In certain high-illumination situations, sunglasses allow for better visual function in a number of ways.
Improvement of contrast sensitivity
On a bright, sunny day, irradiance from the sun ranges from 10,000–30,000 foot-lamberts. These high light levels tend to saturate the retina and therefore decrease finer levels of contrast sensitivity. The major function of dark (gray, green, or brown) sunglasses is to allow the retina to remain at its normal level of contrast sensitivity. Most dark sunglasses absorb 70%–80% of the incident light of all wavelengths.
Reduction of glare sensitivity
Various types of sunglasses can reduce glare sensitivity. Because light reflected off a horizontal surface is polarized in the horizontal plane, properly oriented polarized lenses reduce the intensity of glare from road surfaces, glass windows, metal surfaces, and lake and river surfaces. Graded-density sunglasses are deeply tinted at the top and gradually become lighter toward the lens center. They are effective in removing glare from sources above the line of sight, such as the sun. Wide-temple sunglasses work by reducing glare from temporal light sources.
Use of photochromic lenses
When short-wavelength light (300–400+ nm) interacts with photochromic lenses, the lenses darken by means of a chemical reaction that converts silver ions to elemental silver. This process is similar to the reaction that occurs when photographic film is exposed to light. Unlike that in photographic film, however, the chemical reaction in photochromic lenses is reversible. Current photochromic lenses incorporate complex organic compounds in which UV light changes the molecules into different configuration states (ie, cis to trans); this process darkens the lenses (Fig 4-37). Photochromic lenses can darken enough to absorb approximately 80% of the incident light; when the amount of illumination falls, they can lighten to absorb only a small part of the incident light. Note that these lenses take some time to darken and, in particular, take longer to lighten than to darken. This discrepancy can be problematic in patients who move frequently between outdoor and indoor environments. Because automobile glass and the window glass in many residences and commercial buildings absorb light in the UV spectrum, most photochromics do not darken inside cars or buildings. In colder weather, patients should also be warned that these lenses darken more than usual, especially during a cloudy day. Nevertheless, photochromic lenses are excellent UV absorbers.
The spectrum of ultraviolet (UV) light is divided into 3 types: UVA contains wavelengths of 400–320 nm, UVB contains wavelengths of 320–290 nm, and UVC contains wavelengths below 290 nm. The ozone layer of the atmosphere absorbs almost all UVC coming from the sun. Most exposure to UVC is from manufactured sources, including welding arcs, germicidal lamps, and excimer lasers. Of the total solar radiation falling on the earth, approximately 5% is UV light, of which 90% is UVA and 10% UVB.
Figure 4-37 Photochromic lenses.
(Courtesy of Tommy Korn, MD.)
The amount of UV light striking the earth varies with season (greatest in the summer), latitude (greatest near the equator), time of day (greatest at noon), and elevation (greatest at high elevation). UV light can also strike the eye by reflection. Fresh snow reflects between 60% and 80% of incident light; sand (beach, desert) reflects approximately 15% of incident light; and water reflects approximately 5% of incident light.
Laboratory experiments have shown that UV light damages living tissue in 2 ways. First, chemicals such as proteins, enzymes, nucleic acids, and cell-membrane components absorb UV light. When they do so, their molecular bonds (primarily the double bonds) may become disrupted. Second, these essential biochemicals may become disrupted by the action of free radicals (such as the superoxide radical). Free radicals can often be produced by UV light in the presence of oxygen and a photosensitizing pigment. For a fuller discussion of free radicals, see BCSC Section 2, Fundamentals and Principles of Ophthalmology.
Because it may take many years for UV light to damage eye tissue, a tight linkage between cause and effect is difficult to prove. Therefore, proof that UV light damages the eye comes primarily from acute animal experiments and epidemiologic studies covering large numbers of patients.
Some surgeons routinely prescribe UV-absorbing glasses after surgery. Intraocular lenses incorporating UV-absorbing chromophores are now the norm. For further information regarding the effects of UV radiation on various ocular structures, see BCSC Section 8, External Disease and Cornea, and Section 12, Retina and Vitreous.
Almost all dark sunglasses absorb most incident UV light. The same is true for certain coated clear-glass lenses and clear plastic lenses made of CR-39 or polycarbonate. One suggestion has been that certain sunglasses (primarily light blue ones) may cause light damage to the eye. Proponents of this theory contended that the pupil dilates behind dark glasses and that if the sunglasses do not then absorb significant amounts of UV light, they will actually allow more UV light to enter the eye than if no sunglasses were worn. In fact, dark sunglasses reduce light levels striking the eye on a bright, sunny day to the range of 2000–6000 foot-lamberts. Such levels are approximately 10 times higher than those of an average lighted room. At such light levels, the pupil is significantly constricted. Thus, contrary to the preceding argument, dark sunglasses used on a bright day allow pupillary dilation of only a fraction of a millimeter and do not lead to light injury of the eye.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.