Figure 1. The electromagnetic spectrum.
By Karl Citek, OD, PhD, FAAO
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Paraphrasing Dorothy in the Wizard of Oz, "Melanomas, pterygia, conjunctivitis, keratitis, cataracts, and macular degenerations, oh my!" But instead of scary imaginary lions, and tigers, and bears that lived in the Kingdom of Oz, these are just some of the scary (and very real) eye conditions that can be caused by exposure to ultraviolet (UV) radiation.
This course will describe the occurrence of UV in natural and artificial lighting environments, the effects of UV on biological tissues in and around the eye, the use of materials and products that reduce skin and ocular exposure to UV, and how to treat patients who have been exposed to UV.
For the interested reader, an extensive overview of electromagnetic radiation is also included as an appendix, which may be consulted regarding definitions of concepts, terms, and units.
The electromagnetic spectrum (EMS) comprises radiant energy that extends from short-wavelength, high-energy gamma rays to long-wavelength, low-energy radio waves. (See Figure 1)
Figure 1. The electromagnetic spectrum.
The zones of the EMS are based on their source, function, or effect. There is significant overlap between the zones, depending on the intensity of the energy and/or exposure duration. In general:
Figure 2. Release of a photon from a radioactive atom.
The adjective, “actinic,” is a general term, defined as pertaining to radiation capable of producing a chemical change. While the term is often used to describe the action of UV, it is not specific or unique to any particular wavelength or group of wavelengths (bandwidth). Any effect of radiation may be beneficial, harmless, or catastrophic for biological tissues, depending on whether the action has long-term consequences. Long-term phenomena are non-reversible and cumulative in nature. For example, the denaturation of protein fibers that occurs over time in the crystalline lens cannot be repaired by the eye itself, leading to the formation of a cataract.
Draper’s law states that energy must be absorbed in order to have an effect on the medium through which it travels. Figure 3 shows the relationship between wavelength, as measured in nanometers (nm), and energy, as measured in units of electron-volts (eV).
Figure 3. Photon energy (eV) with respect to wavelength (nm). C=O, carbon-oxygen double bond; C=C, carbon-carbon double bond; C-H, carbon-hydrogen bond; C-N, carbon-nitrogen bond. Blue line, boundary between UV and VIS (approx. 400 nm); purple line, wavelength of excimer laser, 193.3 nm.
Photons with particular minimum energies are capable of breaking various bonds between the carbon, nitrogen, and oxygen atoms that are ubiquitous in biological tissues. Structural changes of molecules can occur with energy as low as 3.1 eV, which is equivalent to a wavelength of 400 nm and visible as the color violet.
Long-term (chronic) exposure to such long-wavelength UV, combined with photochemical reactions caused by short-wavelength VIS up to 500 nm (blue-green), at environmental levels has been characterized as the “Blue Light Hazard.” This can contribute to retinal damage, such as age-related macular degeneration (AMD).
Effect of Radiation on the Eyes
While much of the radiant energy that strikes our bodies and eyes is harmlessly reflected or transmitted, even a low percentage of absorption can produce significant effects. Consider the last time you had your picture taken by someone using a camera with a flash. Even though very little of the incident light from the flash was absorbed by the photoreceptors in your eyes, the light was bright enough to produce an afterimage that probably lasted for several minutes. The “red eye” in the resulting photo was the reflection of the flash from your retina. Any remaining light, of course, passed harmlessly through the retina to be absorbed, without consequence, by the retinal pigment epithelium and the choroid.
X-rays
If the exposure had been made using x-rays, you would have seen nothing, but photographic film behind you would have an image of your bones and any other dense masses in your body. Soft tissues (e.g., skin, muscle, internal organs) readily transmit x-rays, while bones, teeth, and other dense structures reflect x-rays. However, small amounts of x-rays are absorbed by all biological tissues, producing cellular changes that ultimately can result in cancer following prolonged and/or repeated exposure. Cataract is the most common effect on the eyes caused by prolonged or repeated exposure to x-rays. That is why the x-ray technician leaves the area even when the x-ray source is not facing her.
UV
A harmless ocular effect of radiation occurs, for example, when the crystalline lens is irradiated with a low-intensity “black light.” The lens will fluoresce and emit a greenish glow. (See Figure 4) The fluorescence disappears as soon as the UV source is removed, and the UV will have no deleterious effects if there was only a short period of exposure.
Figure 4. Fluorescence of the crystalline caused by UV-A irradiation (“black light”).
Vision
A beneficial effect of radiation occurs when “normal intensity” light is captured by photoreceptors, resulting in a reversible photochemical change of the photopigments in rods or cones, ultimately producing vision. However, directly viewing a high-intensity source causes a non-reversible photochemical change if the exposure period is long enough. For example, sustained fixation for several minutes of the sun at midday eventually results in photoreceptor death, known as solar retinopathy. Note that viewing the sun directly at sunrise or sunset does not produce the same catastrophic result, since much of the sun’s radiation is scattered and filtered by the atmosphere.
Laser Pointers
Handheld laser pointers produce retinal effects that also depend on exposure duration and intensity. Laser pointers typically are available in green (532 nm) and red (630 to 670 nm). Because the monochromatic coherent beam is quite intense, even for low-power battery-operated pointers, a long-lasting afterimage results following direct exposure of a few seconds. Permanent retinal damage can result after several minutes of sustained viewing. An argon green laser, operating at 514 nm (shorter wavelength and higher energy than handheld pointers) and at much higher intensities (thus requiring much shorter exposure duration), is used for some retinal photocoagulation procedures. Retinal photocoagulation also can be performed with a high-intensity krypton red laser (647 nm).
Ophthalmic Equipment
Certain diagnostic techniques, such as binocular indirect ophthalmoscopy and slit lamp biomicroscopy with a high plus lens, can induce long-lasting afterimages if the light is directed at one location on the patient's retina for an extended period. For example, the recommended maximum permissible viewing time for a biomicroscope set on high illumination with a 78D lens is 36 sec; with a Superfield NC lens it is 57 sec; with a Super 66 lens it is 32 sec; and with a 90D lens it is 52 sec.
Such procedures, as well as overall exposure to light, should also be kept to a minimum for patients with retinitis pigmentosa, because the condition can be exacerbated by exposure to light.
UV radiation may be divided into four zones, based on the observed effects, as described in the US sun eyewear standard (ANSI Z80.3-2001) and elsewhere. Typical transitions for the zones are at wavelengths of 290 nm (UV-C to UV-B), 315 nm (UV-B to UV-A), and 380 nm (UV-A to VIS), as shown in Table 1.
Table 1. UV zones and approximate bandwidths. Black, commonly accepted range; gray, extended range and overlap with neighboring zone.
There are no sharp cut-offs between any of these zones. As a matter of fact, the International Commission on Illumination (CIE) and the Australia/New Zealand sun eyewear standard (AS/NZ 1067: 2003) extend UV-A up to 400 nm. In addition, the CIE, sun eyewear standards of the European Community (EN1836: 1997) and Australia/New Zealand (AS/NZ 1067: 2003), and the occupational eyewear standard in the U.S. (ANSI Z87.1-2003) extend UV-B down to 280 nm.
In general, sun eyewear standards do not address protecting the wearer from UV-C or shorter wavelengths, since these rarely occur in the natural environment. However, ocular (and skin) protection from these short wavelengths is considered in various occupational safety standards.
Some authorities combine far and extreme UV as a single zone, or they ignore extreme UV altogether other than when considering specific industrial environments. Also, some texts extend UV-B up to 320 nm and others extend extreme UV down to 10 nm, where there is significant overlap with x-rays.
The descriptive names are not limited to the particular zones. For example, extended exposure durations to wavelengths up to 340 nm can cause skin reddening (erythema). Similarly, wavelengths below 280 nm are used in fast-acting water disinfection systems. However, exposing water to wavelengths at 320 and 400 nm at environmental levels for 1.5 to 3 hours, respectively, can kill 99.9% of microorganisms such as E. coli and streptococcus.
The effects of UV are understood when one considers that wavelengths below 400 nm are capable both of creating ions and radicals, and of breaking molecular bonds. Consequently, changes can occur to cell structures and even to intracellular components. Breaking bonds of protein molecules causes denaturation, producing opacification (i.e., cataract) in the crystalline lens. (See Figure 5)
Figure 5. Cataract.(http://www.mrcophth.com)
Breaking bonds of collagen molecules, as in the skin, causes cinching of the fibers, producing sagging, leatheriness, and possible wrinkling. Alteration of DNA sequences within cells is the mechanism that can trigger irregular or abnormal cell replication. (See Figure 6)
Figure 6. DNA mutation with UV irradiation. (www.answers.com)
If the DNA cannot repair itself, the cell can become cancerous. Excessive UV exposure also can suppress the immune system, thus making an individual more susceptible to infectious diseases. Likewise, UV exposure can trigger an outbreak of a dormant condition, such as herpes simplex virus.
The penetration and absorption of UV varies by wavelength, depending on the tissue upon which it is incident. The longer the wavelength, the deeper the radiation will penetrate through the skin or the structures of the eye. (See Figures 7 and 8)
Figure 7. UV penetrance of skin.
Figure 8. UV penetrance of eye.
Tears, cornea, aqueous, and vitreous can potentially transmit wavelengths as short as 290 nm. The crystalline lens of a child transmits wavelengths as short as 310 nm, while that of an older adult only transmits wavelengths of 375 nm and above. Thus, the need for ocular UV protection for children and aphakic patients is even more critical than for adults.
The Sun and The Earth’s Atmosphere
The sun is the direct source of UV in the natural environment. The solar constant is the total radiant energy received from the sun, approximately 1367 W/m2, based on an average earth-to-sun distance of about 150 x 106 km (93 x 106 mi). UV radiation comprises almost 8% of the solar constant, and VIS and IR each comprise about 45%; the remaining 2% occurs at wavelengths as long as 25,000 nm.
The earth’s atmosphere filters as much as half of the solar constant, as well as a greater proportion of UV, such that the total direct irradiance at the surface of the earth on a clear day is about 700 to 900 W/m2. A small percentage of additional indirect irradiance can be observed because of scatter and reflection within the atmosphere.
The amount of filtration depends on the sun angle and the thickness of atmosphere (air mass) through which the radiation passes; the greater the air mass, the higher the absorption. When the sun is directly overhead, as on an equinox at the equator, the zenith angle is zero and the air mass is 1. On the same day in Chicago (latitude about 42 degrees north) and Wellington, NZ, (latitude about 42 degrees south), the zenith angle is about 48 degrees and air mass is 1.5. (See Figure 9) Alternately, when the sun is 30 degrees above the horizon (zenith angle of 60 degrees), regardless of the latitude, time of year, or time of day, air mass is 2.
Figure 9. Solar zenith angle. (www.mynasadata.larc.nasa.gov)
At an air mass of 1, UV comprises about 4.4% of the irradiance at the earth’s surface; at an air mass of 2, it is only about 2.7%. (See Figure 10) At zenith angles greater than 62 degrees, atmospheric scatter increases greatly for UV and short-wavelength VIS, such that significantly less of that radiation reaches the viewer. Hence, the beautiful (and non-harmful) orange and red hues that are observed at sunrise and sunset.
Figure 10. Solar irradiance at the upper atmosphere (solar constant) and at the surface of the earth at sea level, with solar zenith angle about 48 degrees (air mass 1.5).
UV Absorption by Various Gases
Various combinations of oxygen and nitrogen account for the greater absorption of solar UV compared to absorption of longer wavelengths. Molecular oxygen (O2), atomic oxygen (O), and nitrogen (N2 and N3) in the atmosphere absorb all wavelengths below 85 nm, while chiefly O absorbs wavelengths between 85 and 200 nm. Ozone (O3) absorbs the remaining wavelengths between 200 and 288 nm. Ozone also absorbs most of the radiation up to 340 nm.
Ozone is a bluish gas that has a distinct “electrical” odor. About 90% of ozone is present in the stratosphere, which extends from an altitude of about 10 km (6 mi) to about 50 km (30 mi). The remaining 10% of ozone is present in the troposphere, which is below 10 km altitude. (See Figure 11) Ozone near the earth’s surface primarily results from industrial pollution. While it has the benefit of reducing UV-B, ozone is toxic when it comes in contact with biological tissues.
Figure 11. Distribution of ozone in the atmosphere and absorption of UV.
Depletion of the ozone layer in the stratosphere results from the release and breakdown of chemicals such as man-made chlorofluorocarbons (CFCs), bromines, and nitrogen oxides. This depletion, referred to as the “ozone hole,” has been accumulating primarily over the earth’s polar regions, especially over Antarctica during the southern spring (September to November). (See Figure 12) In recent years, the ozone hole has extended over populated areas at the southern tip of South America for brief periods.
Figure 12. Ozone hole over Antarctica. (www.microcare.com)
Other regions around the globe also have experienced a variation in the ozone layer. Ozone depletion results in an increased irradiance of wavelengths below 340 nm and, possibly, of UV-C. The immediate danger of exposure to short-wavelength UV-B and UV-C is increased risk of sunburn and keratitis, and the long-term danger is increased incidence of skin cancer. Long-term decrease of atmospheric ozone by 1% has been hypothesized to increase the incidence of skin cancer by 2% worldwide.
Effects of Direct Exposure to UV
In a normal atmospheric environment with no ozone depletion, exposure to solar UV varies with the time of day, season, location (latitude and altitude), and climatic and atmospheric conditions, such as water vapor, dust, and airborne pollutants. (See Figure 13)
Figure 13. Sources of environmental UV.
Effects of Cloud Cover and Altitude
Cloud cover offers little protection from solar UV, as thin high clouds allow 90% or more of UV to reach the earth’s surface, and actually scatter and reflect more UV than a clear or slightly hazy sky. Heavy overcast clouds still allow as much as 33% of UV to reach the surface. Note that clouds will readily block IR, such that the characteristic “warmth” that a sun-bather feels – which, in and of itself is harmless, but might suggest to the sun bather to limit the amount of exposure – is absent. Nonetheless, skin tanning, or even burning, can occur in an unprotected individual on a cloudy day.
UV irradiance also varies directly with altitude, increasing by 4% for every 300 m (1000 ft) rise in elevation. This is especially significant for visitors to mountains and other high altitude locations, who may not be aware of the need for extra protection from UV. Short-term (acute) exposure to these higher UV levels is the source of photokeratitis and photoconjunctivitis historically referred to as “snow blindness.” Thus, recreational skiers and mountain climbers should be advised to wear goggles designed for the given lighting and weather conditions. (See Figure 14) In addition, they should repeatedly apply the appropriate strength of waterproof sunscreen to exposed skin of the face, neck, ears, or any other part of the body, even on an overcast day.
Figure 14. Skier wearing orange tinted goggles.
Shallow water, as when swimming or snorkeling near the surface, is not an effective UV filter. As much as 40% of UV can penetrate water to a depth of 50 cm. Patients should apply waterproof sunscreen before entering the water and reapply it immediately after leaving the water.
Exposure to environmental UV-B has proven effects on the structures of the anterior segment of the eye, as shown by studies such as that on 838 watermen on the Chesapeake Bay. For the affected individuals, much of the incident UV-B likely was reflected from surfaces in the environment, such as water. Chronic UV-B exposure can lead to the development of climatic droplet keratopathy (CDK), which is a form of spheroid degeneration and may be known by names such as Labrador keratopathy and keratinoid degeneration, among others. The condition is marked by the presence of yellow, oily-looking subepithelial droplets at Bowman’s membrane or the anterior stroma, usually in the periphery. (See Figure 15) Similar spheroid degeneration in the conjunctiva leads to the development of a pinguecula, which may present by itself or in association with CDK. Patients with pingueculae or CDK are often asymptomatic, or present with mild dry eye symptoms.
Figure 15. Climatic droplet keratopathy. (www.onjoph.com)
Chronic exposure to UV-B also can cause a pterygium, a highly vascularized, triangular abnormal growth of the conjunctiva onto the peripheral cornea. (See Figure 16) Most pterygia form in the nasal quadrant, because the cornea focuses temporally-incident UV on the nasal limbus and sclera. Symptoms include typical dry eye complaints.
Figure 16. Pterygium.
Chronic UV-B exposure can also hasten the onset of cataracts, because the crystalline lens is the optical component of the eye that will absorb most of the frontally-incident UV. Symptoms can include reduced visual acuity and contrast sensitivity, excessive glare (especially at night), and tritanomalous (blue-yellow) color deficiency.
Development and exacerbation of AMD can occur because of chronic exposure to visible blue light and UV-A. The mechanism of damage likely is similar to that which produces solar retinopathy, in that the photochemical process within the photoreceptors is overwhelmed, thus not allowing proper regeneration of the photopigment. Symptoms include reduced visual acuity and contrast sensitivity, central visual field loss, and image shape distortion (metamorphopsia).
Wide-brim visors, caps, and hats, as well as standard spectacles and most over-the-counter sunglasses, are sufficient to protect the eye from UV coming from in front of the patient. However, because most spectacle and sunglass frames have relatively flat fronts (low wrap), they do not protect the wearer from UV coming from the side. In addition, UV coming from behind the wearer can reflect from the back surface of the spectacle lens directly into the eye. For best protection, patients should wear goggles or spectacles with side-shields, or, for better cosmesis, frames that are contoured to the wearer’s head (high wrap). (See Figure 17) Contact lenses that provide UV absorption can reduce the likelihood of pterygia and intraocular effects, but offer little protection to the conjunctiva, sclera, or eyelids, unless they are used in conjunction with spectacles or goggles.
Figure 17. Patient wearing non-prescription sun eyewear with high wrap.
Different surfaces reflect radiant energy at different levels. Figure 18 shows the reflectance spectra for samples of typical grass, sand, concrete, and asphalt measured in our lab. Table 2 shows the percentage of reflectance of VIS and UV for these and other common surfaces. Variations in the composition or nature of the surface, such as different types of grass, will result in slightly different values.
Figure 18. Reflectance spectra of various common surfaces.
Table 2. Percentage of reflectance of VIS and UV from several common surfaces.
Comfortable viewing occurs for most observers when luminance is between about 350 and 2,000 cd/m2. The luminance of bright direct sunlight can be as high as 40,000 to 70,000 cd/m2, and that of indirect sunlight (reflecting from the atmosphere and objects in the environment) may be one-fourth to one-third as great. Thus, even 1% reflectance of VIS can provide sufficient light for an observer to be able to see comfortably in many outdoor environments.
Likewise, 1% reflectance of UV under such conditions could be harmful to unprotected skin and eyes, especially with chronic exposure. For example, wearing a wide-brim hat, but no goggles or other eyewear, on a sunny day on a field of fresh snow is almost useless, since most of the incident UV actually reflects from the snow, and thus from below or to the side of the patient’s line of sight.
Figure 19 shows the diurnal and seasonal variation in UV-B exposure at sea level on a clear day between 40 and 50 degrees north latitude, where most of the earth’s population lives.
Figure 19. Diurnal and seasonal variations in UV irradiance, in sunburn units (SBU), for temperate northern latitudes.
Note that “solar noon” is defined as the time when the sun reaches its highest position in the sky, and is distinguished from “clock noon” based on the observer’s location within a time zone and local ordinances governing daylight saving time. The greatest potential exposure occurs from about 2 hours before until about 2 hours after solar noon, between the summer solstice (around June 21) and the autumnal equinox (around September 22) in the northern hemisphere.
One “sunburn unit” (SBU) is equivalent to radiant energy per unit time of about 0.038 J/cm2-hr, representing the amount of UV-B necessary to produce noticeable reddening of the skin 24 hrs after exposure. In temperate northern latitudes in late June, an observer should expect to experience about 1.8 SBU each hour around midday.
Another gauge of UV exposure is given by the UV Index, which was established by the World Health Organization and is endorsed by the U.S. Environmental Protection Agency, National Oceanic and Atmospheric Administration, and similar governmental bodies in other countries. The UV Index is a concise measure that factors in time of day and year, geographic location, weather conditions, and pollution. The UV Index is reported to the nearest integer, and varies from 1 (low) to greater than 11 (extreme). (See Figure 20) One UV Index unit is equivalent to 25 mW/m2 (about 0.009 J/cm2-hr, or about 1/4 SBU).
Figure 20. Description of UV Index and range of values.
The UV Index is regularly reported during weather forecasts in the U.S. and most other countries around the world. Figure 21 shows representative plots of the UV Index over one-year periods for selected U.S. cities at different latitudes and altitudes with varying atmospheric conditions.
Figure 21. Variations in UV Index over one year for Anchorage, New York, Denver, and San Juan, Puerto Rico. (www.cpc.ncep.noaa.gov)
Defining exposure time that causes burning solely on the basis of the amount of radiant energy can be misleading. One must also consider the exposed person’s skin color and tone – specifically, the amount of melanin in the skin. Table 3 summarizes skin type categories and their responses to direct environmental UV exposure.
Table 3. Skin type categories, UV exposure, and exposure duration before burning or tanning.
The UV Index, together with skin type, can be used by individuals to estimate their susceptibility to sunburn. (See Figure 22)
Figure 22. Combination of UV Index with skin type to estimate amount of UV exposure before burning. (www.geo.mtu.edu)
The average American is exposed to about 25,000 J/m2 of UV-B per year, ranging from about 19,000 to 24,000 J/m2 for females and 23,000 to 33,000 J/m2 for males, based on age and geographic location. (See Figure 23) For individuals who vacation away from home for about 3 weeks, the average exposure increases by almost 8,000 J/m2. These values are almost two times higher than for many Europeans, mainly because of differences in weather conditions and latitude.
Figure 23. Annual UV-B exposure for residents in the US based on age, gender, and general geographic location.
The most common source of artificial UV is in fluorescent lighting. The lamp contains a small amount of mercury and an inert gas, such as argon, krypton, or neon, at low pressure. An electric current excites the mercury atoms, causing them to release energy at a wavelength of 254 nm, which is absorbed by a phosphorescent coating on the inside of the bulb. Variations in lamp color, such as “cool white,” “warm white,” and “daylight,” depend on the chemical composition of the coating. There is virtually no danger of UV exposure for any fluorescent lamp, since cracked or broken lamps will allow the gas to escape and the lamp will not function.
“Black light” fluorescent tubes emit short-wavelength VIS (blue-violet) and low intensity UV-A (about 365 nm) that is harmless for brief exposure durations. They are used in the entertainment industry, in insect control (e.g., “bug lights”), and in non-destructive testing. For example, eye care professionals commonly use Burton lamps, which typically combine two black light tubes with a magnifying lens, to assess rigid gas permeable contact lens fits. (See Figure 24)
Figure 24. Burton lamp. (www.spectacle.berkeley.edu)
Other uses of black lights include identification of certain minerals by geologists. (See Figure 25)
Figure 25. Fluorescent mineral illuminated with white light (top) and with UV (bottom).
Black lights can also be used by pet owners to determine the location of urine and vomit stains and by store-owners for the detection of counterfeit bills. Use of black lights (and other scientific equipment and techniques) by forensic scientists can reveal blood, urine, and semen stains, and certain drugs, such as cocaine. This has been recently portrayed in the popular TV series, CSI:. (See Figure 26) Some people even carry black lights to check how well the hotel rooms they are about to occupy have been cleaned.
Figure 26. Character on CSI: using black light.
Metal halide lamps, also known as high intensity discharge (HID) lamps, are very efficient sources of white light. (See Figure 27) The light produced is more similar to sunlight than that from any other common lamp. HID lamps are used in warehouses, sports stadiums, school gymnasiums, and large department, discount, and bulk stores (e.g., Home Depot, Costco).
Figure 27. Comparison of various light sources.
Dangers of Defective HID Lamps
The typical metal halide lamp has a central arc tube made of quartz that contains argon gas, mercury, thorium iodide, sodium iodide, and scandium iodide under high pressure. The lamp emits a significant amount of UV-B and some UV-C in addition to VIS. The lamp must be enclosed in borosilicate glass (a.k.a. Pyrex) to absorb the UV radiation. Unfortunately, if the glass cover cracks or breaks, most lamps of this type will still function and an observer can be exposed to as much UV radiation in 8 min as during a full day of bright sun, producing typical exposure effects such as skin burn, keratitis, and conjunctivitis.
Reports in the literature indicate that ocular damage may take several weeks to months to resolve following exposures to defective HID lamps of up to three hours. Recently, five hours of exposure to several teachers in Oregon has resulted in conjunctival damage and photophobia that are still present over one year after exposure. The direct long-term effects of hastening the onset of cataracts and retinopathy are not known at this time, since many other environmental and personal factors, such as time spent outdoors, genetic predisposition, and history of smoking, will contribute to those conditions.
More expensive metal halide lamps are available with cut-off switches that will not allow the lamp to operate if the outer glass covering is cracked or broken. These are recommended in areas where individuals are potentially directly exposed for more than several minutes; this is an issue only, for example, for store clerks who spend much time in one location, but not for shoppers who move around the store. Otherwise, a secondary safety covering to prevent breakage is recommended, or a different type of light source, such as standard fluorescent, should be used.
Sunbeds, also known as tanning beds, use an array of fluorescent lamps with broadband UV-A and UV-B emission. (See Figure 28)
Figure 28. Sunbed advertisements.
People with lighter skin tend to use sunbeds more often than those with darker skin, but most individuals with skin type I or II (see Table 3 above) will likely not achieve the anticipated results and should be counseled against using sunbeds. For all skin types, repeated and uncontrolled exposures will cause premature skin aging and can be carcinogenic. The proper protective eyewear must be worn at all times by all users, regardless of skin type.
Electric arc welding emits UV radiation throughout its spectrum, as well as very high intensity VIS. Most of the ocular damage, known as “welder’s flash,” occurs in the moment, often less than 1 second, between the striking of the arc and the lowering of the very dark filter over the eyes. Because of the acute exposure, the damage is limited to the outer layers of the cornea and conjunctiva, typically appearing within 6 hours after exposure and resolving by itself within one day. Of course, the patient can experience minor ocular discomfort, such as foreign body sensation or a “sandy, gritty” sensation. Unprotected skin that is exposed when the arc is on will easily burn, similar to a sunburn. Therefore, welders should be advised to wear shields that fully cover the face and neck, as well as protective gloves and clothing to cover the body and, especially, hands and arms.
Curing and Measurement Systems
Glues, epoxies, and plastics used in various industries require UV radiation for proper curing (hardening). The typical source is a xenon arc lamp, which has broadband emission throughout the UV, VIS, and IR spectra, emitting UV radiation down to 200 nm. While many curing systems completely filter UV when closed and in use, workers are advised to wear proper eye and skin protection if they need to insert or remove materials when the unit is on. (See Figure 29)
Figure 29. Technician using UV curing system wearing protective eyewear and gloves.
Xenon arc lamps also are used in spectrophotometers that measure transmission and reflection of optical and other materials, and as broadband sources in science labs. Protective precautions similar to those for curing system should be employed.
In recent years, the dental industry has converted to compounds used in making dentures and repairing teeth that can be cured with high intensity blue light. While blue light itself can potentially be dangerous to the eyes, the risk is limited to long-term retinal damage (with the exception of intentional misuse, that is, purposeful direct viewing of the high intensity visible source). There are no documented negative effects of blue light on the skin or the anterior structures of the eye in normal patients.
The excimer laser used in refractive surgery (e.g., PRK and LASIK) operates at a wavelength of 193.3 nm, which is in the UV-C range. The laser can “sculpt” the upper layers of the stroma very precisely by breaking carbon bonds in the collagen fibers, but it cannot penetrate to the deeper stromal layers or any structures inside the eyeball. (See Figure 30)
Figure 30. Patient undergoing LASIK procedure.
By comparison, the energy from an Nd:YAG laser, operating at 1064 nm (in the IR), is readily transmitted by the cornea and absorbed by the iris (when performing iridotomy) and the lens capsule (when performing capsulotomy following opacification secondary to cataract surgery).
Therapeutic lamps that emit UV-A and UV-B radiation are used in the treatment of skin disorders such as psoriasis, vitiligo, and dermatitis. The UV-B radiation is often narrowband, centered at about 311 nm to avoid the erythemal effects of broadband UV-B sources.
In photochemotherapy, a photosensitizing drug or agent is applied topically, ingested, or injected, thereby enhancing the effect of broadband UV-A exposure. In all cases, exposure areas must be strictly controlled to avoid burns and potential cell and tissue damage to otherwise healthy skin. Exposure duration also must be limited to reduce the potential inhibition of the therapeutic benefits.
Lamps with UV-C emission are very effective at killing microorganisms. They are often used in pools, spas, and water purification systems in place of chemical disinfectants, such as chlorine. To avoid exposure to anyone in or near the water, the lamps are completely enclosed within tubing in conjunction with the filtering and circulation system. Lamp efficiency, as determined by hours of usage, and clouding or contamination of the tubing must be carefully monitored to ensure proper operation.
A combination technique, termed “halosol,” can be used to treat very polluted water in areas with adequate sunshine. Large doses of sodium hypochlorite or iodine solution are added to the water. The chemical disinfectants are then decomposed following exposure to solar UV-A radiation for several hours. In this time, solar UV-A and UV-B can contribute to further disinfection of the water.
The Strategic Defense Initiative of the 1980’s (affectionately dubbed “Star Wars”) envisioned using UV lasers to disable and blind pilots and astronauts. However, the laser intensities needed are impractically high (given the many kilometers of distance between the source and the target), the pilot or astronaut would need to purposely look at the beam to be affected, and visors, goggles, and cockpit windows (usually made of special glass or plastic) do not fluoresce when exposed to UV – actually, most optical materials are good UV absorbers, even without the addition of specific coatings or dyes.
Recently, perimeter warning systems have been installed around sensitive military installations and cities around the US, but these usually use green lasers at low intensity to warn a pilot that he is flying too close to the site; the laser is not intended to actually blind the pilot. The pilot can easily avoid the laser by moving his head or flying away from the source, which is the intention of the system.
Other military uses of UV sources include biological agent detection, decontamination of personnel and equipment, water and air purification, efficient light production (similar to standard fluorescent lighting), and non-line-of-sight covert communications. (See Figure 31)
Figure 31. Use of UV in non-line-of-sight communication.
An additional application, yet to be perfected, is as a non-lethal weapon to immobilize a subject using a high-voltage current, similar to the action of a taser. In theory, a UV laser ionizes a trail of air, up to 100 m in length, along which the current travels. Unfortunately, if the intensity is high enough to actually ionize air, the laser itself can be very damaging to anyone standing in its path.
Exposure to UV-C, usually from industrial settings due to welding, will cause an intense burn of the upper layers of exposed skin, and similar damage to the outer layers of the cornea. The cornea can heal itself relatively quickly, within about one day, and the effects of welder’s flash are not long-standing, even after repeated exposures. Patients occasionally complain of mild to moderate photophobia, which will resolve once the cornea heals. Treatment, if any, involves the application of over-the-counter ocular lubricants and cold compresses. Non-preserved lubricants should be used in patients with allergies to preservatives. Prophylactic antibiotics usually are not necessary unless there is evidence of epithelial damage.
Workers in other occupations that use UV curing systems must wear protective eyewear with side-shields and should wear clothing and accessories, including gloves and facemasks, to cover all potentially exposed skin. While the intensities of the curing systems may not be as high as in welding, the exposure duration is potentially longer, and reported signs and symptoms are similar to welder’s flash. Consequently, the treatment regimen is the same.
Corneal and conjunctival damage will occur with exposure to UV from a cracked or broken metal halide lamp, as indicated above. Since many patients who are exposed in this fashion are indoors and unaware of the problem during the exposure period, they will probably not be wearing protective eyewear or clothing. Following exposure, photophobia may be moderate to severe. Burn effects of the skin can be soothed with ointments and salves containing aloe (as when treating a sunburn). Sunburn of the eyelids can have a clinical appearance similar to blepharitis. Photoaging effects of the skin, such as sagging, discoloration, and leatheriness, can be treated with topical lotions and creams containing retinoic acid. In severe cases, the patient should be referred to a dermatologist.
Ocular effects can be treated as for a welder’s flash, including lubricants and cold compresses as needed. If significant corneal damage does occur, a bandage contact lens is preferred to patching. The contact lens also can be used as a time-release delivery system of a broad-spectrum antibiotic. Punctal plugs are recommended if healing is expected to take more than a few days. Temporary collagen plugs will dissolve in 4 days to 2 weeks, depending on the brand and style. Permanent plugs are suggested for longer periods, and should be monitored at least every few weeks for progression of the condition, along with reassessment of any antibiotic and bandage contact lens therapy.
Since many of the symptoms of UV-C (and UV-B) exposure are similar to dry eye syndrome, patients should be counseled to avoid low humidity and excessive air-borne particulates (such as smoke); to wear sunglasses and wide-brimmed visors or hats outdoors; and to supplement their diets with antioxidants and omega-3 fatty acids, such as those found in flax seed (both grain and oil) and fish oil. In low humidity climates, or in excessively heated or air-conditioned homes and offices, patients should be advised to use humidifiers in rooms where they will be spending most of their time.
UV-B has both positive and negative effects for humans. Vitamin D is necessary for proper growth and maintenance of bones. UV-B triggers synthesis of Vitamin D in the skin, and exposure of parts of the face, arms, or legs for 10 to 15 minutes at least twice per week is sufficient for most people. Sunscreen should be avoided for these brief periods, and the exposure duration may need to be extended by as much as twice if there is significant cloud cover, pollution, or low sun angle, such as around the winter solstice. (Vitamin D can be ingested as part of a balanced diet or as a dietary supplement, especially for individuals living in areas where sunlight is significantly reduced for part of the year, such as extreme northern or southern latitudes.)
UV-B also stimulates the production of melanocyte-stimulating hormone (MSH), which is important for weight loss, energy production, and skin tanning. However, unprotected exposure to UV-B for more than 15 minutes on a clear day in the summer can result in the familiar acute effects of sunburn. Peak erythemal activity, regardless of skin type, occurs for radiation with wavelength of 297 nm. (See Figure 32)
Figure 32. Action spectra of UV-A and UV-B.
Longer “brief” exposure durations can lead to damage of the anterior segment of the eye, such as conjunctivitis and/or keratitis, again regardless of skin type. This is especially prevalent under certain environmental conditions, such as snow-skiing at high elevation on a clear day. Treatment and management will depend on the exposure intensity and duration, the type and amount of damage caused, and is identical to that listed above for UV-C exposure.
Chronic exposure (months to years) to UV-B can cause CDK, pinguecula, pterygium, cataract, and skin cancer. For corneal and conjunctival conditions, treatment and management in asymptomatic and low-symptom patients (i.e., mild dry eye complaints only) involve monitoring, counseling patients to minimize their further exposure to UV, and prescribing for the dry eye symptoms.
For cataract, treatment and management include monitoring signs and symptoms, and eventual referral for cataract extraction once visual function is no longer sufficient to meet the patient’s visual requirements. The extracted crystalline lens should be replaced with a plastic intraocular lens (IOL) that contains UV-absorbing dye, especially if the patient will still be spending time outdoors. High wrap sun eyewear, or goggles or spectacles with side-shields in occupational settings, also are recommended for outdoor use.
Certain skin cancers actually develop many years after UV-B exposure that caused sunburn, especially if repeated exposure occurred prior to the age of 18. Any unusual pigmented areas of the skin, especially on the face, neck, and hands (as easily observed by eye care professionals), should be monitored for size, shape, and appearance. Presence of any pigmented areas of irregular shape or recent increase in size should immediately be referred to a dermatologist.
Exposure to UV-A can lead to premature skin aging, commonly seen as skin sagging, deep wrinkling, thickening, and leatheriness. High intensity exposure can result in erythema. Chronic exposure, even at low intensities, can lead to or exacerbate retinal damage, which presents clinically as AMD. Intense acute exposure can cause cataracts in young patients.
Treatment and management of erythema and other short-term skin conditions involve relief of symptoms, as with UV-B exposure described above. Skin areas with pigmentation significantly different than the surrounding skin should be monitored, as also described above.
Treatment and management of AMD include lifestyle changes, such as cessation of smoking, and diet changes. All patients, especially those with family history of AMD, should increase dietary intake of antioxidants such as lutein and zeaxanthin, which commonly are found in leafy green vegetables, such as spinach, kale, and broccoli, and several dietary supplements, such as Ocuvite®. (See Figure 33) Photodynamic therapy with verteporfin (using a laser with wavelength of 689 nm) reduces subfoveal neovascularization in AMD, thereby reducing or even reversing some of the visual effects of AMD.
Many systemic and topical medications have the unwanted and potentially harmful side effect of increased photosensitivity and risk of severe sunburn when the patient is exposed to UV radiation. These include tetracyclines, antihistamines, sulfa drugs, diuretics, hypoglycemic agents, and some oral contraceptives. Patients taking therapeutic dosages of these medications, especially when traveling on vacation or business in unfamiliar locations, should be advised to avoid direct or reflected sunlight, and to wear sufficient skin protection at all times. While ocular side effects have not been demonstrated, patients still should be advised to wear proper eye protection.
Other than the 15- to 30-min weekly exposure to UV-B required for Vitamin D synthesis, any exposure to UV radiation can be harmful, even years after the exposure incident. “Minimal erythemal doses” have been defined as the amount of time necessary to produce skin reddening, but these levels depend greatly on the individual’s skin type. (See Table 3)
In occupational settings, the recommended maximum exposure depends on the EMS zone being considered. For UV-C and UV-B, it is 0.1 uW/cm2 (= 0.001 W/m2); for UV-A, 1 mW/cm2 (10 W/m2); for VIS, 1 cd/cm2 (= 10,000 cd/m2); and for IR, 10 mW/cm2 (= 100 W/m2). The maximum permissible dosage over 8 hours for UV exposure actually varies with wavelength, as shown in Figure 34.
Figure 34. Occupational UV-C and UV-B exposure limits in an 8-hour period. Note that a lower limit is potentially more harmful.
Activity-Specific Spectacles and Goggles
For full protection of the eyelids and conjunctiva, the frame must be large enough to cover that area of the wearer’s face. Ski goggles and safety eyewear often have side-shields to prevent exposure from the side or behind (via reflection from the back surface of the spectacle lens). High wrap contoured frames with highly curved lenses, such as those marketed by Nike and Oakley, have more cosmetic appeal and provide sufficient coverage if the frame is properly selected to match the contour of the wearer’s head.
On the other hand, fashionable low base curve lenses offer little or no protection to the eye or the adnexa from UV radiation incident from the side or behind the wearer, regardless of the UV protection properties of the lens material. (See Figure 35)
Figure 35. Light from behind the wearer reflecting from the back surface of a low-curvature spectacle lens into the wearer’s eye.
Absorption of UV by Ophthalmic Materials
All common optical materials absorb UV-C radiation. This is a not an issue except for the extreme condition of stratospheric ozone depletion. Nonetheless, several manufacturers of over-the-counter sun eyewear claim such protection, which may give the consumer the mistaken impression that the eyewear provides adequate protection in industrial settings where UV-C may be present, such as welding and use of certain curing systems. The practitioner must advise the patient that only filters rated and approved for the specific industrial application are to be used in those situations.
Many clear optical materials block wavelengths to between 380 and 400 nm, either because of UV-absorbing tints or dyes incorporated into the material, or coatings applied to the lens surfaces. These materials include CR-39, polycarbonate, and many proprietary plastics and glass. Lenses intended as sun eyewear incorporate additional tints, dyes, and coatings, but filter VIS as well as UV. Most non-prescription sun eyewear from high-end manufacturers, such as Nike, Oakley, Bollé, and Maui Jim absorb or reflect wavelengths below 400 nm. Even moderately priced eyewear, such as RayBan and Walgreen’s brand, provide adequate ocular protection from UV. (See Figure 36)
Figure 36. Transmittance spectra for various lens materials. PC, polycarbonate; SR, scratch-resistant coating only; AR, anti-reflective coating; PG, PhotoGray.
However, certain optical materials should be avoided for general use. For example, acrylic is often used as a lightweight safety faceshield and splashguard, and as demo lenses in sample frames. Unfortunately, it is also used in many toy and inexpensive or knock-off (counterfeit) sunglasses. Patients should be warned to purchase sun eyewear only from reputable vendors.
Photochromic Lenses
Photochromic lens materials darken when exposed to UV radiation, and are recommended for prescription spectacles when the patient may be exposed to varying light conditions, or when it may be difficult or inconvenient for the patient to switch between clear and sun eyewear. Glass photochromics, such as Photogray, contain silver halide molecules that separate when they absorb UV radiation. Plastic photochromics, such as Transitions, contain a high molecular weight organic molecule. The molecule changes shape when a chemical bond within it is broken after UV radiation is absorbed. For both types of materials, the molecular process is reversed when the lens is no longer exposed to UV.
The current generation of full-spectrum photochromics transmit as much as 88% of VIS in the lightened state, and down to about 20% or less of VIS in the darkened state. Heat inhibits UV absorption by the photochromic molecules, so that the lenses do not completely darken in a very warm environment. Nonetheless, additional absorbing dyes in the lenses sufficiently protect the eyes in any lightened or darkened state. Anti-reflective (AR) coating, which is beneficial in reducing reflections and glare and improving cosmesis in any “clear” lens, will reduce the photochromic effect by only about 5%. A cosmetic “flash” mirror coating will produce a similar reduction in the photochromic effect.
Polarizing Lenses
Polarizing filters selectively block light and UV that are not polarized with the same orientation as the filter. For example, sunlight reflecting from a flat road surface or body of water will be horizontally polarized. Lenses with vertical polarization block that reflected light, thereby reducing glare and increasing visibility. Likewise, reflected UV radiation also will be polarized and blocked by the filter. Unfortunately, sunlight reflecting from a vertical or sloped surface is at least partially vertically polarized, and readily will be transmitted by the polarizing filters. Consequently, polarizing filters should be avoided for patients who ski or snowboard.
Tints and Dyes
Other tints and dyes block UV radiation and a significant portion of short-wavelength VIS. Such lens treatments include melanin, “blue-blockers,” and the photochromic Corning CPF series. As shooters and skiers know, blocking blue light reduces glare and provides a perceptual contrast enhancement. (See Figure 37)
Figure 37. Biathlon participant wearing yellow tinted lenses.
Because of their yellow, amber, or orange appearance (depending on tint density), the lenses also tend to alter color perception. Patients should be warned of this consequence if they drive (especially if they have a color deficiency) or intend to perform any tasks where color vision is critical.
Contact Lenses
Certain outdoor sports activities are not conducive to wearing spectacles. Likewise, a patient who wears contact lenses may need to remove sun eyewear if it is too dark in a given situation. Several manufacturers offer both rigid gas permeable and soft contact lenses that are clear and include UV absorbers to protect the internal structures of the eye.
Recently, Nike and Bausch & Lomb have released soft contact lenses with sports enhancing tints, marketed as MaxSight. Unlike cosmetic tints, the amber and grey-green tints extend across the entire lens, and virtually all wavelengths below 500 nm (including UV) are blocked. (See Figure 38)
Figure 38. Left: Baseball player wearing Nike/Bausch & Lomb MaxSight amber lens. Right: Spectral transmittance curve of MaxSight amber lens.
Intraocular Lenses (IOLs)
Patients who undergo cataract surgery can now have a UV absorber included in the implanted IOL. Depending on the lens manufacturer, a blue-blocking absorber can also be included, resulting in a yellowish lens appearance and ocular light filtration similar to that prior to the surgery. Without the UV absorber, the retina of the pseudophakic eye potentially can be exposed to wavelengths as low as 290 nm, just like an aphakic eye. Clear or tinted spectacles with adequate UV protection, along with hats, caps, or visors with wide brims, should be worn outdoors during daytime hours.
Autodarkening Welding Goggles
To avoid “welder’s flash” when manually lowering a filter cannot be done quickly enough to avoid UV exposure, autodarkening filters have been developed. These are battery operated, often with multiple sensors to detect a change in light level. The optical material increases in opacity (shade number) when an electric current is applied across it. The speed at which the shade changes depends on the amount of change, as well as the starting and ending shades. For small changes in shade number, it must take no longer than 1 sec; for changes that switch from a light shade to a very dark shade, it should take less than 1 ms. The switching limits are specifically defined in occupational eyewear standards, such as ANSI Z87.1-2003.
Sunscreen Lotions
Many sunscreens and sun lotions are marketed to protect the consumer from excessive environmental UV exposure. They may be opaque, such as zinc oxide, or greasy to reflect and/or absorb radiation. Sunscreens are rated in terms of the sun protection factor (SPF), which is calculated by comparing the amount of time needed to produce a sunburn when the skin is protected versus when it is unprotected. For example, if an unprotected light-skinned individual normally turns red in 15 min, then a sunscreen with SPF 4 would allow that person to be exposed for 60 min before redness occurs, and a sunscreen with SPF 8 would allow exposure for 2 hours before the skin turns red. For an individual with darker skin, who would turn red in 30 min without protection, the time to redness would increase to 2 hours using SPF 4 and 4 hours using SPF 8.
Many people are under the mistaken impression that a suntan will protect them from burning. In fact, a suntan has an equivalent SPF of 2 to 3, which would offer minimal protection even for a dark-skinned individual. Most people will receive sufficient protection with a sunscreen with SPF 15, especially if it is reapplied on a regular basis with continued exposure and following swimming or exercising. However, persons with skin type I or II should consider using sunscreens with SPF 45 or higher.
Note that SPF applies only to UV-B radiation; there is no comparable rating for UV-A. Many sunscreens offer broad-spectrum, but often incomplete, protection from UV-A. Sunscreens containing mineral oil or metallic compounds, such as zinc oxide and titanium oxide, provide the best complete UV-A (and UV-B) protection. Patented formulations marketed by L’Oreal, under the names Mexoryl SX and Mexoryl XL, claim full UV-A and UV-B protection with SPF up to 60. These products have been sold successfully in Europe and Japan, but have not yet been approved by the Food and Drug Administration for sale in the US.
To protect the skin of the eyelids and upper cheeks, apply a waterproof sunscreen that does not run and is specifically recommended for use near the eyes, such as DDF-Titanium & Zinc Sunblock. Even if it does get in the eye, it is only a minor irritant with no long-term effects; simply flush the eye sufficiently with water. There is no truth or basis to the urban myth that has circulated on the Internet in recent years claiming that getting waterproof sunscreen in the eye causes blindness.
Clothing and Hats
Clothing is rated by the UV protection factor (UPF), which is the percentage of UV radiation that the garment transmits. For example, fabric with UPF 20 allows one-twentieth to pass, while fabric with UPF 50 allows only one-fiftieth to pass, thus blocking 95% and 98% of incident UV radiation, respectively. Fabrics that usually offer better UV protection are loose fitting, tightly woven or knitted, darker in color, heavier in weight, and not stretched, wet, or worn out.
A hat or visor with at least a 10-cm (4-inch) brim is also recommended. To adequately block UV radiation incident from the side or below the eyes, spectacles should have side-shields. Alternately, and in many cases more cosmetically acceptable, spectacles should have a small vertex distance and high wrap, fitting close to the shape of the head. In addition, lens materials must have tints, dyes, or coatings that completely absorb and/or reflect all UV radiation.
UV radiation has some beneficial but many harmful effects on the body and the eyes. Under most circumstances, most individuals are exposed to adequate dosages of UV to trigger the beneficial effects simply by walking outdoors – they must make no special effort to receive the benefit, such as Vitamin D production by the skin.
Alternately, everyone should be mindful of reducing exposure to UV, even on days that are not completely bright and clear. Chronic exposure to environmental levels of UV may cause skin-aging effects, hasten the onset of cataracts, and exacerbate retinal conditions, such as AMD. Acute exposure to high levels of UV, as in certain industrial settings and at the top of a mountain when skiing, may cause conjunctivitis, keratitis, and photophobia.
Recommendations for maximum and complete protection from UV include the use of:
What is Energy?
Energy is defined as the ability to do work. Energy comes in many different forms, such as mechanical (or kinetic), chemical, electrical, magnetic, or radiant. These forms of energy are interchangeable. For example, the chemical energy stored within a battery is released as electrical energy, which in turn is used to move the hands of a clock. Likewise, electrical energy is converted to chemical energy as the battery of your laptop computer recharges. The common unit of energy is the joule (J).
Energy that is radiant in nature comes from changes in the excitation state of atomic or sub-atomic particles. Radiation can result from the flow of electrons (i.e., electricity), the presence of a magnetic field, or a chemical reaction. Conversely, radiant energy can induce electrical flow or a chemical change. Radiation has important consequences for biological responses, producing phenomena such as vision, synthesis of molecules within tissues, and changes in cell and tissue structure.
Light and Radiation
The visible spectrum (VIS), also known as “light,” is that portion of the EMS to which the photoreceptors of the human eye – rods and cones – are sensitive. The VIS includes wavelengths between about 380 nm and 760 nm. Other species have different sensitivity spectra; for example, most insects and birds can see UV radiation.
The perceptual phenomenon of color arises from the fact that the normal human eye has three different types of cones that each have different sensitivity to any particular wavelength. Light near the short-wavelength end of VIS (380 nm) is perceived as “violet,” while light near the long-wavelength end (760 nm) is perceived as “red,” with the remainder of the color spectrum – blue, green, yellow, and orange – in between.
All other portions of the EMS are properly referred to as “radiation,” rather than light, because human photoreceptors are not sensitive to those energies. Infrared (IR) radiation, with wavelengths longer than red light (wavelength greater than 760 nm), is commonly referred to as “heat.” Environmental IR has virtually no effect on the tissues of the eye; retinal temperature would need to increase by at least 10 degrees C to have any harmful effect. Similarly, radiation of longer wavelengths (microwaves and radio waves) at environmental levels has no skin or ocular effects.
On the other hand, UV radiation, with wavelengths shorter than violet light (wavelength less than 380 nm), may have serious short- and long-term effects on biological tissues.
To Give and To Get: Sources and Receivers of Energy
A source emits energy through a medium, and a receiver within the medium has energy incident upon it. The source is said to have radiance, while the receiver is irradiated. If the energy is visible to the human eye as light, the source has luminance, while the receiver is illuminated.
Power is the rate at which energy is expended over time, measured in watts (W), where 1 W = 1 J/s. If the energy is visible to the human eye, power is measured in lumens (lm). The intensity of a visible source is measured in lumens per solid angle, or candelas (cd), and its luminance is given by intensity per area of the source, in cd/m2. Luminance is perceived as brightness by a human observer. For comfortable viewing, a source should provide 350 to 2000 cd/m2. On a bright, hazy day or a day with uniform high clouds, sunlight can have luminance as high as 70,000 cd/m2. On a heavy overcast day, luminance may not exceed 30,000 cd/m2. Objects that reflect sunlight, such as streets, sidewalks, grass, sand, and trees, often reflect less than 10% of the incident light, such that they often can be viewed comfortably by most individuals with the naked eye or a medium sunglass tint.
The receiver is the interface between the medium of the source and the medium behind the receiver, such as the surface of a lens separating air from plastic. In most cases, multiple receivers in several media potentially act on the emitted energy. For example, sunlight passes through the front and back surfaces of a car windshield (air-to-glass-to-air), and then through the front and back surfaces of a spectacle lens (air-to-plastic-to-air), before it enters the eye of the viewer. Within the viewer’s eye, the light passes from the tears to the cornea to the aqueous to the crystalline lens to the vitreous before it is potentially captured by the photoreceptors in the retina.
Exposure to radiant energy, or irradiance, is expressed in terms of power over area of the receiver, such as W/m2 or J/s/m2 (which may also be written as J/s-m2). For energy visible to the human eye, illuminance is given by lumens per area of the receiver, in lm/m2 (lux) or lm/ft2 (foot-candle, or fc), where 1 fc = 10.76 lux. For most office and classroom activities, illuminance of the viewed object (e.g., patient chart or reading assignment) should be between 200 and 1000 lux (about 20 to100 fc), depending on the viewer’s age, target (e.g., letter) size and contrast, and critical nature of the task (i.e., consequence of making mistakes). Simple orientation and mobility and non-critical tasks can be accomplished with illuminance as low as 20 lux (2 fc). At the opposite extreme, tasks with low contrast and/or very small targets, for prolonged durations, or of a very precise and exacting nature (e.g., microsurgery), especially by older individuals, may require specific lighting with illuminance as high as 20,000 lux.
Dual Nature of Radiation: Waves vs. Particles
Energy in the EMS can have characteristics of either a wave or a particle, depending on the phenomenon being considered. For example, an anti-reflective (AR) coating on a spectacle lens produces destructive interference of reflected light waves, thus increasing lens transmittance. On the other hand, a solar cell produces electricity because particles of light (photons) dislodge electrons from the atoms within the panel.
The wave properties of radiation are expressed in terms of wavelength and frequency, which describe the variation of the energy over space and time, respectively. Wavelength is the distance between corresponding points, such as the peaks or troughs, of successive waves. Wavelength, specified by the Greek letter lambda, is measured in distance units, typically nanometers (1 nm = 10-9 m) for energy in and near the visible spectrum. Frequency is given by the number of waves (cycles) that pass a given location in a period of time. Frequency, specified by the Greek letter nu, is measured in units of inverse time, such as 1/s (= s-1), which is also referred to as cycles per second or Hertz (Hz). For radiant energy, the product of wavelength and frequency is a constant, given by the equation
Speed = wavelength * frequency
where c is measured in units of speed. For electromagnetic radiation, c in a vacuum (or air) is the familiar “speed of light,” approximately 3 x 108 m/s (about 186,000 mi/s).
The particle properties of radiation are expressed in terms of the amount of energy, E, of a photon, given by either equation
where frequency (nu), speed (c), and wavelength (lambda) are described above and h is Planck’s constant, 6.63 x10-34 J-s. The energy of a photon is measured in electron-volts (eV), where 1 eV = 1.602 x 10-19 J.
Ions, Radicals, and Ultrasound
Energies with very short wavelengths, such as x-rays, gamma rays, and most UV, are referred to as “ionizing radiation,” from their ability to strip electrons from atoms, thus creating positively charged particles known as ions. These energies are also responsible for creating radicals, which are uncharged atoms with at least one unpaired excited electron, resulting in an asymmetric distribution of charge within the atom. Both ions and radicals potentially can bind to, and destructure or destabilize, neutral atoms and molecules, resulting in significant chemical and structural changes. Compounds known as antioxidants, such as Vitamin C and lutein, can selectively remove radicals from the blood and tissues of the body.
Energies with relatively longer wavelengths, that is, visible and above, are referred to as “non-ionizing radiation,” because they do not cause changes at the atomic level. Nonetheless, they can cause changes in molecular structures, and their effects can be very significant and profound. Even energies at very long wavelengths can affect biological tissues if the energy is of sufficient intensity and concentration. For example, conductive keratoplasty, used to surgically correct hyperopia and presbyopia, alters corneal curvature by direct application of radio waves (nu = 350 kHz, lambda = 1.167 mm) via a small probe inserted into the upper stroma. The radio waves have a thermal effect (see Absorption, Transmission, and Reflection below) on the stromal collagen, such that they produce cinching of the fibers that leads to a change in curvature.
By contrast, ultrasonography uses mechanical energy to image structures below the skin surface and within the eyeball, or to break apart molecules, as in phacoemulsification of the crystalline lens during cataract surgery. High intensity ultrasound, especially at longer exposure durations, will produce corneal edema and compromise of the corneal endothelium. However, external application of ultrasound at no more than 0.25 W/cm2 for less than 5 min, or 0.0337 W/cm2 for longer periods, will not produce any harmful or irreversible ocular effects. Standard clinical A- and B-scan ultrasound instruments fall well within these limits.
Absorption, Transmission, and Reflection
Depending on the characteristics of the energy (such as wavelength and intensity) and of the receiver (such as surface curvature and transparency), one or more of the following actions can occur at the receiver:
After reflection, the receiver may become a (secondary) source. For example, if a light bulb illuminates a page of text, the paper is a source of light for the viewer. For vision to occur, the viewer’s retina must be the final receiver. Objects that appear colored, or that appear bright or dark when illuminated with a particular type of light, selectively reflect parts of the VIS to the viewer. Energy that is not reflected will be absorbed and/or transmitted.
For non-ionizing radiation, energy that is absorbed by the receiver may be altered in any of the following ways:
Energy that is transmitted by a receiver will not affect the receiver, but the energy nonetheless may be altered by the receiver. For example, refraction occurs when light that is incident at an angle to a surface changes direction as it passes through the surface. Also, the speed of light decreases as it passes from a rarer medium, such as air or vacuum, into a denser medium, such as plastic or water. The ratio of the speed in a vacuum to the speed in the denser medium is the index of refraction of the denser medium. (Note that air at 20 degrees C [68 degrees F] and one atmosphere of pressure [sea level] has a refractive index of 1.0003. Thus, measurements made in air, rather than in a vacuum, introduce very little error for most applications.) Index of refraction depends directly on the wavelength of light that is used to measure it.
Common optical materials have a higher refractive index for shorter wavelengths than for longer wavelengths; thus arises the memory aid, “blue bends best.” The refractive index is higher still for UV compared to VIS, resulting in greater reflection and refraction than predicted by the refractive index as determined using a visible wavelength. (See Figure A)
Figure A. Wavelength versus index of refraction relationships for various materials.
American National Standard for Ophthalmics – Nonprescription Sunglasses and Fashion Eyewear – Requirements, ANSI Z80.3-2001, American National Standards Institute.
American National Standard – Occupational and Educational Personal Eye and Face Protection Devices, ANSI Z87.1-2003, American National Standards Institute.
Australian/New Zealand Standard, Sunglasses and Fashion Spectacles, AS/NZS 1067:2003, Standards Australia.
European Standard, Personal Eye Protection – Sunglasses and Sunglare Filters for General Use, EN 1836:1997, European Committee for Standardization.
Fannin TE and Grosvenor T. Chap. 7: Absorptive Lenses and Lens Coatings. In Clinical Optics, Second Ed. Boston: Butterworth-Heinemann, 1996.
Pitts DG. Chap. 4: The Electromagnetic Spectrum. In Environmental Vision: Interactions of the Eye, Vision, and the Environment, Pitts DG and Kleinstein RN, eds. Boston: Butterworth-Heinemann, 1993.
Pitts DG. Chap. 6: Ocular Effects of Radiant Energy. In Environmental Vision: Interactions of the Eye, Vision, and the Environment, Pitts DG and Kleinstein RN, eds. Boston: Butterworth-Heinemann, 1993.
Pitts DG. Chap. 9: Principles in Ocular Protection. In Environmental Vision: Interactions of the Eye, Vision, and the Environment, Pitts DG and Kleinstein RN, eds. Boston: Butterworth-Heinemann, 1993.
Javitt JC and Taylor HR. Chap. 55: Ocular Protection from Solar Radiation. In Duane’s Clinical Ophthalmology, Volume 5. Tasman W, ed. Philadelphia: Lippincott Williams & Wilkins, 1991.
Disclosure and Conflict of Interest Statement
In addition to his adacemic duties at Pacific University, Dr. Citek serves, or has served, as a consultant to various companies, including, Nike, Bausch & Lomb, Essilor, Paragon Vision Sciences, and SOLA. He is not receiving compensation from any company for this course, nor is he receiving any benefit, direct or indirect, from the mention or sale of any product described in this course.
Contact this author:
Karl Citek, OD, PhD, FAAO Pacific University College of Optometry 2043 College Way, Forest Grove OR 97116Pacific University College of Optometry provides On-Line CE as a service to optometrists. The college does not endorse or recommend any products, equipment, or services that might be discussed in the courses. Courses are prepared by individuals believed to be experts in their areas of specialization who are compensated for their efforts. The College relies on their expertise to produce accurate and timely courses. Questions or concerns about courses should be directed to the individual authors and/or the Continuing Education Department at the College of Optometry at kundart@pacificu.edu.
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