the retina responsible for the best sight.
• Acceleration of the progression of the pigmentary retinopathies (retinitis pigmentosa), iritis (intraocular inflammation) or cystoid macular edema (central fluid accumulation in the retina).

Certain drugs and conditions can increase UV phototoxicity. Tetracycline, sulfonamides, chlorothiazides, phenothiazines and psoralen compounds are photosensitizing drugs that enhance the absorption of UV light. Special care must be exercised to reduce exposure to UV radiation in patients receiving these drugs.

Not all UV exposure is equal. Astronauts, pilots, lifeguards, ski instructors, farmers and fishermen have high occupational exposure. Avocational and recreational exposure, although varied, also warrants protection.

Sun protection for the skin divides UV light according to its wavelength into UVA (315 to 400 nm), UVB (280 to 315 nm) and UVC (less than 280 nm) radiation. UVC radiation is assumed to be absorbed by the ozone layer. UVB rays are the tanning and cancer-causing rays. The sun protection factor (SPF) used in sunscreens for the skin is a measure of the control over UVB rays. An SPF of 4 means that a person whose skin normally burns in 15 minutes can be exposed for 1 hour before burning.

Recently, UVA sunscreens have also been marketed. Compounds with Parsol 1789, cinnamates and benzophenones also screen UVA rays.

Opaque blockers such as zinc oxide and titanium dioxide block all light absorption.

Sunglasses should block all UVA, UVB and UVC rays. The consumer must beware. Claims of "100% UV protection" are meaningless unless they specify the wavelength either in nanometres (less than 400 nm) or in the UVA/UVB/UVC classification.

If sunglasses filter 100% of UVB radiation but no UVA radiation, all light with a wavelength greater than 315 nm is transmitted, and the sunglasses provide inadequate protection. A photometer is available at most optical shops that can determine the UV light transmission. When buying sun-glasses, specify filtration of rays with a wavelength of less than 400 nm.

Whether sunglasses are gray, brown, green or yellow depends on personal preference, for comfort, contrast and circumstances. It is important for people with red - green discrimination problems to select a colour that does not make their colour perception worse. Wearing the wrong colour may impair driving ability or skills requiring colour performance. A neutral gray performs best under such circumstances.

Photochromic lenses that change under varying light conditions may pose problems for driving. The UV light that triggers the colour change is absorbed by the car windshield. This may impair the performance of the sunglasses while the person is driving.

Polarizers and antireflective coatings reduce glare and enhance contrast.

Special filters that include visible blue light are also available for patients with certain retinal problems. They interfere with normal colour perception.

Children less than 6 months of age should be kept out of the sun. It is recommended that hats and canopies be used but not topical sunscreens.

Older children should be dressed appropriately, and hats and sunscreens should be used for high-exposure activities. If an activity involves sufficient exposure to warrant the use of a sunscreen, sunglasses that filter light with a wavelength of less than 400 nm should be used or a hat with a brim worn. There is no evidence that these measures enhance one's sensitivity to light.

Newer intraocular lens implants filter UV radiation. This eliminates any concern about UV eye protection after cataract surgery.

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