Ultraviolet and Near-Blue Light Effects on the Eye

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IntroductionElectromagnetic radiation from the sun continuously bombards the Earth's atmosphere. Gamma and x-rays make up the higher frequencies of this spectrum and are absorbed by the atmosphere. Likewise, most wavelengths classified as ultraviolet radiation (UVR) are also absorbed, except where there are areas of atmospheric depletion known as ozone layer holes. The ozone layer absorbs wavelengths up to 286 nanometers (nm), thus shielding living beings from exposure to radiation with the highest energy. However, we are exposed to wavelengths above 286 nanometers, most of which falls within the visual spectrum. The shorter wavelengths pose the greatest hazard because they inversely contain more energy. Exposure to these wavelengths has been called the blue-light hazard because these wavelengths appear blue to the human eye.1Sources of Light RadiationNot all light radiation is solar in origin. We are exposed to ambient light throughout the day and night from many different sources. These, too, have the potential to cause damage to the eye. Using a spectroradiometer, Okuno et al measured the effective radiance of various sources of light known to cause distinct eye problems.1 Radiance is a measurement for the number of watts of power applied to a specific surface area. Shielded metal arc welding can cause photokeratitis and led the top of the list for most radiance in the workplace. Also included was the interior of a glass furnace. Interestingly, it was found that halogen lamps produce more effective radiance than conventional incandescent lamps.Many sources of light used daily in ophthalmic practice have also been documented to produce phototoxicity. Michels and Sternberg have previously reviewed the pathophysiology and guidelines for prevention of light toxicity from the operating microscope.2 Other sources of potential hazardous exposure include the indirect ophthalmoscope and slit lamp biomicroscope.3,4 A rhesus monkey model was used to prove these hazards when the lighting filament was directly imaged on the retina.5 However, the situation is different in the human eye, where the indirect light source is constantly being shifted to different locations on the retina.6 Although the potential for damage exists, it is practically only theoretical.Natural Protective MechanismsAlthough the eye makes up only 2% of the body's surface, various natural structures are in place to protect its integrity and functional capabilities. These include the surrounding ocular adnexa such as the eyebrows, orbital rim, eyelashes, eyelids, and blink responses that help to decrease the amount of light entering the eye from straight on, above and below.7 The pupillary response also plays a role in decreasing the amount of light energy entering the eye. The cornea and lens further protect the retina from light radiation exposure. The cornea is able to absorb wavelengths up to 300 nm, whereas the natural crystalline lens absorbs light wavelengths between 300 and 400 nm. Keates et al8 found between 91.2% and 94.4% transmittance of light of wavelengths between 300 and 400 nm through the lens capsule with a positive correlation between age and greater transmittance. These findings underscore the fact that the major protective barrier to near-UVR (between 300 nm and 400 nm) is provided by the crystalline lens, not by the lens capsule. There is an accumulation of insoluble lens proteins in the brunescent nuclear cataract believed to be due to photo-oxidation by ultraviolet light. It has been theorized that this yellow filter further protects the retina against blue light in the visible spectrum. Replacement of this natural filter to high-energy wavelengths with an implant potentially allows damaging near-UVR and blue light to reach the retina, depending on the spectral transmittance of the intraocular lens.

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